Simple mass balance approach for assessment of flood control sumps in an urban watershed: case study of heavy metal loading

Levee sump systems are used by many riverine communities for temporary stor age of urban wet weather Rows. The complex hydraulics and transport of stor mwater pollutants in sump systems, however, have not been systematically st udied. The objective of this work is to present a case study, utilizing a r elatively simple and low-cost methodology, for assessing the hydraulic perf ormance of flood control sumps in an urban watershed. Two sumps of highly v ariable physical and hydraulic characteristics were selected for analysis. HEC-1 software was used to estimate the Row hydrograph for each outfall to a sump as part of the overall Row balance, resulting in a total runoff hydr ograph for a precipitation event. To validate HEC-1 results, a water balanc e was used to estimate the total runoff using sump operational data. The re sults suggest that HEC-1 calculation provide a satisfactory estimate of the total runoff and its time-distribution to the sump. The hydraulic model wa s then used to estimate nonpoint loads of selected heavy metals to the sump and to the river. Although Row of stormwater through a sump system is regu lated solely by flood-control requirements, these sumps may function as sed imentation basins that provide purification of stormwater. An example calcu lation of removal of heavy metals in a sump using a mass balance approach i s presented.