Water surface Elevation between two adjacent cross sections

[Anonymous]

Hello! We are currently working on a project of a river to be modeled in HEC-RAS. After we ran the unsteady flow simulation using a 100 year return period discharge, an abrupt change in water surface elevation occurred at two adjacent stations. The water surface elevation at the upstream cross section 11.8 meters and at the downstream is 7.9 meters. The slope of the bed between the 2 sections is almost horizontal. Based from observations this is not normal for a river, what can be the problem. Thank you!

[Anonymous]

Would probably need more context/data to answer the question, but aiming to help…It sounds safe to say the simulation is unstable, and a number of things could be ‘the problem’. The bed slope (being horizontal) is important, as are spacing between XS’s (maybe tighter is needed?) and transition of area available for conveyance between adjacent XS’s (maybe the adjacent XS’s aren’t similar enough?), HTab hydraulic parameters (maybe set finer around this trouble spot), controls/obstructions, discharge inputs (if any other than an inflow hydrograph), etc.

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