Integrating river cross section measurements with digital terrain models for improved flow modelling applications

Abstract

An important element in hydraulic modelling is the topography data of the riverbed and the floodplain area. The latter can be obtained directly from digital terrain models (DTM), but the measurement of the riverbed topography is not straightforward. Since in the aerial survey only the elevation of the water surface is recorded, information about the riverbed topography cannot be obtained from the DTM. Therefore a method for the integration of river topography data in a DTM was developed. While the DTM contains in most cases regularly spaced data points, the resolution of the cross section profiles is generally much higher in the lateral than in the longitudinal direction of the river. An algorithm was developed which combines lateral river profiles with a DTM to produce a grid that can be used for flow modelling. The cross section data are interpolated to the grid points of the DTM that are part of the channel by using a bilinear technique with bounding breaklines, which can be obtained from the river cross section profiles. Additionally the algorithm can be adapted so that only the submerged part of the riverbed is subject to interpolation. It preserves the original DTM in the exposed part of the gravel bars. The algorithm was applied to a restored reach of the river Thur in Switzerland. The corrected grid represents well the complex morphological features of the river such as gravel bars and widenings. The accuracy of the algorithm was tested by comparing the interpolated elevation of the exposed part of the gravel bar to measurements obtained from an aerial survey. The mean height difference is in the order of about 9 cm. Further the interpolated grid was used for a two dimensional flow simulation and the resulting water level was compared to the one recorded in the original DTM.