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

Abstract

Levee sump systems are used by many riverine communities for temporary storage of urban wet weather flows. The complex hydraulics and transport of stormwater pollutants in sump systems, however, have not been systematically studied. The objective of this work is to present a case study, utilizing a relatively simple and low-cost methodology, for assessing the hydraulic performance of flood control sumps in an urban watershed. Two sumps of highly variable physical and hydraulic characteristics were selected for analysis. HEC-1 software was used to estimate the flow hydrograph for each outfall to a sump as part of the overall flow balance, resulting in a total runoff hydrograph for a precipitation event. To validate HEC-1 results, a water balance was used to estimate the total runoff using sump operational data. The results suggest that HEC-1 calculation provide a satisfactory estimate of the total runoff and its time-distribution to the sump. The hydraulic model was then used to estimate nonpoint loads of selected heavy metals to the sump and to the river. Although flow of stormwater through a sump system is regulated solely by flood-control requirements, these sumps may function as sedimentation basins that provide purification of stormwater. An example calculation of removal of heavy metals in a sump using a mass balance approach is presented.