Representing soakaways in a physically distributed urban drainage model – Upscaling individual allotments to an aggregated scale

The increased load on urban stormwater systems due to climate change and growing urbanization can be partly alleviated by using soakaways and similar infiltration techniques. However, while soakaways are usually small-scale structures, most urban drainage network models operate on a larger spatial scale and the simulation of individual soakaways in these models is therefore often not readily feasible. This study describes the coupling of a soakaway model to a physically distributed urban drainage model, and investigates different upscaling methods. The soakaway component calculates the infiltration rate based on water depth and soil properties for each time step, and controls the removal of water from the urban drainage model. The model is intended to be used to assess the impact of soakaways on urban drainage networks. The model is tested using field data and shown to simulate the behavior of individual soakaways well. Six upscaling methods to aggregate individual soakaway units with varying saturated hydraulic conductivity (K) in the surrounding soil have been investigated. In the upscaled model, the weighted geometric mean hydraulic conductivity of individual allotments is found to provide the best match to an individual allotment model when comparing total outflow volume and peak flow rate. The error introduced by upscaling is found to be in the range of a few percent, and is relatively small compared to the uncertainty of typical field estimates of hydraulic conductivity.