Incremental Damage Analysis

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Damage to Upper Falls Dam. (Image Source: Wikimedia)

In some jurisdictions, the Inflow Design Flood (IDF) can be established using incremental damage analysis or an incremental hazard evaluation. In this analysis, "the IDF is determined through an iterative process known as an incremental hazard evaluation. In other words, to evaluate the incremental increase in consequences due to dam failure, the evaluation begins with the normal full reservoir level with normal stream flow conditions prevailing. That condition should be routed through the dam and downstream areas, with the assumption that the dam remains in place. The same flow should then be routed through the dam with the assumption that the dam fails. The incremental increase in downstream water surface elevation between the with-failure and without-failure conditions should then be determined (in other words, how much higher would the water downstream be if the dam failed than if the dam did not fail?). The amount of damage that could result should then be identified. If the incremental rise in flood water downstream indicates an additional threat to downstream life and/or property, assess the need for remedial action." .

"If the study under normal flow conditions indicates no adverse consequences, the same analyses should be done for several larger flood flows to determine the greatest unacceptable threat to downstream life and/or property. Under each incrementally larger inflow condition, identify the consequences of failure. For each larger assumed flood inflow condition (which can be percentages of the PMF): 1)assume the dam remains in place during the non-failure conditions, and 2) assume the dam fails when the peak reservoir elevation is attained for the assumed inflow condition."

"The above procedure should be repeated until the flood inflow condition is identified such that a failure at that flow or larger flows (up to the PMF) will no longer result in an additional hazard to downstream life and/or property. The resultant flood flow is the IDF for the project. The maximum IDF is always the PMF, but in many cases the IDF will be substantially less than the PMF. It is important to investigate the full range of flood flow conditions to verify that a failure under flood flows larger than the selected IDF up through the PMF will not result in any additional hazard. In addition, once the design for remedial repairs is selected, the IDF should be verified for that design."

Best Practices Resources

Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 2- Selecting and Accommodating Inflow Design Floods for Dams, FERC

Citations:

Revision ID: 7403
Revision Date: 07/21/2023