Impact of River Planform and Groyne Lowering on Groyne‐Field Bathymetry

Abstract

Groyne fields, near‐bank pockets between successive groynes in engineered rivers, act as zones of sediment storage. While groyne‐field bathymetry is known to vary considerably in space and time, the controls on this variability and the consequences of groyne modification thereof remain poorly quantified. Our objective is to quantify the spatial and temporal variability of groyne‐field bathymetry and to assess the impact of groyne lowering on bed‐level change. We combine three data sets from the Waal (Rhine, Netherlands): multibeam bathymetry from approximately 500 groyne fields for spatial analysis; three‐monthly multi‐sensor surveys of three reference fields over three years for temporal dynamics; and multi‐annual repeat surveys across 40 groyne fields to assess the effects of groyne lowering. Wavelet analysis reveals that river curvature controls bathymetry at spatial scales above 2 km: curvature‐driven helical flow causes groyne fields in outer bends to have lower bed levels and steeper transition slopes than those in inner bends. As curvature decreases, local factors such as groyne geometry become comparably important. The greatest bed‐level changes are decimeter scale and occur near groynes, where flow separation drives alternating scour and deposition. Interior groyne‐field changes are centimeter‐scale and often within measurement uncertainty. Contrary to earlier research, peak flows do not consistently induce aggradation. On multi‐year timescales, fields with lowered groynes erode significantly more than unmodified groyne fields, because prolonged groyne submergence exposes the affected groyne fields to higher flow velocities.