Optimal sizing of exfiltration trenches

Optimal sizing of exfiltration trenches for both flood control and water-quality control is investigated for conventional and partially submerged trenches. An analytic formulation is presented, showing that the minimum required trench length for flood control can be determined directly from parameters that characterize the design rainfall, catchment, trench, and porous medium surrounding the trench. It is also shown that the minimum required trench length for water-quality control can be determined directly from the design water-quality depth, exfiltration time that is credited to water-quality control, and parameters that characterize the trench and catchment. Using these analytic relationships, a six-step procedure is presented for sizing trenches that simultaneously meet both flood-control and water-quality control objectives. Implementation of the design methodology is illustrated for both conventional and partially saturated trenches.