Infercap the Walter simlation

Engialfa

hello i have a dem and point cloud with a dam, but i only have the water surface, (survey done with drone) what will be the best workflow to try to simulate the natural terrain under the water level and try to calculate the volume  approximate stored water.  trying to use the average slope of the main waterline and slopes of the surrounding land.

Any sugestiona.
tks
Pedro Reia
Portugal.