Localised geomorphic response to channel-spanning leaky wooden dams

Abstract. The introduction of leaky wooden dams (or engineered log jams) into river corridors in low order steams in upper catchments has recently become a popular form of natural flood management, particularly in NW Europe. Leaky wooden dams are designed to emulate processes such as those of naturally occurring large wood in river systems, aiming to reduce downstream flood risk through the attenuation of water during higher flows, decreasing in-channel velocities and increasing channel-floodplain connectivity. Leaky wooden dams effectively act as channel roughness agents that disrupt the fluvial and hydrological regime and attenuate the peaks in high river flows thus mitigating downstream flood risk. Despite their widespread installation, there is a paucity of data and understanding concerning the longer-term fluvial geomorphological response to leaky wooden dam installation. Here we present detailed quantification of both the geomorphic and sedimentary response to the installation of two leaky wooden dams in a catchment in Dalby Forest (North Yorkshire, UK) using high-resolution terrestrial laser scanning and detailed bathymetric surveys over a 2.5-year period. This period included two major storms with a recurrence interval of 3.9 years and 3.4 years, and a further four smaller storm events (1.22–2.3 years). Results show that when leaky wooden dams are engaged by the river flow, local topographic complexity significantly increases as sediment transport pathways are perturbed. The flow field complexity additionally changes channel bed grain-size distribution with trends of fining upstream and coarsening downstream of the structure observed. The leaky wooden dam was also observed to generate scour pools downstream of the structure, and coarsen the armour layer through winnowing of fines. Monthly observations revealed that channel topography and bed sediment patterns self-organise in response to sustained low flows and are perturbed by higher flow events. The findings highlight how frequent monitoring of different leaky wooden dam designs and structures under various flow conditions is vital to understand their longer-term impacts. Moreover it is critical that such observations are extended over longer-term periods in order to fully assess the efficacy of the structures as the channels respond to installations and the evolution of the geomorphic response. Finally, additional work is also required to better consider how individual leaky wooden dams influence local geomorphology and alter sediment transport connectivity throughout the catchment.