Impact of engineered logjams in enhancing performance of full floodplain restorations

Changes in storm frequency and intensity, linked to increases in drought and flooding, are expected to continue to rise due to climate change. To promote provision of ecosystem services and mitigate climate change impacts, international interest has grown in use of nature-based solutions for climate adaptive management of surface water. Nature-based solutions for natural flood management include large wood additions, construction of engineered logjams, and implementation of full floodplain restorations, in which stream water formerly directed to an incised channel is allowed access to a restored floodplain with increased hydraulic roughness. Here, we examine the role of engineered logjams in enhancing water storage on a full floodplain restoration site (Lowther Estate, Penrith, Cumbria, UK), at which water formerly directed to an artificial diversion channel has been allowed to access its historic route across a restored floodplain. A time record of discharge entering and exiting the site is obtained from calibrated flumes. We present results evaluating the floodplain restoration performance in relation to vegetative drag and event magnitude. The potential is examined for engineered logjams in combination with existing vegetation to enhance water storage during major flood events, while allowing reduced storage during routine conditions. Reduction in hydrograph peak magnitude and increase in time delay of the peak are evaluated using a 1D network model exploring jam spacing and structural common metric, with the accumulation of wood pieces acting as a porous obstruction. The effect of jam spacing and structure is explored and related to the observed range of local wood piece characteristics, yielding recommendations for design and maintenance of full floodplain restoration interventions.