If the a constant value of the bottom friction over the model area was selected, you have to specify only one value for C or M. If you choose to specify spatially varying bed roughness, a data file containing one item (C or M) 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 bed resistance data file must be the same as in the HD data file.