Modelling of planimetric/historical changes of the Geul river (The Netherlands)

This report presents the analyses and conclusions of a study on the planform evolution of the Geul river in the last 80 years. The results are compared with those of studies carried out by the Vrije Universiteit Amsterdam (VUA). The Geul river is to tributary of the Meuse River (The Netherlands). It has been straightened and protected against the erosion in the past, but since 1980 some stretches are allowed to meander freely. A meandering stretch near the Belgian border has been analysed. Historical planform changes are analysed with aerial photographs between 1935 and 2003. Channel migration is observed through the movement of the centreline of the river for each photograph. Channel width evolution, the presence of trees and its effects are also studied from the photos. In order to simulate the planform changes of the Geul river, the river planform model Miandras (Crosato, 1990) has been applied. Flow parameters (discharge, Chézy factor...) are analysed and discussed. After the calibration of some of them, erodibility coefficients needed for Miandras computations are obtained. The results of the aerial photos analysis show that bank erosion and channel width increase with time, as expected, due to the bank protection removal. Bank stability analysis is more affected by the properties of riverbanks than by the flow properties. The flowing ground water from the hills to the rivers and though the valley has an importance influence of the stability. Centreline coordinates obtained from the aerial photos are affected by inaccuracies in the photos, and the river migration obtained from them does not correspond with the one observed tn field analysis. This renders aerial photos unsuitable to monitor small displacements of this type of rivers. The results obtained with Miandras reproduce the migration trend as derived from the field observations for a short scale of time. This means that a prediction for a period of few years is possible