In-Channel Natural Flood Management Approach to Flood Risk Management: Modelling Applications on a Small Catchment in the UK

Leaky barriers, instream runoff and stream flow attenuation methods were modelled using a 1D HEC-RAS hydraulic model for a Natural Flood Management (NFM) monitoring test reach in the Corvedale catchment, UK. Indirect representations of leaky barriers used in the model included: a WINFAP- FARL (Flood Attenuation by Reservoirs and Lakes) index approach; a combined FARL and Manning's n increased roughness approach; and clusters of leaky barriers as culvert units. The computed discharges and water depths were compared against recorded monitoring data at the test reach. For two rainfall events which represent partial overbank and full overbank conditions, the model results indicated that the greatest time lag attenuation of the peak flow was achieved through the culvert approach for the partial overbank condition. The culvert approach was verified through a comparison of observed and modelled data for both water depths and storage volumes. The verification shows that the culvert modelling represents both the localized backwater effect of the leaky barriers and the temporary water storage response offered by the leaky barriers at the test reach. The global attenuation indexes of WINFAP- FARL and combined FARL- Manning's n adjustment emulated the cumulative effect of the enhanced ground infiltration of 86 leaky barriers and resulting reduction in streamflow by up to 24%. Further work is ongoing to test these approaches and alternative approaches for different observed rainfall and design flow events.