City of Azle, TX Part 14.3 Integrated Design Criteria

(Ordinance 2012-10 adopted 8/21/12)

This chapter gives details on criteria to meet the three focus areas of water quality, streambank protection and flood mitigation, as well as information supportive of hydrology and stormwater conveyance.

§ 14.3.1 Hydrologic Methods.

14.3.1.1 Types of Hydrologic Methods

There are a number of empirical hydrologic methods available to estimate runoff characteristics for a site or drainage sub-basin. However, the following methods have been selected to support hydrologic site analysis for the design methods and procedures included in this manual:

*	Rational Method
*	SCS Unit Hydrograph Method
*	Snyder’s Unit Hydrograph Method
*	USGS & TXDOT Regression Equations
*	iSWM Water Quality Protection Volume Calculation
*	Water Balance Calculations

Table 14.3.1 lists the hydrologic methods and the circumstances for their use in various analysis and design applications. Table 3.2 provides some limitations on the use of several methods.

In general:

*

The Rational Method is acceptable for small, highly impervious drainage areas, such as parking lots and roadways draining into inlets and gutters.

*

The U.S. Geological Survey (USGS) and Texas Department of Transportation (TXDOT) regression equations are acceptable for drainage areas with characteristics within the ranges given for the equations shown in Table 3.2. These equations should not be used when there are significant storage areas within the drainage basin or where other drainage characteristics indicate general regression equations are not appropriate.

Local Provisions: NONE

Table 14.3.1. Applications of the Recommended Hydrologic Methods
Method	Rational Method	SCS Method	Modified Rational	Snyder’s Unit Hydrograph	USGS/TXDOT Equations	ISWM Water Quality Volume Calculation
Water Quality Protection Volume (WQv)						*
Streambank Protection Volume (SPv)		*		*
Flood Mitigation Discharge (Qv)		*		*	*
Storage Facilities		*	*	*
Outlet Structures		*		*
Gutter Flow and Inlets	*
Storm Drain Pipes	*	*		*
Culverts	*	*		*	*
Bridges		*		*
Small Ditches	*	*		*
Open Channels		*		*	*
Energy Dissipation		*		*

Table 14.3.2. Constraints on Using Recommended Hydrologic Methods
Method	Size Limitations1	Comments
Rational	0 - 100 acres	Method can be used for estimating peak flows and the design of small site or subdivision storm sewer systems.
Modified Rational2	0 - 200 acres	Method can be used for estimating runoff volumes for storage design.
Unit Hydrograph (SCS)3	Any Size	Method can be used for estimating peak flows and hydrographs for all design applications.
Unit Hydrograph (Snyder’s)4	1 acre and larger	Method can be used for estimating peak flows and hydrographs for all design applications.
TXDOT Regression Equations	10 to 100 mi2	Method can be used for estimating peak flows for rural design applications.
USGS Regression Equations	3 - 40 mi2	Method can be used for estimating peak flows for urban design applications.
iSWM Water Quality Protection Volume Calculation	Limits set for each Structural Control	Method can be used for calculating the Water Quality Protection Volume (WQv).

1	Size limitation refers to the drainage basin for the stormwater management facility (e.g., culvert, inlet).
2	Where the Modified Rational Method is used for conceptualizing, the engineer is cautioned that the method could underestimate the storage volume.
3	This refers to SCS routing methodology included in many readily available programs (such as HEC-HMS or HEC-1) that utilize this methodology.
4	This refers to the Snyder’s methodology included in many readily available programs (such as HEC- HMS or HEC-1) that utilize this methodology.

Local Provisions
Table 14.3.2A. City of Azle Constraints on Using Recommended Hydrologic Methods
Method	Size Limitations1	Comments
Rational	0 - 200 acres	Method for estimating peak flows and the design of small site or subdivision storm sewer systems.
Modified Rational	0 - 25 acres	Method can be used for detention planning and conceptual design.
Unit Hydrograph (SCS)3	Any Size	Method can be used for estimating peak flows and hydrographs for all design applications.
Unit Hydrograph4 (Snyder’s)	100 acres and larger	Method can be used for estimating peak flows and hydrographs for all design applications.
TXDOT Regression Equations	10 to 100 mi2	Method can be used for estimating peak flows for rural design applications.
USGS Regression Equations	3 - 40 mi2	Method can be used for estimating peak flows for rural design applications.
* City of Azle requires that the “C” coefficients presented in Table 14.3.2A be used in the Modified Rational Method.
* Rainfall distribution for the SCS Unit Hydrograph shall be based on the Frequency Rainfall Data provided in Section 5.0 of the Hydrology Technical Manual centered at the midpoint of the rainstorm (12th hour of a 24-hour storm) unless otherwise approved by the Storm Water Manager.
* Figure 14.5.1 in Section 14.5.0 presents a sample computation sheet for the presentation of unit hydrograph method results. This form should be completed even if the computations are performed on an acceptable computer programs HEC-1 or HEC-HMS.
* An alternative method to determine Snyder’s Lag is to determine the time of concentration (travel time) by the methodology described in Section 1.4 of the Hydrology Technical Manual and multiply this time of concentration by 0.6.
* The TxDOT and USGS Regression methods should only be used for comparison of the reasonableness of other approved determinations, not for final results or design unless specifically approved by Storm Water Manager.
* iSWM Water Quality Protection Volume (WQv) calculation method is not currently required by City of Azle.
* Fully Developed Conditions - For watershed hydrology, fully developed conditions include:
* All existing developed areas shall reflect current land use or current zoning, whichever yields the greatest runoff.
* All existing undeveloped areas shall reflect anticipated future land use designated by zoning classification, by the City’s Comprehensive Plan, or by an approved concept plan.
* If the anticipated future development is unknown, a minimum weighted runoff coefficient of 0.75 shall be used.
* Table 3.2B presents the Rational Formula Runoff “C” Coefficients for the City of Azle. The basis of these coefficients is the standard zoning classification used by the City (“A-43”, “A-21”, etc.) An example of the determination of these coefficients is presented in Figure 14.3.1A.

Table 14.3.2B. Runoff Coefficients
Description of Land Use	% Impervious	Runoff Coefficient “C”
Residential one-acre lots (1) (2)	35	0.51
Residential " half-acre lots	37	0.52
Residential 10,000 SF lots	49	0.59
Residential " 7,500 SF Lots	55	0.59
Residential " 5,000 SF Lots	61	0.63
Residential " < 5,000 SF Lots	0.65	0.67
Multifamily	> 64	0.69
	≥ 79	0.77
	≥ 93	0.86
Commercial/Industrial/House of Worship/School
20% Open Space (Site Plan required)	80	0.78
Parks, Cemeteries	7	0.34
Railroad Yard Areas	29	0.47
Streets: Asphalt, Concrete and Brick	100	0.90
Drives, Walks, and Roofs	100	0.90
Gravel Areas	43	0.56
Unimproved Areas	0	0.30

Assumptions:
(1)	For Residential Calculations:
	- Current CITY OF AZLE development standards for minimum lot size and maximum lot coverage (structure) for each classification
	- Assumed 10.5' Parkway and 18' driveway
	- Assumed 29' B-B street dimension
	- Calculated by applying 90% runoff from impervious areas and 30% runoff from pervious areas
(2)	Calculated from designated setbacks

14.3.1.2 Rainfall Estimation

Rainfall intensities are provided in Section 5.0 of the Hydrology Technical Manual for the nine (9) counties within the North Central Texas Council of Governments. The intensities are based on a combination of data from Hydro-35 and USGS. These intensities shall be used for all hydrologic analysis within the applicable county.

Local Provisions: NONE

(Ordinance 2012-10 adopted 8/21/12)

§ 14.3.2 Water Quality Protection.

14.3.2.1 Introduction

iSWM requires the use of integrated Site Design Practices as the primary means to protect the water quality of our streams, lakes and rivers from the negative impacts of stormwater runoff from development. The integrated Site Design Practices shall be designed as part of the iSWM plans. In addition to the integrated Site Design Practices, required water quality protection can be achieved by two additional options: (1) by treating the water quality protection volume and (2) assisting with off-site pollution prevention activities. These three approaches are described below.

Local Provisions: The City of Azle has currently opted to implement the streambank protection and flood control goals, and water quality protection components. The City of Azle encourages land developers to consider the use of stormwater controls within new developments that benefit not only flood control and streambank protection, but also water quality protection.

14.3.2.2 Option 1: Integrated Site Design Practices and Credits

The integrated Site Design Practices are methods of development that reduce the “environmental footprint” of a site. They feature conservation of natural features, reduced imperviousness, and the use of the natural drainage system. In this option, points are awarded for the use of different Site Design Practices. A minimum number of points are needed to meet the iSWM requirements for Water Quality. Additional points can be gained to qualify for development incentives.

List of integrated Site Design Practices and Techniques

Twenty integrated Site Design Practices are grouped into four categories listed below. Not all practices are applicable to every site.

*	Conservation of Natural Features and Resources

1. Preserve Undisturbed Natural Areas

2. Preserve Riparian Buffers

3. Avoid Floodplains

4. Avoid Steep Slopes

5. Minimize Silting on Porous or Erodible Soils

*	Lower Impact Site Design Techniques

6. Fit Design to the Terrain

7. Locate Development in Less Sensitive Areas

8. Reduce Limits of Clearing and Grading

9. Utilize Open Space Development

10. Consider Creative Designs

*	Reduction of Impervious Cover

11. Reduce Roadway Lengths and Widths

12. Reduce Building Footprints

13. Reduce the Parking Footprint

14. Reduce Setbacks and Frontages

15. Use Fewer or Alternative Cul-de-Sacs

16. Create Parking Lot Stormwater “Islands”

*	Utilization of Natural Features for Stormwater Management

17. Use Buffers and Undisturbed Areas

18. Use Natural Drainageways Instead of Storm Sewers

19. Use Vegetated Swale Instead of Curb and Gutter

20. Drain Rooftop Runoff to Pervious Areas

More detail on each site design practice is provided in the integrated Site Design Practice Summary Sheets in Section 2.2 of the Planning Technical Manual.

Local Provisions: NONE

Integration of Site Design Practices into Site Development Process

During the site planning process described in Section 14.2.0, there are several steps involved in site layout and design, each more clearly defining the location and function of the various components of the stormwater management system. To be most effective and easier to incorporate, integrated Site Design Practices should be part of this overall development process as outlined in Table 14.3.3.

Table 14.3.3. Integration of Site Design Practices with Site Development Process
Site Development Phase	Site Design Practice Activity
Site Analysis	* Identify and delineate natural feature conservation areas (natural areas and stream buffers) * Perform site reconnaissance to identify potential areas for and types of credits * Determine stormwater management requirements
Conceptual Plan	* Preserve natural areas and stream buffers during site layout * Reduce impervious surface area through various techniques * Identify locations for use of vegetated channels and groundwater recharge * Look for areas to disconnect impervious surfaces * Document the use of site design practices
Preliminary and Final Plan	* Perform layout and design of credit areas - integrating them into treatment trains * Ensure integrated Focus Areas are satisfied * Ensure appropriate documentation of site design credits according to local requirements
Construction	* Ensure protection of key areas * Ensure correct final construction of areas needed for credits * Inspect and maintain implementation of BMPs during construction
Final Inspection	* Develop maintenance requirements and documents * Ensure long term protection and maintenance * Ensure credit areas are identified on final plan and plat if applicable

Point System
All sites that meet iSWM applicability must provide on-site enhanced water quality protection. Under the integrated Site Design Practice option, sites that accumulate a minimum number of points by incorporating integrated Site Design Practices are considered to have provided enhanced water quality protection.
The point system is made up of three components:
1.	The initial percentage of the site that has been previously disturbed sets the minimum requirement. This is shown in the left-hand column of Table 14.3.4.
2.	A minimum required total of Water Quality Protection (WQP) points is needed to meet the basic water quality criteria. This minimum is shown in the center column of Table 14.3.4.
3.	Optional additional points can be accumulated through additional use of Site Design Practices to be eligible for developer incentives. Each developer incentive attained requires ten (10) additional Site Design Practice points above the minimum required points as shown in the right-hand column of Table 14.3.4.
As shown in Table 14.3.4, the initial percentage of site disturbance sets the minimum required points necessary to meet Water Quality Protection criteria. If a developer wishes to go beyond this minimum then the number of additional points required to attain specific development incentives is also given.

Table 14.3.4. Integrated Site Design Point Requirements
Percentage of Site (by Area) with Natural Features Prior to Proposed Development	Minimum Required Points for Water Quality Protection (WQP)	Additional Points Above WQP for Development Incentives
> 50%	50	10 points each
20 - 50%	30	10 points each
< 20%	20	10 points each

The minimum number of points required to achieve WQP, as shown in the center column of Table 14.3.4, depends on the proportion of undisturbed natural features that exist on the site before it is developed. It is assumed that disturbing a site that has little previously disturbed area will cause more relative environmental impact than a site that has already incurred significant site disturbance. Therefore, disturbing a “pristine” site carries a higher restoration/preservation requirement.
For the purpose of this evaluation, undisturbed natural features are areas with one or more of the following characteristics:
*	Unfilled floodplain
*	Stand of trees, forests
*	Established vegetation
*	Steep sloped terrain
*	Creeks, gullies, and other natural stormwater features
*	Wetland areas and ponds
The number of points credited for the use of integrated Site Design Practices is shown in Table 14.3.5. To determine the qualifying points for a site, the developer must reference Table 3.5 and follow the guidance for each practice in the Planning Technical Manual.
Using the area of the site that is eligible for a practice as a basis, points are given for the percent of that area to which the integrated Site Design Practice is applied. For example, if a planned site has four (4) acres of riparian buffer and the developer proposes to preserve two (2) acres, then the site would qualify for 50 percent of the 8 credit points for iSWM Site Design Practice 2 (Preserve Riparian Buffers), because 50 percent of the site design practice was incorporated. The actual points earned for iSWM Site Design Practice 2 would be 4 points (0.50 * 8 pts = 4 pts). To comply with water quality protection and to apply for site design credits, the developer must submit the completed table and associated documentation or calculations to the review authority.

Table 14.3.5. Point System for integrated Site Design Practices
ISWM Practice No.	Practice	Percent of Eligible Area Using Practice	Maximum Points	Actual Points Earned (% practice used * max. points)
Conservation of Natural Features and Resources
1	Preserve/Create Undisturbed Natural Areas		8
2	Preserve or Create Riparian Buffers Where Applicable		8
3	Avoid Existing Floodplains or Provide Dedicated Natural Drainage Easements		8
4	Avoid Steep Slopes		3
5	Minimize Site on Porous or Erodible Soils		3
Lower Impact Site Design
6	Fit Design to the Terrain		4
7	Locate Development in Less Sensitive Areas		4
8	Reduce Limits of Clearing and Grading		6
9	Utilize Open Space Development		8
10	Incorporate Creative Design (e.g. Smart Growth, LEED Design, Form Based Zoning)		8
Reduction of Impervious Cover
11	Reduce Roadway Lengths and Widths		4
12	Reduce Building Footprints		4
13	Reduce the Parking Footprint		5
14	Reduce Setbacks and Frontages		4
15	Use Fewer or Alternative Cul-de-Sacs		3
16	Create Parking Lot Stormwater “Islands”		5
Utilization of Natural Features
17	Use Buffers and Undisturbed Areas		4
18	Use Natural Drainageways Instead of Storm Sewers		4
19	Use Vegetated Swale Design		3
20	Drain Runoff to Pervious Areas		4
Subtotal - Actual site points earned			100
Subtract minimum points required (Table 3.4) -
Points available for development incentives
Add 1 point for each 1% reduction of impervious surface +
Total Points for Development Incentives

Local Provisions: The Water Quality Protection Volume requirement is not required at this time in Azle, except as may be required by Tarrant Regional Water District for new facilities connecting directly to Eagle Mountain Lake.

Development Incentives
The developer can use integrated Site Design Practice points in excess of the minimum required for water quality protection to qualify for development incentives provided by the municipality. Additional points can be earned for redevelopment sites. Each reduction of one (1) percent imperviousness from existing conditions qualifies for one (1) site design point. The total points available for development incentives shall be calculated per Table 14.3.5. Each incentive requires ten (10) additional points above the minimum point required to meet water quality criteria, as stated in Table 14.3.4.
A list of available development incentives includes:
1.	Narrower pavement width for minor arterials
2.	Use of vegetated swales in lieu of curb and gutter for eligible developments
3.	Reduced R.O.W. requirements, i.e. Sidewalk/Utility Easements
4.	Increased density in buildable area, floor area ratios, or additional units in buildable area
5.	Expedited plans review and inspection
6.	Waiver or reduction of fees
7.	Local government public-private partnerships
8.	Waiver of maintenance, public maintenance
9.	Stormwater user fee credits or discounts
10.	Rebates, local grants, reverse auctions
11.	Low interest loans, subsidies, tax credits, or financing of special green projects
12.	Awards and recognition programs
13.	Reductions in other requirements

Local Provisions: The Development Incentives and Integrated Design point system described above are not adopted by the City of Azle. The development policies, however, encourage the incorporation of stormwater controls for achieving stormwater quality goals through the acceptance of perpetual, limited maintenance of preserved streams and by affording flexibility in placing stormwater quality treatment controls in land required for other purposes such as parks of [or] commercial landscape areas.

14.3.2.3 Option 2: Treat the Water Quality Protection Volume

Treat the Water Quality Protection Volume by reducing total suspended solids from the development site for runoff resulting from rainfall of 1.5 inches (85th percentile storm). Stormwater runoff equal to the Water Quality Protection Volume generated from sites must be treated using a variety of on-site structural and nonstructural techniques with the goal of removing a target percentage of the average annual total suspended solids.

A system has been developed by which the Water Quality Protection Volume can be reduced, thus requiring less structural control. This is accomplished through the use of certain reduction methods, where affected areas are deducted from the site area, thereby reducing the amount of runoff to be treated. For more information on the Water Quality Volume Reduction Methods see Section 1.3 of the Water Quality Technical Manual.

Water Quality Protection Volume

The Water Quality Protection Volume (WQ_v) is the runoff from the first 1.5 inches of rainfall.

Thus, a stormwater management system designed for the WQ_v will treat the runoff from all storm events of 1.5 inches or less, as well as a portion of the runoff for all larger storm events. For methods to determine the WQ_v, see Section 1.2 of the Water Quality Technical Manual.

Local Provisions: For reference only.

Recommended Stormwater Control Practices
Below is a list of recommended structural stormwater control practices. These structural controls are recommended for use in a wide variety of applications and have differing abilities to remove various kinds of pollutants. It may take more than one control to achieve a certain pollution reduction level. A detailed discussion of each of the controls, as well as design criteria and procedures, can be found in the Site Development Controls Technical Manual. Refer to Table 14.3.6 for details regarding primary and secondary controls.
*	Bioretention
*	Enhanced swales (dry, wet, wetland)
*	Alum treatment
*	Detention
*	Filter strips
*	Sand filters, filter boxes, etc.
*	Infiltration wells and trenches
*	Ponds
*	Porous surfaces
*	Proprietary systems
*	Green roofs
*	Rainwater harvesting
*	Wetlands
*	Submerged gravel wetlands

Local Provisions: For design guidance and technical reference.

Using Other or New Structural Stormwater Controls

Innovative technologies will be allowed and encouraged. Any such system will be required to provide sufficient documentation as to its effectiveness and reliability. Communities can allow controls not included in this manual at their discretion. However, these communities shall require third party proof of performance, maintenance, application requirements, and limitations.

More specifically, new structural stormwater control designs will not be accepted for inclusion in the manual until independent performance data shows that the structural control conforms to local and/or State criteria for treatment, conveyance, maintenance, and environmental impact.

Suitability of Stormwater Controls to Meet Stormwater Management Goals

The stormwater control practices recommended in this manual vary in their applicability and ability to meet stormwater management goals:

Primary Controls

Primary Structural Stormwater Controls have the ability to fully address one or more of the Steps in the integrated Focus Areas if designed appropriately. Structural controls are recommended for use with a wide variety of land uses and development types. These structural controls have a demonstrated ability to effectively treat the Water Quality Volume (WQ_v) and have been shown to be able to remove 70% to 80% of the annual average total suspended solids (TSS) load in typical post-development urban runoff when designed, constructed, and maintained in accordance with recommended specifications. Several of these structural controls can also be designed to provide primary control for downstream streambank protection (SP_v) and flood mitigation. These structural controls are recommended stormwater management facilities for a site wherever feasible and practical.

Secondary Controls

A number of structural controls are recommended only for limited use or for special site or design conditions. Generally, these practices either: (1) do not have the ability on their own to fully address one or more of the Steps in the integrated Focus Areas, (2) are intended to address hotspot or specific land use constraints or conditions, and/or (3) may have high or special maintenance requirements that may preclude their use. These types of structural controls are typically used for water quality treatment only. Some of these controls can be used as pretreatment measures or in series with other structural controls to meet pollutant removal goals. Such structural controls are not recommended for residential developments.

Table 14.3.6 summarizes the stormwater management suitability of the various stormwater controls in addressing the integrated Focus Areas. The Site Development Controls Technical Manual provides guidance on the use of stormwater controls as well as how to calculate the pollutant removal efficiency for stormwater controls in series. The Site Development Controls Technical Manual also provides guidance for choosing the appropriate stormwater control(s) for a site as well as the basic considerations and limitations on the use of a particular stormwater control.

Table 14.3.6. Suitability of Stormwater Controls to Meet integrated Focus Areas
Category	Integrated Stormwater Controls	TSS/Sediment Removal Rate	Water Quality Protection	Streambank Protection	On–Site Flood Control	Downstream Flood Control
Bioretention Areas	Bioretention Areas	80%	P	S	S	–
Channels	Enhanced Swales	80%	P	S	S	S
	Channels, Grass	50%	S	S	P	S
	Channels, Open	–	–	–	P	S
Chemical Treatment	Alum Treatment System	90%	P	–	–	–
Conveyance System Components	Culverts	–	–	–	P	P
	Energy Dissipation	–	–	P	S	S
	Inlets/Street Gutters	–	–	–	P	–
	Pipe Systems	–	–	P	P	P
Detention	Detention, Dry	65%	S	P	P	P
	Detention, Extended Dry	65%	S	P	P	P
	Detention, Multi-purpose Areas	–	–	P	P	P
	Detention, Underground	–	–	P	P	P
Filtration	Filter Strips	50%	S	–	–	–
	Organic Filters	80%	P	–	–	–
	Planter Boxes	80%	P	–	–	–
	Sand Filters, Surface/Perimeter	80%	P	S	–	–
	Sand Filters, Underground	80%	P	–	–	–
Hydrodynamic Devices	Gravity (Oil-Grit) Separator	40%	S	–	–	–
Infiltration	Downspout Drywell	80%	P	–	–	–
	Infiltration Trenches	80%	P	S	–	–
	Soakage Trenches	80%	P	S	–	–
Ponds	Wet Pond	80%	P	P	P	P
	Wet ED Pond	80%	P	P	P	P
	Micropool ED Pond	80%	P	P	P	P
	Multiple Ponds	80%	P	P	P	P
Porous Surfaces	Green Roof	85%	P	S	–	–
	Modular Porous Paver Systems	2	S	S	–	–
	Porous Concrete	2	S	S	–	–
Proprietary Systems	Proprietary Systems1	1	S/P	S	S	S
Re-Use	Rain Barrels	–	P	–	–	–
Wetlands	Wetlands, Stormwater	80%	P	P	P	P
	Wetlands, Submerged Gravel	80%	P	P	S	–

P	=	Primary Control: Able to meet design criterion if properly designed, constructed and maintained.
S	=	Secondary Control: May partially meet design criteria. Designated as a Secondary control due to considerations such as maintenance concerns. For Water Quality Protection, recommended for limited use in approved community-designated areas.
–	=	Not typically used or able to meet design criterion.
1	=	The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data, if used as a primary control. Third-party sources could include Technology Acceptance Reciprocity Partnership, Technology Assessment Protocol - Ecology, or others.
2	=	Porous surfaces provide water quality benefits by reducing the effective impervious area.

14.3.2.4 Option 3: Assist with Off-Site Pollution Prevention Programs and Activities

Some communities have implemented pollution prevention programs/activities in certain areas to remove pollutants from the runoff after it has been discharged from the site. This may be especially true in intensely urbanized areas facing site redevelopment where many of the BMP criteria would be difficult to apply. These programs will be identified in the local jurisdiction’s approved TPDES stormwater permit and/or in a municipality’s approved watershed plan. In lieu of on-site treatment, the developer can request to simply assist with the implementation of these off-site pollution prevention programs/activities.

Developers should contact the municipality to determine if there are any plans to address runoff pollutants within the region of proposed development. If no plans exist, consider proposing regional alternatives that would address pollution prevention.

Local Provisions: Off-site pollution prevention activities are not currently required by the City of Azle.

(Ordinance 2012-10 adopted 8/21/12)

§ 14.3.3 Acceptable Downstream Conditions.

As part of the iSWM Plan development, the downstream impacts of development must be carefully evaluated for the two focus areas of Streambank Protection and Flood Mitigation. The purpose of the downstream assessment is to protect downstream properties from increased flooding and downstream channels from increased erosion potential due to upstream development. The importance of the downstream assessment is particularly evident for larger sites or developments that have the potential to dramatically impact downstream areas. The cumulative effect of smaller sites, however, can be just as dramatic and, as such, following the integrated Focus Areas is just as important for the smaller sites as it is for the larger sites.

The assessment shall extend from the outfall of a proposed development to a point downstream where the discharge from a proposed development no longer has a significant impact, in terms of flooding increase or velocity above allowable, on the receiving stream or storm drainage system. The local jurisdiction shall be consulted to obtain records and maps related to the National Flood Insurance Program and the availability of Flood Insurance Studies and Flood Insurance Rate Maps (FIRMs) which will be helpful in this assessment. The assessment shall be a part of the preliminary and final iSWM plans, and must include the following properties:

*	Hydrologic analysis of the pre- and post-development on-site conditions
*	Drainage path that defines extent of the analysis
*	Capacity analysis of all existing constraint points along the drainage path, such as existing floodplain developments, underground storm drainage systems culverts, bridges, tributary confluences, or channels
*	Off-site undeveloped areas are considered as “full build-out” for both the pre- and post-development analyses
*	Evaluation of peak discharges and velocities for three 24-hour storm events
	*	Streambank protection storm
	*	Conveyance storm
	*	Flood mitigation storm
*	Separate analysis for each major outfall from the proposed development

Once the analysis is complete, the designer must answer the following questions at each determined junction downstream:

*	Are the post-development discharges greater than the pre-development discharges?
*	Are the post-development velocities greater than the pre-development velocities?
*	Are the post-development velocities greater than the velocities allowed for the receiving system?
*	Are the post-development flood heights more than 0.1 feet above the pre-development flood heights?

These questions shall be answered for each of the three storm events. The answers to these questions will determine the necessity, type, and size of non-structural and structural controls to be placed on-site or downstream of the proposed development.

Section 2.0 of the Hydrology Technical Manual gives additional guidance on calculating the discharges and velocities, as well as determining the downstream extent of the assessment.

Local Provisions:
Downstream Assessment
Downstream impacts due to a development must be analyzed and mitigated for the 1-, 10-, and 100-year floods for the entire Zone of Influence, as determined by the development engineer’s analysis. The Zone of Influence for any proposed development must be defined by the development engineer, based on a drainage study that determines the specific location along the drainage route where “no adverse impacts” from the new development exist. Storm drainage from a development must be carried to an “adequate outfall” or “acceptable outfall.”
Zone of Influence
A “zone of influence” from a proposed development extends to a point downstream where the discharge from a proposed development no longer has a significant impact upon the receiving stream or storm drainage system. The Zone of Influence for any proposed development must be defined by the development engineer by a drainage study that: (1) determines the extent of the downstream drainage route subject to impacts from a proposed development, and (2) delineates what existing conditions are in place or what proposed mitigation is planned so that “no adverse impacts” from the new development will occur.
A drainage study will include the necessary hydrologic and hydraulic analyses to clearly demonstrate that the limits of the Zone of Influence have been identified, and that along the drainage route to that location, these parameters are met:
	* No new or increased flooding of existing insurable (FEMA) structures (habitable buildings).
	* No significant (0.1') increases in flood elevations over existing roadways for the 1-, 10- and 100-year floods.
	* No significant rise (0.1' or less) in 100-year flood elevations, unless contained in existing channel, roadway, drainage easement and/or R.O.W.
	* Where provisions of the City’s floodplain ordinance may be more restrictive, the floodplain ordinance shall have authority over the above provisions.
	* No significant increases (maximum of 5%) in channel velocities for the 1-, 10- and 100-year floods. Post-development channel velocities cannot be increased by more than 5% above pre-development velocities, nor exceed the applicable maximum permissible velocity shown in Table 3.3 in the Hydraulics Technical Manual. Exceptions to these criteria will require certified geotechnical/geomorphologic studies that provide documentation that the higher velocities will not create additional erosion. If existing channel velocities exceed six (6) feet per second, no additional increase in velocities will be allowed.
	* No increases in downstream discharges caused by the proposed development that, in combination with existing discharges, exceeds the existing capacity of the downstream storm drainage system.
	* For watersheds of 100 acres or less at any proposed outfall, the downstream assessment may use the ten percent rule of thumb (as delineated in Section 2.0 of the Hydrology Technical Manual) or a detailed study in order to determine the Zone of Influence.
	* For all other watersheds, the Zone of Influence will be defined by a detailed hydrologic and hydraulic analysis.
Adequate Outfall
Storm drainage from a development must be carried to an “adequate outfall” or “acceptable outfall.” An adequate outfall is one that does not create adverse flooding or erosion conditions downstream and is in all cases subject to the Storm Water Manager approval.
Drainage Studies
Studies of the proposed development and drainage areas, including a downstream assessment of properties that could be impacted by the development, will accompany the conceptual, preliminary, and final site plans. The “zone of influence” and “adequate outfall point” for the proposed development will be identified in the study and iSWM Site Plan. An adequate outfall is one that does not create adverse flooding or erosion conditions downstream and is in all cases subject to the approval of the Storm Water Manager.
These studies will include adequate hydrologic analysis to determine the existing, proposed, and fully developed runoff for the drainage area that is affected by the proposed development. They will also include hydraulic studies that help define the “Zone of Influence” and any upstream or downstream off-site effects. The study, as part of the development site plan, shall address existing downstream, off-site drainage conveyance system(s) and define the drainage path from the outfall of the on-site stormwater facilities, to the off-site drainage system(s) and/or appropriate receiving waters.

(Ordinance 2012-10 adopted 8/21/12)

§ 14.3.4 Streambank Protection.

The second focus area is in streambank protection. There are three options by which a developer can provide adequate streambank protection downstream of a proposed development. The first step is to perform the required downstream assessment as described in Section 14.3.3. If it is determined that the proposed project does not exceed acceptable downstream velocities or the downstream conditions are improved to adequately handle the increased velocity, then no additional streambank protection is required. If on-site or downstream improvements are required for streambank protection, easements or right-of-entry agreements will need to be obtained in accordance with Section 14.3.7. If the downstream assessment shows that the velocities are within acceptable limits, then no streambank protection is required. Acceptable limits for velocity control are contained in Tables 14.3.10 and 14.3.11.

Option 1. Reinforce/Stabilize Downstream Conditions

If the increased velocities are greater than the allowable velocity of the downstream receiving system, then the developer must reinforce/stabilize the downstream conveyance system. The proposed modifications must be designed so that the downstream system is protected from the post-development velocities. The developer must provide supporting calculations and/or documentation that the downstream velocities do not exceed the allowable range once the downstream modifications are installed.

Allowable bank protection methods include stone riprap, gabions, and bio-engineered methods. Sections 3.2 and 4.0 of the Hydraulics Technical Manual give design guidance for designing stone riprap for open channels, culvert outfall protection, riprap aprons for erosion protection at outfalls, and riprap basins for energy dissipation.

Local Provisions: NONE

Option 2. Install Stormwater Controls to Maintain Existing Downstream Conditions

The developer must use on-site controls to keep downstream post-development discharges at or below allowable velocity limits. The developer must provide supporting calculations and/or documentation that the on-site controls will be designed such that downstream velocities for the three storm events (Streambank Protection, Conveyance, and Flood Mitigation) are within an allowable range once the controls are installed.

Local Provisions: NONE

Option 3. Control the Release of the 1-yr, 24-hour Storm Event

Twenty-four hours of extended detention shall be provided for on-site, post-developed runoff generated by the 1-year, 24-hour rainfall event to protect downstream channels. The required volume for extended detention is referred to as the Streambank Protection Volume (denoted SP_v). The reduction in the frequency and duration of bankfull flows through the controlled release provided by extended detention of the SP_v will reduce the bank scour rate and severity.

To determine the SP_v refer to Section 3.0 of the Hydrology Technical Manual.

Local Provisions: This option protects a stream from increased runoff discharge rates and velocities that tend to occur with development.

(Ordinance 2012-10 adopted 8/21/12)

§ 14.3.5 Flood Mitigation.

14.3.5.1 Introduction

Flood analysis is based on the design storm events as defined in Section 14.1.3: for conveyance storm and the flood mitigation storm.

The intent of the flood mitigation criteria is to provide for public safety; minimize on-site and downstream flood impacts from the three storm events; maintain the boundaries of the mapped 100-year floodplain; and protect the physical integrity of the on-site stormwater controls and the downstream stormwater and flood mitigation facilities.

Flood mitigation must be provided for on-site conveyance system, as well as downstream outfalls as described in the following sections.

14.3.5.2 

Flood Mitigation Design Options

There are three options by which a developer may address downstream flood mitigation. These options closely follow the three options for Streambank Protection. When on-site or downstream modifications are required for downstream flood mitigation, easements or right-of-entry agreements will need to be obtained in accordance with Section 14.3.7.

The developer will provide all supporting calculations and/or documentation to show that the existing downstream conveyance system has capacity (Q_f) to safely pass the full build-out flood mitigation storm discharge.

Option 1. Provide Adequate Downstream Conveyance Systems

When the downstream receiving system does not have adequate capacity, then the developer shall provide modifications to the off-site, downstream conveyance system. If this option is chosen the proposed modifications must be designed to adequately convey the full build-out stormwater peak discharges for the three storm events. The modifications must also extend to the point at which the discharge from the proposed development no longer has a significant impact on the receiving stream or storm drainage system. The developer must provide supporting calculations and/or documentation that the downstream peak discharges and water surface elevations are safely conveyed by the proposed system, without endangering downstream properties, structures, bridges, roadways, or other facilities.

Option 2. Install Stormwater Controls to Maintain Existing Downstream Conditions

When the downstream receiving system does not have adequate capacity, then the developer shall provide stormwater controls to reduce downstream flood impacts. These controls include on-site controls such as detention, regional controls, and, as a last resort, local flood protection such as levees, floodwalls, floodproofing, etc.

The developer must provide supporting calculations and/or documentation that the controls will be designed and constructed so that there is no increase in downstream peak discharges or water surface elevations due to development.

Option 3. In lieu of a Downstream Assessment, Maintain Existing On-Site Runoff Conditions

Lastly with Option 3, on-site controls shall be used to maintain the pre-development peak discharges from the site. The developer must provide supporting calculations and/or documentation that the on-site controls will be designed and constructed to maintain on-site existing conditions.

It is important to note that Option 3 does not require a downstream assessment. It is a detention-based approach to addressing downstream flood mitigation after the application of the integrated site design practices.

For many developments however, the results of a downstream assessment may show that significantly less flood mitigation is required than “detaining to pre-development conditions.” This method may also exacerbate downstream flooding problems due to timing of flows. The developer shall confirm that detention does not exacerbate peak flows in downstream reaches.

Local Provisions: NONE

(Ordinance 2012-10 adopted 8/21/12)

§ 14.3.6 Stormwater Conveyance Systems.

14.3.6.1 Introduction

Stormwater system design is an integral component of both site and overall stormwater management design. Good drainage design must strive to maintain compatibility and minimize interference with existing drainage patterns; control flooding of property, structures, and roadways for design flood events; and minimize potential environmental impacts on stormwater runoff.

Stormwater collection systems must be designed to provide adequate surface drainage while at the same time meeting other stormwater management goals such as water quality, streambank protection, habitat protection, and flood mitigation.

Design

Fully developed watershed conditions shall be used for determining runoff for the conveyance storm and the flood mitigation storm.

Local Provisions: NONE

14.3.6.2 Hydraulic Design Criteria for Streets and Closed Conduits

Introduction
This section is intended to provide criteria and guidance for the design of on-site flood mitigation system components including:
*	Street and roadway gutters
*	Stormwater inlets
*	Parking lot sheet flow
*	Storm drain pipe systems

Streets and Stormwater Inlets
Design Frequency
*	Streets and roadway gutters: conveyance storm event
*	Inlets on-grade: conveyance storm event
*	Parking lots: conveyance storm event
*	Storm drain pipe systems: conveyance storm event
*	Low points: flood mitigation storm event
*	Street R.O.W.: flood mitigation storm event
*	Drainage and Floodplain easements: flood mitigation storm event

Local Provisions: The iSWM Inlet Design Methodology (iSWM Hydraulics Technical Manual) is adopted. Under the City of Azle classification system, inlets have been classified into two major groups namely: Inlets in Sumps and Inlets on Grade with Gutter Depression. The only curb inlets that are allowed by the City of Azle are those in sumps and depressed inlets on grade. Grate inlets and combination inlets are not allowed.

Figures presented in Section 14.5.0 can be used to document all closed conduit calculations even if calculations are performed on an acceptable computer program unless otherwise approved by Storm Water Manager.

A “rooftop” section should be used for concrete streets and a parabolic section for asphalt streets. Please note that the nomograph in Figure 1.2 of the iSWM Hydraulics Technical Manual does not completely address cases where the crown elevation is lower that the top of curb elevation. For those cases a combination of Figure 1.2 and 1.3 can be used or a standard hydraulics program such as EPA-SWMM, HEC-RAS or FlowMaster can be applied.

The design storms presented in the regional portion of Section 14.1.3 of this document are replaced by the design storms required by the City of Azle as follows:

Storm Sewer System

The design storm is a minimum of the 100-year storm for the combination of the closed conduit and surface drainage system.

Runoff from the 5-year storm must be contained within the permissible spread of water in the gutter. The 100-year storm flow must be contained within the R.O.W. Adequate inlet capacity shall be provided to intercept surface flows before the street R.O.W. capacity is exceeded. Note: The capacity of the underground system may be required to exceed the 5-year storm in order to satisfy the 100-year storm criteria.

The closed conduit HGL must be equal to or below the gutter line for pipe systems and one (1) foot or more below the curb line at inlets. For situations where no R.O.W. exists, the 100-year HGL must be below finished ground. The 100-year HGL will be tracked carefully throughout the system and described in the hydraulic calculations tables in Section 14.5.0 and in the construction drawings.

Inlets in Sumps

Curb opening inlets in sumps (Type CO-S) are addressed in Section 1.2.7 of the Hydraulics Technical Manual. Drop inlets in sumps (Y Inlet) are addressed in Section 1.2.9 of the Hydraulics Technical Manual.

In sag or sump conditions, the storm drain and sump inlets should be sized to intercept and convey minimum of the 25-year storm and a positive structural overflow is required to provide for the remainder of the 100-year storm. The positive overflow structure must be concrete or other acceptable non-earthen structure with a minimum bottom width of 4 feet extending from the sump inlet to the storm sewer outfall. It must be designed to pass at least 20 cfs with 1' of freeboard from the top of curb to the adjacent finish floor elevations (minimum finish floor elevations for all lots adjacent to said overflows must be shown on the plat).

All flumes that pass through sidewalks shall have a bolted-down, rustproof, 3/8-inch (min.) steel plate with a pedestrian-rated walking surface. The plate shall be recessed into the concrete sidewalk from face of curb to the property line. The plate must be secured to the concrete with bolts and flush with the top of sidewalk. A center support may be added depending on the width of the flume. Fences must be kept behind the curb line of the flume. Where a structural overflow is not feasible, a variance must be requested from Storm Water Manager. If no structural overflow is constructed, the sump inlets must be designed with a 50% clogging factor. In a cul-de-sac where no structural overflow is feasible, additional on-grade inlet capacity may be provided upstream of the sump in lieu of additional sump inlets.

An explanation of the Inlets in Sumps Calculation Sheet is included in Section 14.5.3.1.

Inlets on Grade with Gutter Depression (Type CO-D)

The hydraulic efficiency of stormwater inlets varies with gutter flow, street grade, street crown, and with the geometry of the inlet depression. The design flow into any inlet can be greatly increased if a small amount (5 to 10 percent) of gutter flow is allowed to flow past the inlet. When designing inlets, freedom from clogging or from interference with traffic often takes precedence over hydraulic considerations. See Section 14.5.3.1 for computation sheet for Type CO-D inlet.

The depression of the gutter at a curb-opening inlet (See Figure 14.5.3) below the normal level of the gutter increases the cross-flow towards the opening, thereby increasing the inlet capacity. Also, the downstream transition out of the depression causes backwater which further increases the amount of water captured. Depressed inlets should be used on all public streets and alleys. Recessed depressed inlets should be used on all arterials.

The capacity of a depressed curb inlet on grade will be based on the methodology presented in Section 1.2.7 of the iSWM Hydraulics Technical Manual.

Drop Inlets (Area Drains)

1. Drop inlets serving a drainage area of 10 to 25 acres will be designed with a 50% clogging factor.

2. Grading plans to direct flow into drop inlets will be included in the construction plans and Community Facilities Agreement documents. Where earthen swales or other means of collecting and directing runoff into drop inlets are needed, they should be contained in appropriately sized drainage easements.

3. Consideration should be given to a structural overflow in the same manner as described for sump inlets.

4. Drop inlets shall be located where they can be easily accessed for inspection and maintenance by the City.

Headwalls

1. A headwall will be used to collect a drainage area of 25 acres or more flowing to one spot.

2. Areas that have been channelized or discharged from a storm drain system will use a headwall to reintroduce the flow to a new storm drain system. These provisions do not apply to special multi-stage outlet structures draining detention facilities.

Design Criteria

Streets and R.O.W.: Depth in the street shall not exceed top of curb or maximum flow spread limits for the conveyance storm. The flood mitigation storm shall be contained within the right-of-ways or easements.

Parking Lots: Parking lots shall be designed for the conveyance storm not to exceed top of curb with maximum ponding at low points of one (1) foot. The flood mitigation storm shall be contained on-site or within dedicated easements.

Flow Spread Limits: Inlets shall be spaced so that the spread of flow in the street for the conveyance storm shall not exceed the guidelines listed below, as measured from the gutter or face of the curb:

Table 14.3.7. Flow Spread Limits
Street Classification	Allowable Encroachment
Collectors, Arterial, and Thoroughfares (greater than 2 lanes)	8 feet or one travel lane, both sides for a divided roadway
Residential Streets	curb depth or maximum 6 inches at gutter

Local Provisions: Spread of water refers to the amount of water that is allowed to collect in streets during a storm of 5-year design frequency. In order that excess stormwater will not collect in streets or thoroughfares during a storm of the design frequency, the following spread of water values shall be used for the various types of streets.

Arterials (Divided)

1. Permissible Spread of Water - The permissible spread of water in gutters of major divided thoroughfares shall be limited so that one traffic lane on each side remains clear during the 5-year storm. Gutter flow shall be based on maximum storm duration of 15 minutes.

2. Conditions - Inlets shall preferably be located at street intersections, at low points of grade or where the gutter flow exceeds the permissible spread of water criteria. Inlets shall be located, when possible, on side streets when grades permit. In no cases shall the gutter depression at inlets exceed the standard. In super-elevated sections, inlets placed against the center medians shall have no gutter depression and shall intercept gutter flow at the point of vertical curvature to prevent flow from crossing the thoroughfares on the surface in valley gutters or otherwise.

Arterials (Not Divided)

1. Permissible Spread of Water - The permissible spread of water in gutters of major undivided thoroughfares shall be limited so that two traffic lanes will remain clear during the 5-year storm. The 100-year storm shall be contained within the R.O.W.

2. Conditions - Inlets shall preferably be located at street intersections, low points of grades, or where the gutter flow exceeds the permissible spread of water criteria. Inlets shall be located, when possible, on the side streets when grades permit. In no case shall the gutter depression at inlets exceed. [sic]

3. Super-elevated Sections - Intercept gutter flow at P.V.C. or P.V.T. to prevent flow from crossing thoroughfare. Unless expressly approved by the Storm Water Manager, stormwater will not be allowed to cross major thoroughfares on the surface in valley gutters or otherwise.

Collector Streets

1. Permissible Spread of Water - The permissible spread of water in gutters of collector streets shall be limited so that one standard lane of traffic will remain clear during the 5-year storm. The 100-year storm shall be contained within the R.O.W.

2. Conditions - Inlets shall preferably be located at street intersections, low points of grade or where the gutter flow exceeds the permissible spread of water criteria. Inlets shall be located, when at all possible, on the side streets when grade permits. Inlets with the standard gutter depression shall be used. In no case shall the gutter depression at inlets exceed the standard.

Minor Streets (Residential)

1. Permissible Spread of Water - The permissible spread of water in gutters for minor streets shall be limited by the height of the curb for 5-year storms. The 100-year storm shall be contained within the R.O.W.

2. Conditions - Inlets shall be located at street intersections, low points of grade or where the gutter flow exceeds the permissible spread of water criteria. Inlets with depressed standard gutter depression shall be used in all cases unless special grading problems are involved. In no case shall the gutter depression at inlets exceed the standard.

Must use roadway sections as approved by City of Azle.

Storm Drain Pipe Design
Design Frequency
*	Pipe Design: conveyance storm event within pipe with hydraulic grade line (HGL) below throat of inlets
*	R.O.W. and Easements: flood mitigation storm event must be contained within the R.O.W. or easement

Local Provisions: City of Azle pipe design frequency is the 100-year storm less any gutter, roadway, and flume flows.

Design Criteria
*	For ordinary conditions, storm drainpipes shall be sized on the assumption that they will flow full or practically full under the design discharge but will not be placed under pressure head. The Manning Formula is recommended for capacity calculations.
*	The maximum hydraulic gradient shall not produce a velocity that exceeds 15 feet per second (fps). Table 3.8 shows the desirable velocities for most storm drainage design. Storm drains shall be designed to have a minimum mean velocity flowing full at 2.5 fps.

Table 14.3.8. Desirable Velocity in Storm Drains
Description	Maximum Desirable Velocity
Culverts (All types)	15 fps
Storm Drains (Inlet laterals)	No Limit
Storm Drains (Collectors)	15 fps
Storm Drains (Mains)	12 fps

*	The minimum desirable physical slope shall be 0.5% or the slope that will produce a velocity of 2.5 feet per second when the storm sewer is flowing full, whichever is greater.
*	If the potential water surface elevation exceeds 1 foot below ground elevation for the design flow, the top of the pipe, or the gutter flow line, whichever is lowest, adjustments are needed in the system to reduce the elevation of the hydraulic grade line.
*	Access manholes are required at intermediate points along straight runs of closed conduits. Table 14.3.9 gives maximum spacing criteria.

Table 14.3.9. Access Manhole Spacing Criteria (HEC 22, 2001)
Pipe Size (inches)	Maximum Spacing (feet)
12 - 24	300
27 - 36	400
42 - 54	500
60 and up	1000

Local Provisions: This section replaces the Closed Conduit System sections 1.2.9, most of 1.2.10, and 1.2.11 of the iSWM Hydraulics Technical Manual. Storm Drain Outfalls located within Section 1.2.10 (page HA-45) is adopted.

Velocities and Grades

Storm drains should operate with velocities of flow sufficient to prevent excessive deposits of solid materials; otherwise objectionable clogging may result. The controlling velocity is near the bottom of the conduit and considerably less than the mean velocity of the sewer. Storm drains shall be designed to have a minimum mean velocity flowing full of 2.5 fps. The table of Minimum Grades for Storm Drains indicates the minimum grades for concrete pipe (n = 0.013), flowing at 2.5 fps.

Velocities in sewers are important mainly because of the possibilities of excessive erosion on the storm drain inverts. Table 14.3.8 shows the desirable velocities for most storm drainage design. Velocities in excess of those shown on this table must be approved by the Storm Water Manager. Supercritical flow in main lines should be avoided unless approved by the Storm Water Manager

Table 14.3.9A. Minimum Grades For Storm Drains
Pipe Size (Inches)	Concrete Pipe Slope (Ft./Ft.)
18	0.0018
24	0.0013
27	0.0011
30 - 96	0.0010

Materials

Only reinforced concrete pipe is allowed under pavement for public storm drains in the City of Azle:

In selecting roughness coefficients for concrete pipe, consideration will be given to the average conditions at the site during the useful life of the structure. The “n” value of 0.015 for concrete pipe should be used primarily in analyzing old sewers where alignment is poor and joints have become rough. If, for example, concrete pipe is being designed at a location where it is considered suitable, and there is reason to believe that the roughness would increase through erosion or corrosion of the interior surface, slight displacement of joints or entrance of foreign materials. A roughness coefficient will be selected which in the judgment of the designer, will represent the average condition. Any selection of “n” values below the minimum or above the maximum, either for monolithic concrete structures, concrete pipe or HDPE, will have to have written approval of the Storm Water Manager.

The recommended coefficients of roughness [are] listed in Table 14.3.9B below and are for use in the nomographs contained herein, or by direct solution of Manning’s Equation.

Table 14.3.9B. Manning’s Coefficients for Storm Drain Conduits*
Type of Storm Drain	Manning’s n
Concrete Pipe (Design n = 0.013)	0.012 - 0.015
Concrete Boxes (Design n = 0.015)	0.012 - 0.015
Corrugated Metal Pipe, Pipe-Arch and Box (Annular or Helical Corrugations - see Table 1.8 in iSWM Hydraulics Technical Manual. NOTE: CITY OF AZLE DOES NOT ALLOW CMP FOR NEW CONSTRUCTION	0.022 - 0.037
Profile Wall Thermoplastic High Density Polyethylene (HDPE) or Polyvinyl Chloride (PVC) NOTE: CITY OF AZLE DOES NOT ALLOW HDPE OR PVC FOR NEW CONSTRUCTION	0.010 - 0.013
*NOTE: Actual field values for conduits may vary depending on the effect of abrasion, corrosion, deflection, and joint conditions.

Manholes
Manholes shall be located at intervals not to exceed 1000 feet for pipe 48 inches in diameter and larger. Manholes must be installed at the upstream end of a system and whenever a storm drain leaves the pavement, unless the outfall is within 50 feet of the roadway and directly accessible. Manholes shall preferably be located at street intersections, sewer junctions, changes of grade and changes of alignment. When the storm drain is a concrete box instead of an RCP, four-foot diameter manhole risers may be installed instead of vaults to provide access. In all cases, steps shall be installed to the flowline of the pipe.
See Section 14.5.3 for the City of Azle requirements on Stormwater Inlets, Minor Head Losses at Structures, Storm Drain Design Examples, and General Construction Standards for Closed Conduit Systems.
Full or Part Full Flow in Storm Drains
All storm drains shall be designed by the application of the Continuity Equation and Manning’s Equation either through the appropriate charts or nomographs or by direct solutions of the equations as follows:
Q	=	Runoff in cubic feet per second.
A	=	Cross-sectional area of pipe or channel.
V	=	Velocity of flow.
n	=	Coefficient of roughness of pipe or channel.
r	=	Hydraulic radius = A/P
Sf	=	friction slope in feet per foot in pipe or channel.
P	=	Wetted perimeter.
The size of pipe required to transport a known quantity of storm runoff is obtained by substituting known values in the formula. In practice, the formula is best utilized in the preparation of a pipe flow chart which interrelates values of runoff, velocity, slope, and pipe geometry. With two of these variables known or assumed. The other two are quickly obtained from the chart. A pipe flow nomograph for circular conduits flowing full graphs is shown in iSWM Hydraulics Technical Manual Figure 1.17. Nomographs for flow in conduits of other cross-sections are available in TxDOT Hydraulic Design Manual, dated March 2004, Chapter 6, and Section 2. For circular conduits flowing partially full, graphs are presented in iSWM Hydraulics Technical Manual Figure 1.19a.
Hydraulic Gradient and Profile of Storm Drain
In storm drain systems flowing full (or partially full as discussed above) all losses of energy through resistance with flow in pipes, by changes of momentum or by interference with flow patterns at junctions, must be accounted for by accumulative head losses along the system from its initial upstream inlet to its outlet. The purpose of accurate determinations of head losses at junctions is to include these values in a progressive calculation of the hydraulic gradient along the storm drain system. In this way, it is possible to determine the water surface elevation which will exist at each structure. The rate of loss of energy through the storm drain system shall be represented by the hydraulic grade line, which measures the pressure head available at any given point within the system.
The hydraulic grade line (HGL) shall be established for all storm drainage design in which the system operates under a head. The hydraulic grade line is often controlled by the conditions of the sewer outfall; therefore, the elevation of the tailwater must be known. The hydraulic gradient is constructed upstream from the downstream end, taking into account all of the head losses that may occur along the line. The iSWM Hydraulics Technical Manual Table 1.10 provides a table of coincident design frequencies to assist with tailwater determination. The hydraulic gradient shall begin at the higher of the tailwater or depth of flow in the pipe at the downstream end.
All head losses shall be calculated if the storm drain system is in a subcritical flow regime whether the system is flowing partially full or surcharged. Hydraulic calculations shall reflect partially full pipe where appropriate. Supercritical flow is allowed in main lines only with the approval of the Storm Water Manager. If the system is in supercritical regime the section should be marked “SUPERCRITICAL FLOW.” The presence of supercritical regime should be confirmed by analyzing from downstream as well as upstream.
The friction head loss shall be determined by direct application of Manning’s Equation or by appropriate nomographs or charts as discussed in the first paragraph of this subsection. Minor losses due to turbulence at structures shall be determined by the procedure of last subsection of this section (“Minor Head Losses at Structures”) or in the iSWM Hydraulics Technical manual. All HGL calculations will be carried upstream to the inlet.
The hydraulic grade line shall in no case be above the surface of the ground or street gutter for the design storm. Allowance of head must also be provided for future extensions of the storm drainage system. In all cases the maximum HGL must be 12" below top of curb at any inlet.
Minor Head Losses at Structures
Section 14.5.3.2 contains detailed information on the calculation of minor head losses at structures. Figures 14.5.6 and 14.5.7 provide details of minor losses for manholes, wye branches, and bends in the design of closed conduits. Minimum head loss used at any structure shall be 0.10 foot.
Storm Drain Design Examples
Section 14.5.3.3 presents an example of storm drain design.
Hydrologic Methodology with MWH InfoWorks/SWMM Programs
InfoWorks SD by MWH Soft and the Stormwater Management Model (SWMM) family of programs have been applied to several complex storm sewer systems in the City of Azle. These programs include several hydrologic sub-area runoff procedures. In addition to the hydrologic methods described in Section 14.3.0, the City of Azle accepts the following procedures when applying these programs:
*	With case-by-case approval by the Storm Water Manager, the SWMM Method in which the flow is routed using a single linear reservoir, whose routing coefficient depends on surface roughness (Manning’s n), surface area, ground slope and catchment width.
*	A version of the Unit Hydrograph Method in which a triangular unit hydrograph [is] developed using the time to peak (time of concentration times 0.6), total runoff time (time to peak times 2.67) and the peak of the unit hydrograph (2 divided by total runoff time).

14.3.6.3 Hydraulic Design Criteria for Structures

Introduction

This section is intended to provide design criteria and guidance on several on-site flood mitigation system components, including culverts, bridges, vegetated and lined open channels, storage design, outlet structures, and energy dissipation devices for outlet protection.

Open Channels

Design Frequency

*	Open channels, including all natural or structural channels, swales, and ditches shall be designed for the flood mitigation storm event
*	Channels shall be designed with multiple stages. A low flow channel section containing the streambank protection flows and a high flow section that contains the conveyance and flood mitigation storms will improve stability and better mimic natural channel dimensions.

Local Provisions: 100-year design storm for fully developed watershed conditions. Channels may be designed with multiple stages (e.g., a “low-flow” or “trickle” channel section for common recurring flows, and a high flow section that contains the design discharge). The “low-flow” or “trickle” channel shall convey 2% of the design 100-year discharge.

Design Criteria

*	Trapezoidal channels shall have a minimum channel bottom width of 6 feet.
*	Channels with bottom widths greater than 6 feet shall be designed with a minimum bottom cross-slope of 12 to 1 or with compound cross-sections.
*	Channel side slopes shall be stable throughout the entire length and the side slope shall depend on the channel material. Channel side slopes and roadside ditches with a side slope steeper than 3:1 shall require detailed geotechnical and slope stability analysis to justify slopes steeper than 3:1. However, any slope that is less than 3:1 needs a detailed analysis to prove that it can be done.
*	Trapezoidal or parabolic cross-sections are preferred over triangular shapes.
*	For vegetative channels, design stability shall be determined using low vegetative retardance conditions (Class D). For design capacity, higher vegetative retardance conditions (Class C) shall be used.
*	For vegetative channels, flow velocities within the channel shall not exceed the maximum permissible velocities given in Tables 14.3.10 and 14.3.11.
*	If relocation of a stream channel is unavoidable, the cross-sectional shape, meander, pattern, roughness, sediment transport, and slope shall conform to the existing conditions insofar as practicable. Energy dissipation will be necessary when existing conditions cannot be duplicated.
*	Streambank stabilization shall be provided, when appropriate, as a result of any stream disturbance such as encroachment and shall include both upstream and downstream banks as well as the local site.
*	HEC-RAS, or similarly capable software approved by the entity with jurisdiction, shall be used to confirm the water surface profiles in open channels.
*	The final design of artificial open channels shall be consistent with the velocity limitations for the selected channel lining. Maximum velocity values for selected lining categories are presented in Table 14.3.10. Seeding and mulch shall only be used when the design value does not exceed the allowable value for bare soil. Velocity limitations for vegetative linings are reported in Table 14.3.11. Vegetative lining calculations and stone riprap procedures are presented in Section 3.2 of the Hydraulics Technical Manual.
*	For gabions, design velocities range from 10 fps for 6-inch mattresses up to 15 fps for 1-foot mattresses. Some manufacturers indicate that velocities of 20 fps are allowable for basket installations. The design of stable rock riprap lining depends on the intersection of the velocity (local boundary shear) and the size and gradation of the riprap material. More information on calculating acceptable riprap velocity limits is available in Section 3.2.7 of the Hydraulics Technical Manual.

Local Provisions:
Normal Depth (Uniform Flow):
For uniform flow calculations, the theoretical channel dimensions, computed by the slope-area methods outlined in the iSWM manual, are to be used only for an initial dimension in the design of an improved channel. Exceptions will be for small outfall channels (with the approval of the Storm Water Manager) with the following options:
	* Completely contained on the development site for on-site drainage;
	* Where no off-site drainage easement is required (i.e. not crossing or adjacent to another property that could be flooded if design storm occurs).
	* No nearby downstream restrictions.
Backwater Profile (Gradually Varied Flow):
City of Azle requires a hand computed or HEC-RAS backwater/frontwater analysis on any proposed open channel to determine the actual tailwater elevations, channel capacity and freeboard, and impacts on adjacent floodplains. If a stream or creek has an effective FEMA model, the engineer will be required to use a computer program for the analysis. If the current effective FEMA model for the stream is a HEC-2 model, the engineer has the option to either use that model, or convert to HEC-RAS for analysis of proposed conditions.
Supercritical Flow Regime
Supercritical flow will not be allowed except under unusual circumstances, with special approval of the City staff. However, for lined channels, the hand computed front water or HEC-RAS analysis should include a mixed-flow regime analysis, to make sure no supercritical flow occurs. City of Azle requires that the computed flow depths in designed channels be outside of the range of instability, i.e. depth of flow should be at least 1.1 times critical depth.
Channel Transitions or Energy Dissipation Structures or Small Dams
A HEC-RAS model or complete hand computed backwater analysis is a standard requirement for design of channel transitions (upstream and downstream), energy dissipation structures, and small dams. A backwater analysis will be required by the City of Azle, either hand computed or HEC-RAS, to determine accurate tailwater elevation, head losses, headwater elevations and floodplains affected by the proposed transition into and out of an improved channel, any on-stream energy dissipating structures, and small dams (less than 6 feet). If the current effective FEMA model for the stream is a HEC-2 model, the engineer has the option to either use that model, or convert to HEC-RAS for analysis of proposed conditions. For larger dams, a hydrologic routing will be required, as well as hydraulic analysis, to determine impacts of the proposed structure on existing floodplains and adjacent properties.
General Criteria
Earthen Channels
	1. An earthen channel shall have a trapezoidal shape with side slopes not steeper than a 4:1 ratio and a channel bottom at least four (4) feet in width.
	2. One (1) foot of freeboard above the 100-year frequency ultimate development water surface elevation must be available within all designed channels at all locations along the channel.
	3. The side slopes and bottom of an earthen channel shall be smooth, free of rocks, and contain a minimum of six (6) inches of topsoil. The side slopes and channel bottom shall be revegetated with grass. No channel shall be accepted for maintenance by the City until a uniform (e.g., evenly distributed, without large bare areas) vegetative cover with a density of 70% has been established.
	4. The Storm Water Manager may require each reach of a channel to have a ramp for maintenance access. Ramps shall be at least ten (10) feet wide and have 15% maximum grade. Twelve-foot (12') channel width is required if ramp is deemed necessary by Storm Water Manager.
	5. Minimum channel slope is 0.0020 ft/ft unless approved by the Storm Water Manager.
	6. Erosion protection to be provided at outfall to the receiving stream.
Lined Channels
	1. Channels shall be trapezoidal in shape and lined with reinforced concrete in accordance with City Standards and Specifications with side slopes of two (2) foot horizontal to one (1) foot vertical or otherwise to such standards, shape and type of lining as may be approved by the Storm Water Manager. The lining shall extend to and include the water surface elevation of the 100-year design storm plus one foot freeboard above the 100-year water surface elevation.
	2. The channel bottom must be a minimum of four (4) feet in width. (Overflow structures for storm sewer system sumps may have a minimum bottom width of 6 feet.)
	3. The maximum water flow velocity in a lined channel shall be fifteen (15) feet per second except that the water flow shall not be supercritical in an area from 100' upstream from a bridge to 25' downstream from a bridge. Hydraulic jumps shall not be allowed from the face of a culvert to 50' upstream from that culvert. In general channels having supercritical flow conditions are discouraged.
	4. Whenever flow changes from supercritical to subcritical channel protection shall be provided to protect from the hydraulic jump that is anticipated (see comment in Item 3).
	5. The design of the channel lining shall take into account the super-elevation of the water surface around curves and other changes in direction.
	6. A chain-link fence six (6) feet in height or other fence may be required by the Storm Water Manager and shall be constructed on each side of the concrete or gabion channel lining.
	7. The Storm Water Manager may require a geotechnical study and/or an underground drainage system design for concrete lined channels.
Roadside Ditches
Design Storms
	1. A roadside ditch (“rural”) street section is permissible only as specifically approved by the Storm Water Manager. No median ditches are allowed.
	2. The design storm for the roadside ditches shall be the 100-year storm. The 100-year flow shall not exceed the right-of-way capacity defined as the natural ground at the right-of-way line or top of roadside ditch.
	Design Considerations
	1. For grass-lined sections, the maximum design velocity shall be 6.0 feet per second during the 100-year design storm (Higher velocities justified by a sealed geotechnical study).
	2. A grass-lined or unimproved roadside ditch shall have minimum 2-foot bottom width and side slopes no steeper than four horizontal to one vertical. There shall be a four-foot strip at maximum 2% cross slope between the edge of pavement and the beginning of the ditch.
	3. Minimum grades for roadside ditches shall be 0.0050 foot/foot (0.50%).
	4. Manning’s roughness coefficient for analysis and design of roadside ditches are presented in Section 3.2.3 in the iSWM Hydraulics Technical Manual.
	5. Erosion protection will be provided at the upstream and downstream ends of all culverts.
	6. Maximum depth will not exceed 4 feet from centerline of pavement except as specifically approved by Storm Water Manager.
	7. If the ditch extends beyond the right-of-way line, an additional drainage easement shall be dedicated extending at least 2 feet beyond the top of bank. Utility easements must be separate and beyond any drainage easements.
	8. Hydraulic analysis of roadside ditches will require a HEC-RAS analysis.
Culverts in Roadside Ditches
	1. Culverts will be placed at all driveway and roadway crossings and other locations where appropriate.
	2. Erosion protection will be provided at all driveway and roadway crossings and other locations where appropriate.
	3. Roadside culverts are to be sized based on drainage area, assuming inlet control. Calculations are to be provided for each block based on drainage calculations. The size of culvert used shall not create a head loss of more than 0.20 feet greater than the normal water surface profile without the culvert.
	4. Roadside ditch culverts will be no smaller than 24 inches inside diameter or equivalent for roadway crossings and 18 inches for driveway culverts.
	5. A driveway culvert schedule shall be included on the face of the plat. It shall include for each lot approximate culvert flowline depth below top of pavement, number and size of pipe required, and horizontal distance from edge of pavement to center of culvert (based on horizontal control requirements above).
Channel Velocity Limitations
Maximum allowable:
	* Lined Channels - Maximum velocities = 15 fps. Exceptions can be granted by the Storm Water Manager, with justifiable, technical reasons.
	* Grass Lined Channels - Maximum velocities = 6 fps. Higher values can be justified by a sealed geotechnical study/analysis of soil type and conditions.
Critical Flow Calculations
Section 3.2.5 Critical Flow Calculations of the iSWM Hydraulics Technical manual is for reference only.
Vegetative Design
Section 3.2.6 Vegetative Design of the iSWM Hydraulics Technical manual is for reference only.
Stone Riprap Design
Riprap design is to be by Method #2 (Gregory Method) described in Section 3.2.7 of the iSWM Hydraulics Technical Manual. A properly designed geotextile material is required under the granular bedding. Regardless of computed thickness the minimum allowable riprap thickness is 12 inches.
Section 3.2.7 of the iSWM Hydraulics Technical Manual, Stone Riprap Design Method #1: Maynard and Reese, is for reference only.
Grouted Riprap
The City of Azle will allow grouted stone riprap as an erosion control feature. However, the design thickness of the stone lining will not be reduced by the use of grout. See the U.S. Army Corps of Engineers’ design manual ETL 1110-2-334 on design and construction of grouted riprap.
Uniform Flow - Example Problems
Section 3.2.9 Uniform Flow - Example Problems in the iSWM Hydraulics Technical manual are for reference only.
Rectangular, Triangular, and Trapezoidal Open Channel Design
Section 3.2.11 Rectangular, Triangular, and Trapezoidal Open Channel Design - Example Problems in the iSWM Hydraulics Technical manual are for reference only.

Manning Roughness Coefficients for Design
Table 14.3.9C. City of Azle Manning’s Roughness Coefficients for Design
Lining Type	Manning’s n	Comments
Grass Lined	0.035 0.050	Use for velocity check
		Use for channel capacity check (freeboard check)
Concrete Lined	0.015
Gabions	0.030
Rock Riprap	0.040	N = 0.03950 1/6 where d50 is the stone size of which 50% of the sample is smaller
Grouted Riprap	0.028	FWHA

Table 14.3.10. Roughness Coefficients (Manning’s n) and Allowable Velocities for Natural Channels
Channel Description	Manning’s n	Max. Permissible Channel Velocity (ft/s)
MINOR NATURAL STREAMS
Fairly regular section
1. Some grass and weeds, little or no brush	0.030	3 to 6
2. Dense growth of weeds, depth of flow materially greater than weed height	0.035	3 to 6
3. Some weeds, light brush on banks	0.035	3 to 6
4. Some weeds, heavy brush on banks	0.050	3 to 6
5. Some weeds, dense willows on banks	0.060	3 to 6
For trees within channels with branches submerged at high stage, increase above values by	0.010
Irregular section with pools, slight channel meander, increase above values by	0.010
Floodplain - Pasture
1. Short grass	0.030	3 to 6
2. Tall grass	0.035	3 to 6
Floodplain - Cultivated Areas
1. No crop	0.030	3 to 6
2. Mature row crops	0.035	3 to 6
3. Mature field crops	0.040	3 to 6
Floodplain - Uncleared
1. Heavy weeds scattered brush	0.050	3 to 6
2. Wooded	0.120	3 to 6
MAJOR NATURAL STREAMS
Roughness coefficient is usually less than for minor streams of similar description on account of less effective resistance offered by irregular banks or vegetation on banks. Values of “n” for larger streams of mostly regular sections, with no boulders or brush	Range from 0.028 to 0.060	3 to 6
UNLINED VEGETATED CHANNELS
Clays (Bermuda Grass)	0.035	5 to 6
Sandy and Silty Soils (Bermuda Grass)	0.035	3 to 5
UNLINED NON-VEGETATED CHANNELS
Sandy Soils	0.030	1.5 to 2.5
Silts	0.030	0.7 to 1.5
Sandy Silts	0.030	2.5 to 3.0
Clays	0.030	3.0 to 5.0
Coarse Gravels	0.030	5.0 to 6.0
Shale	0.030	6.0 to 10.0
Rock	0.025	15

For natural channels with specific vegetation type, refer to Table 14.3.11 for more detailed velocity control.

Table 14.3.11. Maximum Velocities for Vegetative Channel Linings
Vegetation Type	Slope Range (%)1	Maximum Velocity2 (ft/s)
Bermuda grass	0 - 5	6
Bahia		4
Tall fescue grass mixtures3	0 - 10	4
Kentucky bluegrass	0 - 5	6
Buffalo grass	5 - 10 > 10	5 4
Grass mixture0 - 51 5 - 10	4 3
Sericea lespedeza, Weeping lovegrass, Alfalfa	0 - 54	3
Annuals5	0 - 5	3
Sod		4
Lapped sod		5

¹Do not use on slopes steeper than 10% except for side-slope in combination channel.

²Use velocities exceeding 5 ft/s only where good stands can be maintained.

³Mixtures of Tall Fescue, Bahia, and/or Bermuda

⁴Do not use on slopes steeper than 5% except for side-slope in combination channel.

⁵Annuals - used on mild slopes or as temporary protection until permanent covers are established.

Source: Manual for Erosion and Sediment Control in Georgia, 1996.

Vegetative Design
*	A two-part procedure is required for final design of temporary and vegetative channel linings.
*	Part 1, the design stability component, involves determining channel dimensions for low vegetative retardance conditions, using Class D as defined in Table 14.3.12.
*	Part 2, the design capacity component, involves determining the depth increase necessary to maintain capacity for higher vegetative retardance conditions, using Class C as defined in Table 14.3.12.
*	If temporary lining is to be used during construction, vegetative retardance Class E shall be used for the design stability calculations.
*	If the channel slope exceeds 10%, or a combination of channel linings will be used, additional procedures not presented below are required. References include HEC-15 (USDOT, FHWA, 1986) and HEC-14 (USDOT, FHWA, 1983).

Local Provisions: For reference only.

Table 14.3.12. Classification of Vegetal Covers as to Degrees of Retardance
Retardance Class	Cover	Condition
A	Weeping Lovegrass	Excellent stand, tall (average 30")
	Yellow Bluestem Ischaemum	Excellent stand, tall (average 36")
B	Kudzu	Very dense growth, uncut
	Bermuda grass	Good stand, tall (average 12")
	Native grass mixture Little bluestem, bluestem, blue gamma other short and long stem Midwest grasses	Good stand, unmowed
	Weeping lovegrass	Good stand, tall (average 24")
	Lespedeza sericea	Good stand, not woody, tall (average 19")
	Alfalfa	Good stand, uncut (average 11")
	Weeping lovegrass	Good stand, unmowed (average 13")
	Kudzu	Dense growth, uncut
	Blue gamma	Good stand, uncut (average 13")
C	Crabgrass	Fair stand, uncut (10 - 48")
	Bermuda grass	Good stand, mowed (average 6")
	Common lespedeza	Good stand, uncut (average 11")
	Grass-legume mixture: summer (orchard grass redtop, Italian ryegrass, and common lespedeza)	Good stand, uncut (6 - 8")
	Centipede grass	Very dense cover (average 6")
	Kentucky bluegrass	Good stand, headed (6 - 12")
D	Bermuda grass	Good stand, cut to 2.5"
	Common lespedeza	Excellent stand, uncut (average 4.5")
	Buffalo grass	Good stand, uncut (3 - 6")
	Grass-legume mixture: fall, spring (orchard grass, redtop, Italian ryegrass, and common lespedeza)	Good stand, uncut (4 - 5")
	Lespedeza serices	After cutting to 2" (very good before cutting)
E	Bermuda grass	Good stand, cut to 1.5"
	Bermuda grass	Burned stubble

Note: Covers classified have been tested in experimental channels. Covers were green and generally uniform.

Source: HEC-15, 1988.

Culverts
Design Frequency
Culverts are cross drainage facilities that transport runoff under roadways or other improved areas.
*	Culverts shall be designed for the flood mitigation storm or in accordance with TxDOT requirements, whichever is more stringent. Consideration when designing culverts includes: roadway type, tailwater or depth of flow, structures, and property subject to flooding, emergency access, and road replacement costs.
*	The flood mitigation storm shall be routed through all culverts to be sure building structures (e.g., houses, commercial buildings) are not flooded or increased damage does not occur to the highway or adjacent property for this design event.

Local Provisions: 100-year storm for fully developed watershed conditions, or in accordance with TxDOT requirements, whichever is more stringent. For multiple barrel culverts the City of Azle encourages the placement of one of the barrels at the flowline of the stream with the other barrels at a higher elevation to encourage a single flow path for lower flow and reduce sediment and debris accumulation. Where practical the low-flow portion of the low barrel(s) should convey 2% of the design 100-year discharge.

Design Criteria
	Velocity Limitations
	*	The maximum velocity shall be consistent with channel stability requirements at the culvert outlet.
	*		The maximum allowable velocity for corrugated metal pipe is 15 feet per second. There is no specified maximum allowable velocity for reinforced concrete pipe, but outlet protection shall be provided where discharge velocities will cause erosion conditions.
	*	To ensure self-cleaning during partial depth flow, a minimum velocity of 2.5 feet per second is required for the streambank protection storm when the culvert is flowing partially full.
	Length and Slope
	*	The maximum slope using concrete pipe is 10% and for CMP is 14% before pipe-restraining methods must be taken.
	*	Maximum vertical distance from throat of intake to flowline in a drainage structure is 10 feet.
	*	Drops greater than 4 feet will require additional structural design.
	Headwater Limitations
	*	The allowable headwater is the depth of water that can be ponded at the upstream end of the culvert during the design flood, which will be limited by one or more of the following constraints or conditions:
		1.	Headwater will be non-damaging to upstream property.
		2.	Culvert headwater plus 12 inches of freeboard shall not exceed top of curb or pavement for low point of road over culvert, whichever is lower.
		3.	Ponding depth will be no greater than the elevation where flow diverts around the culvert.
		4.	Elevations will be established to delineate floodplain zoning.
	*	The headwater shall be checked for the flood mitigation storm elevation to ensure compliance with floodplain management criteria and the culvert shall be sized to maintain flood-free conditions on major thoroughfares with 12-inch freeboard at the low-point of the road.
	*	Either the headwater shall be set to produce acceptable velocities or stabilization/energy dissipation shall be provided where these velocities are exceeded.
	*	In general, the constraint that gives the lowest allowable headwater elevation establishes the criteria for the hydraulic calculations.
	Tailwater Considerations
	*	If the culvert outlet is operating with a free outfall, the critical depth and equivalent hydraulic grade line shall be determined.
	*	For culverts that discharge to an open channel, the stage-discharge curve for the channel must be determined. See Section 2.1.4 of the Hydraulics Technical Manual on methods to determine a stage-discharge curve.
	*	If an upstream culvert outlet is located near a downstream culvert inlet, the headwater elevation of the downstream culvert will establish the design tailwater depth for the upstream culvert.
	*	If the culvert discharges to a lake, pond, or other major water body, the expected high water elevation of the particular water body will establish the culvert tailwater.
	Other Criteria
	*	In designing debris control structures, the Hydraulic Engineering Circular No. 9 entitled Debris Control Structures or other approved reference is required to be used.
	*	If storage is being assumed or will occur upstream of the culvert, refer to Section 2.0 of the Hydraulics Technical Manual regarding storage routing as part of the culvert design.
	*	Reinforced concrete pipe (RCP), pre-cast and cast in place concrete boxes are recommended for use (1) under a roadway, (2) when pipe slopes are less than 1%, or (3) for all flowing streams. RCP and fully coated corrugated metal pipe or high-density polyethylene (HDPE) pipe may also be used in open space areas.
	*	Culvert skews shall not exceed 45 degrees as measured from a line perpendicular to the roadway centerline without approval.
	*	The minimum allowable pipe diameter shall be 18 inches.
	*	Erosion, sediment control, and velocity dissipation shall be designed in accordance with Section 4.0 of the Hydraulics Technical Manual.

Local Provisions: City of Azle requires a backwater analysis, by hand, or HEC-RAS to evaluate proposed structure for final design. The Culvert Hydraulics Checklist Appendix A - City of Azle Detailed Checklists (Form CITY OF AZLE-4) should be completed for each design.

Nomographs

Nomographs are not allowed by City of Azle for final sizing of culverts. The reference for nomographs is FHWA HDS-5. A backwater analysis using HEC-RAS is required.

Culvert Design Example

Section 3.3.5 Culvert Design Example of the iSWM Hydraulics Technical manual is adopted with the following modifications. The (nomographs) procedure is acceptable for preliminary sizing only.

Design Procedures for Beveled-Edged Inlets

Section 3.3.6 Design Procedures for Beveled-Edged Inlets of the iSWM Hydraulics Technical manual is adopted with the following modifications. The procedure is acceptable for preliminary sizing only.

Flood Routing and Culvert Design

Section 3.3.7 Flood Routing and Culvert Design of the iSWM Hydraulics

Technical Manual is for reference only.

Erosion, Sediment Control, Velocity Dissipation

See iSWM Hydraulics Technical Manual Section 3.2.7, Gregory Method for culvert outfall protection for riprap sizing, gradation, and bedding. Use Section 4.0 of that Manual for spatial dimensions of riprap and other energy dissipation design.

Bridges
Design Frequency
Bridges are cross drainage facilities with a span of 20 feet or larger.
*	Flood mitigation storm for all bridges

Local Provisions: 100-year storm for fully developed watershed conditions or in accordance with TxDOT requirements, whichever is more stringent.

Design Criteria
*	A freeboard of two feet shall be maintained between the computed design water surface and the low chord of all bridges.
*	The contraction and expansion of water through the bridge opening creates hydraulic losses. These losses are accounted for through the use of loss coefficients. Table 14.3.13 gives recommended values for the Contraction (Kc) and Expansion (Kc) Coefficients.

Table 14.3.13 Recommended Loss Coefficients for Bridges
Transition Type	Contraction (Kc)	Expansion (Ke)
No losses computed	0.0	0.0
Gradual transition	0.1	0.3
Typical bridge	0.3	0.5
Severe transition	0.6	0.8

Additional design guidance is located in Section 3.4 of the Hydraulics Technical Manual.

Local Provisions: A backwater analysis using HEC-RAS is used for final design of the proposed structure. For bridges up to 100' width (measured at low chord), 2' of freeboard required; for bridge >100' width, 1' of freeboard required. Exceptions on freeboard must be approved by City of Azle. Complete Bridge Hydraulics Documentation Checklist (Appendix A - City of Azle Detailed Checklists, Form CITY OF AZLE-5.

Backwater analysis will be required using HEC-RAS, for any proposed bridge, to determine accurate tailwater elevations, velocities, head losses, headwater elevations, profiles and floodplains affected by the proposed structure. If the current effective FEMA model is a HEC-2 model, the engineer has the option to either use that model, or convert to HEC-RAS for analysis of proposed conditions.

Detention Structures

Design Frequency

Detention structures shall be designed for the three storms (streambank protection, conveyance, and flood mitigation storms) for the critical storm duration that results in the maximum (or near maximum peak flow.

Local Provisions: 1-, 10-, and 100-year storm for the critical storm duration (i.e. 3-hour, 6-hour or 24-hour duration) that results in the maximum (or near maximum) peak flow. Analysis should consider both existing watershed plus developed site conditions and fully developed watershed conditions.

Design Criteria
*	Dry detention basins are sized to temporarily store the volume of runoff required to protection up to the flood mitigation storm, if required.
*	Extended detention dry basins are sized to provide extended detention of the stream volume over 24 hours and can also provide additional storage volume for normal detention (peak flow reduction) of the flood mitigation storm event.
*	Routing calculations must be used to demonstrate that the storage volume and outlet structure configuration are adequate. See Section 2.0 of the Hydraulics Technical Manual for procedures on the design of detention storage.
*	Detention Basins shall be designed with an 8-foot-wide maintenance access.
*	No earthen (grassed) embankment slopes shall exceed 4:1.
*	A freeboard of 1 foot will be required for all detention ponds.
*	A calculation summary shall be provided on construction plans. For detailed calculations of unit hydrograph studies, a separate report shall be provided to the municipality for review and referenced on the construction plans. Stage-storage-discharge values shall be tabulated and flow calculations for discharge structures shall be shown on the construction plans.
*	An emergency spillway shall be provided at the flood mitigation maximum storage elevation with sufficient capacity to convey the flood mitigation storm assuming blockage of the outlet works with six inches of freeboard. Spillway requirements must also meet all appropriate state and Federal criteria.
*	A landscape plan shall be provided for all detention ponds.
*	All detention basins shall be stabilized against significant erosion and include a maintenance plan.
*	Design calculations will be provided for all spillways and outlet structures.
*	Maintenance agreements shall be included for all detention structures.
*	Storage may be subject to the requirements of the Texas Dam Safety Program (see iSWM Program Guidance) based on the volume, dam height, and level of hazard.
*	Earthen embankments 6 feet in height or greater shall be designed per Texas Commission on Environmental Quality guidelines for dam safety (see iSWM Program Guidance).
*	Vegetated slopes shall be less than 20 feet in height and shall have side slopes no steeper than 2:1 (horizontal to vertical) although 3:1 is preferred. Riprap-protected slopes shall be no steeper than 2:1. Geotechnical slope stability analysis is recommended for slopes greater than 10 feet in height. Vegetated slopes with a side slope steeper than 2:1 shall require detailed geotechnical and slope stability analysis to justify slopes steeper than 2:1.
*	Areas above the normal high water elevations of the detention facility should be sloped toward the basin to allow drainage and to prevent standing water. Careful finish grading is required to avoid creation of upland surface depressions that may retain runoff. The bottom area of storage facilities should be graded toward the outlet to prevent standing water conditions. A low flow or pilot channel across the facility bottom from the inlet to the outlet (often constructed with riprap) is recommended to convey low flows and prevent standing water conditions.

Local Provisions: Stormwater detention shall be provided to mitigate increased peak flows in Azle waterways in specific circumstances as defined below. The purpose of the mitigation is to minimize downstream flooding impacts from upstream development. In some instances, detention may be shown to exacerbate potential flooding conditions downstream. Therefore, the “Zone of Influence” criteria shall be applied in addition to these criteria. Design data for dams will be submitted to the City of Azle on Form CITY OF AZLE-6.

1. Detention Basins shall be required for all Development greater than 1 acre in size or when downstream facilities within the “Zone of Influence” are not adequately sized to convey a design storm based on current City criteria for hydraulic capacity.

2. Calculated proposed stormwater discharge from a site shall not exceed the calculated discharges from existing conditions, unless sufficient downstream capacity above existing discharge conditions is available.

3. The Modified Rational Method is allowed for planning and conceptual design for watersheds of 200 acres and less. For final design purposes the Modified Rational Method is allowed only for watersheds of 25 acres and less (see Table 1.2 in the iSWM Hydrologic Manual).

4. Detention Basins draining watersheds over 25 acres shall be designed using a detailed unit hydrograph method acceptable to the City of Azle. These include Snyder’s Unit Hydrograph (>100 acres) and SCS Dimensionless Unit Hydrograph (any size). The SCS method is also allowed for basins with watersheds less than 25 acres (see Table 1.2 in the iSWM Hydrologic Manual).

5. Detention Basins shall be designed for the 1-year, 10-year and 100-year storm for the critical storm duration (i.e. 3-hour, 6-hour, or 24-hour storm duration) that results in the maximum (or near maximum) peak flow. Analysis of additional storm (i.e. 5-year, 25-year, etc.) may be required where storm sewers are included in the watershed.

6. No earthen (grassed) embankment slopes shall exceed 4:1. Concrete lined or structural embankment can be steeper with the approval of the Storm Water Manager.

7. A calculation summary shall be provided on construction plans. For detailed calculations of unit hydrograph studies, a separate report shall be provided to the City for review and referenced on the construction plans. Stage-storage-discharge values shall be tabulated and flow calculations for discharge structures shall be shown on the construction plans.

8. An emergency spillway shall be provided at the 100-year maximum storage elevation with sufficient capacity to convey the fully urbanized 100-year storm assuming blockage of the closed conduit portion outlet works with six inches of freeboard. Spillway requirements must also meet all appropriate state and Federal criteria.

9. All detention basins shall be stabilized against significant erosion and include a maintenance plan.

10. State TCEQ rules and regulations regarding impoundments shall be followed. According to current (2009) guidelines, dams fall under the jurisdiction of the TCEQ Dam Safety Program if they meet one or more of the following criteria:

i. They have a height greater than or equal to 25 feet and a maximum storage capacity greater than or equal to 15 acre-feet;

ii. They have a height greater than 6 feet and a maximum storage capacity greater than or equal to 50 acre-feet.

iii. They are a high or significant hazard dam as defined in the regulations (relating to Hazard Classification Criteria), regardless of height or maximum storage capacity; or

iv. They are used as a pumped storage or terminal storage facility.

11. Design calculations will be provided for all spillways.

12. Maintenance agreements will be provided.

13. In accordance with Texas Water Code §11.142, all permanent surface impoundments not used solely for domestic or livestock purposes must obtain a water rights permit from the TCEQ. A completed permit for the proposed use, or written documentation stating that a permit is not required, must be obtained.

14. Detention basin outlet structures shall be designed to minimize the likeliness of clogging and shall include features to prevent activation of the emergency spillway if such activation would create an uncontrolled discharge. The use of orifice plates or non-standard structures is subject to the approval of the Storm Water Manager.

15. Dry detention basin design should consider multiple uses such as recreation. As such pilot channels should follow the edges of the basin to the extent practical. The bottom of the basin shall have a minimum grade of 1% per Figure 5.9 in Section 14.5.0, although swales may have minimum grades of 0.5%. Concrete flumes shall be used for main pilot channels shallower than 0.5% slope.

Items 6, 7, 9, 10, 11, 12 and 14 also apply to amenity ponds.

Outlet Structures

Extended detention (ED) orifice sizing is required in design applications that provide extended detention for downstream streambank protection or the ED portion of the water quality protection volume. The release rate for both the WQ_v and SP _v shall discharge the ED volume in a period of 24 hours or longer. In both cases an extended detention orifice or reverse slope pipe must be used for the outlet. For a structural control facility providing both WQ _v extended detention and SP_v control (wet ED pond, micropool ED pond, and shallow ED wetland), there will be a need to design two outlet orifices - one for the water quality control outlet and one for the streambank protection drawdown.

Design Frequency

Water quality storm

Streambank protection storm

Conveyance storm

Flood mitigation storm

Local Provisions: NONE

Design Criteria
*	Estimate the required storage volumes for water quality protection, streambank protection, conveyance storm, and flood mitigation.
*	Design extended detention outlets for each storm event.
*	Outlet velocities shall be within the maximum allowable range based on channel material as shown in Tables 14.3.10 and 14.3.11.
*	Design necessary outlet protection and energy dissipation facilities to avoid erosion problems downstream from outlet devices and emergency spillway(s).
*	Perform buoyancy calculations for the outlet structure and footing. Flotation will occur when the weight of the structure is less than or equal to the buoyant force exerted by the water.
Additional design guidance is located in Section 2.2 of the Hydraulics Technical Manual.

Local Provisions: NONE

Energy Dissipation
Design Frequency
All drainage system outlets, whether for closed conduits, culverts, bridges, open channels, or storage facilities, shall provide energy dissipation to protect the receiving drainage element from erosion.
*	Conveyance storm
*	Flood mitigation storm

Local Provisions: 100-year design storm for fully developed watershed conditions.

Design Criteria
*	Energy dissipaters are engineered devices such as riprap aprons or concrete baffles placed at the outlet of stormwater conveyance systems for the purpose of reducing the velocity, energy and turbulence of the discharged flow.
*	Erosion problems at culvert, pipe and engineered channel outlets are common. Determination of the flow conditions, scour potential, and channel erosion resistance shall be standard procedure for all designs.
*	Energy dissipaters shall be employed whenever the velocity of flows leaving a stormwater management facility exceeds the erosion velocity of the downstream area channel system.
*	Energy dissipater designs will vary based on discharge specifics and tailwater conditions.
*	Outlet structures shall provide uniform redistribution or spreading of the flow without excessive separation and turbulence.
*	Energy dissipaters are a required component of the iSWM Construction Plan.
Recommended Energy Dissipaters for outlet protection include the following:
*	Riprap apron
*	Riprap outlet basins
*	Baffled outlets
*	Grade Control Structures
The reader is referred to Section 4.0 of the Hydraulics Technical Manual and the Federal Highway Administration Hydraulic Engineering Circular No. 14 entitled, Hydraulic Design of Energy Dissipaters for Culverts and Channels, for the design procedures of other energy dissipaters.
Additional design guidance is located in Section 4.0 of the Hydraulics Technical Manual.

Local Provisions:

Channel Transitions, Energy Dissipation Structures, or Small Dams

A backwater analysis is required by the City of Azle, either hand computed or HEC-RAS, to determine accurate tailwater elevation and velocities, head losses, headwater elevations, velocities and floodplains affected by the proposed transition into and out of 1) An improved channel, 2) Any on-stream energy dissipating structures, and 3) Small dams (less than 6 feet). If the current effective FEMA model for the stream is a HEC-2 model, the engineer has the option to either use that model, or convert to HEC-RAS for analysis of proposed conditions. For larger dams, a hydrologic routing will be required, as well as hydraulic analysis, to determine impacts of the proposed structure on existing floodplains and adjacent properties.

Examples of Open Channel Transition Structures

See drawings in Appendix C - Miscellaneous Details and Specifications for Harris County Flood Control District Straight Drop Structure, Bureau of Reclamation Baffled Chute (Basin IX) and Gabion Drop Structure. The computer program associated with FHWA Hydraulic Engineering Circular No. 14 is “HY8Energy” dated May 2000. This program provides guidance in the selection and sizing of a broad range of energy dissipaters including some of those listed in Section 4 of the iSWM Hydraulics Technical Manual.

(Ordinance 2012-10 adopted 8/21/12)

§ 14.3.7 Easements, Plats, and Maintenance Agreements.

Easements

Easements are required for all drainage systems that convey stormwater runoff across a development and must include sufficient area for operation and maintenance of the drainage system. Types of easements to be used include:

*	Drainage easements - are required for both on-site and off-site public storm drains and for improved channels designed according to current municipality standards.
*	Floodplain easements - shall be provided on-site along drainageways that are in a Special Flood Hazard Area as designated on the effective FEMA FIRM maps. No construction shall be allowed within a floodplain easement without the written approval of the municipality.
*	Temporary drainage easements are required off-site for temporary channels when future off-site development is anticipated to be enclosed underground or follows an altered alignment. Temporary drainage easements will not be maintained by the municipality and will not terminate until permanent drainage improvements meeting municipality standards are installed and accepted. Temporary drainage easements will require written approval from the municipality.
*	Drainage and utility easements can be combined for underground storm drains and channels, subject to adequate easement width as approved by the municipality.
*	Drainage easements shall include adequate width for access and maintenance beyond the top of bank for improved channels.
*	Retaining walls are not permitted within or adjacent to a drainage easement in a residential area in order to reduce the easement width. Retaining walls adjacent to the channel are allowed in nonresidential areas only if the property owner provides an agreement for private maintenance.
*	The minimum finished floor elevation for structures adjacent to a Special Flood Hazard Area shall be a minimum of one (1) foot above the fully developed flood mitigation stormwater surface elevation or two (2) feet above the effective FEMA base flood elevation.
*	Improved channels shall have drainage easements dedicated to meet the requirements of the width of the channel, the one-foot freeboard, any perimeter fencing, and any underground tiebacks or anchors.
*	Easements for detention ponds and permanent control BMPs shall be negotiated between the municipality and the property owner.
*	The entire reach or each section of any drainage facility must be readily accessible to maintenance equipment. Additional easement(s) shall be required at the access point(s) and the access points shall be appropriately designed to restrict access by the public (including motorcycles).

Minimum easement width requirements for storm drain pipe are shown in Table 14.3.14 and shall be as follows:

*	The outside face of the proposed storm drain line shall be placed five (5) feet off either edge of the storm drain easement. The proposed centerline of overflow swales shall normally coincide with the centerline of the easement.
*	For pipe sizes up to 54", a minimum of five (5) additional feet shall be dedicated when shared with utilities.
*	Box culvert minimum easement width shall be determined using Table 14.3.14 based on an equivalent box culvert width to pipe diameter.
*	For parallel storm drain systems with a combined width greater than 8 feet the minimum easement shall be equal to the width of the parallel storm drain system plus twenty (20) additional feet.
*	Drainage easements will generally extend at least twenty-five (25) feet past an outfall headwall to provide an area for maintenance operations. Drainage easements along a required outfall channel or ditch shall be provided until the flowline reaches an acceptable outfall. The minimum storm drain shall not be on property line, except where a variance has been granted.

Table 14.3.14. Closed Conduit Easements
Pipe Size	Minimum Easement Width Required
39" and under	15 Feet
42" through 54"	20 Feet
60" through 66"	25 Feet
72" through 102"	30 Feet

Local Provisions:

Easements for Open Channels and Detention Ponds:

* Drainage easements shall be required for both on-site and off-site public stormwater drainage improvements, including standard engineered channels, storm drain systems, detention and retention facilities and other stormwater controls. (Public Water). Drainage easements shall include a five-foot (5') margin on both sides beyond actual top of bank for improved earthen channels. Retaining walls are not permitted within or adjacent to a drainage easement in a residential area in order to reduce the easement width. Retaining walls adjacent to the channel are allowed in nonresidential areas only if the property owner provides an agreement for private maintenance.

* Floodplain easements shall be provided on sites along natural or improved earthen drainageways (other than standard engineered channels); to encompass the ultimate developed 100-year floodplain plus a 10' buffer on either side. The buffer shall be part of the floodplain easement itself and not a separate easement. Floodplain easements are not routinely maintained by the City.

* Natural creeks shall have a dedicated floodplain easement containing the inundation area of a 100-year frequency storm based on ultimate developed conditions, plus a ten-foot buffer horizontally adjacent to the inundation area. The minimum finished floor elevation for lots impacted by natural creeks shall be a minimum of two (2) feet above the 100-year ultimate developed water surface elevation. In addition, a riparian area along the creek may be placed in a drainage easement for perpetual, limited maintenance by the City of Azle, subject to the approval of the City of Azle and an agreement to preserve natural conditions and habitat within the riparian area.

* Concrete Lined Channels and Gabion Lined Channels shall have drainage easements dedicated to meet the requirements of the width of the channel, the one-foot freeboard, and the fence, if required by Storm Water Manager.

* Private drainage easements, not dedicated to the City, may be required for private stormwater drainage improvements serving multiple lots or for stormwater controls on a property. (No Public Water)

* Access easements shall be provided for access to public stormwater drainage improvements where necessary for maintenance.

* Dam easements shall be provided, to encompass any proposed dams (including any dams already existing) and spillway structures. The 100-year water surface of any impounded lake shall be covered by a floodplain easement as described above. Dams and spillways shall comply with applicable City policy and state regulations.

* No construction shall be allowed within a floodplain easement without the written approval (floodplain permit) of the City of Azle, and then only after detailed engineering plans and studies show that no flooding will result, and that no obstruction to the natural flow of water will result.

* In certain circumstances where detention is in place or a master drainage plan has been adopted, a development may plan to receive less than ultimate developed flow conditions from upstream with the approval of the Storm Water Manager.

* Any parallel utility easements must be separate and outside of drainage easements for channels. Drainage and utility easements may be combined for underground storm drains, subject to the easement width requirements provided in this section and Section 14.3.3.

* Easements for stormwater controls including detention basins, sediment traps and retention ponds, shall be negotiated between the City and the Property Owner, but will normally include essential access to all embankment areas and inlet and outlet controls.

* The entire reach or each section of any drainage facility must be readily accessible to maintenance equipment. Additional easement(s) shall be required at the access point(s) and the access points shall be appropriately designed to restrict access by the public (including motorcycles).

* Drainage easements for structural overflows, swales, or berms shall be of sufficient width to encompass the structure or graded area.

City of Azle Easement Requirements for Closed Conduit Systems

* Box culverts shall have an easement width equal to the width of the box plus twenty (20) additional feet. The edge of the box should be located five (5) feet from either edge of the easement.

* Drainage easements shall encompass the entire width of an overflow flume plus five feet on each side. For an easement containing both a concrete flume and a storm drain, the wider of the two easement criteria shall control.

* Alternatively, a drainage right-of-way or HOA lot (not part of any adjacent lot) may be dedicated for the width of the flume provided that an additional easement is dedicated for any storm drain pipe to meet the total width requirements specified above.

Plats
All platting shall follow established development standards established by the local municipality. Plats shall include pertinent drainage information that will be filed with the plat. Elements to be included on the plat include:
*	All public and private drainage easements not recorded by separate instrument
*	Easements to be recorded by separate instrument shall be documented on the plat
*	All floodplain easements
*	Legal disclosure for drainage provisions upon sale or transfer of property
*	Documentation of maintenance responsibilities and agreements including transfer of responsibility upon sale of the property

Local Provisions: NONE

Maintenance Agreements

All drainage improvements constructed within a development and any existing or natural drainage systems to remain in use shall require a maintenance agreement that identifies responsible parties for maintenance. Both private and public maintenance responsibility shall be negotiated between the municipality and the owner and documented in the agreement. The maintenance agreement shall be written such that it remains in force upon sale of transfer of the property.

Local Provisions:

City Maintenance

The City of Azle will provide for perpetual maintenance, in accordance with adopted city maintenance standards, of all public drainage facilities located within dedicated easements and constructed to the City of Azle standards. In addition, limited perpetual maintenance may be provided by the City of Azle for riparian areas preserved in their natural state, subject to the approval of the City of Azle. Access shall be provided and dedicated by the developer to all public stormwater facilities in developments for maintenance and inspection by the City of Azle. City of Azle requires maintenance agreements only for private facilities.

Private Maintenance

* Private drainage facilities include those drainage improvements which are located on private property and which handle only private water.

* Private drainage facilities may also include detention or retention ponds, dams, and other stormwater controls which collect public water, as well as drainageways not constructed to City standards but which convey public water. Such facilities must be designed in accordance with sound engineering practices and reviewed and inspected by the City.

* An agreement for perpetual maintenance of private drainage facilities serving public water shall be executed with the City prior to acceptance of the final plat. This agreement shall run with the land and can be tied to commercial property or to an owner’s association, but not to individual residential lots.

* Access shall be provided by the developer/owner to all private drainage facilities where there may be a public safety concern for inspection by the City of Azle.

* Also see Section 14.5.1.3.

(Ordinance 2012-10 adopted 8/21/12)

§ 14.3.8 Stormwater Control Selection.

14.3.8.1 Control Screening Process

Outlined below is a screening process for structural stormwater controls that can effectively treat the water quality volume, as well as provide water quantity control. This process is intended to assist the site designer and design engineer in the selection of the most appropriate structural controls for a development site and to provide guidance on factors to consider in their location. This information is also contained in the iSWM Technical Manual - Site Development Controls section.

The following four criteria shall be evaluated in order to select the appropriate structural control(s) or group of controls for a development:

*	Stormwater treatment suitability
*	Water quality performance
*	Site applicability
*	Implementation considerations

In addition, the following factors shall be considered for a given site and any specific design criteria or restrictions need to be evaluated:

*	Physiographic factors
*	Soils
*	Special watershed or stream considerations

Finally, environmental regulations shall be considered as they may influence the location of a structural control on site or may require a permit.

The following steps provide a selection process for comparing and evaluating various structural stormwater controls using a screening matrix and a list of location and permitting factors. These tools are provided to assist the design engineer in selecting the subset of structural controls that will meet the stormwater management and design objectives for a development site or project.

Step 1. Overall Applicability

The following are the details of the various screening categories and individual characteristics used to evaluate the structural controls.

Table 14.3.15 - Stormwater Management Suitability

The first category in the matrix examines the capability of each structural control option to provide water quality treatment, downstream streambank protection, and flood control. A blank entry means that the structural control cannot or is not typically used to meet an integrated Focus Area. This does not necessarily mean that it should be eliminated from consideration, but rather it is a reminder that more than one structural control may be needed at a site (e.g., a bioretention area used in conjunction with dry detention storage).

Ability to treat the Water Quality Volume (WQ_v): This indicates whether a structural control provides treatment of the water quality volume (WQ_v). The presence of “P” or “S” indicates whether the control is a Primary or Secondary control, respectively, for meeting the TSS reduction goal.

Ability to provide Streambank Protection (SP_v): This indicates whether the structural control can be used to provide the extended detention of the streambank protection volume (SP_v). The presence of a “P” indicates that the structural control can be used to meet SP_v requirements. An “S” indicates that the structural control may be sized to provide streambank protection in certain situations, for instance on small sites.

Ability to provide Flood Control (Q): This indicates whether a structural control can be used to meet the flood control criteria. The presence of a “P” indicates that the structural control can be used to provide peak reduction of the flood mitigation storm event.

Table 14.3.16 - Relative Water Quality Performance

The second category of the matrix provides an overview of the pollutant removal performance for each structural control option when designed, constructed, and maintained according to the criteria and specifications in this manual.

Ability to provide TSS and Sediment Removal: This column indicates the capability of a structural control to remove sediment in runoff. All of the Primary structural controls are presumed to remove 70% to 80% of the average annual TSS load in typical urban post-development runoff (and a proportional removal of other pollutants).

Ability to provide Nutrient Treatment: This column indicates the capability of a structural control to remove the nutrients nitrogen and phosphorus in runoff, which may be of particular concern with certain downstream receiving waters.

Ability to provide Bacteria Removal: This column indicates the capability of a structural control to remove bacteria in runoff. This capability may be of particular concern when meeting regulatory water quality criteria under the Total Maximum Daily Load (TMDL) program.

Ability to accept Hotspot Runoff: This last column indicates the capability of a structural control to treat runoff from designated hotspots. Hotspots are land uses or activities that produce higher concentrations of trace metals, hydrocarbons, or other priority pollutants. Examples of hotspots might include: gas stations, convenience stores, marinas, public works storage areas, garbage transfer facilities, material storage sites, vehicle service and maintenance areas, commercial nurseries, vehicle washing/steam cleaning, landfills, construction sites, industrial sites, industrial rooftops, and auto salvage or recycling facilities. A check mark indicates that the structural control may be used on hotspot site. However, it may have specific design restrictions. Please see the specific design criteria of the structural control for more details in the Site Development Controls Technical Manual. Local jurisdictions may have other site uses that they designate as hotspots. Therefore, their criteria should be checked as well.

Table 14.3.17 - Site Applicability

The third category of the matrix provides an overview of the specific site conditions or criteria that must be met for a particular structural control to be suitable. In some cases, these values are recommended values or limits and can be exceeded or reduced with proper design or depending on specific circumstances. Please see the specific criteria section of the structural control for more details.

Drainage Area: This column indicates the approximate minimum or maximum drainage area considered suitable for the structural control practice. If the drainage area present at a site is slightly greater than the maximum allowable drainage area for a practice, some leeway can be permitted if more than one practice can be installed. The minimum drainage areas indicated for ponds and wetlands should not be considered inflexible limits and may be increased or decreased depending on water availability (baseflow or groundwater), the mechanisms employed to prevent outlet clogging, or design variations used to maintain a permanent pool (e.g., liners).

Space Required (Space Consumed): This comparative index expresses how much space a structural control typically consumes at a site in terms of the approximate area required as a percentage of the impervious area draining to the control.

Slope: This column evaluates the effect of slope on the structural control practice. Specifically, the slope restrictions refer to how flat the area where the facility is installed must be and/or how steep the contributing drainage area or flow length can be.

Minimum Head: This column provides an estimate of the minimum elevation difference needed at a site (from the inflow to the outflow) to allow for gravity operation within the structural control.

Water Table: This column indicates the minimum depth to the seasonally high water table from the bottom or floor of a structural control.

Table 14.3.18 - Implementation Considerations

The fourth category in the matrix provides additional considerations for the applicability of each structural control option.

Residential Subdivision Use: This column identifies whether or not a structural control is suitable for typical residential subdivision development (not including high-density or ultra-urban areas).

Ultra-Urban: This column identifies those structural controls appropriate for use in very high-density (ultra-urban) areas, or areas where space is a premium.

Construction Cost: The structural controls are ranked according to their relative construction cost per impervious acre treated, as determined from cost surveys.

Maintenance: This column assesses the relative maintenance effort needed for a structural stormwater control, in terms of three criteria: frequency of scheduled maintenance, chronic maintenance problems (such as clogging), and reported failure rates. It should be noted that all structural controls require routine inspection and maintenance.

Local Provisions: The Site Development Controls iSWM Technical Manual contains an exhaustive discussion and detailed examples of stormwater controls that can be implemented in land development to meet the goals of protecting water quality, minimizing streambank erosion, and reducing flood volumes. It is an excellent planning and design resource document and has valuable design examples that the City of Azle encourages local developers to consider in their site planning. Although it is primarily oriented toward water quality issues, these stormwater controls bring additional and valuable benefits for flood control and streambank protection. Many of the listed stormwater control features and techniques enhance the aesthetics and value of land developments, as well as providing a drainage function.

Since the City of Azle is currently emphasizing the streambank protection and flood control components of the integrated stormwater management approach, the Stormwater Control Section (Section 14.3.8) of applicable features that may be implemented in local developments and redevelopments.[,] The City of Azle does not mandate the use of any of these stormwater controls, but recognizes the inherent values of their application in overall stormwater management.

Therefore, the City of Azle adopts for design guidance and technical reference sections of the iSWM Technical Manual. There are, however, no City of Azle requirements for achieving Stormwater Quality or Channel Protection volumes.

Table 14.3.15. Stormwater Treatment Suitability
Category	Category	Category
Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas
Channels	Channels	Channels	Channels	Channels	Channels
Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment
Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components
Detention	Detention	Detention	Detention	Detention	Detention
Filtration	Filtration	Filtration	Filtration	Filtration	Filtration
Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices
Infiltration	Infiltration	Infiltration	Infiltration	Infiltration	Infiltration
Ponds	Ponds	Ponds	Ponds	Ponds	Ponds
Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces
Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems
Re-Use	Re-Use	Re-Use	Re-Use	Re-Use	Re-Use
Wetlands	Wetlands	Wetlands	Wetlands	Wetlands	Wetlands

P = Primary Control: Able to meet design criterion if properly designed, constructed and maintained.

S = Secondary Control: May partially meet design criteria. May be a Primary Control but designated as a Secondary due to other considerations. For Water Quality Protection, recommended for limited use in approved community-designated areas.

– = Not typically used or able to meet design criterion.

¹ = The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data if used as a primary control.

Table 14.3.16. Water Quality Performance
Category	Category	Category
Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas
Channels	Channels	Channels	Channels	Channels	Channels
Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment
Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components
Detention	Detention	Detention	Detention	Detention	Detention
Filtration	Filtration	Filtration	Filtration	Filtration	Filtration
Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices
Infiltration	Infiltration	Infiltration	Infiltration	Infiltration	Infiltration
Ponds	Ponds	Ponds	Ponds	Ponds	Ponds
Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces
Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems
Re-Use	Re-Use	Re-Use	Re-Use	Re-Use	Re-Use
Wetlands	Wetlands	Wetlands	Wetlands	Wetlands	Wetlands

S = Meets suitability criteria

– = Not typically used or able to meet design criterion.

1 = The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data if used as a primary control.

2 = Porous surfaces provide water quality benefits by reducing the effective impervious area.

Table 14.3.17. Site Applicability
Category	Category	Category
Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas
Channels	Channels	Channels	Channels	Channels	Channels	Channels
Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment
Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components
Detention	Detention	Detention	Detention	Detention	Detention	Detention
Filtration	Filtration	Filtration	Filtration	Filtration	Filtration	Filtration
Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices
Infiltration	Infiltration	Infiltration	Infiltration	Infiltration	Infiltration	Infiltration
Ponds	Ponds	Ponds	Ponds	Ponds	Ponds	Ponds
Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces
Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems
Re-Use	Re-Use	Re-Use	Re-Use	Re-Use	Re-Use	Re-Use
Wetlands	Wetlands	Wetlands	Wetlands	Wetlands	Wetlands	Wetlands

– = Not typically used or able to meet design criterion.

¹ = The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data if used as a primary control.

² = Porous surfaces provide water quality benefits by reducing the effective impervious area.

³ = Drainage area can be larger in some instances

Table 14.3.18. Implementation Considerations
Category	Category	Category
Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas
Channels	Channels	Channels	Channels	Channels	Channels
Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment
Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components
Detention	Detention	Detention	Detention	Detention	Detention
Filtration	Filtration	Filtration	Filtration	Filtration	Filtration
Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices
Infiltration	Infiltration	Infiltration	Infiltration	Infiltration	Infiltration
Ponds	Ponds	Ponds	Ponds	Ponds	Ponds
Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces
Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems
Re-Use	Re-Use	Re-Use	Re-Use	Re-Use	Re-Use
Wetlands	Wetlands	Wetlands	Wetlands	Wetlands	Wetlands

S = Meets suitability criteria

– = Not typically used or able to meet design criterion.

¹ = The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data if used as a primary control.

Step 2. Step 2 Specific Criteria

The last three categories in the Structural Control Screening matrix provide an overview of various specific design criteria and specifications, or exclusions for a structural control that may be present due to a site’s general physiographic character, soils, or location in a watershed with special water resources considerations.

Table 14.3.19 - Physiographic Factors
Three key factors to consider are low relief, high relief, and karst terrain. In the North Central Texas, low relief (very flat) areas are primarily located east of the Dallas metropolitan area. High relief (steep and hilly) areas are primarily located west of the Azle metropolitan area. Karst and major carbonaceous rock areas are limited to portions of Palo Pinto, Erath, Hood, Johnson, and Somervell counties. Special geotechnical testing requirements may be needed in karst areas. The local reviewing authority should be consulted to determine if a project is subject to terrain constraints.
*	Low relief areas need special consideration because many structural controls require a hydraulic head to move stormwater runoff through the facility.
*	High relief may limit the use of some structural controls that need flat or gently sloping areas to settle out sediment or to reduce velocities. In other cases, high relief may impact dam heights to the point that a structural control becomes infeasible.
*	Karst terrain can limit the use of some structural controls as the infiltration of polluted waters directly into underground streams found in karst areas may be prohibited. In addition, ponding areas may not reliably hold water in karst areas.

Table 14.3.20 - Soils

The key evaluation factors are based on an initial investigation of the NRCS hydrologic soils groups at the site. Note that more detailed geotechnical tests are usually required for infiltration feasibility and during design to confirm permeability and other factors.

Table 14.3.21 - Special Watershed or Stream Considerations

The design of structural stormwater controls is fundamentally influenced by the nature of the downstream water body that will be receiving the stormwater discharge. In addition, the designer should consult with the appropriate review authority to determine if their development project is subject to additional structural control criteria as a result of an adopted local watershed plan or special provision.

In some cases, higher pollutant removal or environmental performance is needed to fully protect aquatic resources and/or human health and safety within a particular watershed or receiving water. Therefore, special design criteria for a particular structural control or the exclusion of one or more controls may need to be considered within these watersheds or areas. Examples of important watershed factors to consider include:

High Quality Streams (Streams with a watershed impervious cover less than approximately 15%). These streams may also possess high quality cool water or warm water aquatic resources or endangered species. The design objectives are to maintain habitat quality through the same techniques used for cold-water streams, with the exception that stream warming is not as severe of a design constraint. These streams may also be specially designated by local authorities.

Wellhead Protection: Areas that recharge existing public water supply wells present a unique management challenge. The key design constraint is to prevent possible groundwater contamination by preventing infiltration of hotspot runoff. At the same time, recharge of unpolluted stormwater is encouraged to maintain flow in streams and wells during dry weather.

Reservoir or Drinking Water Protection: Watersheds that deliver surface runoff to a public water supply reservoir or impoundment are a special concern. Depending on the available treatment, a greater level of pollutant removal may be necessary for the pollutants of concern, such as bacteria pathogens, nutrients, sediment, or metals. One particular management concern for reservoirs is ensuring stormwater hotspots are adequately treated so they do not contaminate drinking water.

Local Provisions: NONE

Table 14.3.19. Physiographic Factors
Category	Category	Category
Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas	Bioretention Areas
Channels	Channels	Channels	Channels	Channels
Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment	Chemical Treatment
Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components	Conveyance System Components
Detention	Detention	Detention	Detention	Detention
Filtration	Filtration	Filtration	Filtration	Filtration
Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices	Hydrodynamic Devices
Infiltration	Infiltration	Infiltration	Infiltration	Infiltration
Ponds	Ponds	Ponds	Ponds	Ponds
Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces	Porous Surfaces
Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems	Proprietary Systems
Re-Use	Re-Use	Re-Use	Re-Use	Re-Use
Wetlands	Wetlands	Wetlands	Wetlands	Wetlands
	Wetland, Submerged Gravel

¹ = The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data if used as a primary control.

Table 14.3.20. Soils
Category	Integrated Stormwater Controls	Soils
Bioretention Areas	Bioretention Areas	Clay or silty soils may require pretreatment
Channels	Enhanced Swales
	Channels, Grass
	Channels, Open
Chemical Treatment	Alum Treatment System
Conveyance System Components	Culverts
	Energy Dissipation
	Inlets/Street Gutters
	Pipe Systems
Detention	Detention, Dry	Underlying soils of hydrologic group “C” or “D” should be adequate to maintain a permanent pool. Most group “A” soils and some group “B” soils will require a pond liner.
	Detention, Extended Dry
	Detention, Multi-purpose Areas
	Detention, Underground
Filtration	Filter Strips
	Organic Filters
	Planter Boxes	Type A or B
	Sand Filters, Surface/Perimeter	Clay or silty soils may require pretreatment
	Sand Filters, Underground
Hydrodynamic Devices	Gravity (Oil-Grit) Separator
Infiltration	Downspout Drywell	Infiltration rate > 0.5 inch/hr
	Infiltration Trenches	Infiltration rate > 0.5 inch/hr
	Soakage Trenches	Infiltration rate > 0.5 inch/hr
Ponds	Wet Pond	“A” soils may require pond liner “B” soils may require infiltration testing
	Wet ED Pond
	Micropool ED Pond
	Multiple Ponds
Porous Surfaces	Green Roof
	Modular Porous Paver Systems	Infiltration rate > 0.5 inch/hr
	Porous Concrete
Proprietary Systems	Proprietary Systems1
Re-Use	Rain Barrels
Wetlands	Wetlands, Stormwater	“A” soils may require pond liner
	Wetlands, Submerged Gravel

¹ = The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data if used as a primary control.

Table 14.3.21. Special Watershed Considerations
Category	Integrated Stormwater Controls	Special Watershed Considerations
		High Quality Stream	Aquifer Protection	Reservoir Protection
Bioretention Areas	Bioretention Areas	Evaluate for stream warming	Needs to be designed with no exfiltration (i.e. outflow to groundwater)
Channels	Enhanced Swales		Hotspot runoff must be adequately treated	Hotspot runoff must be adequately treated
	Channels, Grass
	Channels, Open
Chemical Treatment	Alum Treatment System
Conveyance System Components	Culverts
	Energy Dissipation
	Inlets/Street Gutters
	Pipe Systems
Detention	Detention, Dry
	Detention, Extended Dry
	Detention, Multi-purpose Areas
	Detention, Underground
Filtration	Filter Strips
	Organic Filters
	Planter Boxes
	Sand Filters, Surface/Perimeter	Evaluate for stream warming	Needs to be designed with no exfiltration (i.e. outflow to groundwater)
	Sand Filters, Underground
Hydrodynamic Devices	Gravity (Oil-Grit) Separator
Infiltration	Downspout Drywell
	Infiltration Trenches		Maintain safe distance from wells and water table. No hotspot runoff	Maintain safe distance from bedrock and water table. Pretreat runoff
	Soakage Trenches
Ponds	Wet Pond
	Wet ED Pond	Evaluate for stream warming	May require liner if “A” soils are present Pretreat hotspots 2 to 4 ft. separation distance from water table
	Micropool ED Pond
	Multiple Ponds
Porous Surfaces	Green Roof
	Modular Porous Paver Systems
	Porous Concrete
Proprietary Systems	Proprietary Systems1
Re-Use	Rain Barrels
Wetlands	Wetlands, Stormwater	Evaluate for stream warming	May require liner if “A” soils are present Pretreat hotspots 2 to 4 ft. separation distance from water table
	Wetlands, Submerged Gravel

¹ = The application and performance of proprietary commercial devices and systems must be provided by the manufacturer and should be verified by independent third-party sources and data if used as a primary control.

Step 3. Step 3 Location and Permitting Considerations

In the last step, a site designer assesses the physical and environmental features at the site to determine the optimal location for the selected structural control or group of controls. Table 14.3.22 provides a condensed summary of current restrictions as they relate to common site features that may be regulated under local, state, or federal law. These restrictions fall into one of three general categories:

*	Locating a structural control within an area when expressly prohibited by law
*	Locating a structural control within an area that is strongly discouraged, and is only allowed on a case-by-case basis. Local, state, and/or federal permits shall be obtained, and the applicant will need to supply additional documentation to justify locating the stormwater control within the regulated area.
*	Structural stormwater controls must be set back a fixed distance from a site feature.

This checklist is only intended as a general guide to location and permitting requirements as they relate to siting of stormwater structural controls. Consultation with the appropriate regulatory agency is the best strategy.

Local Provisions: NONE

Table 14.3.22. Location and Permitting Checklist
Site Feature	Location and Permitting Guidance
Jurisdictional Wetland (Waters of the U.S.)	* Jurisdictional wetlands must be delineated prior to siting structural control.
U.S. Army Corps of Engineers Regulatory Permit	* Use of natural wetlands for stormwater quality treatment is contrary to the goals of the Clean Water Act and should be avoided. * Stormwater should be treated prior to discharge into a natural wetland. * Structural controls may also be restricted in local buffer zones. Buffer zones may be utilized as a non-structural filter strip (i.e., accept sheet flow). * Should justify that no practical upland treatment alternatives exist. * Where practical, excess stormwater flows should be conveyed away from jurisdictional wetlands.
Stream Channel (Waters of the U.S.) U.S. Army Corps of Engineers Section 404 Permit	* All Waters of the U.S. (streams, ponds, lakes, etc.) should be delineated prior to design. * Use of any Waters of the U.S. for stormwater quality treatment is contrary to the goals of the Clean Water Act and should be avoided. * Stormwater should be treated prior to discharge into Waters of the U.S. * In-stream ponds for stormwater quality treatment are highly discouraged. * Must justify that no practical upland treatment alternatives exist. * Temporary runoff storage preferred over permanent pools. * Implement measures that reduce downstream warming.
Texas Commission on Environmental Quality Groundwater Management Areas	* Conserve, preserve, protect, recharge, and prevent waste of groundwater resources through Groundwater Conservation Districts * Groundwater Conservation District pending for Middle Trinity. * Detailed mapping available from Texas Alliance of Groundwater Districts.
Texas Commission on Environmental Quality Surface Water Quality Standards	* Specific stream and reservoir buffer requirements. * May be imperviousness limitations * May be specific structural control requirements. * TCEQ provides water quality certification - in conjunction with 404 permit * Mitigation will be required for imparts to existing aquatic and terrestrial habitat.
100-year Floodplain Local Stormwater review Authority	* Grading and fill for structural control construction is generally discouraged within the 100-year floodplain, as delineated by FEMA flood insurance rate maps, FEMA flood boundary and floodway maps, or more stringent local floodplain maps. * Floodplain fill cannot raise the floodplain water surface elevation by more than limits set by the appropriate jurisdiction.
Stream Buffer Check with appropriate review authority whether stream buffers are required	* Consult local authority for stormwater policy. * Structural controls are discouraged in the streamside zone (within 25 feet or more of streambank, depending on the specific regulations).
Utilities Local Review Authority	* Call appropriate agency to locate existing utilities prior to design. * Note the location of proposed utilities to serve development. * Structural controls are discouraged within utility easements or rights-of-way for public or private utilities.
Roads TxDOT or DPW	* Consult TxDOT for any setback requirement from local roads. * Consult DOT for setbacks from State maintained roads. * Approval must also be obtained for any stormwater discharges to a local or state-owned conveyance channel.
Structures Local Review Authority	* Consult local review authority for structural control setbacks from structures. * Recommended setbacks for each structural control group are provided in the performance criteria in this manual.
Septic Drainfields Local Health Authority	* Consult local health authority. * Recommended setback is a minimum of 50 feet from drainfield edge or spray area.
Water Wells Local Health Authority	* 100-foot setback for stormwater infiltration. * 50-foot setback for all other structural controls.

(Ordinance 2012-10 adopted 8/21/12)