Tuesday, March 30, 2010

Dynamic versus Level Pool Reservoir Drawdown for Dam Breach Modeling

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

This is a summary from a paper (Goodell,Christopher;Wahlin, Brian. “Dynamic and Level Pool Reservoir Drawdown: A Practical Comparison for Dam Breach Modeling.” 33rd IAHR Congress Proceedings, Vancouver Canada, 2009) on level pool versus dynamic reservoir drawdown for dam breach modeling. In RAS you can define your reservoir with a series of cross sections (which uses dynamic routing) or a storage area (which uses level pool routing). Dynamic routing is generally assumed to be more accurate, but the size and shape of a reservoir can sometimes make level pool reservoir adequate.

A key component to dam breach modeling is the reservoir drawdown. This has a significant impact on the magnitude and shape of the breach outflow hydrograph, and ultimately the extent of flood inundation in the downstream reach. Drawdown of the reservoir can be modeled with the precise and physically correct dynamic routing method, which uses the full St. Venant equations of Conservation of Mass and Conservation of Momentum. However, this requires detailed bathymetric data for the reservoir, which is frequently very difficult and expensive to obtain for existing reservoirs. Furthermore, dynamic routing is complex and prone to numeric instabilities. A level pool drawdown is a more simplistic, numerically stable approach that can be used successfully under certain circumstances and requires only a simple stage-storage curve for the reservoir.

Two primary characteristics emerge as indicators of a given reservoir’s ability to be described by a level pool analysis. The Compactness Factor, Fc, is simply the ratio of the dam height (H) to the reservoir length (L). The longer and shallower the reservoir, the lower the Compactness Factor and the more the reservoir acts like a river during its drawdown. Thus dynamic routing would be more appropriate in this situation. Short, relatively deep reservoirs are more compact, have a larger Fc value, and can be adequately described using a level pool analysis.

The Translation Factor, Ft, describes the relationship between the speed of the breach development and the ability of the reservoir to supply water to replace the water leaving through the breach. The easier the reservoir can deliver water to the breach, the more it can be described by a level pool analysis. Fast breach developments and long reservoirs are more appropriate to be modeled by dynamic routing. The Translation Factor is computed as:

Ft = ct/L

Where: c = shallow water wave celerity =clip_image002.

d = representative reservoir depth.

and t = time.

A third parameter can be used to help graphically display the results of the various simulations. The Drawdown Number, Dn, is defined as the product of the Translation Factor and the Compactness Factor.

clip_image002




It becomes apparent that for high Drawdown Numbers, the level pool analysis produces results very close to dynamic routing. By enveloping the data points, a 5% threshold Drawdown Number is shown to be 0.41. That means that a reservoir with a Drawdown Number of 0.41 or greater will produce peak outflow results within 5% of a dynamic routing simulation. The 10% threshold Drawdown Number of 0.24 is also indicated on the plot.

You can see the full paper in the referenced proceedings. Also, the Hbox software has an automated utility for determining the appropriateness of level pool reservoir drawdown based on thd Drawdown Number analysis.

4 comments:

  1. Question: "Is it always level pool that produces the higher estimate? Could it then be argued that level pool, even if inaccurate, is more conservative? (I can see a 100% discrepancy would be unreasonable however!)"

    Yes, the Level Pool consistently gives a higher (more conservative) result than the dynamic routing method for dam breach drawdown. And certainly, given the high stakes of a dam breach event and the uncertainties involved in that type of model, being conservative with your assumptions and approach is warranted. However, in my discussions with Dam Safety officials, too much conservativeness can be problematic. Especially when you find that your results prompt undue safety precautions that could really disrupt things. For example, closing of a major highway or interstate in response to a dam failure warning. As with anything else, there’s a fine balance between appropriate conservativeness and too much. Personally, I believe there is enough conservativeness already built in to the hydrology, the breach parameters, n-values, etc, that I don’t need to add more by going with Level Pool when dynamic is more appropriate.

    Also, you need to consider flood wave travel times. Although the level pool analysis produced consistently higher peaks than its companion dynamic routing analysis, the flood wave travel times were not as consistent. Some faster, some slower. That is problematic, because if you think you have “x” amount of time to evacuate, but really you only have “x-10”, then your emergency action plan is flawed and lives will be at more risk.

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  2. Hi Chris,

    HEC-RAS model - 'L' shape Pond is connected - both end connected to channel via RCB culverts - The direction of flow in both culverts is North -South.

    I am working ON HEC-RAS model for LOMR application. The thing changed since CLOMR approval. On north side of the street a channel is running and draining into a pond on south via two RCB culverts. These RCB’s are crossing the street and sized 3-8X3 and 2-8X3, spaced about 100’. The pond is static pond and 15’ deep permanent pool. This 100’ wide pond is running along the street for about 350’ then makes 90° turn, runs another 170’, then widens. The remainder length is 730’ and width is 230’. At the end of this pond another culvert 5-8’X4’ drains into main creek. Basically the channel is connected via pond and culverts on both ends. How should this modeled in the HEC-RAS? Because the two culverts draining into pond and the flow is diverted to 90 degree turn inside the pond, taking cross sections is a challenge. Maybe this should be modeled as storage area? Please let me know, if you have any further questions.

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  3. Chris,
    Is it typical to have negative flows at the inline structure that is being breached? All other parameters being generally equal, I am experiencing this only with dynamic routing and not with level pool routing. The negative flows are only for a few minutes during the breach. There could be a negative wave, but I'm not clear on why I am only seeing it in the dynamic model.

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    Replies
    1. Not likely. You probably have some instabilities which are manifesting as negative flow values.

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