Wednesday, March 24, 2010

Dam Breach Modeling Q & A

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

Some questions and answers related to dam breach modeling in HEC-RAS…

Question. The Sunny Day model has a consistent water surface elevation from the very start of the model – it only decreases once the breach occurs. How is HEC-RAS setting this starting WSEL?

Answer. You define the starting water surface elevation either by equalizing the flow from your outlet works with the reservoir inflow, or by setting an initial conditions water surface elevation in your flow editor and a pilot flow through the dam equal to the reservoir inflow at the beginning of the simulation.

Question. My breach models show a dramatic decrease in max Q from the cross-section immediately downstream of the dam to the end of the model. I know that HEC-RAS has an inherent storage routine that attenuates the flow throughout the model but is it reasonable to have a result that shows a beginning max Q of 12,370 cfs and an ending Q of 275 cfs (the reach is approx. 3.8 miles long with a slope of 0.02 ft/ft upstream and 0.001 ft/ft downstream)? This is an arroyo about 800-900 ft. wide, Manning’s at .055.

Answer. I would be skeptical of those results. Perhaps there is an error somewhere in the simulation, or you have a lot of flow leaving the system. Sometimes, if your model is not properly constructed, you can develop a large “wall of water” in profile view. A lot of times this is due to poorly defined HTAB parameters. This will create an artificial pool of water behind the wall, which will drastically attenuate your flood wave. Look in the profile plot and animate through your simulation. If you see an unexplainable wall of water backing up flow, that would be the cause.

Question. My models are stable but still have inherent errors (max iterations) and critical depth defaults to varying degrees. Does this have a significant effect on the model results? Changing parameters at this point to reduce inherent errors most likely will cause instability.

Answer. Max iterations are not necessarily a problem as long as the associated errors are small and it is not causing visible instabilities or obvious errors in your results. I try to get rid of all max iterations where possible. If not possible, I try to get the errors below 0.1 ft as much as I can (my own rule of thumb). RAS does not typically default to critical depth in unsteady flow (like it does in steady flow). But it sounds like you have areas that have flow close to critical depth. This can cause instability problems. If you believe flow should be close to critical depth in these locations, try turning on the Mixed Flow option and adjusting your LPI factor. If you do not believe flow should be near critical in these locations (most of the time in natural streams you should not see critical or supercritical flow), then you may be underestimating your Manning’s n values. Manning’s n values for the front end of dam breach flood waves and steep reaches are frequently underestimated. Check Jarrett’s equation if in a steep reach. Your reach slope of 2% is quite high. An n value of 0.055 is possibly too low during the low flow period preceding the dam breach flood.

Question. Does the number of vertices defining a cross section matter, in another words, does the model run better with cross sections that have fewer vertices but still accurately define the section, vs. similar sections that have many redundant vertices?


Answer. Better definition is usually advantageous. RAS does not like to have long horizontal portions of cross sections which is common for coarsely-defined cross sections. It can cause numerical problems. These days, having the maximum number of points in a cross section (500) typically does not noticeably slow down computation speed. I recommend getting as much detail as you can in your cross sections.

8 comments:

  1. Does anyone know if it is acceptable to model catch basins or manholes as a "storage area" in Hec-Ras?

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  2. Definately. If you can produce an elevation-volume curve for the manhole or catch basin, you can model it as a storage are in HEC-RAS. The tricky part is modeling how the water gets there from the river/stream. For a catch basin, I suppose the best technique would be to use a lateral structure that defines the conveyance from the river/stream to the catch basin. Manholes are trickier, since they are typically associated with storm water designs (underground conveyances, curb and gutter, etc.). HEC-RAS is not meant to be used for stormwater design. If you're talking about flooding of an urban area and including the added storage manholes provide, then yes, definately use them.

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  3. When I am running a level pool reservoir, the water level behind the dam (Upstream) is reducing. In practical this will not happen. How can I eliminate this problem

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  4. You must make sure that an equilibrium condition is properly defined. In other words, inflow to the reservoir equals outflow from the reservoir. It sounds to me that you have some kind of outlet flow, but nothing coming into the reservoir. Try a lateral inflow hydrograph to the storage area with a constant discharge = to whatever outflow you have.

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  5. Is it feasible to model an upstream dambreak into a downstream reservoir+dam with dam gates on timed opening(case is a PMF+dambreak)in HEC-RAS. Like DAMBRK (Boss3) Options 12 or 14)

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  6. Hi Ron. Yes, definately. RAS is very flexible in that way.

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  7. Hi I'm modeling a dam break, using a storage area to model the lake upstream the dam (two cross sections - Inline Structure - downstream cross sections). This is my first time doing this, so I've received support for the parameters I have to use.
    The thing is that the model runs correctly for a period of time (32hs), but when I extend the period something happens and the simulation goes unstable. I don't change but the time period and the error comes up on the cross section just upstream the dam.
    Before I noticed this, I cut the river lenght for the simulation to be faster and simplified the cross sections beyond the width needed...Still discarding options and trying to trace the error....Sorry if it is a dumb question, my name is Alex hydraulic civil engineer asking for a door...the rest is on me.
    Thanks in advance, greetins from Mza, AR!

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  8. Hi Alex- It could be any number of things. Can you email me your model? Use the Chris G. link. Send the .prj, .p, .u, and .g files.

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