Optimal Configuration of an Underdrain Delivery System for a Stormwater Infiltration Trench

Abstract

Performance of a green infrastructure (GI) tree trench comprising planters, an underground rock infiltration bed, inlet structures collecting runoff, and a perforated underdrain pipe delivering water from the inlet structure into the rock bed was evaluated. Over 23 months of monitoring, the system removed 94% of the contributing rainfall primarily through infiltration; therefore, the current design achieved the design goal. The planters were found to be hydrologically isolated, so the tree trench underdrain delivery pipe and rock bed control system performance. Overflow occurred prior to the full utilization of the rock bed storage, indicating that the underdrain pipe limited inflow. Over the course of a rain event, the switch of flow control from the conduit to the pipe wall orifices reduced the maximum flow rate, generating overflow. Flow equations considering water heads variations were derived, and maximization of flow rate was presented based on the equations. The general recommendations for similar sites are to lower the delivery pipe to the bottom of rock bed and considering orifice density as the key element in design.