EVALUATING THE ABILITY OF UPSTREAM NATURAL FLOOD MANAGEMENT TO IMPROVE DOWNSTREAM URBAN DRAINAGE PERFORMANCE UNDER DIFFERENT STORM TRACKS

Abstract

The wider benefits of Natural Flood Management (NFM) are increasingly used to engage local stakeholders and justify physical implementation. Previous studies have highlighted the potential for upstream NFM interventions to contribute to water level management strategies by mitigating downstream water levels and promoting free discharge at surface drainage outfalls from urban areas. This study extends the scope of this possible benefit by examining the potential for upstream NFM interventions to desynchronise rural and urban responses under various storm tracks, thereby improving the performance of downstream surface drainage networks. The methodology uses a coupled modelling approach (consisting of Dynamic TOPMODEL, HEC-RAS and Infoworks ICM). Five design events, each applied with eight different storm tracks, are used to evaluate how a catchment-scale NFM in the upper Calder might influence performance of surface drainage in the downstream town of Todmorden. The results suggest that all the considered storm tracks had substantial influence on whether urban rainfall occurs during the period when outfalls are inundated. While upstream NFM interventions create modest flow attenuation, slight delay in rural response can significantly improve drainage performance and reduce instances of nuisance flooding.