HEC-RAS Output Quirks

[DMcGrath]

I'm relatively new to HEC-RAS, and I have built a model of a small system to simulate the passing of a discharge (100-Yr) hydrograph of a large stormwater basin. In the first few time increments, the model computes huge flows (about 1000x what the input hydrograph, sometimes negative). After that it settles down and gives a relatively believable output. Is it OK to just ignore the rough beginning of my output, knowing it is wrong? The entire time of the simulation is about 10 hours, and the beginning rough spot is only the first 30 minutes (when the actual flow is less than 5% of the peak). What I really need to know from the model is the peak elevation only, to be used for some flood high-water mapping.

Thanks,
Dan

 

[redneckengr]

You may have a problem with your initial conditions assuming your inflow hydrograph always has enough flow. HEC-Ras hates to run "dry". Try adding some initial flow and minimum flow to your hydrograph. I would start small, say 50cfs, and work up. Sounds like your model runs once there is enough "water" in your channel. Since you are concerned with the peak elevation adding a little at the end and beginning should not affect your peak elevation.