Local scour at grade control structures in the Steiner-Aa-river / Canton Schwyz

Figure 1: Area of investigation: Succession of grade-control structures. (click to enlarge)

Figure 2: Comparison between calculated scour depths and measured scour depths. (click to enlarge)

The Steiner-Aa-river is characterised by grade control structures along its whole length. After a flood event in spring 2002, an increase of the scour dimensions behind five drop structures near the river mouth (Fig. 1) could be observed.

The goal of the present investigation was to compare these measured scour hole dimensions with scour depths calculated with the approaches of Kotulas (1967) and Tschopp-Bisaz (1972). Due to the relatively small potential of available sediment in the lower reach of the Steiner-Aa, both, clear-water conditions as well as live-bed conditions were applied.

In Figure 2, the comparison between the calculations and the measurements are shown concerning the maximum scour depth. As it can be seen from Figure 2, the scour dimensions behind the structures 1,3 and 4 can properly be explained by the calculations, assuming clear-water conditions. In contrast, for the scour behind the structures 2 and 5, the calculated clear-water scour depths slightly overestimate the observed scour dimensions. These differences can be ascribed to uncertainties resulting from the effective drop height difference, the grain size distribution at the bottom of the scour hole, the effective sediment transport during the flood event, and the tail water depth behind the last structure.