Modelling the efficacy of woody debris dams in slowing and reducing peak discharge

As part of Natural Flood Management, networks of engineered woody debris dams are deployed in upper catchments. There is currently a great deal of interest shown in hydro-environmental modelling (hydro-environmental, referring here to the dynamic parameters, i.e. water levels and velocities, and environmental in the context of the catchment characteristics) to overcome upscaling from plot to catchment. However, there is no standard hydraulic unit to simulate woody debris dams in the modelling domain. This study develops and validates a hydraulic modelling unit that accounts for the physical properties of the woody debris dams and tests this unit with real-world empirical data. Pier-loss bridge units were used to simulate a network of woody debris dams. Woody debris dam blockage area and gap sizes were investigated, and seasonal changes and designs of woody debris dams were simulated by altering the pier-loss bridge legs and soffits. The modelling software package, Jacobs Flood Modeller v6.1 (FM) enabled field data to be imported as boundary conditions so the model could represent the real-world. Two storm events were simulated with data obtained from on-site automated monitoring equipment. Results show pier-loss bridge units within FM, 1D simulations, effectively represented varied woody debris dam designs in attenuating peak discharge.