Hydro-geomorphological modelling of leaky wooden dam efficacy from reach to catchment scale with CAESAR-Lisflood 1.9j

Abstract. Leaky wooden dams (LDs) are woody structures installed in headwater streams that aim to reduce downstream flood risk through increasing in-channel roughness and decreasing river longitudinal connectivity in order to desynchronise flood peaks within catchments. Hydrological modelling of these structures omits sediment transport processes since the impact of these processes on flow routing is considered negligible in comparison to in-stream hydraulics. Such processes are also excluded on the grounds of computational expense. Here we present a study that advances our ability to model leaky wooden dams through a roughness-based representation in the landscape evolution model CAESAR-Lisflood, introducing a flexible and representative approach to simulating the impact of LDs on reach and broader catchment-scale processes. The hydrological and geomorphological sensitivity of the model is tested against grid resolution and variability in key parameters such as leaky dam gap size and roughness. The influence of these parameters is also tested in isolation from grid resolution whilst evaluating the impact of simulating sediment transport on computational expense, model domain outputs, and internal geomorphological evolution. The findings show that simulating sediment transport increased the volume of water stored in the test reach (channel length of 160 m) by up to an order of magnitude, whilst it reduced discharge by up to 31 % during a storm event (6 h, 1-in-10-year event). We demonstrate how this is due to the leaky dam acting to induce geomorphic change and thus increasing channel roughness. When considering larger grid resolutions, however, our results show that care must be due to overestimations of localised scour and deposition in the model and that behavioural approaches should be adopted when using CAESAR-Lisflood in the absence of robust empirical validation data.