I am very new to all things Hec-Ras. I have two streams with debris dams placed in them. I am trying to measure and monitor the change in channel characteristics over time due to the inclusion of these dams. I have measured cross-sectional profiles of the stream upstream from the dam to where the slope meets the height of the dam. Now, the cross sections are not measured in elevation, but depth from the height of the dam. When trying to add the depth of the channel cross-sections in hec ras I entered them as depth from 0 (0 being the height of the crest). so I have these cross sections, but I now want to calculate the volume of water the dam can hold before it is breached? Can this be done on Hec-ras or am I going about this all wrong?
You can do this in HEC-RAS, but the cross section points should be station-elevation rather than station-depth. One approach would be to use an arbitrary elevation for the crest, say 1000, and then subtract the depths from that value (you can also just subtract them from 0 if you don't mind negative elevations).
When you run the model, you can use the "Set Changes in WS and EG..." option in the Flow Data Editor to fix the water surface elevation at the crest elevation for each of the cross sections behind the dam.
In the output table, you can use the Define Table option to select the Volume variable, which computes a cumulative volume. Or you can use the computed flow area at each cross section along with the reach lengths to compute the volume.