HEC RAS Dam Break
HEC RAS Dam Break
(OP)
I am in the process of running my first nonsteady state HEC RAS (4.1.0) model for a small DAM BREACH. This is an in stream detention pond with low baseflow (roughly 2 cfs). I am trying to develop a sunny day breach at full pool. The model functions well under steady state and I have developed a number of boundary and initial conditions with the steady state model at 5 cfs. However, the output of the nonsteady state model demonstrates large fluctuations in the energy grade line while under boundary conditions, and once the DAM BREACH is initiated and boundary conditions are removed the water surface elevations are highly suspect. I have tried a variety of scenarios, but nothing thus far has resulted in reliable results. I am in the process of trouble shooting. Depending on the computation interval, the model also commonly goes unstable after the breach. Any advice would be greatly appreciated.





RE: HEC RAS Dam Break
Unsteady-state models hate sudden changes between sections and over time, so some generalizations need to be made. Here are a few suggestions to start with:
1. Channel profile is too variable, average out the longitudinal slopes.
2. Manning’s numbers are too high (0.8?) and too variable, find an average characteristic Manning’s reach wide.
3. Time step is too long (1 hour). Time step should be <1/20 of the rising limb of the hydrograph (<1 minute, <10 seconds is more typical).
4. Initial flow (5cfs) and corresponding initial depth is too low. Try higher flows (50 cfs is a good starting point). As long as initial flow is <10% of peak, the model is still valid, though some states have a maximum initial flow rule.
5. Section spacing might be too large for a few sections (<200 feet is typical). Use interpolates were necessary.
6. An upstream constant inflow hydrograph is needed at the upstream boundary equal to the initial condition.
RE: HEC RAS Dam Break
I have tried to model this by both placing a lateral structure in front of cross sections and with a storage area. Under both scenarios, the water surface tends to elevate at extremes well above the dam height. This occurs right at the dam and continues upstream. This then appears to lead to a surplus of energy and causes numerous problems. Does anyone have an explanation for this? Or more importantly a solution?
Thank you for your help,
RE: HEC RAS Dam Break
Good luck.
RE: HEC RAS Dam Break