Method of computation. Peak discharge and runoff gauge by computer
using the following:
A.
Applicants may select a combination of runoff control techniques
which are most suitable to control stormwater runoff from the site.
All controls shall be subject to approval of the Municipal Engineer.
The Municipal Engineer may request specific information on design
and/or operating features of the proposed stormwater controls in order
to determine their suitability and adequacy in terms of the standards
of this section.
B.
The applicant shall consider the effect of the proposed stormwater
management techniques on any special soil condition or geological
hazards which may exist on the development site. In the event such
conditions are identified on the site, the Municipal Engineer may
require in-depth studies by a competent geotechnical engineer.
C.
Any BMP which is a dam, culvert, stream, enclosure or outfall, as
defined in 25 Pa. Code Chapter 105, shall be designed according to
the requirement in those regulations.
D.
Any stormwater BMP which does not constitute a dam under 25 Pa. Code
Chapter 105 and is designed to store runoff and requiring a berm or
earthen embankment (i.e., detention basin) shall be designed to satisfy
the following:
(1)
Berms and earthen embankments shall be designed to provide an emergency
spillway to handle flow up to and including the one-hundred-year post-development
conditions.
(2)
Berms and earthen embankments shall be designed to provide a minimum
1.0 foot of freeboard above the maximum pool elevation, computed when
the facility functions for the one-hundred-year post-development inflow.
(3)
Water obstructions shall convey runoff from the twenty-five-year
design storm with a minimum of 1.0 foot of freeboard measured below
the lowest point along the top of the roadway without damage to the
drainage structure or the roadway.
(4)
Roadway crossings located within designated floodplain areas must
be able to convey runoff from a one-hundred-year design storm.
(5)
Drainage conveyances must be able to convey, without damage to the
drainage structure or roadway, runoff from the twenty-five-year design
storm without surcharging.
(6)
Adequate erosion and sediment control protection shall be provided
along all open channels and at all points of discharge.
(7)
All BMP facilities will be sized for the two-, ten- and twenty-five-year
storm and release ratings.
E.
Stormwater runoff from all development and post-development sites
shall be calculated using either the Rational Method or a Soil Cover
Complex methodology.
F.
Any stormwater runoff calculations involving areas greater than 200
acres, including on- and off-site areas, shall use a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. Table I summarizes acceptable computation methods. All methods
will be selected by the design professional based on the individual
limitations and suitability of each method for a particular site.
These assumptions shall be used in runoff calculations:
(1)
Average antecedent moisture conditions.
(2)
Type II distribution storm.
(3)
Meadow in good condition shall be used in predevelopment runoff calculations
for all areas of existing cultivation.
(4)
All areas other than cultivation shall use the land cover conditions
which existed during the past 10 consecutive years.
(5)
All areas to be disturbed during construction and subsequently returned
to open space will be assumed to be reduced one hydrologic group category
level for post-development runoff.
(6)
If the initial condition of the site is undeveloped land, the land
use shall be considered as "meadow" unless the natural land cover
is proven to generate lower curve numbers or Rational "C" value, such
as forested lands.
G.
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. If a hydrologic computer model such
as TR-20, PSRM or HEC-HMS is used for stormwater runoff calculations,
then the duration of rainfall shall be 24 hours.
H.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods from the design storm curves from Pennsylvania
Department of Transportation Design Rainfall Curves (1986). Times
of concentration for overland flow shall be calculated using the methodology
presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS,
and TR-55 (as amended or replaced from time to time by NRCS). Times
of concentration for channel and pipe flow shall be computed using
Manning's Equation.
I.
Runoff curve numbers (CN) for both existing and proposed conditions
are to be used in the Soil Cover Complex Method.
J.
Runoff coefficient (c) for both existing and proposed conditions
is to be used in the Rational Method.
K.
Where uniform flow is anticipated, the Manning Equation shall be
used for hydraulic computations and to determine the capacity of open
channels, pipes and storm sewers.
L.
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.
M.
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 20 acres in size, 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 which
shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
N.
The municipality may require that computed existing runoff rates
be reconciled with field observations and conditions. If the design
professional can substantiate through actual physical calibration
that more appropriate runoff and time-of-concentration values should
be utilized at a particular site, then appropriate variations may
be made upon review and recommendations of the Municipal Engineer.
Calibration shall require detailed gauge and rainfall data for the
particular site in question.
|
Table I
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|
Method
|
Method Developed by
|
Applicability
| |
|
TR-20 or commercial package based on TR-20
|
USDA-NRCS
|
When use of full model is desirable or necessary
| |
|
TR-55 or commercial package based on TR-55
|
USDA-NRCS
|
Applicable for plans within the model's limitations
| |
|
HEC-1
|
U.S. Army Corps of Engineers
|
When full model is desirable or necessary
| |
|
PSRM
|
Penn State University
|
When full model is desirable or necessary
| |
|
Rational Method or commercial package based on Rational Method
|
Emil Kuiching (1889)
|
For sites less than 200 acres
| |
|
Other methods
|
Various
|
As approved by the Municipal Engineer
| |
O.
Rainfall frequency data. (Available from United States Department
of Commerce, National Weather Service and Pennsylvania Department
of Environmental Protection, Research Publication Number 70.)
P.
Maintenance of natural drainageways. All natural streams, channels,
swales, drainage systems and/or areas of surface water concentration
shall be maintained in their existing condition unless an alteration
is approved by Union Township. All encroachment activities shall comply
with the requirements of Chapter 105 (Water Obstructions and Encroachments)
of Title 25 (Pennsylvania Code), Rules and Regulations of the Pennsylvania
Department of Environmental Protection.
Q.
Methods of stormwater runoff detention and control.
(1)
The following is a listing of detention and control methods which
may be utilized in stormwater management systems, if appropriate.
The choice of control techniques is not limited to the ones appearing
on this list.
(a)
Detention basins.
(b)
Rooftop storage.
(c)
Parking lot and street ponding.
(d)
Seepage pits, seepage trenches or other infiltration structures.
(e)
Porous pavement and concrete lattice block surfaces.
(f)
Grassed channels and vegetated strips.
(g)
Cisterns and underground reservoirs.
(h)
Routed flow over grass.
(i)
Decreased impervious area coverage.
(2)
The use of other control methods which meet the criteria in this
section will be permitted when approved by the Union Township Engineer.
Various combinations of methods should be tailored to suit the particular
requirements of the type of development and the topographic features
of the project area.
A.
In order to implement the provisions of the Washington County Stormwater
Management Plan, Union Township is hereby divided into stormwater
management districts, which shall be designated:
(1)
Peters Creek No. 1 Gastonville Watershed.
(2)
Peters Creek No. 2 Trax Watershed.
(3)
Piney Fork Watershed.
(4)
Unnamed watershed.
(5)
Peters Creek No. 3 Hackett Watershed.
(6)
Peters Creek No. 4 Finleyville Watershed.
(7)
Mingo Creek No. 2 Watershed.
(8)
Mingo Creek No. 1 Watershed.
(9)
Foreman Run Watershed.
(10)
Crookham Watershed.
(11)
Huston Run Watershed.
(12)
Coal Bluff Watershed.
(13)
Shire Oaks Watershed.
(14)
Elrama Watershed.
(15)
Lobbs Run Watershed.
B.
The location and boundaries of the stormwater management districts
are shown on an Official Map, which is available for inspection from
the Union Township Secretary.
C.
When a project or land disturbance activity is located in more than
one stormwater management district, stormwater may not be transferred
from a district with stricter stormwater management criteria to a
district with less strict criteria, unless the need for such a transfer
is identified in the Washington County Stormwater Management Plan,
the regional water quality management plan or the state water plan.
A.
Stormwater rate and quality.
(1)
Peters Creek No. 1 Gastonville Watershed. There shall be no increase
in the rate of stormwater discharge from any activity covered by this
chapter than would have occurred from the land prior to the activity,
using predevelopment as the prior condition. The peak release rate
of stormwater from detention facilities shall be 95% of the predevelopment
rate. The water quality discharge standards shall be the removal of
80% of the total suspended solids load (TSS) for all storage structures
that have a volume greater than 1,000 cubic feet (for twenty-five-year
storm).[1]
(2)
Peters Creek No. 2 Trax Watershed. There shall be no increase in
the rate of stormwater discharge from any activity covered by this
chapter than would have occurred from the land prior to the activity,
using predevelopment as the prior condition. The peak post-development
release rate of the stormwater from detention facilities shall be
90% of the predevelopment rate. The water quality discharge standards
shall be the removal of 80% of the total suspended solids load (TSS)
for all storage structures that have a volume greater than 1,000 cubic
feet (for twenty-five-year storm).
(3)
Piney Fork Watershed. There shall be no increase in the rate of stormwater
discharge from any activity covered by this chapter than would have
occurred from the land prior to the activity, using predevelopment
as the prior condition. The peak post-development release rate of
the stormwater from detention facilities shall be 100% of the predevelopment
rate. The quality discharge standards shall be the removal of 80%
of the total suspended solids load (TSS) for all storage structures
that have a volume greater than 1,000 cubic feet (for twenty-five-year
storm).
(4)
Unnamed watershed. There shall be no increase in the rate of stormwater
discharge from any activity covered by this chapter than would have
occurred from the land prior to the activity, using the predevelopment
as the prior condition. The peak post-development release rate of
the stormwater from detention facilities shall be 100% of the predevelopment
rate. The water quality discharge standards shall be the removal of
80% of the total suspended solids load (TSS) for all storage structures
that have a volume greater than 1,000 cubic feet (for twenty-five-year
storm).
(5)
Peters Creek No. 3 Hackett Watershed. There shall be no increase
in the rate of stormwater discharge from any activity covered by this
chapter than would have occurred from the land prior to the activity,
using predevelopment as the prior condition. The peak post-development
release rate of the stormwater from detention facilities shall be
95% of the predevelopment rate. The water quality discharge standards
shall be the removal of 80% of the total suspended solids load (TSS)
for all storage structures that have a volume greater than 1,000 cubic
feet (for twenty-five-year storm).
(6)
Peters Creek No. 4 Finleyville Watershed. There shall be no increase
in the rate of stormwater discharge from any activity covered by this
chapter than would have occurred from the land prior to the activity,
using the predevelopment as the prior condition. The peak post-development
release rate of the stormwater from detention facilities shall be
95% of the predevelopment rate. The water quality discharge standards
shall be the removal of 80% of the total suspended solids load (TSS)
for all storage structures that have a volume greater than 1,000 cubic
feet (for twenty-five-year storm).
(7)
Mingo Creek No. 2 Watershed. There shall be no increase in the rate
of stormwater discharge from any activity covered by this chapter
than would have occurred from the land prior to the activity, using
the predevelopment as the prior condition. The peak post-development
release rate of the stormwater from detention facilities shall be
100% of the predevelopment rate. The water quality discharge standards
shall be the removal of 80% of the total suspended solids load (TSS)
for all storage structures that have a volume greater than 1,000 cubic
feet (for twenty-five-year storm).
(8)
Mingo Creek No. 1 Watershed. There shall be no increase in the rate
of stormwater discharge from any activity covered by this chapter
than would have occurred from the land prior to the activity, using
the predevelopment as the prior condition. The peak post-development
release rate of the stormwater from detention facilities shall be
100% of the predevelopment rate. The water quality discharge standards
shall be the removal of 80% of the total suspended solids load (TSS)
for all storage structures that have a volume greater than 1,000 cubic
feet (for twenty-five-year storm).
B.
Erosion and sedimentation. All land disturbance activities shall
be conducted in such a way as to minimize accelerated erosion and
resulting sedimentation. Measures to control erosion and sedimentation
shall, at a minimum, meet the standards of the Conservation District
and Chapter 102 (Erosion Control) of Title 25, Rules and Regulations
of the Pennsylvania Department of Environmental Protection.
