A.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses except as modified by stormwater
management facilities or open channels consistent with this chapter.
B.
The existing ports of concentrated drainage that discharge onto adjacent
property shall not be relocated and shall be subject to any applicable
discharge criteria specified in this chapter.
C.
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas. If diffused flow is proposed to be concentrated and
discharged onto adjacent property, the developer must document that
adequate downstream conveyance facilities exist to safely transport
the concentrated discharge or otherwise prove that no erosion, sedimentation,
flooding or other harm will result from the concentrated discharge.
D.
Where a development site is traversed by watercourses other than
permanent streams, a drainage easement shall be provided conforming
substantially to the line of such watercourses. The terms of the easement
shall prohibit excavation, the placing of fill or structures and any
alterations that may affect adversely the flow of stormwater within
any portion of the easement. Also, maintenance and mowing of vegetation
within the easement shall be required.
E.
Any stormwater management facilities regulated by this chapter that
would be located on state highway rights-of-way shall be subject to
approval by the Pennsylvania Department of Transportation (Pennsylvania
DOT).
F.
The Pennsylvania DER, Chapter 105, Rules and Regulations, apply to the construction, modification, operation or maintenance of both existing and proposed dams, water obstructions and encroachments throughout the watershed, including work in wetlands. Inquiries on permit requirements or other concerns should be addressed to the Soils and Waterways Section of the applicable DER Regional Office.
A.
General. The following general standards shall be applied to all
development within the Borough to promote flow attenuation, erosion
and sediment control and flood control.
(1)
All site development in the Borough creating 7,500 square feet or
more of impervious cover shall submit a drainage plan consistent with
this chapter to the municipality for review. This criteria shall apply
to the total proposed development even if development is to take place
in stages. Impervious cover shall include, but not be limited to,
any roof, parking or driveway areas and any new streets and sidewalks.
Any areas designed to initially be gravel or crushed stone shall be
assumed to be impervious for the purposes of comparison to the waive
criteria.
(2)
Runoff from impervious areas must be drained to pervious areas of
the property or properly conveyed to existing storm sewers, provided
that adequate capacity exists.
(3)
Roof drains must not be connected to streets or sanitary or storm
sewers or roadside ditches.
(4)
Runoff from a site should not be concentrated or increase runoff
discharged onto adjacent property without the written consent of the
adjacent landowners.
B.
Detention/infiltration standards.
(1)
Post-development rates of runoff from any regulated activity shall
not exceed the peak rates of runoff prior to development for the two-
and ten-year design storms. That is, facilities for the control of
stormwater from a development site must be designed for both the two-year
and ten-year design storms.
(2)
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., are encouraged, where soil
conditions permit, to reduce the size or eliminate the need for detention
facilities or for conveyance to storm sewers.
A.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation.
B.
Any stormwater management facility required or regulated by this chapter shall be designed to provide an emergency spillway to handle the flow of the one-hundred-year storm post-development. The height of embankment must be set as 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. However, criteria for design and construction of stormwater management facilities are not the same criteria that are used in the permitting of dams under the DER Dam Safety Program. Depending upon the physical characteristics of a dam, a dam permit may be required and the design will have to meet the provisions of Chapter 105 of the Dam Safety and Encroachments Act. Depending on the physical characteristics of a dam, the design could require that anywhere from a fifty-year to a PMF storm event be considered.
C.
Any drainage conveyance facility that doesn't fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of roadway. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
D.
Storm sewers must be able to convey post-development runoff from
a twenty-five-year design storm without surcharging inlets.
E.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method or a Soil-Cover-Complex Methodology.
A.
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 Borough may approve the use of any
generally accepted full hydrograph approximation technique and shall
use a total runoff volume that is consistent with the volume from
a method that produces a full hydrograph.
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. If a hydrologic computer model such as PSRM
or HEC-1 is used for stormwater runoff calculations, then the duration
of rainfall shall be 24 hours.
C.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods. Times of concentration for overland flow shall
be calculated using the methodology presented in Chapter 3 of Urban
Hydrology for Small Watersheds, SCS, TR-55 (as amended or replaced
from time to time by SCS). Times of concentration for channel and
pipe flow shall be computed using Manning's equation.
D.
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.
E.
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.
The methodology for determining required stormwater controls
for a regulated activity is outlined below:
A.
Compute.
(1)
Predevelopment hydrograph at the site discharge point for the two-
and ten-year, twenty-four-hour storm.
(2)
Post-development hydrograph at the site discharge point incorporating
any nondetention techniques such as pervious areas, swales, infiltration
trenches, etc.
(3)
Note: Hydrographs may be obtained from SCS methods such as TR-55
or from use of the modified rational formulas.
B.
Compare. Post-development hydrographs with predevelopment hydrographs. If the peak rate of runoff and the shape of the hydrographs are nearly identical, stormwater management has been achieved. Detention will not be required. If not, proceed as per Subsection C.
C.
Design. Detention/retention facilities, in conjunction with any nondetention
techniques, such that post-development peak rates from the site will
not exceed predevelopment levels for both the two- and ten-year storms.
