Coexistence of two dune scales in a lowland river

Abstract. A secondary scale of bedforms, superimposed on larger, primary dunes, has been observed in fluvial systems worldwide. This notwithstanding, very little is known about the morphological behaviour and characteristics of this secondary scale. This study aims to better characterize and understand how two dune scales coexist in fluvial systems, and how both scales adapt over time and space, considering their interdependence. The study is based on analysis of a biweekly multibeam echosounding dataset that spans a period of approximately three years and 34 kilometers in the Waal River, a lowland sand-bedded river. Results reveal that the secondary dune scale is ubiquitous across space and time, and not limited to specific flow or transport conditions. Whereas primary dunes lengthen during low flows, secondary dune height, lee slope angle and length correlate with discharge. Secondary dune size and migration strongly depend on the primary dune lee slope angle and height. Secondary dunes can migrate over the lee slope of low-angled primary dunes, and their height is inversely correlated to the upstream primary dune height and lee slope angle. In the Waal River, a lateral variation in bed grain size, attributed to shipping, largely affects dune morphology. Primary dunes are lower and less often present in the southern lane, where grain sizes are smaller. Here, secondary bedforms are more developed. At peak discharge, secondary bedforms even become the dominant scale, whereas primary dunes entirely disappear but reestablish during lower flows.