In a Grassed Waterway the grass is always greener – and surface runoff a challenge to measure

Grassed waterways (GWW) are a nature-based solution in agricultural catchments to reduce surface runoff and soil erosion. However, continuous measurements of surface runoff in a GWW remain challenging, limiting knowledge of how to construct a measurement station to obtain reliable data. Furthermore, these limitations restrict our understanding of hydrological processes and the effectiveness of GWWs. In this study, we present a monitoring station designed to measure surface runoff and quantify soil erosion from a 6 ha agricultural sub-catchment, and discuss the opportunities and limitations of monitoring runoff, sediments, and nutrients in a managed GWW. This study is part of the overall 66 ha catchment at the HOAL (Hydrological Open-Air Laboratory), Austria.  

We developed an H-Flume-like structure that reliably quantifies flow without disturbing the GWW’s function. Non-contact radar probes measure the height and velocity of runoff in the structure, allowing discharge calculations during runoff events. When a specified runoff height is detected, an automatic water sampler collects water for further analysis, such as sediment quantification. Thermal and optical cameras are mounted on the structure to capture images from upslope, the structure itself, and downslope, providing several perspectives for visual documentation of runoff processes and sediment transport.

While complementary measurements and modelling support the understanding of the overall effectiveness of the GWW in the HOAL catchment, this station provides valuable information on the timing of runoff, peak flow reduction, and catchment connectivity. The integrated sensor network at this station and throughout the HOAL - including rain gauges, soil moisture sensors, and additional runoff stations - enables a process-based understanding of how grassed waterways affect surface runoff, pluvial floods, and sediment and nutrient transport towards the stream.

This methodology remains under active development, and we encourage community input on improvements to the current methodologies and suggestions for additional observations. This presentation aims to share our current design, present preliminary results, and foster collaborative discussion on advancing monitoring of vegetated, nature-based erosion control structures.