Effects of Managing Drainage Ditches for Biodiversity on Drainage and Water Storage Functions

Abstract Drained lowland arable landscapes rely on extensive networks of drainage ditches to maintain agricultural productivity and mitigate flood risk. These human-made channels are not merely hydraulic features but also, in many cases, represent complex ecosystems shaped by vegetative growth, which may significantly influence flow dynamics. Despite the recognized role of vegetation in altering water velocity and flow patterns, systematic assessments of how vegetation management strategies affect both flood resilience and biodiversity provision are lacking. This becomes increasingly important as proposals are put forward for reprofiling ditches and modifying management regimes to promote and diversify vegetational growth in the interests of increasing landscape-scale biodiversity. Here, we employ computational fluid dynamics modeling to evaluate four common vegetation management approaches (e.g., vegetation clearance) observed in drained lowland arable landscapes. Two distinct modeling frameworks are compared: a vegetation roughness-based representation and a detailed stem-resolved approach. We find that the spatial distribution and intensity of vegetation management critically determines hydraulic performance, and selective management can generate heterogeneous flow conditions that can be beneficial for biodiversity, while more intensive clearance enhances hydraulic efficiency during high-flow events. The results show that vegetation configuration and management intensity exert a strong control on mean velocity, shear stress distribution, and conveyance capacity. Selective vegetation management generates heterogeneous hydraulic conditions that support biodiversity objectives, whereas intensive clearance improves hydraulic efficiency under high flow conditions. Among the tested scenarios, a selectively managed configuration, retaining vegetation while maintaining a clear central flow path, provides the most effective balance between flood conveyance performance and ecological potential.