A.
Applicants proposing regulated activities in the municipality which do not fall under the exemption criteria shown in § 257-6 shall submit a stormwater management site plan consistent with this chapter and the applicable watershed stormwater management plan to the municipality for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.
B.
No regulated activity within the municipality shall commence until
the municipality issues approval of a SWM plan which demonstrates
compliance with the requirements of this chapter.
C.
The applicant is required to design the site to minimize surface
discharge of stormwater and the creation of impervious surfaces in
order to maintain, as much as possible, the natural hydrologic regime.
D.
The SWM site plan must be designed consistent with the sequencing provisions of § 257-13 to ensure maintenance of the natural hydrologic regime, to promote infiltration, and to protect groundwater and surface water quality and quantity. The SWM site plan designer must proceed sequentially in accordance with Article III of this chapter.
E.
Stormwater drainage systems shall be designed in order to preserve
natural flow conditions to the maximum extent practicable.
F.
Existing drainage discharge onto adjacent property shall not be altered
in any manner without written permission, and a maintenance access
agreement with, the affected property owner(s). Such discharge shall
be subject to any applicable discharge criteria specified in this
chapter and still must meet the requirements of Act 167.
G.
Areas of existing diffused drainage discharge, whether proposed to
be concentrated or maintained as diffused drainage areas, shall be
subject to any applicable discharge criteria in the general direction
of existing discharge, except as otherwise provided by this chapter.
If diffused drainage discharge is proposed to be concentrated and
discharged onto adjacent property, the applicant must document that
adequate downstream conveyance facilities exist to safely transport
the concentrated discharge or otherwise prove that no erosion, sedimentation,
flooding, or other impacts will result from the concentrated discharge.
H.
Where a development site is traversed by a stream, drainage easements
of 25 feet shall be provided on either side of, and conform to the
line of such streams.
I.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., is encouraged where soil
conditions permit in order to reduce the size or eliminate the need
for detention facilities or other structural BMPs.
J.
All stormwater runoff from new development or redevelopment shall
be pretreated for water quality prior to discharge to surface or groundwater.
Rooftop runoff may go directly to an infiltration BMP or be evapotranspirated.
K.
All regulated activities within the municipality shall be designed,
implemented, operated, and maintained to meet the purposes of this
chapter, through these two elements:
L.
The BMPs shall be designed, implemented, and maintained to meet state
water quality requirements and any other more stringent requirements
as determined by the municipality.
N.
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VII.
O.
All BMPs used to meet the requirements of this chapter shall conform
to the state water quality requirements and any more stringent requirements
as set forth by the municipality.
Q.
In selecting the appropriate BMPs or combinations thereof, the applicant
shall consider the following:
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site's soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high water table.
(5)
Proximity to building foundations and wellheads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream bank erosion.
(10)
Efficiency of the BMPs to mitigate potential water quality problems.
(11)
The volume of runoff that will be effectively treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
(16)
Enhancement of aesthetic and property values.
R.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices in a manner that does not
aggravate existing stormwater problems. The municipality reserves
the right to disapprove any design that would result in construction
in or continuation of a stormwater problem area.
T.
Stormwater hotspots. Stormwater runoff from hotspots shall be pretreated
prior to surface or groundwater infiltration to prevent pollutant
runoff. Industrial sites referenced in 40 CFR 125 are examples of
hotspots.
(1)
Examples of hotspots:
(a)
Vehicle salvage yards and recycling facilities.
(b)
Vehicle fueling stations.
(c)
Vehicle service and maintenance facilities.
(d)
Vehicle and equipment cleaning facilities.
(e)
Fleet storage areas (bus, truck, etc.).
(f)
Industrial sites based on Standard Industrial Classification
Codes.
(g)
Marinas (service and maintenance areas).
(h)
Outdoor liquid container storage.
(i)
Outdoor loading/unloading facilities.
(j)
Public works storage areas.
(k)
Facilities that generate or store hazardous materials.
(l)
Commercial container nursery.
(m)
Contaminated sites/brownfields.
(n)
Other land uses and activities as designated by an appropriate
review authority.
(2)
The following land uses and activities are not normally considered
hotspots:
(3)
While streets and highways, average daily traffic volume (ADT) greater
than 30,000, are not considered stormwater hotspots, it is important
to ensure that highway stormwater management facilities are designed
to adequately protect receiving streams and/or groundwater.
(4)
The Environmental Protection Agency's (EPA) NPDES stormwater
program requires some industrial sites to prepare and implement a
stormwater pollution prevention plan.
U.
The following standards for protection of downgradient properties
from off-site conveyance must be accomplished:
(1)
For any location where a new concentrated discharge of stormwater
from any frequency rainfall event, up to and including the one-hundred-year,
twenty-four-hour event, will flow onto a downgradient property, the
following are required:
The following permit requirements may apply to certain regulated
earth disturbance activities and must be met prior to commencement
of regulated earth disturbance activities, as applicable:
A.
All regulated earth disturbance activities subject to permit requirements
by PADEP under regulations at Title 25 Pennsylvania Code, Chapter
102.
B.
Work within natural drainageways subject to permit by PADEP under
Title 25 Pennsylvania Code, Chapter 105.
C.
Any stormwater management facility that would be located in or adjacent
to surface waters of the commonwealth, including wetlands, subject
to permit by PADEP under Title 25 Pennsylvania Code, Chapter 105.
D.
Any stormwater management facility that would be located on or discharging
to a state highway right-of-way, or require access to or from a state
highway shall be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under Title 25 Pennsylvania
Code, Chapter 105.
A.
No regulated earth disturbance activities within the municipality
shall commence until the municipality receives an approval from the
PADEP in compliance with Title 25, Chapter 102 of the Pennsylvania
Code, of an erosion and sediment control plan for construction activities
if applicable.
B.
PADEP has regulations regarding an erosion and sediment control under
Title 25 Pennsylvania Code, Chapter 102.
C.
In addition, under Title 25 Pennsylvania Code, Chapter 92, a PADEP
NPDES construction activities permit is required for regulated earth
disturbance activities.
D.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office or County Conservation District must be provided to the municipality. The issuance of an NPDES construction permit [or permit coverage under the statewide General Permit (PAG-2)] satisfies the requirements of § 257-22A.
E.
A copy of the erosion and sediment control plan and any required
permit, as required by PADEP regulations, shall be available on the
project site at all times.
F.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
At a minimum, they shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
The design of all regulated activities shall include the following
to minimize stormwater impacts to reduce the surface discharge of
stormwater, reduce the creation of unnecessary impervious surfaces,
prevent the degradation of waters of the commonwealth, and maintain
as much as possible the natural hydrologic regime of the site.
B.
The applicant shall demonstrate that the design process follows the
sequence noted below. The goal of the sequence is to minimize the
increases in stormwater runoff and impacts to water quality resulting
from the proposed regulated activity:
(1)
The following items in this subsection shall be addressed prior to
development of other stormwater management site plan design elements:
(a)
Prepare an Existing Resource and Site Analysis Map (ERSAM) showing
environmentally sensitive areas including, but not limited to, steep
slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools,
stream buffers, and hydrologic soil groups. Land development, any
existing recharge areas, and other requirements outlined in the municipal
SALDO shall also be included.
(d)
Identify site-specific existing conditions drainage areas, discharge
points, recharge areas, and hydrologic soil groups A and B (areas
conducive to infiltration).
(2)
The following items in this subsection may be addressed in any order, provided that all items in Subsection B(1) have been completed.
(a)
Satisfy the infiltration objective (§ 257-14) and provide for stormwater pretreatment prior to infiltration.
(c)
Provide stream bank erosion protection in accordance with § 257-16, Stream bank erosion requirements.
(d)
Prepare final project design to maintain existing conditions
drainage areas and discharge points, to minimize earth disturbance
and impervious surfaces, and, to the maximum extent possible, to ensure
that the remaining site development has no surface or point discharge.
(e)
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements (§ 257-17).
(f)
Manage any remaining runoff prior to discharge through detention,
bioretention, direct discharge, or other structural control.
A.
Providing for infiltration consistent with the natural hydrologic regime is required. Design of the infiltration facilities shall consider infiltration to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created. If it cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of the municipality that this cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc. vs. a financial hardship as defined in § 257-9). If it can be physically accomplished, the volume of runoff to be infiltrated shall be determined from Subsection B(2), depending on demonstrated site conditions, and shall be the greatest volume that can be physically infiltrated. For example:
B.
Infiltration BMPs shall meet the following minimum requirements:
(1)
Infiltration BMPs intended to receive runoff from developed or redeveloped
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum depth of 24 inches between the bottom of the BMP and
the top of the limiting zone.
(b)
An infiltration rate sufficient to accept the additional stormwater
volume and dewater completely as determined by field tests conducted
by the applicant's design professional.
(c)
The infiltration facility shall be capable of completely draining
the retention (infiltration) volume (Rev) within
three days (72 hours) from the end of the design storm.
(2)
The size of the infiltration facility and Rev shall be based upon the following volume criteria:
(a)
Modified Control Guideline One (MCG-1) of the Pennsylvania Stormwater
Best Management Practices (PA BMP) Manual. The retention (infiltration)
volume (Rev) to be captured and infiltrated
shall be the net two-year twenty-four-hour volume. The net volume
is the difference between the postdevelopment runoff volume and the
predevelopment runoff volume. The postdevelopment total runoff volume
for all storms equal to or less than the two-year twenty-four-hour
duration precipitation shall not be increased. For modeling purposes,
existing (predevelopment) nonforested pervious areas must be considered
meadow in good condition or its equivalent, and 20% of existing impervious
area, when present, shall be considered meadow in good condition.
(b)
Infiltrating the entire Rev volume in Subsection B(2)(a) (above) may not be feasible on every site due to site-specific limitations such as shallow depth to bedrock or the water table. If it cannot be physically accomplished, then the following criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
|
At least the first one-inch of runoff from new or replacement
impervious surfaces shall be infiltrated.
| |
|
Rev = one (inch)* impervious area (square
feet) + 12 (inches) = cubic feet (cf)
| |
|
[An asterisk (*) in equations denotes multiplication.]
|
(c)
Only if infiltrating the entire Rev volume in Subsection B(2)(b) (above) cannot be physically accomplished, then the following minimum criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
[1]
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire water quality volume (WQv) (Section 257-16.A); however, in all cases at least
the first 0.5 inch of the WQv shall be infiltrated.
The minimum infiltration volume (Rev) required
would, therefore, be computed as:
|
Rev = I * impervious area (square feet)
+ 12 (inches) = cubic feet (cf)
| ||||
|
An asterisk (*) in equations denotes multiplication.
| ||||
|
Where:
| ||||
|
I
|
=
|
The maximum equivalent infiltration amount (inches) that the
site can physically accept or 0.50 inch, whichever is greater.
| ||
[2]
The retention volume values derived from the methods in Subsection B(2)(a), (b), or (c) is the minimum volume the applicant must control through an infiltration BMP facility. If site conditions preclude capture of runoff from portions of the impervious area, the infiltration volume for the remaining area should be increased an equivalent amount to offset the loss.
C.
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 design professional and at minimum
address soil permeability, depth to bedrock, and subgrade stability.
The general process for designing the infiltration BMP shall be:
(1)
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.
(2)
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.
(3)
Design the infiltration structure for the required retention (Rev) volume based on field determined capacity at the level
of the proposed infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
design professional, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
D.
Infiltration facilities should, to the greatest extent practicable,
be located to avoid introducing contaminants via groundwater, and
be in conformance with an approved source water protection assessment
or source water protection plan.
E.
Roadway drainage systems should provide an opportunity to capture
accidental spills. Road deicing material storage facilities shall
be designed to avoid salt and chloride runoff from entering waterways
and infiltration facilities. The qualified design professional shall
evaluate the possibility of groundwater contamination from the proposed
infiltration facility and perform a hydrogeologic justification study
if necessary.
F.
The antidegredation analysis found in PA Code Chapter 93 shall be
applied in HQ or EV streams.
G.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. The municipality
may require a detailed hydrogeologic investigation.
H.
The applicant should provide safeguards against groundwater contamination
for land uses that may cause groundwater contamination should there
be a mishap or spill.
A.
The applicant shall comply with the following water quality requirements
of this article. To control postconstruction stormwater impacts from
regulated activities and conform to state water quality requirements,
BMPs which replicate predevelopment stormwater infiltration and runoff
conditions must be provided in the site design such that postconstruction
stormwater discharges do not degrade the physical, chemical, or biological
characteristics of the receiving waters. This may be achieved by the
following:
(1)
Infiltration: replication of preconstruction stormwater infiltration
conditions;
(2)
Treatment: use of water quality treatment BMPs to provide filtering
of chemical and physical pollutants from the stormwater runoff; and
(3)
Stream bank and stream bed protection: management of volume and rate
of postconstruction stormwater discharges to prevent physical degradation
of receiving waters (e.g., from scouring).
B.
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 257-14 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the calculated water quality volume (WQv) is greater than the volume required to be infiltrated as described in § 257-14B(2), then the difference between the two volumes shall be treated for water quality by an acceptable stormwater management practice(s). The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of stormwater runoff from the developed areas of the site. To achieve this requirement, the following criterion is established:
(1)
From Control Guideline (CG-1) in the PA BMP Manual, the water quality
volume shall be the net two-year twenty-four-hour volume. The net
volume is the difference between the postdevelopment runoff volume
and the predevelopment runoff volume. The postdevelopment total runoff
volume for all storms equal to or less than the two-year twenty-four-hour
duration precipitation shall not be increased. For modeling purposes,
existing (predevelopment) nonforested pervious areas must be considered
meadow in good condition or its equivalent, and 20% of existing impervious
area, when present, shall be considered meadow in good condition.
(2)
This volume requirement can be managed by the permanent volume of
a wet basin or the detained volume from other BMPs. Where appropriate,
wet basins shall be utilized for water quality control and shall follow
the guidelines of the PA BMP manual referenced in Appendix G.[1]
[1]
Editor's Note: Appendix G is on file in the Township offices..
(3)
Release of water can begin at the start of the storm (i.e., the invert
of the water quality orifice is at the invert of the facility). The
design of the facility shall provide for protection from clogging
and unwanted sedimentation.
C.
The temperature of receiving waters shall be protected through the
use of BMPs that moderate temperature.
D.
If a perennial or intermittent stream passes through, or a water
body (i.e., lake, pond, wetland) is present on the site, the applicant
shall create a riparian buffer extending a minimum of 50 feet, to
either side of the top-of-bank of the channel, lake, or wetland. The
buffer area shall be planted with native vegetation and maintained
in a vegetated state (refer to Appendix B, Pennsylvania Native Plant
List, contained in the PA BMP Manual).
(1)
The following provisions also apply to riparian buffers on lots in
existence at the time of adoption of this chapter:
(a)
If the applicable rear or side yard setback is less than 50
feet, the buffer width may be reduced to 25% of the setback or 25
feet, whichever is greater:
(2)
Permitted uses within the buffer include the following, subject to
municipal approval and provided that they comply with all federal,
state, and local regulations:
E.
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.
A.
In addition to controlling the water quality volume (in order to
minimize the impact of stormwater runoff on downstream stream bank
erosion), the primary requirement to control stream bank erosion is
to design a BMP to detain the proposed conditions two-year, twenty-four-hour
design storm 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) to
release the proposed conditions one-year storm for a minimum of 24
hours from a point in time when the maximum volume of water from the
one-year storm is stored in a proposed BMP (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 quality
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. When the
calculated orifice size is below three inches, gravel filters (or
other methods) are recommended to discharge low-flow rates subject
to the municipal engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function. All facilities shall make use of measures to extend
the flow path and increase the travel time of flows in the facility.
A.
Each watershed has been divided into either stormwater management
districts or release rate districts as shown on the respective Management
District or Release Rate Maps in Appendix A.[1]
(1)
In addition to the watershed-specific requirements specified in Tables 257-17.1 and 257-17.2 for each of the watersheds, the erosion and sedimentation control (§ 257-12), the nonstructural project design (§ 257-13), the infiltration (§ 257-14), the water quality (§ 257-15), and the stream bank erosion (§ 257-16) requirements shall be implemented.
(2)
Standards for managing runoff from each subarea in a watershed for
the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year
design storms are shown in Tables 257-17.1 and 257-17.2. Development
sites located in each of the management/release rate districts must
control proposed conditions runoff rates to existing conditions runoff
rates for the design storms in accordance with the tables.
[1]
Editor's Note: Appendix A is on file in the Township offices..
B.
General. Proposed conditions rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff from existing conditions
for the design storms specified on the Stormwater Management District
Watershed Map (Appendix A)[2] and in this section.
[2]
Editor's Note: Appendix A is on file in the Township offices..
C.
District boundaries. The boundaries of the stormwater management
districts are shown on an Official Map that is available for inspection
at the municipal and county planning offices. A copy of the Official
Map at a reduced scale is included in Appendix A.[3] The exact location of the stormwater management district
boundaries as they apply to a given development site shall be determined
by mapping the boundaries using the two-foot topographic contours
(or most accurate data required) provided as part of the SWM site
plan.
[3]
Editor's Note: Appendix A is on file in the Township offices..
D.
Sites located in more than one district or watershed. For a proposed
development site located within two or more stormwater management
district subareas, the peak discharge rate from any subarea shall
meet the management district criteria for which the discharge is located.
The natural hydrology of each respective subarea shall be maintained,
and drainage shall not be redirected from one subarea to another.
Under circumstances where the applicant shows this cannot be accomplished,
a waiver is required by the municipality.
|
Table 257-17.1
Peak Rate Control Standards in the Crum Creek Watershed
| |||
|---|---|---|---|
|
District
|
Proposed Condition Design Storm
(reduce to)
|
Existing Condition Design Storm
| |
|
A
|
2-year
|
1-year
| |
|
5-year
|
5-year
| ||
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
|
B
|
2-year
|
1-year
| |
|
5-year
|
2-year
| ||
|
10-year
|
5-year
| ||
|
25-year
|
10-year
| ||
|
50-year
|
25-year
| ||
|
100-year
|
100-year
| ||
|
Table 257-17.2
Peak Rate Control Standards by Stormwater Management District
in the Darby-Cobbs Creek Watershed
| |||
|---|---|---|---|
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| |
|
A
|
2-year
|
1-year
| |
|
5-year
|
5-year
| ||
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
100-year
|
100-year
| ||
|
B-1
|
2-year
|
1-year
| |
|
10-year
|
5-year
| ||
|
25-year
|
10-year
| ||
|
50-year
|
25-year
| ||
|
100-year
|
100-year
| ||
|
B-2
|
2-year
|
1-year
| |
|
5-year
|
2-year
| ||
|
25-year
|
5-year
| ||
|
50-year
|
10-year
| ||
|
100-year
|
100-year
| ||
|
C
|
Conditional Direct Discharge District
| ||
E.
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. On-site drainage facilities shall be designed to
safely convey off-site flows through the development site.
F.
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 peak rate control standards noted above. Unimpacted
areas for which the discharge point has not changed are not subject
to the peak rate control standards.
G.
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 Marple Township Engineer:
A.
Stormwater runoff from all development sites with a drainage area
of greater than five acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. Table 257-18.1 summarizes acceptable computation methods.
The method selected by the design professional shall be based on the
individual limitations and suitability of each method for a particular
site. The use of the Rational Method to estimate peak discharges for
drainage areas greater than five acres shall be permitted only upon
approval of the municipality.
|
Table 257-20.1
Acceptable Computation Methodologies for SWM Site Plan
| |||
|---|---|---|---|
|
Method
|
Developed By
|
Applicability
| |
|
TR-20
(or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary.
| |
|
TR-55
(or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans where limitations described
in TR-55.
| |
|
HEC-1/HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary.
| |
|
Rational Method
(or commercial computer package based on Rational Method)
|
Emil Kuichling (1889)
|
For sites up to 5 acres, or as approved by the municipality
and/or Municipal Engineer.
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer.
| |
B.
All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths for
the various return period storms. Rainfall depths shall be according
to NOAA Atlas 14 values consistent with a partial duration series.
When stormwater calculations are performed for routing procedures
or water quality functions, the duration of rainfall shall be 24 hours.
C.
The following criteria shall be used for peak rate runoff calculations:
(1)
For development sites not considered redevelopment, the ground cover
used in determining the existing conditions flow rates shall be as
follows:
(a)
Wooded sites shall use a ground cover of "woods in good condition."
Portions of a site having more than one viable tree measuring a diameter
at breast height (DBH) of six inches or greater per 1,500 square feet
shall be considered wooded where such trees existed within three years
of application.
(b)
The undeveloped portion of the site including agriculture, bare
earth, and fallow ground shall be considered as "meadow in good condition,"
unless the natural ground cover generates a lower curve (CN) number
or Rational "c" value (i.e., woods) as listed in Tables C-1 or C-2
in Appendix C of this chapter.[1]
[1]
Editor's Note: Appendix C is on file in the Township offices..
(2)
For redevelopment sites, the ground cover used in determining the
existing conditions flow rates for the developed portion of the site
shall be based upon actual land cover conditions.
D.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration (duration) and
storm events with rainfall intensities obtained from NOAA Atlas 14
partial duration series estimates, or the latest version of the PennDOT
Drainage Manual (PDM Publication 584). Times-of-concentration shall
be calculated based on the methodology recommended in the respective
model used. Times of concentration for channel and pipe flow shall
be computed using Manning's equation.
G.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5). Hydraulic computations
to determine the capacity of open channels shall be consistent with
methods and computations contained in the Federal Highway Administration
Hydraulic Engineering Circular Number 15 (Publication No. FHWA-NHI-05-114
HEC 15). Values for Manning's roughness coefficient (n) shall
be consistent with Table C-3 in Appendix C.[4]
[4]
Editor's Note: Appendix C is on file in the Township offices..
H.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
I.
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 an acceptable
method. The design storm hydrograph shall be computed using a calculation
method that produces a full hydrograph. The municipality may approve
the use of any generally accepted full hydrograph approximation technique
that shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
A.
All wet basin designs shall incorporate biologic controls consistent
with the West Nile Guidance found in Appendix G, PADEP document 363-0300-001 "Design Criteria
– Wetlands Replacement/Monitoring," or contact the Pennsylvania
State Cooperative Wetland Center (www.wetlands.psu.edu/) or the Penn
State Cooperative Extension Office (www.extension.psu.edu/extmap.html).
B.
Any stormwater basin required or regulated by this chapter designed
to store runoff and requiring a berm or earthen embankment shall be
designed to provide an emergency spillway to handle flow up to and
including the one-hundred-year proposed conditions. The height of
embankment must provide a minimum one foot of freeboard above the
maximum pool elevation computed when the facility functions for the
one-hundred-year proposed conditions inflow. Should any stormwater
management facility require a dam safety permit under PADEP Chapter
105, the facility shall be designed in accordance with Chapter 105
and meet the regulations of Chapter 105 concerning dam safety, Chapter
105 may require the passing of storms larger than one-hundred-year
event.
C.
Any drainage conveyance facility and/or channel not governed by Chapter
105 regulations must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-year storm event.
The larger the events (fifty-year and one-hundred-year) must also
be safely conveyed in the direction of natural flow without creating
additional damage to any drainage structures, nearby structures, or
roadways.
D.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from the facility.
E.
Roadway crossings or structures located within designated floodplain
areas must be able to convey runoff from a one-hundred-year design
storm consistent with Federal Emergency Management Agency National
Flood Insurance Program – Floodplain Management Requirements.
F.
Any facility located within a PennDOT right-of-way must meet PennDOT
minimum design standards and permit submission requirements.
G.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular Number 11 (Publication No. FHWA-IP-89-016) and
the PADEP Erosion and Sediment Pollution Control Program Manual (Publication
No. 363-2134-008).
