In addition to the water quality volume, to minimize the impact of stormwater runoff on downstream stream bank erosion, a BMP must be designed to detain the proposed conditions 2-year, 24-hour design storm to the existing conditions 1-year flow using the SCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) so that the proposed conditions 1-year storm takes a minimum of 24-hours to drain from the facility from a point where the maximum volume of water from the 1-year storm is captured (e.g. the maximum water surface elevation is achieved in the facility).
Release of water may begin at the start of the storm (e.g. the invert of the water quality orifice is at the invert of the facility). The design of the facility shall minimize clogging and sedimentation. Orifices smaller than four (4) inches in diameter are not allowed unless the design engineer can verify that the smaller orifice is protected from clogging by use of trash racks, etc. In that case, smaller orifices may be permitted. Trash racks are required for any primary orifice.
Whenever a watercourse is located within a development site, it shall remain open in the natural state and location and shall not be piped, impeded, or altered (except for road crossings). It is the responsibility of the developer to restore existing eroded stream/channel banks within a subdivision or land development site and obtain all permits necessary from PADEP, to do so. The developer must submit pictorial documentation of existing stream/channel banks to determine whether existing banks must be stabilized.
(Ord. 186, passed 9-12-2017)