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Virginia Administrative Code
Title 4. Conservation And Natural Resources
Agency 50. Virginia Soil And Water Conservation Board
Chapter 20. Impounding Structure Regulations

4VAC50-20-52. Incremental damage analysis.

A. The proposed potential hazard classification for an impounding structure may be lowered based on the results of an incremental damage analysis utilizing one of the following methodologies:

1. Section III of the United States Department of Interior, Bureau of Reclamation's ACER Technical Memorandum No. 11, 1988. An impact shall be deemed to occur where there are one or more lives in jeopardy as a result of a dam failure; or

2. An approach to determining hazard classification found in any document that is on the list of acceptable references set out in 4VAC50-20-320. The owner's engineer shall reference the methodology utilized in the submittal to the department.

B. The proposed spillway design flood for the impounding structure may be lowered based on the results of an incremental damage analysis. Once the owner's engineer has determined the required spillway design flood through application of Table 1, further analysis may be performed to evaluate the limiting flood condition for incremental damages. Site-specific conditions should be recognized and considered. In no situation shall the allowable reduced level be less than the level at which the incremental increase in water surface elevation downstream due to failure of an impounding structure is no longer considered to present an additional downstream threat. This engineering analysis will need to present water surface elevations at each structure that may be impacted downstream of the dam. An additional downstream threat to persons or property is presumed to exist when water depths exceed two feet or when the product of water depth (in feet) and flow velocity (in feet per second) is greater than seven.

The spillway design flood shall also not be reduced below the minimum threshold values as determined by Table 1.

C. The proposed potential hazard classification for the impounding structure and the required spillway design flood shall be subject to reclassification by the board as necessary to reflect the incremental damage assessment, changed conditions at the impounding structure, and changed conditions in the dam break inundation zone.

Statutory Authority

§ 10.1-605 of the Code of Virginia.

Historical Notes

Derived from Virginia Register Volume 24, Issue 25, eff. September 26, 2008; amended Virginia Register Volume 29, Issue 2, eff. November 8, 2012.

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