1240815MAINTENANCE MANUAL rcd 3 12 2026

PART VI – Underground Pipe System BMP#2 Functionality The underground detention system is a subsurface storage facility that receives and temporarily stores stormwater runoff from roofs of structures, parking lots, and grass areas. Discharge of the stored runoff from the underground system occurs through gravity by a discharge structure. An underground system may be either a structural chamber and/or an excavated pit filled with aggregate. The system may also be a set of pipes surrounded in stone or a set of pipes without stone. Due to its storage capacity, an underground system may be used to reduce the total stormwater quantity design storm runoff volume that a site would ordinarily discharge to downstream stormwater management facilities. Underground systems can be used to reduce the increased volume of stormwater runoff caused by a whole site. Underground systems can also be used to indirectly enhance water quality by reducing the amount of stormwater quality design storm runoff volume to be treated by the other, downstream stormwater management facilities. Underground systems can also be used to meet the groundwater recharge requirements of the NJDEP Stormwater Management Rules. Proper care and attention in the long-term maintenance of the stormwater management measure is critically important to the safety and health of the public. Type of BMP – Dry Stormwater Management Measure An underground system is a type of dry stormwater management measure. Dry stormwater management measures must fully drain within 72 hours of the most recent rainfall. Standing water in excess of 72 hours is a sign of failure. It may also contribute to mosquito breeding and other health and safety issues. The design drain time shall be closely monitored to ensure that potential failure is recognized early. Underground systems can detain, infiltrate, and recharge stormwater runoff; however, underground systems are not designed to treat stormwater runoff for water quality; therefore, no TSS removal is assigned to the system. Basic Design Information Hydrology Design Targets -This underground system is designed with a subsoil permeability rate of 5.8 inches/hour (pre-construction) and 2.9 inches/hour (post-construction). -The design drain time is 24.30 hours. -The elevation of the seasonal high water table of this system was observed to

pre-construction) and 2.9 inches/hour (post-construction). -The design drain time is 24.30 hours. -The elevation of the seasonal high water table of this system was observed to be 7 feet below the systems bottom surface, at EL.410.50 feet. Hydraulic Design Targets -This underground system is designed to infiltrate the runoff from the (Water Quality Design Storm / groundwater recharge storm), which generates 0.005 acre feet of runoff. 21