Mitigating soil erosion and muddy floods with vegetative barriers in Belgium's open agricultural landscapes

Mitigating soil erosion and muddy floods with vegetative barriers in Belgium's open agricultural landscapes

Glenn Desplentere, Amaury Frankl

Climate and Earth Lab, Department of Geography, Ghent University, Gent, Belgium

Vegetative barriers are widely implemented nature-based solutions in open agricultural landscapes to mitigate the off-site impacts of soil erosion and reduce the occurrence of muddy floods. Although their effectiveness and functioning are well documented in laboratory settings, relatively few studies have evaluated their performance under field conditions. Using hilly Flanders (Belgium) as a case study, this research assessed (i) the quality of field implementation, (ii) the effectiveness in reducing peak runoff, and (iii) potential negative geomorphological feedbacks, across 244 small agricultural catchments treated with vegetative barriers. The barriers consisted of woodchips, coconut-fibre bales, willow brushwood, and straw bales. To assess whether field-installed barriers function as intended, 40 randomly selected barriers were inspected in situ. Barrier capacity to attenuate peak runoff was evaluated using a semi-quantitative scoring grid, based on criteria including structural continuity, evidence of bypass flow, and the presence of soil piping. Peak runoff was simulated for all barriers under the assumption of fully functional performance. Long-term runoff time series were generated from historical rainfall records, and peak flows were derived using peak-over-threshold (POT) frequency analysis. Simulated peak inflows were then compared with barrier outflow capacity to assess effectiveness. Results indicate a generally poor implementation quality: only a limited fraction of barriers were fully functional, while more than half were classified as dysfunctional (i.e., unable to mitigate peak flow). Woodchip barriers performed best overall, whereas straw-bale barriers exhibited the lowest reliability. Modelled peak runoff suggests that approximately half of all barriers are unable to buffer peak flows during high-intensity rainfall events, whereas well-maintained barriers show a clear capacity to attenuate peak discharges. In addition, negative geomorphological feedbacks were observed: ephemeral gullying occurred on nearly half of cropland areas downstream of barriers. While nature-based solutions are increasingly promoted to mitigate off-site erosion impacts and reduce muddy-flood risk, these findings highlight substantial management challenges and potential unintended geomorphological effects. Field-based evidence such as presented here is crucial, as modelling approaches may otherwise overestimate barrier effectiveness. Muddy floods can be viewed as a symptom of structurally and hydrologically degraded soils; vegetative barriers can contribute to mitigation, but only when carefully implemented and maintained, and when guided by a robust geomorphological understanding of the treated catchments.