A proper understanding of the City’s adopted goals, and policies for stormwater management is essential for the proper application of this Manual.
14.5.1.1 
Program Goals.
The City’s primary goal is to manage stormwater so that things don’t get worse as new areas are developed - while making improvements in the areas of the city that are already developed. We can accomplish this goal by:
1. 
Developing detailed watershed plans that promote orderly growth and result in an integrated system of public and private stormwater infrastructure
2. 
Adopting development policies and standards that prevent flooding, preserve streams and channels, and minimize water pollution without arresting either new or infill development
3. 
Fully complying with regulatory permit requirements
4. 
Operating the stormwater system in a more efficient and effective manner
5. 
Informing the public about stormwater issues in the community
6. 
Securing funding that is adequate for meeting these needs and is recognized by the public as fair and equitable
14.5.1.2 
Planning and Design Objectives
1. 
Establish and implement drainage policy and criteria so that new development does not increase flooding problems, cause erosion or pollute downstream water bodies.
2. 
Facilitate the development of comprehensive watershed planning that promotes orderly growth and results in an integrated system of public and private stormwater infrastructure.
3. 
Minimize flood risks to citizens and properties, and stabilize or decrease streambank and channel erosion on creeks, channels, and streams.
4. 
Improve stormwater quality in creeks, rivers, and other water bodies, remove pollutants, enhance the environment and mimic the natural drainage system, to the extent practicable, in conformance with the Texas Pollutant Discharge Elimination System (TPDES) permit requirements.
5. 
Support multi-use functions of stormwater facilities for trails, green space, parks, greenways or corridors, stormwater quality treatment, and other recreational and natural features, provided they are compatible with the primary functions of the stormwater facility.
6. 
Encourage a more standardized, integrated land development process.
14.5.1.3 
Design Guidelines
1. 
All development within the City of Azle City Limits or its Extraterritorial Jurisdiction (ETJ) shall include planning, design, and construction of storm drainage systems in accordance with this Stormwater Management Design Manual, Plan Commission Rules and Regulations, and Policy for the Installation of Community Facilities. Please see definition of development and project size imitations for specific design requirements under “Abbreviations and Definitions” in the Foreword.
2. 
Conceptual, Preliminary and Final Drainage Studies and Plans shall be required for all proposed developments within the City of Azle City limits or its ETJ, in conformance with this Stormwater Management Design Manual, Plan Commission Rules and Regulations, and Policy for the Installation of Community Facilities. The checklists for each stage of this three-tier process are included in Appendix A - City of Azle Detailed Checklists.
3. 
All drainage related plans and studies shall be prepared and sealed by a Licensed Professional Engineer with a valid license from the State of Texas. The engineer shall attest that the design was conducted in accordance with this Stormwater Management Design Manual.
4. 
All drainage studies and design plans shall be formulated and based upon ultimate, fully developed watershed or drainage area runoff conditions. The rainfall frequency criteria for stormwater facilities, as enumerated within this Stormwater Management Design Manual, shall be utilized for all drainage studies and design plans.
5. 
Stormwater must be carried to an “adequate or acceptable outfall.” An adequate outfall is one that does not create or increase flooding or erosion conditions downstream and is in all cases subject to the approval of the Storm Water Manager.
6. 
Proposed stormwater discharge rates and velocities from a development shall not exceed the runoff from existing, pre-development conditions, unless a detailed study is prepared that demonstrates that no unacceptable adverse impacts will be created. Adverse impacts include: new or increased flooding of existing insurable (FEMA) structures, significant increases in flood elevations over existing roadways, unacceptable rises in FEMA base flood elevations, and new or increased streambank erosion.
7. 
Stormwater runoff may be stored in detention and retention basins to mitigate potential downstream problems caused by a proposed development. Proposed detention or retention basins shall be analyzed both individually and as a part of the watershed system, to assure compatibility with one another and with the City’s overall Stormwater Management Master Plan for that watershed (if available). Storage of stormwater runoff, near to the points of rainfall occurrence, such as the use of parking lots, ballfields, property line swales, parks, road embankments, borrow pits and on-site ponds is desirable and encouraged.
8. 
Streambank stabilization and protection features to reduce or prevent erosion and sedimentation for creeks, streams, and channels shall be required, as specified in this Manual.
9. 
All proposed developments within the City of Azle City Limits or Extraterritorial Jurisdiction (ETJ) shall comply with all local, county, state and federal regulations and all required permits or approvals shall be obtained by the developer.
10. 
The policy of the City of Azle is to avoid substantial or significant transfer of stormwater drainage runoff from one basin to another and to maintain historical drainage paths whenever possible.
11. 
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. 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.
12. 
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.
(Ordinance 2012-10 adopted 8/21/12)
14.5.2.1 
Hydrograph Method Computation Sheet
Figure 14.5.1 presents a sample computation sheet for 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.
Figure 14.5.1 - Computation Sheet - Hydrology by Unit Hydrograph Method
(Ordinance 2012-10 adopted 8/21/12)
14.5.3.1 
Stormwater Inlets Computation Sheets
Explanation of the Inlets in Sumps Computation Sheet
In order to facilitate the computations required in determining the various hydraulic properties for curb opening inlets Type CO-S and drop inlets Type D-S in sump use Computation Sheet Figure 14.5.2.
Table Column
Description
Column 1
Inlet number and designation.
Column 2
Slope of gutter in ft. per ft.
Column 3
Crown slope of pavement in ft. per ft. For parabolic crowns enter type of street section.
Column 4
Total gutter flow in c.f.s. For inlets other than the first inlet in a system, gutter flow is the sum of runoff from contributing area plus carry-over flow from inlet or inlets upstream.
Column 5
Depth of gutter flow in feet from the spread of water calculations in Figure 1.2 (iSWM Hydraulics Technical Manual), Section 1.2.4 or from direct solution of Manning’s equation for triangular gutters.
Column 6
Depth of gutter depression in ft.
Column 7
Depth of water at inlet opening in ft. Column 5 plus Column 6.
Column 8
Capacity of curb opening inlet or drop inlet in c.f.s. per ft. of length of opening or perimeter around inlet from Figures 1.10, 1.12 or 1.14 in the iSWM Hydraulics Technical Manual or by direct solution.
Column 9
Assumed length of inlet opening or perimeter in feet.
Column 10
Capacity of inlet in c.f.s. Column 8 times Column 9.
Column 11
Carry-Over flow passing inlet (into overflow swale) in c.f.s. Column 4 minus Column 10.
Column 12
Percent of flow captured by inlet. Column 10 divided by Column 4 times 100.
Figure 14.5.2. Computation Sheet for Curb Opening and Drop Inlets
Explanation of the Inlets on Grade with Gutter Depression (Type CO-D) Computation Sheet
In order to facilitate the computations required in determining the various hydraulic properties for Curb Opening Inlets Type CO-D on grade (depressed), Figure 14.5.4 Computation Sheet has been prepared.
Table Column
Description
Column 1
Design Point for Inlet
Column 2
Inlet number(s)
Column 3
Location of inlet by storm drain station number
Column 4
Drainage area designation for incremental area
Column 5
Drainage area size (acres)
Column 6
Runoff coefficient “c” provided in Table 5.1 located in Section 3.6.2 under the “Storm Drain Pipe Design” section
Column 7
Time of concentration (minutes)
Column 8
Longitudinal slope (ft/ft)
Column 9
Cross slope of the pavement (ft/ft)
Column 10
Cross slope of the gutter measured from the cross slope of the pavements. The cross slope is equal to the gutter depression (in) divided by the width of the depressed gutter (in).
Column 11
Depth of gutter flow “yo” in approach gutter from spread of water determinations in iSWM Figure 1.3 or from direct solution of Manning’s equation for triangular gutters: yo = 1.245 Qo3/8 (n3/8/So3/16) (1/z)3/8. When the crown is overtopped, a composite analysis will be required.
Column 12
Spread of flow is calculated using Figure 1.2 in the iSWM Hydraulics Technical Manual or from direct solution of Manning’s Equation
Column 13
Equivalent cross slope is computed by using Figure 1.3 and 1.4 in the iSWM Hydraulics Technical Manual to determine the ratio of flow in the depressed gutter section to the total flow
Column 14
Street crown section type (straight crown (“rooftop”) or parabolic)
Column 15
Manning’s roughness coefficient (n) for pavement values located in Section 3.6.2 under the “Storm Drain Pipe Design” section
Column 16
5-year rainfall intensity (in/hr), From Section 5.0 in the iSWM Hydrology Technical Manual Tarrant County Rainfall Table
Column 17
5-year runoff, Q = cAi (cfs)
Column 18
5-year carryover flow from upstream inlet (cfs)
Column 19
5-year total gutter flow (Column 16 + Column 17) (cfs)
Column 20
100-year rainfall intensity (in/hr), From Section 5.0 in the iSWM Hydrology Technical Manual Tarrant County Rainfall Table
Column 21
100-year runoff, Q = cAi (cfs)
Column 22
100-year carryover flow from upstream inlet (cfs)
Column 23
100-year total gutter flow (Column 20 + Column 21) (cfs)
Column 24
Total right-of-way capacity (normally 2.5" over top of curb) (cfs)
Column 25
This indicates the controlling storm for inlet spacing, depending on which criteria (5-year in street or 100-year in R.O.W.) may be exceeded. This indicates whether the inlet is sized for the 5-year or 100-year flows
Column 26
Length required for total interception of the design storm determination in Figure 1.8 of the iSWM Hydraulics Technical Manual or by direct solution of Manning’s Equation
Column 27
Actual length (L) in feet of the inlet which is to be provided (10', 15', or 20')
Column 28
Ratio of the length of inlet provided (L) to the length of the inlet required for 100% interception (LT). Column 24 divided by Column 25.
Column 29
The efficiency of the provided inlet determined by Figure 1.9 in the iSWM Hydraulics Technical Manual.
Column 30
Discharge (Qi) in cubic feet per second in which the inlet in question actually intercepts in the design storm. Column 18 multiplied by Column 27.
Column 31
Carry-over flow (q) is the amount of water which passes the inlet in a 5-year storm. A substantial portion of the 5-year flow should be picked up by the inlet. The carry-over flow should be accounted for in further downstream inlets.
Column 32
Carry-over flow (q) is the amount of water which passes the inlet in a 100-year storm. The carry-over flow should be accounted for in further downstream inlets and should be reflected in the inlet bypass flow (Column 17) in the Storm Drain Hydraulics Table (minor variances may occur due to travel time routing in the Hydraulics Table).
Figure 14.5.4 - Computation Sheet for On Grade Curb Inlet
14.5.3.2 
Minor Head Losses at Structures
The following head losses at structures shall be determined for manholes, wye branches or bends in the design of closed conduits. See Figures 14.5.5 and 14.5.6 for details of each case. Minimum head loss used at any structure shall be 0.10 foot.
The basic equation for most cases, where there are both upstream and downstream velocity, takes the form as set forth below with the various conditions of the coefficient “Kj” shown in Table 14.5.3.
hj = (v2/2g) – Kj(v12/2g)
hj = Junction or structure head loss in feet.
v1 = Velocity in upstream pipe in fps
v2 = Velocity in downstream pipe in fps
Kj = Junction or structure coefficient of loss.
In the case where the manhole is at the very beginning of a line or the line is laid with bends or on a curve, the equation becomes the following without any velocity of approach.
Table 14.5.1. Junction or Structure Coefficient of Loss
Case No.
Reference Figure
Description of Condition
Coefficient Kj
I
5.10
Inlet on Main Line
0.50
II
5.10
Inlet on Main Line with Branch Lateral
0.25
III
5.10
Manhole on Main Line with 45° Branch lateral
0.50
IV
5.10
Manhole on Main Line with 90° Branch Lateral
0.25
V
5.11
45° Wye Connection or cut-in
0.75
VI
5.11
Inlet or Manhole at Beginning of Line
1.25
VII
5.11
Conduit on Curves for 90° *
Curve radius = diameter
Curve radius = 2 to 8 diam.
Curve radius = 8 to 20 diam.
0.50
0.25
0.10
VIII
5.11
Bends where radius is equal to diameter
90° Bend
60° Bend
45° Bend
22-1/2° Bend
Manhole on line with 60° Lateral
Manhole on line with 22-1/2° Lateral
0.50
0.43
0.35
0.20
0.35
0.75
*Where bends other than 90° are used, the 90° bend coefficient can be used with the following percentage factor applied.
60° Bend - 85%; 45° Bend - 70%; 22-1/2° Bend - 40%
The values of the coefficient “Kj” for determining the loss of head due to obstructions in pipes are shown in Table 5.4 and the coefficients are used in the following equation to calculate the head loss at the obstruction:
Hj = Kjv22/2g
Table 14.5.2 Head Loss Coefficients Due To Obstructions
A/Ao*
Kj
A/Ao*
Kj
1.05
0.10
3.0
15.0
1.1
0.21
4.0
27.3
1.2
0.50
5.0
42.0
1.4
1.15
6.0
57.0
1.6
2.40
7.0
72.5
1.8
4.00
8.0
88.0
2.0
5.55
9.0
104.0
2.2
7.05
10.0
121.0
2.5
9.70
 
 
*A/Ao = Ratio of area of pipe to area of opening at obstruction.
The values of the coefficient “Kj” for determining the loss of head due to sudden enlargements and sudden contractions in pipes are shown in Table 14.5.3, and the coefficients are used in the following equation to calculate the head loss at the change in section:
Hj = Kj v2/2g where,
V = Velocity in smaller pipe
Table 14.5.3 Head Loss Coefficients Due To Sudden Enlargements and Contractions
D2*/D1
Sudden Enlargements Kj
Sudden Contractions Kj
1.2
0.10
0.08
1.4
0.23
0.18
1.6
0.35
0.25
1.8
0.44
0.33
2.0
0.52
0.36
2.5
0.65
0.40
3.0
0.72
0.42
4.0
0.80
0.44
5.0
0.84
0.45
10.0
0.89
0.46
 
0.91
0.47
*D2/D1 = Ratio of larger to smaller diameter
14.5.3.3 
Storm Drain Design Examples
All storm drains shall be designed by the application of the Manning Equation either directly or through appropriate charts or nomographs. In the preparation of hydraulic designs, a thorough investigation shall be made of all existing structures and their performance on the waterway in question.
An example of the use of the method used in the manual for the design of a storm drainage system is outlined below and shown on Figure 14.5.7 Computation Sheet. The design theory has been presented in the preceding sections with their corresponding tables and graphs of information.
Preliminary Design Considerations
*
Prepare a drainage map of the entire area to be drained by proposed improvements. Contour maps serve as excellent drainage area maps, when supplemented by field reconnaissance. The scale of the map shall not be less than 1" = 200' for project area although smaller scale maps [may be used] for large off-site drainage areas.
*
Prepare a layout of the proposed storm drainage system, locating all inlets, manholes, mains, laterals, ditches, culverts, etc.
*
Outline the drainage area for each inlet in accordance with present and future street development.
*
Indicate on each drainage area the code identification number and the direction of surface runoff by small arrows. Provide a runoff table showing area, “C” factor for each portion and composite “e”, Ta, I5, Q5, I100 and Q100.
*
Show all existing underground utilities.
*
Establish design rainfall frequency.
*
Establish minimum inlet time of concentration.
*
Establish the typical cross-section of each street.
*
Establish permissible spread of water on all streets within the drainage area.
*
Plot profile of existing natural ground along the centerline of the proposed storm drain.
*
Extend downstream plan and profile beyond the end of the pipe to a point of acceptable outfall.
Figure 14.5.7 - Computations Sheet for Storm Drains
Runoff Computations
Storm drain hydraulics are shown on Figure 14.5.7, Storm Drain Hydraulic Calculations Computation Sheet. The first 18 columns of the computation sheet cover the tabulation for runoff calculations:
Column 1
Enter the downstream storm drain station number.
Column 2
Enter the upstream storm drain station number. This is the design point. Design should start at the farthest upstream point.
Column 3
Enter the distance (in feet) between the storm drain stations.
Column 4
Enter the designation of the drainage area(s) at the design point in Column 2 corresponding to the designations shown on the drainage area map.
Column 5
Enter the area in acres for the drainage area identified in Column 4.
Column 6
Enter the total drainage area in acres within the system corresponding to storm drain station shown in Column 2.
Column 7
Enter the runoff coefficient “C” for the drainage area shown in Column 5.
Column 8
Multiply Column 5 by Column 7 for each area.
Column 9
Determine the total “CA” for the drainage system corresponding to the inlet or manhole shown in Column 2.
Column 10
Determine inlet time of concentration (See iSWM Hydrology Technical Manual Section 1.2.4).
Column 11
Determine flow time in the storm drain in minutes. The flow time is equal to the distance in Column 3 divided by 60 times the velocity of flow through the storm drain in ft/sec.
Column 12
Total time of concentration in minutes. Column 10 plus Column 11. Note that time of concentration only changes at a downstream junction with another drainage area(s). It remains the same from an inlet or junction to the next inlet or junction picking up additional drainage areas. The junction of two paired inlets with each other is not a downstream junction.
Column 13
The intensity of rainfall in inches per hour for the 5-year storm frequency from the appropriate county rainfall table in the iSWM Hydrology Technical Manual.
Column 14
The intensity of rainfall in inches per hour for the 100-year storm frequency from the appropriate county rainfall table in the iSWM Hydrology Technical Manual.
Column 15
The 5-year storm runoff in cfs. Column 9 times Column 13.
Column 16
The 100-year storm runoff in cfs. Column 9 times Column 14.
Column 17
The proposed inlet bypass during a 100-year storm. This should generally correspond to the carry-over flow “q” in Column 31 of the On-Grade Inlet Capacity Calculations Table (minor variances may occur due to travel time routing in the Hydraulics Table).
Column 18
Design Discharge for the storm drain system (“Qpipe”) in cfs. This should be the greater of a substantial portion of Q5 (Column 15) or Q100 - Qbypass (Column 16 minus Column 17).
Hydraulic Design
After the computation of the quantity of storm runoff entering each inlet, the size and gradient of pipe required to carry off the design storm are determined. Any number of computer programs are available to provide design assistance for pipe sizing to the engineer. However, storm drain hydraulics must be converted and reported in Figure 14.5.7, Storm Drain Hydraulics Calculation Table. The hydraulic grade line (HGL) must be calculated for all storm drain mains and laterals using appropriate head loss equations. In all cases, the storm drain HGL must remain below grade and must be at least one foot below top of curb at any inlet.
In partial flow conditions, the HGL represents the actual water surface within the pipe. Note that for partial flow conditions, the velocity of the flow should be calculated based on actual area of flow, not the full flow area of the pipe or box.
Although the table is presented from upstream to downstream, the calculations are normally performed from the outfall upstream to each inlet. Unless partial flow conditions exist, the beginning hydraulic gradient (Column 22 of the last downstream section) must begin at either the top of pipe or at the hydraulic gradient of the receiving stream at the coincident frequency provided in Table 14.1.10, whichever is higher.
Column 19
Enter the selected pipe size.
Column 20
Enter the appropriate Manning’s roughness coefficient “n” from Table 14.5.1.
Column 21
Enter the required slope of the frictional gradient (hydraulic gradient) determined by Manning’s equation. The pipe shall be designed on a grade such that the inside crown of the pipe coincides or is below the HGL when flowing full. In a partial flow condition, the friction slope is the slope of the water surface and should follow the slope of the pipe.
Column 22
This is the beginning hydraulic gradient of the line. It is equal to the Design HGL (Column 31) for the next downstream segment, or the beginning HGL of the system as described above.
Column 23
This is the upstream HGL before the structure and is calculated as Column 22 plus the friction loss (Column 3 times Column 21).
Column 24
Velocity of flow in incoming pipe (main line) at the junction, inlet or manhole at the design point identified in Column 2.
Column 25
Velocity of flow in outgoing pipe (i.e. the pipe segment being analyzed) at junction, inlet or manhole at design point identified in Column 2.
Column 26
Velocity head of the velocity in Column 24.
Column 27
Velocity head of the velocity in Column 25.
Column 28
Head loss coefficient “Kj”, at junction, inlet or manhole at design point from Table 14.5.2, 14.5.3, or 14.5.4, or from Figure 14.5.6 and 14.5.7.
Column 29
Multiply Column 26 by Column 28.
Column 30
Head Loss at Structure. At a junction or change in pipe size, this is Column 27 minus Column 29. At a bend or inlet, this is Column 27 times Column 28. In all cases this is 0.10' minimum.
EXCEPTION: In a supercritical flow regime with partial flow conditions, head losses are not generated at upstream junctions. These may be designated as “SUPERCRITICAL PARTIAL FLOW” in the head loss calculations, but must be supported by Froude Number in the comments column. Any other proposed deviations from standard head loss calculations due to other unusual flow regimes must be approved by D-TPW on a case-by-case basis.
Column 31
Design HGL at the design point identified in Column 2. Column 23 plus Column 30. This is the beginning HGL (Column 22) for any upstream pipe discharging into that junction.
Column 32
Invert elevation for the pipe being analyzed at the downstream storm drain station in Column 1.
Column 33
Invert elevation for the pipe being analyzed at the design point (upstream storm drain station) in Column 2.
Column 34
Top of curb elevation at the design point in Column 2.
The above procedure is followed for each section of the storm drain. At the outfall, the hydraulic gradient of the line must be at the same elevation or above the gradient of the conduit or channel receiving the storm runoff discharge. See iSWM Hydraulics Technical Manual Section 1.2.10 for guidance on outfall hydraulic gradients.
With the hydraulic gradient established for a particular line, considerable latitude is available for the physical placement of the pipe flow line elevations. The inside top of the pipe must be on or below the hydraulic gradient, thus allowing the pipe to be lowered where necessary to maintain proper cover and to minimize grade conflicts with existing utilities.
14.5.3.4 
General Construction Standards
Utilities
General - In the design of a storm drainage system, the engineer is frequently confronted with the problem of crossings between the proposed storm drain and existing or proposed utilities such as water, gas and sanitary sewer lines.
Water Lines - All existing water lines in the immediate vicinity of the proposed storm drains shall be clearly indicated on both the plan and profile sheets. When design indicates that an intersection of the storm drain line and the water main exists and the proposed storm drain cannot be economically relocated, then the existing water line shall be adjusted and approved by Director of Public Services.
Sanitary Sewers - All existing or proposed sanitary sewers in the immediate vicinity of the proposed storm drains shall be clearly indicated on both plan and profile sheets. When design indicates that an intersection of the storm drain line and the sanitary sewer exist, then either line should be adjusted by relocation. If neither line can be economically relocated, then an alternative design may be considered, provided it is supported by hydraulic calculations and approved by the Storm Water Manager and the Director of Public Services. The alternative design may include a box section in the storm drain to go over or under the sanitary sewer, or a sanitary sewer crossing through the storm drain. If the latter is chosen, the crossing must be installed in a manhole or vault to provide both access and additional capacity. In either alternative, the sanitary sewer must be ductile iron pipe or other material approved by the Director of Public Services.
All Other Utilities - All other utilities in the immediate vicinity of the proposed storm drain shall be clearly indicated on both the plan and profile sheets. Gas lines and other utilities not controlled by elevation shall be adjusted when the design indicates that an intersection of the storm drain line and the utility exists and the proposed storm drain cannot be economically relocated.
Headwalls, Culverts, and Other Structures
For headwalls, culverts and other structures, standard details adopted by the Texas Department of Transportation (TxDOT) shall be used. The appropriate detail sheets should be included in any construction plans. All headwalls and culverts should be extended to or beyond the street right-of-way. TxDOT-approved pedestrian rail shall be used for any headwall within 10' of a sidewalk or other normal pedestrian area.
Minimum Pipe Sizes
Minimum pipe sizes are 24" diameter for mains, 21" diameter for inlet leads, and 18" diameter for driveway culverts less than 60 feet in length. Minimum sizes of box culverts should have equivalent cross-sectional areas to the minimum pipe diameters.
Pipe Connections and Curved Alignment
Prefabricated wye and tee connections and other unusual configurations can usually be fabricated by the pipe manufacturer. Radial pipe is can [sic] also be fabricated by the pipe manufacturer and shall be used through all curved alignments. When field connections or field radii must be used, all joints and gaps must be fully grouted to prevent voids and cave-ins caused by material washout into the storm drain.
Inlets
All curb inlets shall be 5, 10, 15 or 20 feet in length and shall have depressed openings. Recessed inlets shall be provided on arterial streets. Proposed inlet lengths greater than 20 feet must be approved by the Storm Water Manager. Care should be taken in laying out inlets to allow for adequate driveway access between the inlet and the far property line. Standard inlet depth is 4.5' at the lead line and 4.0' at the opposite end, with the bottom sloped to drain to the lead line. Manhole steps shall be installed for any inlet over five feet deep. Lead lines shall be plumbed into the inlet at a manhole opening to expedite mechanical cleaning and inspection.
Drop inlets shall be minimum four-foot square and shall have manhole access and steps. Due to excessive clogging, grate inlets are not allowed on any public storm drain except as specifically approved by the Storm Water Manager.
Streets
To minimize standing water, the minimum street grade shall be 0.50%. Along a curve, this grade shall be measured along the outer gutter line. The minimum grade along a cul-de-sac or elbow gutter shall be 0.70%. Alternatively, elbows may be designed with a valley gutter along the normal outer gutter line, with two percent cross slope from curb to the valley gutter. The minimum grade for any valley gutter shall be 0.50%. A PVI shall be used instead of a vertical curve where the total gradient change is no more than two percent (Δ ≤ 1.0%).
Flow in Driveways and Intersections
At any intersection, only one street shall be crossed with surface drainage and this street shall be the lower classified street. Where an alley or street intersects a street, inlets shall be placed in the intersecting alley or street whenever the combination of flow down the alley or intersecting street would cause the capacity of the downstream street to be exceeded. Inlets shall be placed upstream from an intersection whenever possible. Surface drainage from a 5-year flood may not cross any street classified as a thoroughfare or collector. Not more than 3.0 cfs in a 5-year flood may be discharged per driveway at a business, commercial, industrial, manufacturing, or school site. In all cases, the downstream storm drainage system shall be adequate to collect and convey the flow, and inlets provided as required. The cumulative flows from existing driveways shall be considered and inlets provided as necessary where the flow exceeds the specified design capacity of the street.
(Ordinance 2012-10 adopted 8/21/12)
14.5.4.1 
Stone Riprap Design - Gregory Method Results Table
Table 14.5.4 shall be used to report results of the Gregory channel riprap design method. Table 14.5.5 shall be used to report the results of the Gregory Culvert Outfall Protection Method. A properly designed bedding layer is required under the granular bedding.
Table 14.5.4. ROCK RIPRAP SIZING - GREGORY METHOD
From iSWM Hydraulics Technical Manual, April 2010, Section 3.2.7
Units
Size by Frequency
(Select Largest)
100-year
10-year
2-year
Step 1: Calculate Boundary Shear:
Q = peak discharge
cfs
 
 
 
b = bottom width of channel
feet
 
 
 
y = depth of peak flow
feet
 
 
 
YS = specific weight of stone (150-175 lb/ft3)
lb/ft3
 
 
 
A = cross-sectional area of flow
ft2
 
 
 
WP = wetted perimeter
feet
 
 
 
R = hydraulic radius of channel = A/WP
feet
 
 
 
S = slope of energy gradient
ft/ft
 
 
 
To = average tractive stress on channel bottom = Yw *R*S (Yw = 62.4 lb/ft3)
lb/ft2
 
 
 
= angle of side slope (14 for 4:1 slopes)
degree
 
 
 
= angle of repose of rock, usually 40°)
degree
 
 
 
To' = average tractive stress on channel side slopes = To[1-(sin21/2
lb/ft2
 
 
 
Step 2: Determine the tractive stress in a bend in the channel:
T = the greater of To or To ' from above
lb/ft2
 
 
 
r = centerline radius of bend (10000' if straight)
feet
 
 
 
w = water surface width at upstream end of bend
feet
 
 
 
Tb = local tractive stress in bend = 3.15 (r/w)-1/2
lb/ft2
 
 
 
Step 3: Determine D50 size of riprap stone (size at which 50% of the gradation is finer weight):
 
 
 
 
T = Design shear stress (greatest of To, To' or Tb)
lb/ft2
 
 
 
D50 = required average stone size = T/0.04(Ys - Yw)
feet
 
 
 
Maximum d50 (controlling size)
inches
 
 
 
Step 4: Select minimum riprap thickness from grain size curves (Fig. 3.12 to 3.17 iSWM Hydraulics Technical Manual).
D50 (max) = (Select from smaller side of band at 50% finer gradation)
lb/ft2
 
 
 
Riprap Size = (min thickness is 12")
inches
 
 
 
Step 5: Select riprap gradations table (Fig. 3.18 to 3.19 iSWM Hydraulics Technical Manual)
Riprap Gradation Figure based on riprap thickness in Step 4
Figure
 
 
 
Step 6: Select bedding thickness from grain size curves (Fig. 3.12 to 3.17 iSWM Hydraulics Technical Manual)
Bedding Gradation Figure
Figure
 
 
 
Note: See steps 7-10 in the iSWM Hydraulics Technical Manual Section 3.2.7 for additional guidance.
Table 14.5.5. ROCK RIPRAP SIZING - CULVERT OUTFALL PROTECTION
From iSWM Hydraulics Technical Manual, April 2010 , Section 3.2.7
Units 
Size by Frequency
(Select Largest)
100-year
10-year
2-year
Determine D50 size of riprap stone (size at which 50% of the gradation is finer weight):
 
 
 
 
V = outfall velocity
ft/sec
 
 
 
YS = specific weight of stone (150-175 lb/ft3)
lb/ft3
 
 
 
D50 = V1/2/[1.8*(2g(Ys-Yw)/Yw)1/2] (Yw = 62.4 lb/ft3)
feet
 
 
 
Maximum d50 (controlling size)
inches
 
 
 
(Ordinance 2012-10 adopted 8/21/12)
A Stormwater Facility Maintenance Agreement must be prepared by the engineer for each stormwater control that will not be wholly maintained by the City of Azle, as part of the Operations and Maintenance Plan submittal. This agreement must outline both preventive maintenance tasks as well as major repairs, identify the schedule for each task, assign clear roles to affected parties, and provide a maintenance checklist to guide future owners including an annual self-inspection to be provided to the CITY OF AZLE.
Details of the agreement must be set forth in a series of exhibits:
Exhibit A - Legal Description - This includes the Metes and Bounds, a Surveyor’s Drawing of the area occupied by the facility, and a copy of the Preliminary Plat containing the facility.
Exhibit B - Design Plan and Specifications - these are summary documents intended for the use of future owners in conducting routine maintenance, inspections and major repairs.
Design Data and Calculations - this can be in the form of a letter or statement from the engineer which summarizes critical design calculations related to the functionality of the facility such as storage volume or TSS removal, and attest to the facility conforming to applicable iSWM standards.
Schematic Plan - this should be prepared by the engineer from construction drawings to show the general layout of the facility. Major features requiring regular or special maintenance should be shown and labeled in general terms understandable to a layman. A profile should be given showing critical elevations that control the function and capacity of the facility, and one or more cross-sections should be provided to indicate the general grading of the facility.
Landscaping - Vegetation should be shown consistent with the approved Landscape Plan, either on the Schematic Plan or as a separate drawing.
Exhibit C - Operations and Maintenance Plan - Specific maintenance tasks should be defined for each element of the facility. Maintenance tasks specific to the facility should be described in simple terms consistent with nomenclature contained in the Schematic and Landscape plans. An inspection and maintenance frequency should be established for each task.
Exhibit D - Maintenance Checklist - A checklist consistent with the Operations and Maintenance Plan shall be provided for the use of future owners in performing routine and special maintenance tasks. This list should describe work required and frequency in language that is easy to understand and specific for the facility to be maintained. This form will be completed by the Owner and submitted to the City of Azle annually as part of a regular self-inspection program. See Figure 14.5.10 for an example checklist for a simple detention basin.
Additional guidance for facility maintenance is provided in the iSWM Technical Manual, for several types of stormwater controls. The engineer must certify that the construction has been completed in accordance with the general plans and Schematic Plan. After approval of construction by the City of Azle, an engineer is expected to provide guidance to the owner’s representative in implementing the approved maintenance program and to co-sign the first annual inspection after the construction. A checklist for preparing a Stormwater Facility Maintenance Agreement is provided in Chapter 5, Appendix A, Form CW-8.
(Ordinance 2012-10 adopted 8/21/12)
An engineered overall site grading plan shall be submitted with the subdivision’s paving and drainage plans. The plan shall be consistent with the drainage area map. The plan shall include flow arrows and Type A, B, or C drainage for each lot within the subdivision as described in Federal Housing Administration (FHA) Land Planning Bulletin No. 3, as amended (see Appendix D). Type 1 or 2 block grading as shown in the FHA information is preferred. Type 3 and block 4 grading is allowed only if:
a. 
a swale, flume or channel is constructed at the rear of the lot to intercept runoff; and
b. 
Runoff from 3 or more lots is collected and conveyed within an underground drainage system, swale, flume or channel contained within a dedicated easement.
The engineer may utilize berms and swales to redirect flows. Grass swales shall have a minimum slope of 2% except where contained within a drainage easement, in which case a 1% minimum slope can be allowed. The engineer shall provide more detailed information in addition to the lot grading type (A, B, or C) by indicating spot evaluations on each lot. For Type B lots, side-yard swales should extend from behind the rear building line to the street, in order to collect runoff from the roof. Roof drains, if used in along the rear building line of these lots, should use splash blocks to direct the runoff into the side swales.
The finished floor elevation and surrounding grading must conform to current building codes adopted by the City and provide a minimum height of the finished floor of 12 inches above the surrounding ground. Areas within 10' of the foundation should be sloped to drain away from the foundation. Minimum slopes of 2% for structural improvements and 5% for non-structural elements, respectively, must be maintained away from the footing. See Figure 14.5.11.
If the site is complex and an overall site grading plan cannot be developed in accordance with the HUD standards, an individual grading plan for each lot shall be submitted by an engineer prior to issuing the Building Permit. The individual grading plans shall be coordinated with surrounding lots. For these complex plans, an “as-built” letter shall be submitted prior to final inspection.
Four inches of topsoil shall be provided for all disturbed areas not protected by impervious cover, in order to sustain vegetation after construction has been completed.
Figure 14.5.11 Grading Requirements Next to Building Foundation
Editor’s note–The appendices to chapter 5 are not printed herein, but are on file and available to the public at the city offices.
(Ordinance 2012-10 adopted 8/21/12)