Up-scaling a Model of On-site Stormwater Control Devices

Abstract In this study we investigated whether the catchment-scale effects of on-site stormwater devices can be modelled using aggregated devices and associated source areas rather than modelling each device separately. To address this question, we constructed a detailed model of an 83 ha catchment with spatial resolution at the individual property scale (typically 1000m'). Resulting hydrographs, pollutographs, and loads were compared to results from coarser scale models with various degrees of spatial aggregation of the source areas, devices, and drainage network. The stormwater model MUSIC was used in this study because it allows a range of on-site stormwater flow control and treatment devices to be represented. GIS techniques were developed to generate the detailed model and assist with aggregation. Time-expansion procedures were used to.,increase the temporal resolution of the model, because the conduit travel times in the detailed model were less than the minimum permitted time-step in MUSIC and we were investigating the effect of aggregation on the timing of flows and contaminant loads. This study showed that for a catchment with uniform soil properties and spatial distribution of treatment devices, the outlet hydrograph and pollutograph from the detailed model can be closely approximated using an aggregated representation. The only exception was for peak flow with a single lumped catchment. We expect that similar findings would hold for other models. The aggregation approach can be extended to allow for partial implementation of stormwater controls, devices treating only runoff from impervious areas, variation in soil properties, and separate treatment of runoff from previous and impervious areas. The result that on-site stormwater devices can be represented using a spatially-aggregated system will allow for considerable savings in model setup and computational times.