The potential for natural flood management to maintain free discharge at urban drainage outfalls

Abstract This study examines whether catchment‐scale natural flood management (NFM) interventions could help to manage water levels in downstream urban watercourses and promote free discharge from surface drainage outfalls. A coupled modelling approach consisting of Dynamic TOPMODEL, HEC‐RAS, and Infoworks ICM models is used to characterise the response from a small Cambridgeshire catchment. Four different NFM scenarios (consisting of in‐channel woody debris and wider catchment afforestation) are defined. The attenuation of catchment response created by these measures is evaluated for an historic event and six different design storms. The consequent moderation of water depths at two downstream drainage outfalls is investigated with respect to maintaining free discharge from a surface drainage system. The case study results show that greatest reductions in the time of outfall inundation from NFM occur during frequent storm events (e.g., up to 5.75 hr during a 5‐year event). These reductions diminish with increasing storm severity but, by slightly desynchronising rural and urban responses, upstream interventions continue to have modest benefit for downstream drainage performance (e.g., preventing system capacity being exceeded during a 100‐year event). These results may interest water companies (increasingly involved in catchment‐scale NFM projects) looking to improve performance of surface water drainage.