If it is not possible, necessary, or reasonable to perform full dynamic wave routing though the reservoir, or if the presumed difference between level pool routing and dynamic routing is small, then level pool routing can be performed with HEC-RAS. To model a reservoir using level pool routing in HEC-RAS, the pool area is modeled with a Storage Area (HEC-RAS option for modeling any area with level pool routing). That storage area is connected to a downstream river reach, and that river reach must have a cross section that is inside of the reservoir pool. The first cross section in the reach is tied to the storage area by the fact that they will always have the same water surface elevation during the computations. The dam is modeled as an inline structure, which requires one cross section upstream of the inline structure. However, the cross section upstream of the inline structure is tied to the inline structure boundary condition, and it cannot be the first cross section of the reach. Because of this limitation in HEC-RAS, the result is that the model must have two cross sections upstream of the inline structure: one cross section for the connection to the storage area, and the second cross section for the inline structure boundary condition. Both of the upstream cross sections should be representative of the reservoir area immediately upstream of the dam. The distance between these two cross sections should be short (10 to 20 ft), so that the storage volume between the two cross sections is small. An example diagram of modeling the reservoir with a storage area in HEC-RAS is shown in the figure below.
The engineer must enter an elevation-volume curve as part of the storage-area data describing the reservoir. The minimum elevation of the two upstream cross sections should be roughly equal to the minimum elevation specified for the storage area in order to prevent any instability once the storage area is emptied.
When a dam break is modeled, the breach discharge will be computed by using the same equations as the full dynamic wave method. The only difference is that the water supplied to the dam will come from the storage area, and the storage area elevation will drop as a level pool as water flows out of the breach. As noted above, when a rapidly forming breach occurs, the water surface upstream of the dam will often have a significant slope to it. With the level pool routing method, the water surface in the reservoir is always horizontal. This may or may not produce significant differences in the outflow hydrograph, depending on many factors as outlined in this Section.