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 channel consistent with this chapter.
B. 
The existing points 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 (PennDOT).
F. 
The Pennsylvania Department of Environmental Protection, (25 Pa. Code) 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 dam safety permit requirements or other concerns shall be addressed to DEP's Bureau of Dams, Waterways and Wetlands, Harrisburg, Pennsylvania.
G. 
When it can be shown that, due to topographic conditions, natural drainageways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainageways. Work within natural drainageways shall be subject to approval by PADEP through the joint permit application process or, where deemed appropriate by PADEP, through the general permit process.
A. 
General. The following general standards shall be applied to all development within the Sandy Lick Creek Watershed to promote flow attenuation, erosion and sediment control and flood control.
(1) 
All site development in the Sandy Lick Creek Watershed which does not fall under the exemption criteria shown in this chapter, Appendix A,[1] shall submit a drainage plan consistent with the Sandy Lick Creek Watershed Stormwater Management Plan to the City 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 area 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 waiver criteria.
[1]
Editor's Note: Appendix A is an attachment to this chapter.
(2) 
Runoff from impervious areas must be drained to pervious areas of the property.
(3) 
Roof drains must not be connected to streets, sanitary or storm sewers or roadside ditches.
(4) 
Runoff from a site should not be concentrated or increased 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 design storms specified on Plate I, Volume I and Section V.B.I., Volume II.[2]
[2]
Editor's Note: See the Sandy Lick Creek Watershed Stormwater Management Plan.
(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.
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, (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to the one-hundred-year post-development conditions. The height of embankment must be set as to provide a minimum one 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 DEP 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 and/or channel that does not 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. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. 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. 
Capacity improvements. If the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the existing drainage network, the adequate capacity improvements could be provided by the developer in lieu of stormwater management facilities on the development site. Any capacity improvements would be designed based on development of all areas tributary to the improvements and the capacity criteria specified in this chapter. The type and amount of development that the developer must consider shall be either based on current zoning or established by the City, whichever results in a greater amount of imperviousness. It shall be assumed that all new development upstream of a proposed capacity improvement would implement applicable stormwater management techniques, consistent with this chapter.
F. 
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. 
Any stormwater runoff calculations involving drainage areas greater than 20 acres, including on- and off-site areas, shall use generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table VIII-1 below summarizes acceptable computation methods. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site. The Plan Administrator may approve the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 20 acres.
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 presented in Table B-1 in Appendix B of this chapter.[1] 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. The NRCS's curve, which is shown in Figure B-1 in Appendix B of this chapter, shall be used for the rainfall distribution.[2]
[1]
Editor's Note: Appendix B is an attachment to this chapter.
[2]
Editor's Note: Appendix B is an attachment to this chapter.
C. 
For the purposes of predevelopment flow-rate determination, undeveloped land shall be considered as "meadow" conditions, unless the natural ground cover generates a lower curve number or rational "C" value (i.e., forest).
D. 
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) (Figure B-2). Times of concentration for overland flow shall be calculated using the methodology present in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS TR-55 (as amended or replaced from time to time by the NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's equation.
E. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil-Cover Complex Method shall be obtained from Table B-2 in Appendix B of this chapter.[3]
Table VIII-1
Acceptable Computation Methodologies for Stormwater Management Plans
Method
Method Developed By
Applicability
TR-20 (or commercial 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 within limitations described in TR-55
HEC-1
U.S. Army Corps of Engineers
Applicable where use of full hydrologic computer is desirable or necessary
PSRM
Penn State University
Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1
Rational Method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For sites less than 200 acres, or as approved by the Plan Administrator and City Engineer
Other methods
Varies
Other computation methodologies approved by the Plan Administrator and City Engineer
[3]
Editor's Note: Appendix B is an attachment to this chapter.
F. 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table B-3 in Appendix B of this chapter.[4]
[4]
Editor's Note: Appendix B is an attachment to this chapter.
G. 
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. Values for Manning's roughness coefficient (n) shall be consistent with Table B-4 in Appendix B of this chapter.[5]
[5]
Editor's Note: Appendix B is an attachment to this chapter.
H. 
The design of any stormwater detention facilities intended to meet the performed 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 City may approve the use of any generally accepted full hydrograph approximation technique shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
I. 
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 shown in Figure VIII-1 and outlined below.
A. 
Compute:
(1) 
Predevelopment hydrograph at the site discharge point for the required design storm.
(2) 
Post-development hydrograph at the site discharge point incorporating any "nondetention" techniques such as pervious areas, swales, infiltration trenches, etc.
Note: Hydrographs may be obtained from NRCS methods such as TR-55, 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 to 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 the required design storms.
Figure VIII-1
Stormwater Control Determination Flow Chart