[Ord. 1018, 12/16/2013]
1.
For any of the activities regulated by this chapter, unless preparation
of a stormwater management (SWM) site plan is specifically exempted,
the preliminary or final approval of subdivision and/or land development
plans, the issuance of any building or occupancy permit, and the commencement
of any earth disturbance activity may not proceed until the property
owner or applicant or his/her agent has received written approval
from the municipality of an SWM site plan that demonstrates compliance
with the requirements of this chapter, and a written approval of an
adequate erosion and sediment (E&S) control plan from the municipality
or County Conservation District when required.
2.
An SWM site plan approved by the municipality shall be on site throughout
the duration of the regulated activity.
3.
The municipality may, after consultation with the Department of Environmental
Protection (DEP), approve measures for meeting the state water quality
requirements other than those in this chapter, provided that they
meet the minimum requirements of, and do not conflict with, state
law, including but not limited to the Clean Streams Law.
4.
For all regulated earth disturbance activities, E&S control best
management practices (BMPs) shall be designed, implemented, operated
and maintained during the regulated earth disturbance activities (e.g.,
during construction) to meet the purposes and requirements of this
chapter and to meet all requirements under Title 25 of the Pennsylvania
Code and the Clean Streams Law. The municipality requires an erosion
and sediment control plan for any earth disturbance activities of
1,000 square feet or more. DEP regulations require an erosion and
sediment control plan for any earth disturbance activity of 5,000
square feet or more, under 25 Pa. Code § 102.4(b). In addition,
under 25 Pa. Code Chapter 92, a DEP "NPDES construction activities"
permit is required for regulated earth disturbance activities. A copy
of the erosion and sediment control plan and any required permit,
as required by DEP regulations, shall be available on the project
site at all times. Various BMPs and their design standards are listed
in the Erosion and Sediment Pollution Control Program Manual (E&S
Manual), No. 363-2134-008 (March 31, 2012), as amended and updated.
5.
For all regulated activities, implementation of the water volume controls in § 23-406 is required.
6.
Impervious Areas:
7.
Stormwater discharges onto adjacent property shall not be created,
increased, decreased, relocated, or otherwise altered without written
notification of the adjacent property owner(s). Such stormwater discharges
shall be subject to the requirements of this chapter.
8.
All regulated activities shall include such measures as necessary
to:
A.
Protect health, safety and property;
B.
Meet the water quality goals of this chapter by implementing measures
to:
(1)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(2)
Maintain or extend riparian buffers.
(3)
Avoid erosive flow conditions in natural flow pathways.
(4)
Minimize thermal impacts to waters of this commonwealth.
(5)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
C.
To the maximum extent practicable, incorporate the techniques for
low-impact development practices described in the Pennsylvania Stormwater
Best Management Practices Manual (BMP Manual).
9.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
10.
Infiltration BMPs should be dispersed on site, made as shallow as
practicable, and located to maximize use of natural on-site infiltration
features while still meeting the other requirements of this chapter.
11.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
and not more than 72 hours from the end of the design storm.
12.
The design storm precipitation amounts to be used in the analysis
of peak rates of discharge shall be the rainfall depths listed in
Figure IV-2 of the Borough of Hatboro Specifications and Design Standards,
incorporated herein by reference and as may be amended from time to
time by resolution of the Borough Council.
13.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this chapter and to meet all requirements under Title 25 of the Pennsylvania
Code, the Clean Streams Law, and the Storm Water Management Act.
14.
Various BMPs and their design standards are listed in the BMP Manual.
[Ord. 1018, 12/16/2013]
Approvals issued and actions taken under this chapter do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
[Ord. 1018, 12/16/2013]
1.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office or County Conservation
District must be provided to the municipality.
2.
Approval of earth disturbance activities is required by the municipality
per Table 106.2.
3.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
A.
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity.
B.
Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
[Ord. 1018, 12/16/2013]
1.
The design of all regulated activities should include the following
to minimize stormwater impacts: (See Appendix 23-B for a Nonstructural
Project Design Checklist.[1])
A.
The applicant should find practicable alternatives to the surface
discharge of stormwater, the creation of impervious surfaces, and
the degradation of waters of the commonwealth and must maintain as
much as possible the natural hydrologic regime of the site.
B.
An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
C.
All practicable alternatives to the discharge of stormwater are presumed
to have less-adverse impact on quantity and quality of waters of the
commonwealth, unless otherwise demonstrated.
[1]
Editor's Note: Appendix 23-B is included as an attachment
to this chapter.
[Ord. 1018, 12/16/2013]
1.
Infiltration best management practices (BMPs) shall meet the following minimum requirements, unless the site qualifies for an exemption from the infiltration requirements of this chapter as listed in § 23-106:
A.
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(1)
A minimum soil depth of 24 inches between the bottoms of the
infiltration BMPs and bedrock or other limiting zones.
(2)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's qualified person.
(3)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(4)
All subsurface and contained facilities, such as capture-and-reuse
systems, must have storage available equivalent to the water volume
control amount within three days (72 hours) from the end of the design
storm.
B.
The size of the infiltration facility shall be based upon the following
volume criteria:
(1)
Where practicable and appropriate, the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
inch of runoff (I) over all proposed impervious surfaces.
(2)
The Rev required shall be computed as:
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Rev = (1/12) * (I)
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Where:
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Rev
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=
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Recharge volume (cubic feet).
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I
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=
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Impervious area within the limits of earth disturbance (square
feet).
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An asterisk (*) in equations denotes multiplication.
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2.
Soils. A detailed soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified person and at a minimum address
soil permeability, depth to bedrock, and subgrade stability. The general
process for designing the infiltration BMP shall be:
A.
Analyze hydrologic soil groups as well as natural and man-made features
within the site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
B.
Provide field tests such as double-ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
C.
Design the infiltration structure for the required recharge volume
(Rev) based on field-determined capacity at
the level of the proposed infiltration surface.
D.
If on-lot infiltration structures are proposed by the applicant's
qualified person, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
E.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
[Ord. 1018, 12/16/2013]
1.
The low-impact development practices provided in the BMP Manual shall
be utilized for all regulated activities to the maximum extent practicable.
Water volume controls shall be implemented using the Design Storm
Method in Subsection 1A or the Simplified Method in Subsection 1B
below. For regulated activity areas equal to or less than one acre
that do not require hydrologic routing to design the stormwater facilities,
this chapter establishes no preference for either methodology; therefore,
the applicant may select either methodology on the basis of economic
considerations, the intrinsic limitations on applicability of the
analytical procedures associated with each methodology, and other
factors. All regulated activities greater than one acre must use the
Design Storm Method.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
The post-development total runoff volume for all storms equal
to or less than the two-year, twenty-four-hour storm event shall not
be increased.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities greater than one acre or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces. [Note: An asterisk (*)
in equations denotes multiplication.]
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Volume (cubic feet) = (2/12) * Impervious Surfaces (square feet)
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(2)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into the surface waters of the commonwealth. Removal
options include reuse, evaporation, transpiration, and infiltration.
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Volume (cubic feet) = (1/12) * Impervious Surfaces (square feet)
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(3)
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
[Ord. 1018, 12/16/2013]
1.
If a perennial or intermittent stream passes through the site, the
applicant shall create a stream buffer extending a minimum of 50 feet
to either side of the top-of-bank of the channel. The buffer area
shall be established and maintained with appropriate native vegetation
(refer to Appendix B of the BMP Manual for plant lists). If the applicable
rear or side yard setback is less than 50 feet, the buffer width may
be reduced to 25% of the setback to a minimum of 10 feet. If an existing
buffer is legally prescribed (i.e., deed, covenant, easement, etc.)
and it exceeds the requirements of this chapter, the existing buffer
shall be maintained. This does not include lakes or wetlands.
2.
Applicants shall adhere to the following stream bank erosion/channel
protection requirements:
A.
In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream stream bank erosion),
the primary requirement is to design a BMP to detain the proposed
conditions' two-year, twenty-four-hour storm event to the existing
conditions' one-year flow using the SCS Type II distribution.
Additionally, provisions shall be made (such as adding a small orifice
at the bottom of the outlet structure or a sand filter) so that the
proposed conditions' one-year, twenty-four-hour storm event takes
a minimum of 24 hours to drain from the facility from a point when
the maximum volume of water from the one-year, twenty-four-hour storm
event is captured (i.e., the maximum water surface elevation is achieved
in the facility). Release of water can begin at the start of the storm
(i.e., the invert of the water volume control orifice is at the invert
of the facility).
B.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter where possible, and a trash rack shall
be installed to prevent clogging. On sites with small drainage areas
contributing to this BMP that do not provide enough runoff volume
to allow a twenty-four-hour attenuation with the three-inch orifice,
the calculations shall be submitted showing this condition. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake. It is recommended that the design,
to accommodate maintenance, include a sand or porous media filter.
[Ord. 1018, 12/16/2013]
1.
In addition to the requirements specified in Table 408.1 below, the erosion and sedimentation control (§ 23-403), the nonstructural project design (§ 23-404), the groundwater recharge (§ 23-405), the water volume control (§ 23-406), and the stream bank erosion (§ 23-407) requirements shall be implemented.
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Standards for managing runoff in the Pennypack Creek Watershed
for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year
storm events are shown in Table 408.1. Development sites must control
proposed condition runoff rates to existing condition runoff rates
for the design storms in accordance with Table 408.1.
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Table 408.1
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Peak Rate Control Standards in the Pennypack Creek Watershed
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Proposed Condition Design Storm
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Existing Condition Design Storm
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1-year
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1-year
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2-year
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Reduce to
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1-year
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5-year
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2-year
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10-year
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5-year
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25-year
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10-year
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50-year
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25-year
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100-year
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50-year
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2.
Off-Site Areas. Off-site areas that drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
3.
Site Areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area, only the
proposed impact area utilizing stormwater management measures shall
be subject to the management district criteria. In other words, unimpacted
areas bypassing the stormwater management facilities would not be
subject to the management district criteria.
4.
Alternate Criteria for Redevelopment Sites. For redevelopment sites,
one of the following minimum design parameters shall be accomplished,
whichever is most appropriate for the given site conditions as determined
by the Borough of Hatboro;
[Ord. 1018, 12/16/2013]
1.
Stormwater runoff from all development sites with a drainage area
of greater than two acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. The qualified person must consult with the municipality to
gain approval of design methods prior to design.
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Table 409-1 summarizes acceptable computation methods, and the
method selected by the qualified person shall be based on the individual
limitations and suitability of each method for a particular site.
The Rational Method may be used to estimate peak discharges from drainage
areas that contain less than two acres. The Soil Complex Method shall
be used for drainage areas greater than two acres.
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Table 409.1
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Acceptable Computation Methodologies For Stormwater Management
Plans
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Method
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Method Developed by
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Applicability
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TR-20
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USDA NRCS
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Applicable where use of full hydrology computer model is desirable
or necessary
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TR-55
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USDA NRCS
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Applicable for land development plans within limitations described
in TR-55
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HEC-HMS
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US Army Corps of Engineers
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Applicable where use of full hydrologic computer model is desirable
or necessary
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Rational Method or commercial computer package based on Rational
Method
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Emil Kuichling (1889)
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For sites less than 2 acres or as approved by the municipality
and/or Municipal Engineer
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Other methods
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Varies
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Other computation methodologies approved by the municipality
and/or Municipal Engineer
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*
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NOTE: Successors to the above methods are also acceptable. These
successors include WinTR55 for TR-55 and WinTR20 for TR-20.
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2.
If a hydrologic computer model such as PSRM, HEC-1 or HEC-HMS is
used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The rainfall distribution should reference NRCS
Type II.
3.
Selection of runoff curve numbers (CN) or runoff coefficients (C)
shall be per the following conventions.
A.
For predevelopment or existing conditions:
(1)
Undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number or
Rational "c" value (i.e., forest).
(2)
Bare or disturbed earth surfaces shall be considered as "meadow."
(3)
Impervious surfaces of a site which are in excess of the maximum
impervious cover allowance in the existing condition [refer existing
nonconformity] shall be considered "meadow."
(4)
Open space shall be considered to be in "good" condition.
B.
For post-development or proposed conditions:
(1)
The maximum percentage of impervious surface allowed per the Zoning Ordinance [Chapter 27], regardless of the percentage of impervious surface included in the proposed plan, shall be used. If variances have been obtained to allow for impervious surface in excess of the allowable, the proposed impervious surface area shall be used.
(2)
Any areas proposed as gravel or crushed stone shall be assumed
to be impervious.
(3)
Any areas proposed as any type of pervious paving surface, such
as pervious asphalt, pervious concrete or pervious pavers, shall be
considered impervious.
4.
Calculations using the Rational Method shall use the runoff curve
numbers (C) and rainfall intensities listed in Figures IV-1 and IV-3,
respectively, of the Borough of Hatboro Specifications and Design
Standards, incorporated herein by reference, and as may be amended
from time to time by resolution of the Borough Council. Rainfall intensities
shall be consistent with the appropriate time-of-concentration and
return period.
5.
Calculations using the Soil Cover Complex Method shall use the rainfall
depths listed in Figure IV-2 of the Borough of Hatboro Specifications
and Design Standards, incorporated herein by reference, and as may
be amended from time to time by resolution of the Borough Council.
6.
Times-of-concentration for overland flow shall be calculated using
the methodology presented in Chapter 3 of Urban Hydrology for Small
Watersheds, NRCS, TR-55 (as amended or replaced from time to time
by NRCS). Times-of-concentration for channel and pipe flow shall be
computed using flow velocities as determined by Manning's Equation.
7.
The Manning Equation is preferred for 1-D, gradually-varied, open
channel flow. In other cases, appropriate, applicable methods should
be applied; however, early coordination with the municipality is necessary.
8.
For any size watershed when development of a hydrograph is needed for basin routing or for determination of runoff volume and all parameters of the watershed do not exceed the limitations of the method, the Tabular Hydrograph Method shall be used as outlined in Chapter 5 of TR-55, unless an alternative method is approved by the Borough Engineer.
9.
For any project where the input parameters or the complexity of the
watershed is beyond the limitations of the methodologies presented
above, the SCS Watershed Model (TR-20) or other method approved by
the Borough Engineer shall be used. Procedures and methods associated
with any alternative deign methodology shall be approved by the Borough
Engineer.
10.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using the generally
accepted hydraulic analysis technique or method of the municipality.
11.
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than two acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph.
12.
All stormwater facilities shall be designed to transport a twenty-five-year,
twenty-four-hour design storm. All facilities used to convey off-site
runoff through or around a site shall be designed to transport the
fifty-year, twenty-four-hour design storm. All facilities that convey
stormwater to basins or other structures shall be designed to transport
the one-hundred-year, twenty-four-hour design storm. All conveyance
system designs will be in compliance with PennDOT Design Manual No.
2, as last revised. The capacity of all natural streams shall not
be reduced from the predevelopment conditions.
13.
Where proposed development and improvements to existing stormwater
management facilities will cause adverse impacts on adjacent downstream
properties, the developer shall mitigate such impacts.
14.
If existing stormwater management facilities within a project site
do not meet the design requirements of this chapter, and if such facilities
are affected by new development being proposed within the site, then
the existing stormwater management facilities must be redesigned and
improved to meet the requirements of this chapter.
15.
In all cases where detention, retention, or infiltration is necessary,
completely developed hydrograph routing is required. Hydrographs shall
be developed using the Soil Cover Complex Method.
