Breach analysis using HEC-RAS

[geosavvy]

I am currently working on putting together some standards for dam breach analysis requirements. I have been researching some requirements for other states & counties to see what types of regulations are employed elsewhere. It seems that most jurisdictions will consider a dam to be "high hazard" if it is over about 25 feet, impounds more than 50 acre-feet, or if probable loss of human life or probable loss of critical infrastructure would ocurr in the event of a breach.

So then if a dam qualifies as a "high hazard" dam, a breach analysis is warranted, as well as many other design standards.

My questions has to do with the breach analysis. How can one determine if there is risk of probable loss of human life or critical infrastructure prior to doing the breach analysis? If the proposed dam is under 25 feet and impounds less than 50 acre-feet, then you aren't required to do the analysis unless life or infrastructure is at risk during a breach. It seems like the proverbial "chicken or egg" scenaio.

Does anyone have experience working in a jurisdiction where a breach analysis was required even though the dam was under the "high hazard" height and impunded less than the "high hazard" volume? How was it determined that you had to perform the analysis?

Thank for the help.

 

[DMcGrath]

Virginia is in the final stages of revising the impounding structure regulations, and a breach analysis will be a required part of the dam's eventual hazard classification. Three analyses are required for ALL regulated dams: SDF flow (no breach), sunny day breach (piping failure) and breach at SDF flow. You are supposed to use the dam height and impounded volume as a starting point for the SDF flood. I agree on the whole chicken and egg deal, if you use a higher and higher floods, then you can eventually kill someone, thus essentially "making" it a higher hazard structure. It seems that it could spiral out of control if you let it.

The Ohio DNR has a decent document regarding "Critical Flood Guidelines" and incremental damage assessments at

http://www.dnr.state.oh.us/Portals/7/dsafety/fldguide.pdf

 

[cvg]

your assumption that over 25' or over 50 ac-ft is a high hazard structure should be checked. for most agencies, the hazard rating is not tied to the size of the dam or impoundment volume. Only to the downstream risk.

In fact, for many agencies - a structure might not be considered a (regulated) dam at all unless it met either of those conditions. See the attached chart from the California Division of Safety of Dams.

DMcGrath is correct though - most agencies require a breach analysis and inundation mapping to be part of the emergency action plan for any regulated dam. However, many will also accept an IDA (incremental damage assessment) which can be used to justify a smaller IDF and thus reduce the requirements for your height of dam and spillway capacity.

 

[newnickname]

^true there^

We recently designed a bypass system to carry the breach of a small existing dam in order to construct an adjacent arterial road downstream.