*
|
Rational Method
|
*
|
SCS Unit Hydrograph Method
|
*
|
Snyder’s Unit Hydrograph Method
|
*
|
USGS & TXDOT Regression Equations
|
*
|
iSWM Water Quality Protection Volume Calculation
|
*
|
Water Balance Calculations
|
*
|
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
|
Local Provisions: NONE
|
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.
|
*
|
Conservation of Natural Features and Resources
|
*
|
Lower Impact Site Design Techniques
|
*
|
Reduction of Impervious Cover
|
*
|
Utilization of Natural Features for Stormwater Management
|
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.
|
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 (WQv) 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 (SPv) 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.
|
Local Provisions: Off-site pollution
prevention activities are not currently required by the City of Azle.
|
*
|
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
|
*
|
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?
|
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.
|
Local Provisions: NONE
|
Local Provisions: NONE
|
Local Provisions: This option
protects a stream from increased runoff discharge rates and velocities
that tend to occur with development.
|
Local Provisions: NONE
|
Local Provisions: NONE
|
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).
|
*
|
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.
|
*
|
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 WQv 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
SPv 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.
|
*
|
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).
|
*
|
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.
|
*
|
Stormwater treatment suitability
|
*
|
Water quality performance
|
*
|
Site applicability
|
*
|
Implementation considerations
|
*
|
Physiographic factors
|
*
|
Soils
|
*
|
Special watershed or stream considerations
|
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 (WQv): This indicates whether a structural control
provides treatment of the water quality volume (WQv). 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 (SPv): This indicates whether the structural
control can be used to provide the extended detention of the streambank
protection volume (SPv). The presence of a
“P” indicates that the structural control can be used
to meet SPv 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.
|
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.
|
*
|
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.
|
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.
|