The effects of upstream Natural Flood Management on urban surface drainage performance
The evaluation of rural Natural Flood Management (NFM) has traditionally focused on the ability of interventions to mitigate downstream fluvial flooding by attenuating catchment response. This research expands this focus – investigating whether these same interventions could also mitigate surface flood risk in downstream urban areas. By moderating water levels in receiving watercourses, upstream NFM could promote free discharge from urban drainage outfalls and thereby improve local surface drainage performance. A novel modelling methodology has been developed to characterise the response of three separate catchments – the Bin Brook in Cambridgeshire, the Asker in Dorset and a sub-catchment of the upper Calder in Yorkshire. The upstream, rural response is simulated by coupling Dynamic TOPMODEL (a semi-distributed hydrological model) and HEC-RAS (which solves the shallow flow equations). This offers a freely-available, spatially-informed approach for the evaluation of a range of upstream NFM interventions (located both within and beyond the riparian zone) at a catchment-scale. This modelled rural response then provides the input for a downstream, integrated urban drainage model (Infoworks ICM). This is then be used to examine how any consequent changes in outfall inundation by the urban watercourse alter local drainage performance. Each case study is examined separately before a comparative study of all three is undertaken to identify broader trends. These trends suggest that during more frequent events (e.g. 1 in 10 year), upstream NFM interventions create the greatest reductions for the time low-lying outfalls are submerged by local watercourses. As storm severity increases (heightening risk of drainage surcharge or failure) these reductions diminish. Despite this, the slight delay in rural response continues to allow more water to escape surface systems before outfall inundation occurs, increasing the effective capacity of networks and reducing surface flood volumes. While any improvements to outfall discharge would not, in themselves, justify NFM implementation, these interventions have the potential to contribute to a downstream water level management strategy in certain locations and therefore will be of interest to urban flood risk practitioners.
Natural Flood Management, Drainage, Flooding, Coupled modelling
Networked Digital Library of Theses and Dissertations