Sand and Gravel on the Move: Human Impacts on Bed-Material Load Along the Lower Rhine River

Abstract

Bed material controls the geometry and morphology of the channel bed as well as the suitability of the river to serve as habitat for aquatic organisms. Therefore, knowledge on bed-material load and the human impact thereon is essential for river managers and scientists. In this chapter, we present an overview of human impacts on bed-material load in the lower Rhine River and discuss its implications for river management. Although human activity did not significantly change the overall rate of bed-material load, it strongly changed the character of the transport: (1) the travel times of bed material decreased due to the prohibition of meander migration by bank protection, (2) the distribution of bed material over the Rhine delta changed due to the construction of barrages and the modification of river bifurcations, (3) a continuous exchange of bed material between the banks and the bed was initiated by shipping, and (4) the grain size of the bed material transport increased due to the effects of embankment, meander cut-offs, river narrowing, barrages, and sediment mining. The main morphological problem in large parts of the lower Rhine River is the erosion of bed material from the river bed. This process is probably induced by river narrowing, barrage construction, and sediment mining; and triggered by shipping and dredging. The ongoing bed erosion hinders navigation, infrastructure, ecology, and drinking water supply. River managers input large amounts of sediment to the river to supplement the natural bed-material load, to stabilize the river bed, and to prevent further erosion of bed sediments. At other locations, continuous dredging of bed sediment is necessary to allow year-round navigability. In order to predict the morphological behavior of a river and to develop management strategies, the downstream fluxes of bed material (sand, gravel) through the river and the sources and sinks of this material must be understood. This requires bed-load and suspended-load measurements in combination with sediment budget analyses. The current trend among river managers to reduce the number of transport measurements in favor of relying upon echo soundings is of concern.