The LET limiting is used to limit the flux for small water depth. The limiting starts if the `waterDepth` is lower than `upperH`. The second border is `lowerH`. For all `waterDepth` lower than `lowerH` the flux is set to zero. Currently the borders are hard coded. This should be changed. Instead they should be changeable via the `params.input` file. Also the names of the borders should be changed to be better understandable.