Lengthening of river dunes during low flow in a sandy bed river

River dunes are rhythmic bedforms on the riverbed. These river dunes are a source of bed roughness, making them a key feature for water level predictions especially during high flows. Also, during low flow conditions dunes can cause bottlenecks for shipping as they reduce the navigable flow depth. To obtain more insight in the evolution of river dunes during low flow, we have studied the bed elevation of the Waal River, the Netherlands, using a dataset of biweekly multibeam echo sounding measurements. The dataset covers the entire year 2018 and contains a long, 4-month period of extremely low flow, with flow conditions around the incipient motion for sediment transport.
In the multibeam data, the river dunes were identified using a wavelet analysis approach. Whereafter the dune length and height were determined. Also, mean dune celerity was determined using spatial cross-correlation for sections of 1km.
The results show that during the low flow period the dune length is larger than during median and high flow periods, while the dune height is lower with dune lengths around 120 m and dune heights of 0.4 m. These long and low dunes during low flows are mobile and are moving with a celerity of 2 m/day, while the transport capacity is just above incipient motion. Because the dunes are mobile and relatively constant in shape, these long dunes seem to be in equilibrium with these low flows. These long dunes during low flows are contradicting the current view on dune shape, where the dune length is assumed to scale with the dune height.