The unrealistic vertical adjustment error shown in the figure below is the most common sediment error in general and HEC-RAS/BSTEM in particular. This error indicates that HEC-RAS computed very large bed changes at the indicated cross section in the final time step before the program stopped.
Often this error can be resolved by reducing the computation increment. However, sometimes more systemic model or data problems make the error persist at very small computational increments. The most common causes include:
Excessive toe scour: The most common cause of model failure is excessive toe scour (see previous section). Lower the erodibility substantially or turn off the cohesionless transport methods (go to the BSTEM Options editor under the Options menu in the Sediment Data editor) to stabilize the model.
Bed material does not match transport function: If the bed material is too fine for the transport function, or if the transport function over predicts transport in the reach, the mis-match can lead to large, rapid bed changes, destabilizeing the model. A common error involves assigning fine bank material gradations to the bed.
Equilibrium Load: If the equilibrium load boundary condition is used with bed gradations that include no-trivial silt or clay, this boundary condition can compute enormous fine grained sediment loads at the boundary. Small decreases in transport downsteam can cause large bed changes. User defined rating curves are almost always better sediment boundary condition options, even if they are speculative.
