Promoting Successful Urban Watershed Restoration Through Enhanced Bioretention Cell Modelling

Abstract

Urban runoff and stormwater is one of the top ten leading causes of water quality impairment in lakes, estuaries and streams in the United States (USEPA 2009). Over the last decade, bioretention systems have become a leading stormwater control measure (SCM) that contributes to the restoration of urban streams and watersheds. Bioretention cells increase infiltration of stormwater thereby reducing urban runoff volumes and peak flows which alter the hydrology of local waterways. Although these systems have proven to perform well in many site-scale field studies, less is known about how well these systems work when implemented en masse. Modelling of bioretention allows designers to scale local impacts to the larger watershed. However, current hydrologic models with bioretention capabilities consist of lumped parameters and simplifications that do not fully account for fundamental hydrologic processes. DRAINMOD is an agricultural drainage model that has shown promise when applied to bioretention systems. However, because DRAINMOD was designed for agricultural purposes, it cannot currently accommodate the rapid response time of an urban runoff hydrograph, instead aggregating data to a daily timeframe. For this study, DRAINMOD has been recoded to allow high temporal resolution inputs and outputs, more closely matching the travel times of urban systems. DRAINMOD simulations were conducted both with and without the time scale modifications (original vs. bioretention-specific model) to determine if improvements in site-scale modelling were realized. Future work will compare these results to those of simplistic, lumped-parameter bioretention modelling.