The estimation of a dam breach location, dimensions, and development time are crucial in any assessment of the potential risk from a dam. This is especially true in a risk assessment where dams will be ranked based on the potential for loss of life and property damage. The breach parameters will directly affect the estimate of the peak flow coming out of the dam, as well as any possible warning time available to downstream locations. Unfortunately, the breach location, size, and formation time, are often the most uncertain pieces of information in a dam failure analysis.
When performing a dam breach analysis, one must first estimate the characteristics of the breach. Once the breaching characteristics are estimated, then HEC-RAS can be used to compute the outflow hydrograph from the breach and perform the downstream routing.
The breach dimensions and development time must be estimated for every failure scenario that will be evaluated. This requirement includes different failure modes as well as different hydrologic events. The breach parameters associated with a PMF hydrologic event will be greatly different than the breach parameters for a sunny day failure at a normal pool elevation. Therefore, for each combination of pool elevation (hydrologic event) and failure scenario, a corresponding set of breach parameters must be developed.
A dam's potential breach characteristics can be estimated in several ways, including: comparative analysis (comparing your dam to historical failures of dams of similar size, materials, and water volume); regression equations (equations developed from historical dam failures in order to estimate peak outflow or breach size and development time); and physically based computer models (computer programs that attempt to model the physical breaching process by using sediment transport/erosion equations, soil mechanics, and principles of hydraulics). All of these methods are viable techniques for estimating breach characteristics. However, each of these methods has strengths and weaknesses and should be considered as a way of "estimating" the parameters and not utilized as absolute values.
In addition to the methods described above, site specific information, structural, and geotechnical analyses should be used to refine and support the estimates of the breach parameters for each failure scenario/hydrologic event. Historic breach information, regression equations, and physically based computer models all have limitations that must be well understood when they are applied. In any dam safety study it is important to consider a range of parameter estimates for the breach size and development time for each failure scenario/event, and then perform a sensitivity analysis of the breach parameters to identify their effect on the outflow hydrograph, downstream stages and flows, and warning time to any population at risk.
This section of the manual will cover causes and types of dam failures; estimating breach parameters; recommended approach; and an example application.
As with many aspects of dam failure modeling in risk assessment studies, the level of effort in estimating breach parameters should be consistent with the type of risk assessment. In general, the level of effort and detail will increase from dams that are classified as Low Hazard dams, to dams that are classified as High Hazard dams.