Geomorphological change and river rehabilitation : case studies on lowland fluvial ystems in the Netherlands

Abstract

Integrated spatial planning for river rehabilitation requires insight in the geomorphology of river systems. Procedures are elaborated to implement a functional-geographical approach in geomorphology, in which a view of rivers as four-dimensional systems and the use of a process-based hierarchy of spatio-temporal domains is coupled to methods of land evaluation. Geomorphological mapping and map interpretation are important research techniques. Application is exemplified in case studies on lowland streams and rivers in the Netherlands, in which reference situations, process conditions to be fulfilled, suitability of areas and layout of measures are addressed. The natural developments of bedforms in the meandering sand-bed Keersop stream are strongly influenced by seasonal variations in discharge and aquatic macrophyte cover. Differences in the short-term recovery of the Tongelreep, Keersop and Aa streams to meander rehabilitation are caused by differences in bank material composition, but were also influenced through the design of cross-sectional dimensions and bend curvature. Riverine pastures along the small meandering River Dinkel depend on natural levee overbank deposition and in the long term on meander cutoffs, implicating conservation strategies must be based on geomorphological disturbance processes. Analysis of historical migration rates allowed areas suitable for re-meandering along the small River Vecht to be indicated, on the basis of the spatial variability of bank material resistance to erosion. In the embanked River Rhine depositional zone, four types of fluvial styles occurred before channelisation; landform development was related to the channel width–depht ratio values and the flow velocity over the floodplain. Insights in the Rhine river reach continuum could be incorporated in a cyclical planning procedure, characterised by phases of plan design and plan evaluation, at two different scale levels. Finally, similarities and differences between these case studies are set in a wider perspective and recommendations for river rehabilitation are discussed.