Introducing indices to assess the effects of in-stream large wood on water and sediment connectivity in small streams

Abstract

Large wood (LW) can have significant effects on channel hydraulics by creating hydraulic resistance that decreases reach-average flow velocity and transport capacity, further influencing water and sediment connectivity. Although some case studies have already explicitly dealt with LW considering its effects on connectivity, a conceptualization as well as standardized methods or protocols to assess their effects on water and sediment connectivity in river systems are still missing. The relationship between in-stream LW depositions and their hydraulic function and the related potential effects on water and sediment connectivity can be quantified through drag force. However, such drag analyses are very data-demanding, time consuming and therefore often not practicable. Especially in river management, often simple and fast screening approaches are needed to get a system-wide overview on location, type and the potential effects of LW depositions. In this paper, we introduce a simple LW in-stream dis-connectivity index as well as a LW sediment retention potential index for streams narrower than log length calculated based on visually estimated field-derived parameters. Both indices have been applied in two mixed-load streams in an Austrian national park. The newly developed indices have shown to provide a simple means to quantify and compare the effects of LW on water and sediment connectivity and sediment retention potential between different river systems. Moreover, it has been demonstrated that information derived from the indices as well as on type and location is valuable for place-based river management, e.g. related to river sediment, habitat and flood control. Although we were able to show a clear relationship between the conceptual considerations of the proposed indices and the data derived from field surveys, the presented approach further needs to be tested and evaluated by interested scientists and practitioners in different environmental settings supported and corroborated by more quantitative data.