Assessing multi-scale effects of natural water retention measures on in-stream fine bed material deposits with a modeling cascade

Natural water retention measures (NWRMs) belong to the broad range of nature-based solutions and are receiving increased attention for targeting environmental problems of river catchments as they effectively support the restoration of natural processes and cycles. Challenges involved in NWRMs planning include the development of tools able to project and diagnose the effects at different spatial scales (e.g., catchment or the reach). This paper focuses on NWRMs addressing fine bed material deposits (FBMDs), with the aim of a) assessing factors and scales that control FBMDs; b) simulating the cascading impact of NWRMs on catchment hydrology, sediment loads, reach hydraulics, and FBMD; and c) assessing the effectiveness of NWRMs in targeting FBMDs at different spatial scales (catchment and reach). A modeling cascade was used to propagate the effects. This included a hydrological model (SWAT), a hydraulic model (HEC-RAS), and a spatially distributed Random Forest ensemble to model FBMDs risk for the Aist catchment (650 km(2)) Austria. Our results show that upstream sediment loads are an important predictor controlling the likelihood of sites to be classified as low risk. The effectiveness of NWRMs in reducing FBMDs is variable and depends on the target risk class. Vegetated filter strips that can trap sediments before they enter into the stream are more effective in increasing the extent of the river network towards natural conditions (5% increase compared to baseline). Sediment retention ponds are effective in reducing the extent of reaches where instream habitats are completely clogged by FBMDs (8%). Hydromorphological improvements are effective in targeting sites where FBMDs are transported even during mean flow (10%). A combination of NWRMs could benefit both high-risk as well as low-risk sites. The effectiveness of NWRMs can greatly differ when both reach and catchment diagnostics are analyzed. Thus, multi-scale assessment of the effectiveness can support NWRMs planning by unraveling all the potential benefits and highlighting tradeoffs hidden by analyses limited to a single spatial scale.

Automated summary

This paper investigates the impact of NWRMs on fine bed material deposits and their effectiveness at different spatial scales. The results show that different NWRMs have varying effects on reducing FBMDs, with a combination of measures benefiting both high-risk and low-risk sites.