Title of Regulation: 4VAC50-20. Impounding Structure Regulations (amending 4VAC50-20-50).

Statutory Authority: § 10.1-605 of the Code of Virginia.

Effective Date: March 23, 2016.

Agency Contact: David C. Dowling, Policy and Planning Director, Department of Conservation and Recreation, 600 East Main Street, 24th Floor, Richmond, VA 23219, telephone (804) 786-2291, FAX (804) 786-6141, or email david.dowling@dcr.virginia.gov.

Background:

Chapters 475 and 489 of the 2014 Acts of Assembly directed the Department of Conservation and Recreation, on behalf of the Virginia Soil and Water Conservation Board, to conduct a study that would result in a set of new probable maximum precipitation or PMP values for Virginia. The legislation further states "[s]uch PMP revisions shall be adopted by the Board if it finds that the analysis is valid and reliable and will result in cost savings to owners for impounding structure spillway construction or rehabilitation efforts." The PMP values incorporated into the Impounding Structure Regulations currently are based on U.S. Army Corps of Engineers hydrometeorological studies from as early as the 1970s.

The study entitled "Probable Maximum Precipitation Study for Virginia (November 2015)" was completed on December 1, 2015. The board adopted the study, the PMP values, and authorized this regulatory action on December 9, 2015. The study and related information may be found on the department’s website at http://www.dcr.virginia.gov/dam-safety-andfloodplains/.

Summary:

This action amends 4VAC50-20-50 (Performance standards required for impounding structures) and incorporates by reference the Probable Maximum Study for Virginia (and associated PMP Evaluation Tool and Database) (November 2015).

The amendments (i) remove references to the National Weather Service, NOAA, PMP values and insert references to PMP values "derived from the Probable Maximum Study for Virginia (and associated PMP Evaluation Tool and Database) (November 2015)"; (ii) remove the reference to "flat terrain" from the description of PMP as this set of PMP values does bring orographic variables into the analysis; (iii) update the citation for the Department of Environmental Quality’s Virginia Stormwater Management Program (VSMP) Regulation; and (iv) incorporate by reference the PMP study and evaluation tool.

4VAC50-20-50. Performance standards required for impounding structures.

A. In accordance with the definitions provided by § 10.1-604 of the Code of Virginia and 4VAC50-20-30, an impounding structure shall be regulated if the impounding structure is 25 feet or greater in height and creates a maximum impounding capacity of 15 acre-feet or greater, or the impounding structure is six feet or greater in height and creates a maximum impounding capacity of 50 acre-feet or greater and is not otherwise exempt from regulation by the Code of Virginia. Impounding structures exempted from this chapter are those that are:

1. Licensed by the State Corporation Commission that are subject to a safety inspection program;

2. Owned or licensed by the United States government;

3. Operated primarily for agricultural purposes that are less than 25 feet in height or that create a maximum impoundment capacity smaller than 100 acre-feet;

4. Water or silt-retaining dams approved pursuant to § 45.1-222 or 45.1-225.1 of the Code of Virginia; or

5. Obstructions in a canal used to raise or lower water.

Impounding structures of regulated size and not exempted shall be constructed, operated and maintained such that they perform in accordance with their design and purpose throughout the life of the project. For impounding structures, the spillway(s) spillway capacity shall perform at a minimum to safely pass the appropriate spillway design flood as determined in Table 1. For the purposes of utilizing Table 1, Hazard Potential Classification shall be determined in accordance with 4VAC50-20-40.

B. The spillway design flood (SDF) represents the largest flood that need be considered in the evaluation of the performance for a given project. The impounding structure shall perform so as to safely pass the appropriate SDF. Reductions in the established SDF may be evaluated through the use of incremental damage analysis pursuant to 4VAC50-20-52. The SDF established for an impounding structure shall not be less than those standards established elsewhere by state law or regulations, including but not limited to the Virginia Stormwater Management Program (VSMP) Permit Regulations (4VAC50-60) Regulation (9VAC25-870). Due to potential for future development in the dam break inundation zone that would necessitate higher spillway design flood standards or other considerations, owners may find it advisable to consider a higher spillway design flood standard than is required.

C. PMF: Probable Maximum Flood is the flood that might be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region. The PMF is derived shall be calculated from the current probable maximum precipitation (PMP) available from the National Weather Service, NOAA derived from the Probable Maximum Precipitation Study for Virginia (and associated PMP Evaluation Tool and Database) (November 2015). In some cases, a modified PMF may be calculated utilizing local topography, meteorological conditions, hydrological conditions, or PMP values supplied by NOAA. Any deviation in the application of established developmental procedures must be explained and justified by the owner's engineer. The owner's engineer must develop PMF hydrographs for 6-, 12-, and 24-hour durations. The hydrograph that creates the largest peak outflow is to be used to determine capacity for nonfailure and failure analysis. Present and planned land-use conditions shall be considered in determining the runoff characteristics of the drainage area.

D. 100-Yr: 100-year flood represents the flood magnitude expected to be equaled or exceeded on the average of once in 100 years. It may also be expressed as an exceedence probability with a 1.0% chance of being equaled or exceeded in any given year. Present and planned land-use conditions shall be considered in determining the runoff characteristics of the drainage area.

E. 50-Yr: 50-year flood represents the flood magnitude expected to be equaled or exceeded on the average of once in 50 years. It may also be expressed as an exceedence probability with a 2.0% chance of being equaled or exceeded in any given year. Present and planned land-use conditions shall be considered in determining the runoff characteristics of the drainage area.

F. For the purposes of Table 1 "Existing impounding structure" and "New construction" are defined in 4VAC50-20-30.

G. An existing impounding structure as defined in 4VAC50-20-30, that is currently classified as high hazard, or is subsequently found to be high hazard through reclassification, shall only be required to pass the flood resulting from 0.6 PMP instead of the flood resulting from the 0.9 PMP SDF if the dam owner meets the requirements set out in 4VAC50-20-53.

H. PMP: Probable maximum precipitation means the theoretically greatest depth of precipitation for a given duration that is meteorologically possible over a given size storm area at a particular geographical location at a particular time of year with no allowance made for future long-term climatic trends. In practice, this is derived over flat terrain by storm transposition and moisture adjustment to observed storm patterns. In Virginia, the 0.9 PMP is meant to characterize the maximum recorded rainfall event within the Commonwealth.

DOCUMENTS INCORPORATED BY REFERENCE (4VAC50-20)

ACER Technical Memorandum No. 11, Downstream Hazard Classification Guidelines, December 1988, U.S. Department of the Interior, Bureau of Reclamation

Probable Maximum Precipitation Study for Virginia (and associated PMP Evaluation Tool and Database), Prepared for the Virginia Department of Conservation and Recreation by Applied Weather Associates, LLC, November 2015

Trip Generation, 8th Ed., 2008, Institute of Transportation Engineers, 1627 Eye Street, NW, Suite 600, Washington, DC 20006