Breaching of Condit Dam

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2011. All rights reserved.
Last Friday, Condit Dam, on the White Salmon River was intentionally breached by blowing a hole in the spillway as part of a plan to remove the dam and restore the river to its natural state.  Although the actual breaching of the dam was not all that spectacular (i.e. the dam itself wasn't destroyed-only a relatively small hole was blown through the base of the spillway), the drawdown and subsequent erosion of the reservoir was very interesting.  It was all captured in "time-lapse" in a video posted at the following location:

http://video.nationalgeographic.com/video/player/news/environment-news/us-condit-dam-breach-vin.html

Please enjoy this video.  I'm sure there will be more to come later.

How to use a Storage Area to define a Reservoir.

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.
It’s a little confusing, and not really directly covered in the manuals anywhere. But I get this question a lot. “How do use a Storage Area to define my reservoir in RAS?” First of all, make sure using a Storage Area and level pool routing is an appropriate way to model your reservoir. This is even more important if you are modeling a dam breach on this reservoir. Check this post first to make sure it’s okay.

Once you’re happy with the level pool assumption, draw in (or import)your downstream reach in the geometric editor. Then add in your cross sections. Next, place your inline structure (dam) at the upstream end of the reach. You’ll need to place two dummy cross sections upstream of your inline structure (and downstream of your storage area). These can be copies of the cross section downstream of the dam, but should be as close as possible to the upstream toe of the dam and close enough to each other to minimize the associated volume (relative to the reservoir’s total volume). Next draw (or import) your storage area upstream of the inline structure and it’s two dummy cross sections. It’s not uncommon to have the two cross sections reside within the storage area’s boundaries. That’s okay and it doesn’t affect the computations either way.

To make RAS recognize the connection between the upper dummy cross section and the storage area, you have to “move” the upper end point of the reach inside the storage area. To do this, go to Edit…Move Object, in the main geometry window.



Once in “move” mode, click and drag the upstream end point inside the storage area. RAS should then automatically recognize the connection. Make sure to uncheck “Move Object” in the menu once you’re finished. It should look something like this when done:

Make sure you provide some outlet flow, or your dam will overtop at the beginning of the simulation. This can be done by coding in gates, or by providing pilot flow. If you’re providing pilot flow, you must enter in an “Internal R.S. Initial Stage” for one of the dummy cross sections to set up your starting pool elevation. This is set in the Options menu item in the Unsteady Flow Editor.

Wind-induced waves in reservoirs.

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.
Though HEC-RAS has no function for computing wave heights, its a fairly simply procedure that can be done external to RAS and incorporated into your design, or breach assumptions.

There are equations for wave heights and run up as a function of wind speed, reservoir depth, and fetch length. You simply gather some historical wind data that you can assign probabilities to. Assign a wind direction (to be conservative, you can assume the longest fetch length, unless the wind never blows that way), then compute wave heights and run up for different frequencies of wind magnitudes. Which frequency you design to (100-year, 50-year, etc) is up to you and your client.

Here’s a good place to start:

http://140.194.76.129/publications/eng-manuals/em1110-2-1420/c-15.pdf

Here are some other interesting links that might lead to additional information:

http://wave.oregonstate.edu/

http://www.wldelft.nl/cons/area/wds/im/wave-dynamic-structure.pdf