Remotely sensed data in bed load transport models of mountain rivers

Abstract

While hydraulic models belong to the standard toolkit of engineers, numerical models for bed load transport in mountain rivers, here defined by steep slopes and coarse bed material, still lack reliability. On one hand, this is attributed to the complex processes in mountain rivers due to highly turbulent flows, changing flow conditions and higher form and spill drag due to immobile boulders compared to gravel-bed rivers. And on the other hand, the models highly depend on underlying input terrain data such as river section data, which in an alpine environment is generally difficult to access. The ongoing advances in remote sensing techniques, in particular in the field of topo-bathymetric laser scanning offer high-resolution bathymetry data. Compared to terrestrial laser scanning, topo-bathymetric laser scanning also captures the structure below the water surface. A water-penetrating laser system, using the green region of the electromagnetic spectrum (wavelength of 532 nm), provides valuable information across the whole river section. Depending on the conditions, such as turbidity and white water, the data achieves up to 20 - 50 survey points per square meter. Generally, the laser scanners are carried by aircrafts (manned or unmanned) to deliver large-scale high-resolution bathymetric survey data. The current research investigates the advances of high-resolution and spatially continuous bathymetry laser scanner data on sediment transport models for mountain rivers. Besides the general application as terrain data for bedload transport models, the research interest is also on the derivation of form drag through different parameters such as grain size D84 and standard deviation σz from the point cloud. For this research, available data from a mountain river in South Tyrol (Italy), covering a length of about 1.5 km over the whole river width with about 40 points/m², is applied. The river section has a slope of about 2 %, an anthropogenic-influenced cascade section in the upper part with single exposed boulders and a plan-bed character in the lower section. The mean particle size of the surface layer is about d90 = 0.10 m. With this data, the current research aims to derive extensive grain size and flow resistance information from topo-bathymetric laser scanner data and compare it with the traditional reference measurements from field data. Both data sets, from the remote-sensing and the field measurements, are tested on different approaches for bed-load transport capacity, form drag and critical flow, with particular respect to flow resistance. It is expected, that the higher information on a reach scale greatly improves the estimation of the flow resistance of mountain rivers and thus, improves the estimation of sediment transport rates in alpine environments. The contribution shows the first results of this research.