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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

4VAC50-20-45. Hazard potential classifications based on low volume roadways..

A. All impacted public and private roadways downstream or across an impounding structure shall be considered in determining hazard potential classification. To determine whether a road is impacted by a dam failure, one of the following methodologies shall be utilized:

1. Section IV, Part D of the United States Department of Interior, Bureau of Reclamation's ACER Technical Memorandum No. 11, 1988;

2. An approach to determining impacts to roadways 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 their submittal to the department; or

3. An approach to determine any roadway that would be overtopped, at any depth, by a dam failure under any flood or nonflood condition, including but not limited to probable maximum flood, spillway design flood, or flood from sunny day dam failure, as determined using analysis procedures set out in 4VAC50-20-54.

In all cases, an owner may use an incremental damage analysis conducted in accordance with 4VAC50-20-52 to further refine what roads should be considered impacted.

B. In certain cases, an impounding structure may qualify for a low hazard potential classification in spite of a potential impact to a downstream public or private roadway. If a roadway is found to be impacted in accordance with subsection A of this section, and other factors such as downstream residences, businesses, or other concerns as set forth in this chapter that would raise the hazard potential classification do not exist, such classification may be adjusted in accordance with this section dependent on vehicle traffic volume, based on AADT.

C. For the purposes of determining AADT volume, one of the following techniques may be utilized using data obtained within the last year except as otherwise set out in subdivision 1 of this subsection:

1. The AADT volumes available in the most recent published Daily Traffic Volume Estimates from the Virginia Department of Transportation (VDOT) for the road segment nearest the impounding structure may be utilized. This information is available from VDOT at;

2. Data developed by a local government may be utilized where the locality conducts its own traffic counts;

3. Where AADT volumes are not available from VDOT or a locality, an Average Daily Traffic trip rate that meets the standards set forth in the Institute for Traffic Engineers (ITE) Trip Generation information report, 8th Edition, 2008 (available for ordering online at may be utilized if practicable; or

4. In all cases, average daily traffic volumes may also be established by a traffic count that meets VDOT standards and is conducted or overseen by the owner's engineer or otherwise approved by the department's regional engineer.

D. Where it can be demonstrated that a public or private roadway has limited usage and that the hazard potential classification is being determined based solely upon impacts to roadways, the roadway may be considered to be "limited use" and the impounding structure may be considered a low hazard potential impounding structure despite the presence of the roadway. Such roadways, located either across or below an impounding structure, are those that result in an AADT volume of 400 vehicles or less.

Where a downstream analysis finds that multiple limited use roadways may be impacted by an impounding structure failure, the traffic volumes of those limited use roadways, determined in accordance with subsection B of this section, shall be combined for the purposes of determining the impounding structure's hazard potential classification unless it can be demonstrated that the traffic using each of the roadways is composed of substantially the same vehicle trips, such that the combined number of individual vehicle trips utilizing all of the roadways would result in an AADT of 400 or less.

E. Although a roadway may be considered to have a "limited use" in accordance with subsection D of this section, the Emergency Preparedness Plan for the low hazard impounding structure shall clearly outline a reliable and timely approach for notification of the proper local emergency services by the dam owner regarding the hazards of continued use of the road during an emergency condition.

Statutory Authority

§ 10.1-605 of the Code of Virginia.

Historical Notes

Derived from Virginia Register Volume 29, Issue 2, eff. November 8, 2012.

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