A. Development, which increases runoff, poses a significant threat to the environmental resources of Pennsylvania's groundwater, streams and wetlands. Therefore, attention is being turned to methods of increasing infiltration and groundwater recharge to offset the loss of infiltration and increased runoff due to development.
B. All stormwater management facilities shall be designed to satisfy the following requirements.
(1) All facilities shall be provided with the capability to withstand the discharge associated with the one-hundred-year return rainfall event, without failing or resulting in damage to downstream areas. Some nondetention facilities may be designed to bypass stormwater discharges which are in excess of the appropriate design storm. In this case, conveyance must be provided to transport the one-hundred-year surcharge flow to a downstream facility, natural watercourse or storm drainage system inlet.
(2) All groundwater recharge devices shall be protected from sedimentation. Areas designated for recharge shall not receive runoff until the contributory drainage areas have achieved final stabilization.
(3) Groundwater recharge requirement. The recommended criterion for addressing groundwater recharge is to maintain the annual volume of total runoff at predevelopment levels. This approach is implemented by requiring that a minimum retention volume be completely infiltrated on the site during every storm. The retention volume for Upper Dublin Township is 0.75 inches. Therefore, all rainfall events of less than 0.75 inches of rainfall should be completely retained on the site and infiltrated. If an analysis shows that runoff from the site will be negligible during this event, then the groundwater recharge requirement is satisfied. Controlling runoff from the groundwater recharge design storm is the only requirement for preserving the overall water budget for the watershed.
C. For design of BMPs, refer to the Pennsylvania Handbook of Best Management Practices for Developing Areas, or its successor document.