Modeling Note - Use Average Energy Slope for Headcuts or Models with Sharp Vertical Drops
1. Consider using the Quasi-Unsteady Mixed Flow capability available in recent versions of HEC-RAS.
2. Select the right Average Energy Slope.
The main reason to switch to the Average Energy Slope is to model a headcut or some other thalweg discontinuity. The model below was designed to simulate a headcut caused by a large event through a gravel mining pit in an ephemeral river.
Because the Local Energy Slope method only uses the conveyance at each cross section to back-calculate the energy slope, the transport equation does not "see" the sharp change in the friction slope downstream. But the Average Energy Slope method computes the actual slope in the energy grade line over the discontinuity and passes the transport function a much steeper friction slope. Recent versions of HEC-RAS have also added a Downwind Energy Slope specifically for headcut modeling. Read more about these options at Sediment Transport Energy Slope.
3. Do not average cross section hydraulics.
The average energy slope does not mix well with hydraulic weighting factors in models with sharp thalweg discontinuities (e.g. headcuts or drop structures). HEC-RAS has moved away from these hydraulic weighting factors for most models because they can cause instabilities and the "leap frog" or "saw tooth" error. But because the Average Energy Slope method also uses upstream and downstream hydraulics to compute the transport friction slope, which can exacerbate weighting instabilities.
4. Consider a simplified 2D model.
The finite volume equations used in the HEC-RAS 2D hydraulic model are more stable than the 1D unsteady equations. You may find that a simple 1D model performs better in these settings. (Note: Because of the subgrid bathymetry, be sure to place a break line along the ridge of the slope change.)
