Hec-Ras Unstready Flow Model Dam Failure Analysis

[dbksos]

I am using Hec-Ras for a Dam Failure analysis using an unsteady flow hydrograph.

The dam has a constant 170 cfs flow through low flow outlets.

What is the best way to model this constant flow through the dam?

 

[cvg]

ignore it

 

[Drew08]

It can't be ignored, but the actual value is somewhat arbitrary and won’t significantly affect the results. An unsteady-state model just can’t go “dry” at any time during the simulation.

If you’re entering the breach hydrograph directly as an upstream boundary condition (which it sounds like you are), then enter 170 cfs as a minimum flow in the breach hydrograph boundary condition dialog box(there should be no inline structure representing the breached dam in the model). If you’re modeling the breach directly in RAS using an inline structure, then build a constant upstream inflow hydrograph boundary condition of 170 cfs througout the simulation, and enter 170 cfs as a pilot flow in the inline structure dialog box.

In either case, the first ordinate of the upstream inflow boundary hydrograph, the inline structure pilot flow, and the downstream reach initial flow all have to be equal.

 

[dbksos]

While the 170 cfs is minimal especially compared to the inflow hydrograph, I am hoping (like you mentioned) that a constant flow through the dam will help the model run and keep it from going "dry".

We are modeling the breach directly in RAS using an inline structure. I looked but I did not see where it gives an option to enter in a pilot flow in the inline structure dialog box.

Thanks for the post.

 

[Drew08]

See the attached screenshot. If using the pilot flow, no spillway should be entered. Also, the upstream inflow hydrograph should not be less than the pilot flow throughout the simulation.

 

[dbksos]

Screen shot helped a lot thanks. Although, we have spillway gates entered for the inline structure, but I also plan on trying to use the pilot flow.

Thank you very much for the help.