Drainage representation in flood models: Application and analysis of capacity assessment framework

Drainage systems are an integral part of urban infrastructure to help transport and treat wastewater as well as manage flooding during extreme rainfall events. Although there is a significant cost associated with the creation, operation and maintenance of drainage systems, the representation of these systems in flood models is overly simplified. This simplification is due to data protection regulations, and the complexities associated with drainage network modelling. A new framework developed by Water UK in collaboration with the Environmental Agency and sewerage undertakers for UK Drainage Water Management Plans provides data on the capacity and performance of the drainage system. The output from this framework provides a new method of incorporating a more explicit representation of spatially varied drainage capacity in flood models.This study presents the first application of the UK's capacity assessment framework (CAF) for drainage representation in flood models. We develop a method of using the CAF outputs to represent spatially varied drainage losses across a catchment and assess its impact on flood risk. Three catchments in Leeds are used to quantify the difference generated in flooding when using a national average removal rate (NARR, e.g., 12 mm/hr) and our CAF-derived rainfall removal rates. Although there is variance across catchments, the results show the CAF removal rates increase flood depths, velocities, and flood hazards when compared to the national average due to a more realistic representation of the real system drainage capacity. With the pressures of climate change and continued urban development, a better representation of real drainage systems capacities will become more important and will make local solutions more resilient and relevant to the realities on the ground.