A. Runoff Control.
The rate of storm water runoff from any proposed subdivision or land development shall not exceed the rate of runoff prior to development. Where, in the judgement of the Township Engineer, the quantity of storm water runoff will cause detrimental downstream impact, further controls may be required. This standard shall be maintained for all storms (i.e. both high frequency and low frequency.)
B. Design of Detention Basins.
All detention basins shall be designed as per the procedures developed by the U.S. Department of Agriculture, Soil Conservation Service, as outlined in their Technical Release No. 55, Urban Hydrology for Small Watersheds for Drainage Areas in Excess of 25 Acres. Drainage basins of less than 25 acres shall be designed using the Universal Rational Method.
C. Basin Design Criteria.
The following design criteria shall be used in the design of all detention basins in the 'Township. The basins shall be designed to detain the quantity of water resulting from a 25 year storm frequency return period under full development conditions released at a maximum outflow rate equal to that resulting from a 10 year storm frequency return period under present conditions. Calculations shall also be provided to show that the pond outflow for a 2 year storm has been attenuated to the pre-developed rate. All flows in excess of the above mentioned standard shall flow over an emergency spillway.
1. Outlet Structure - A perforated riser or other type outlet structure specially designed for the outlet condition shall be provided at the outlet of all detention basins. The riser shall be constructed of metal or concrete. The riser shall extend to an elevation 2 feet below the crest elevation of the emergency spillway. The riser shall be designed so that the rate of outflow is controlled by the pipe barrel through the basin berm when the depth of the water within the basin exceeds the height of the riser. The perforations shall be cleanly cut and shall not be susceptible to enlargement. All metal risers shall be suitably coated to prevent corrosion. A trash rack or similar appurtenance shall be provided to prevent debris from entering the riser. All risers shall have a concrete base attached with a watertight connection. The base shall be sufficient weight to prevent floatation of the riser. An anti-vortex device, consisting of a thin vertical plate normal to the basin berm, shall be provided on the top of the riser. A suitable perforated riser design is outlined in the following source:
Erosion and Sediment Control Handbook
Lehigh County Soil and Water Conservation District
Soil Conservation Service
United States Department of Agriculture
2. Maximum Depth of Detention Basins - In general, the maximum depth of water in a detention basin shall not exceed 5 feet.
3. Emergency Spillway - Whenever possible, the emergency spillway for detention basins shall be constructed on undisturbed ground. Emergency spillways shall be constructed of reinforced concrete, vegetated earth, or concrete rubble. All emergency spillways shall be constructed so that the detention basin berm is protected against erosion. The minimum capacity of all emergency spillways shall be such that the combined capacity of the emergency spillway and the principal pipe barrel equal the peak flow rate from the 100 year design storm. The dimensions of the emergency spillways can be determined from the Lehigh County Erosion and Sediment Control Handbook. Emergency spillways shall extend along the upstream and downstream berm embankment slopes. The upstream edge of the emergency spillway shall be a minimum of 3 feet below the spillway crest elevation. The downstream slope of the spillway shall, as a minimum, extend to the toe of the berm embankment. The emergency spillway shall not discharge over earthen fill and/or easily erodible material.
4. Antiseep Collars - Antiseep collars shall be installed around the principal pipe barrel within the normal saturation zone of the detention basin berms. The anitseep collars and their connections to the pipe barrel shall be watertight. The antiseep collars shall extend a minimum of 2 feet beyond the outside of the principal pipe barrel. The maximum spacing between collars shall be 14 times the minimum projection of the collar measured perpendicular to the pipe.
5. Freeboard - Freeboard is the difference between the design flow elevations in the emergency spillway and the top of the settled detention basin embankment. The minimum freeboard shall be 2 feet.
6. Slope of Detention Basin Embankment - The maximum slope of earthen detention basin embankments shall be 3 to 1. The top or toe of any slope shall be located a minimum of 5 feet from any property line. Whenever possible, the side slopes and basin shape shall be amenable to the natural topography. Straight side slopes and rectangular basins shall be avoided whenever possible.
7. Width of Berm - The minimum top width of detention basin berms shall be 10 feet.
8. Slope of Basin Bottom - In order to insure proper drainage of the detention basin, a minimum grade of 2% shall be maintained for all sheet flow. A minimum grade of 0.5% percent shall be maintained for all channel flow.
9. Energy Dissipators - Energy dissipating devices (riprap, end sills, etc.) shall be placed at all basin outlets.
10. Design Information - As part of the Storm Water Management Plan and Report, all design information shall be submitted including, but not limited to, the following:
a. General description of proposed facilities and the operation of the runoff control measures.
b. All computations of the storm water runoff before, during and after construction, including all supporting material.
c. A sketch of the berm embankment and outlet structure inch-eating the embankment top elevation, embankment side slopes, top width of embankment, emergency spillway elevation, perforated riser dimensions, pipe barrel dimensions, and dimensions and spacing of antiseep collars.
d. Design computations for the pipe barrel and perforated riser.
e. A plot of the stage-storage (acre-feet vs. elevation) and all supporting computations.
f. Flood routing computations.
g. A detailed plan of the trash rack and anti-vortex device.
h. A plan, at a scale of 1 inch equals 50 feet showing the grading, landscaping, and fencing around the detention basin.
11. Landscaping and Grading of Detention Basins - All landscaping and grading standards particularly applicable to detention basins are included in Section VIII of this Appendix.
12. The distance from the highest free water surface of any detention basin or drainage facility to a dwelling unit shall be a minimum of 100 feet.
13. The detention basin shall, unless otherwise specified by the Township, be fenced by a chain link fence with a minimum height of 4 feet.
14. The detention basin shall be designed with an impervious liner. The liner may be of the impervious membrane type, placed in accordance with the manufacturer's recommendations, or may be constructed by mixing Bentonite with existing soil available at the site as approved by the Township Engineer.
15. When rock is encountered during the excavation of a pond, it shall be removed to an elevation of at least 12 inches below the proposed basin floor.
(as amended by Ord. 239, 2/9/1990, § 2; and by Res. 1992-29, 8/19/1992)