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.
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Column 31
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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.
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Column 32
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Invert elevation for the pipe being analyzed at the downstream
storm drain station in Column 1.
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Column 33
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Invert elevation for the pipe being analyzed at the design point
(upstream storm drain station) in Column 2.
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Column 34
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Top of curb elevation at the design point in Column 2.
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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.
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Table 14.5.4. ROCK RIPRAP SIZING - GREGORY METHOD
From iSWM Hydraulics Technical Manual, April 2010, Section 3.2.7
| ||||
|---|---|---|---|---|
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Units
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Size by Frequency
(Select Largest)
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100-year
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10-year
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2-year
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Step 1: Calculate Boundary Shear:
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Q = peak discharge
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cfs
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b = bottom width of channel
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feet
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y = depth of peak flow
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feet
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|
|
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YS = specific weight of stone (150-175
lb/ft3)
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lb/ft3
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|
|
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A = cross-sectional area of flow
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ft2
|
|
|
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WP = wetted perimeter
|
feet
|
|
|
|
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R = hydraulic radius of channel = A/WP
|
feet
|
|
|
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S = slope of energy gradient
|
ft/ft
|
|
|
|
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To = average tractive stress on channel
bottom = Yw *R*S (Yw = 62.4 lb/ft3)
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lb/ft2
|
|
|
|
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= angle of side slope (14 for 4:1 slopes)
|
degree
|
|
|
|
|
= angle of repose of rock, usually 40°)
|
degree
|
|
|
|
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To' = average tractive stress on channel
side slopes = To[1-(sin21/2
|
lb/ft2
|
|
|
|
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Step 2: Determine the tractive stress in a bend in the
channel:
| ||||
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T = the greater of To or To ' from above
|
lb/ft2
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|
|
|
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r = centerline radius of bend (10000' if straight)
|
feet
|
|
|
|
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w = water surface width at upstream end of bend
|
feet
|
|
|
|
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Tb = local tractive stress in bend =
3.15 (r/w)-1/2
|
lb/ft2
|
|
|
|
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Step 3: Determine D50 size of riprap stone (size at which
50% of the gradation is finer weight):
|
|
|
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|
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T = Design shear stress (greatest of To, To' or Tb)
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lb/ft2
|
|
|
|
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D50 = required average stone size = T/0.04(Ys - Yw)
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feet
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|
|
|
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Maximum d50 (controlling size)
|
inches
|
|
|
|
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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
|
|
|
|
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Riprap Size = (min thickness is 12")
|
inches
|
|
|
|
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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.
|
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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
|
|
|
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(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)
