This script asks for an input DTM and a poligonal shapefile of a dam (e.g., valley-transverse rectangle-like shapefile) to modify the DTM and backpropagate a flat (water) surface taking as a reference the maximum level of the dam (which corresponds to the highest elevation of the DTM pixels within the input dam shapefile)
Requiers TauDem, available at: https://hydrology.usu.edu/taudem/taudem5/ or https://github.com/dtarb/TauDEM/releases