An Analysis of the influence of the field level hydraulic property approximations in storm water modelling

Abstract

This study analyses the effect of the catchment field approximations for accurate flood hydrograph prediction. Considering the availability of gauged data, the Karasnagala watershed of Attanagalu Oya, located in Gampaha district in the Western province of Sri Lanka was modelled with EPA SWMM 5. Following an event based approach, 3 days Minimum Inter event Time (MIT) and 0 mm/day Minimum inter Event Depth (MED) were used as the threshold. Fifty events were separated from the 1971 to 1982 period. Concave method and constant slope method were compared for baseflow separation in this study. Four major field approximation types were identified as stream geometrical parameters approximations, soil infiltration parameter approximations, approximation of watershed intermittent storages, and subcatchment delineation approximation. Quantitative representation of the catchment intermittent storages and qualitative representation of the connectivity of the catchment intermittent storages were identified as the objectives of the layout parameter optimization. Soil parameter approximations and the stream network geometry parameter approximations were verified by the field observations. Model calibration and verification revealed that EPA SWMM5 can be successfully used to develop regional Karasnagala watershed model with mean ratio of absolute error (MRAE) 0.289 for calibration, and 0.375 MRAE for verification. Incorporation of intermittent storages with optimized model layout obtained best fitting of hydrograph recession MRAE 0.167. Evaluation of available models and the modelling carried out with calibration & verification revealed that EPA SWMM 5 can be used to develop a rural watershed model. Field approximation for subcatchment lumping with a 16 sub basin configuration showed the marginal increment of modelling error when compared with distributed modelling. Stream parameter approximations revealed that the head water streams/lesser order streams parameters sensitivity is higher than the higher order streams. In soil parameter approximations, saturated hydraulic conductivity of the soil was the most influencing parameter.