In this entry, you have to specify whether a non-erodible surface underlying the sandy bed has to be included in the sediment transport simulation or not. If you choose to include a non-erodible surface, the calculated sediment transport rates will be adjusted in such a way that erosion of the bed below the level of the hard surface will not occur. On the other hand, deposition of sediment on top of the non-erodible surface is allowed.
If you choose to included a non-erodible surface, you have to specify its level, which must be coincident or below the bed levels in the bathymetry for your set-up. Spatial constant or spatial varying levels may be chosen.
In the first case, you have to define only one value for the hard-bottom level. If you choose to specify spatially varying non-erodible surface, a data file containing one item (its top level) must be specified. You may do this by typing the path and name of the data file (a dt2 file in the old format and a dfs2 file in the new format) or by clicking the mouse on the button to the right of the Data File entry. The grid spacing and extent of the data file must be the same as in the HD data file.