Abstract
Abstract. This paper addresses the mass balance error observed in runoff hydrographs in urban watersheds by introducing assumptions regarding the contribution of infiltrated rainfall from pervious areas and isolated impervious area (IIA) to the runoff hydrograph. Rainfall infiltrating into pervious areas has been assumed not to contribute to the runoff hydrograph until Hortonian excess rainfall occurs. However, mass balance analysis in an urban watershed indicates that rainfall infiltrated to pervious areas can contribute directly to the runoff hydrograph, thereby offering an explanation for the long hydrograph tail commonly observed in runoff from urban storm sewers. In this study, a hydrologic analysis based on the width function is introduced, with two types of width functions obtained from both pervious and impervious areas, respectively. The width function can be regarded as the direct interpretation of the network response. These two width functions are derived to obtain distinct response functions for directly connected impervious areas (DCIA), IIA, and pervious areas. The results show significant improvement in the estimation of runoff hydrographs and suggest the need to consider the flow contribution from pervious areas to the runoff hydrograph. It also implies that additional contribution from flow paths through joints and cracks in sewer pipes needs to be taken into account to improve the estimation of runoff hydrographs in urban catchments.
Highlights
Urban drainage systems consist of three parts: the overland surface flow system, the sewer network, and the underground porous media drainage system
This paper suggests a framework using the instantaneous unit hydrograph based on the width function (WFIUH) in order to examine the contribution from pervious areas in urban catchments
WFIUH is adapted to account for the contribution of distinct pervious and impervious areas utilizing the spatial distribution of the imperviousness areas in an urban catchment
Summary
Urban drainage systems consist of three parts: the overland surface flow system, the sewer network, and the underground porous media drainage system. It is known that the groundwater discharge accounts for the time-delayed recession curve that is prevalent in certain watersheds (Fetter, 2001). This process has not, been accounted for satisfactorily modeled by surface runoff models alone (Huber and Dickinson, 1992). The well-known urban hydrology and conveyance system hydraulics model, SWMM has a subsurface flow routing subroutine called GROUND in Runoff Block based on physical processes of groundwater. It has a number of parameters for application
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