[Amended 10-1-1979 by Ord. No. 6-79]
A.
Intent. The purpose of this manual is to establish standard procedures for the design and approval of adequate storm drain systems for new land subdivisions within Manor Township.
B.
Exemptions. Developers of subdivisions with land areas of five acres or less are exempt from the design requirements of this manual, provided that not more than 15% of the land area is made impervious by proposed construction.
C.
Scope. The design methods outlined herein apply only to drainage areas of less than 1 1/2 square miles. For large drainage areas, the user is referred to the PennDOT Design Manual, Part 2, Chapter 12, Section 20, "Bridge Waterway Structures" for required analysis of maximum expected discharge and related drainage facilities.
D.
Requirements.
(1)
The importance of properly designed drainage from an economic, safety and public relations standpoint warrants a hydrologic analysis. Requirements for submittals of plans and calculations of proposed facilities are outlined in the following sections. Calculations must include an investigation of existing drainage facilities downstream of the project to insure that they are capable of accepting the additional runoff without causing flooding and erosion.
(2)
The basic concept of the stormwater management policy is to insure that downstream property owners, watercourses, channels or conducts are not adversely affected by an increase in stormwater runoff. Therefore, stormwater runoff from any development, during and after construction, shall be no greater than that flow resulting from the same storm event occurring over the site of the proposed development with the land in its existing, undeveloped condition.
(3)
A comparison analysis of the change in runoff shall be provided in the calculations.
(4)
All increases in stormwater runoff resulting from a proposed development shall be detained on the development site under predetermined and controlled conditions with the rate of drainage therefrom regulated by appropriately installed devices. Methods of detention or flow-delay devices may include but not be limited to the following:
(a)
Wet or dry ponds and detention basins;
(b)
Roof storage and increased roof roughness;
(c)
Parking lot retention;
(d)
Porous pavements, grassed channels and vegetated strips;
(e)
Cisterns and underground reservoirs;
(f)
Increasing the roughness coefficients on the development's surface area;
(g)
Decrease percentage of impervious area.
(5)
The use of other detention methods which prove to meet the objectives and intent of this chapter in accordance with the regulations and standards set forth herein will be permitted subsequent to the approval of the Township Engineer. Various possible combinations of methods may be evaluated on their particular merit for the type and location of development.
(6)
Storm sewer systems within a development shall be designed to handle the peak rate of runoff from a fifty-year frequency storm. Wherever the provisions of federal and state laws impose a greater design frequency, particularly in areas where drainage systems may cross highways, they shall prevail.
(7)
All stormwater retention/detention facilities shall be designed on the basis of providing adequate control for all storm frequencies up to and including the fifty-year storm. All designs shall provide emergency overflow facilities for the one-hundred-year storm, unless positive measures are installed to control the inflow so as not to exceed the safe capacity of the retention/detention facility.
(8)
The retention volume required shall be that necessary to handle runoff of a fifty-year storm from the development, less that volume discharged at the approved release rate.
(9)
The approved peak release rate of stormwater from all retention/detention facilities for any storm event shall be that which was experienced prior to development for the same storm event up to and including the fifty-year storm.
(10)
Stormwater control systems may be planned and constructed in coordination by two or more developments, so long as they are in compliance with the applicable provisions of this chapter.
(11)
All calculations and design parameters shall be subject to the review and approval of the Township Engineer, whose decision in matters involving engineering judgment shall be final.
(12)
In the design of storm drainage facilities, special consideration must be given to adjacent developed or undeveloped properties. In no case may a change be made in the existing topography which would:
(13)
In no case may any slope exceed the normal angle of slippage of the material involved. All slopes must be protected against erosion.
E.
Other approvals.
(1)
Designers are cautioned that the requirements contained herein are only minimum standards established for storm drainage approval by Manor Township. Compliance with this manual and subsequent approval by Manor Township does not preclude the developer's full responsibility in meeting any and all federal, state or county regulations as may be applicable.
(2)
The storm drainage system design should be coordinated with the Sediment and Erosion Control Plan for the project; however, approval of the storm drainage plan does not eliminate any requirements for an approved Sediment and Erosion Control Plan.
