As discussed previously, full dynamic wave (unsteady flow) routing through the reservoir pool is the most accurate methodology and therefore should be performed for dam break analyses of Corps dams whenever practical. To model the reservoir using full dynamic wave routing with HEC-RAS, the user can either model the pool with 1D cross sections throughout the entire reservoir, as would be done for a normal river reach, or the reservoir pool could be modeled as a single 2D Flow Area. The dam is modeled with the Inline Structure option in HEC-RAS. An example plot of modeling the pool with 1D cross sections is shown in the figure below.
The inflow hydrographs (computed with HEC-HMS) can be entered as boundary conditions at the upper most cross section (flow hydrograph), and at any of the cross sections within the reservoir pool (lateral inflow hydrographs).
When modeling the pool with cross sections, the engineer should be aware that after a dam breach occurs, the upper reach will no longer be fully inundated from the reservoir pool, thus acting more like a normal river reach. If the inflowing hydrograph recedes to a very low flow at the tail of the event, there could be some potential model instabilities resulting from the combination of a low flow and irregular channel geometry. One way around this is to increase the base flow on the recession of the upstream hydrographs. Another approach is to smooth out any major irregularities in the channel invert for the cross sections upstream of the dam. Sometimes, the combination of these two suggestions may be necessary to keep a stable solution above the dam for the tail end of the hydrograph.
If the reservoir pool is modeled with a 2D flow area, then the 2D cells can go completely dry without any model stability issues when they dry out.