Final Text
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.
TABLE 1 |
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Applicable to all impounding structures that are 25 feet or greater in height and that create a maximum impounding capacity of 15 acre-feet or greater, and to all impounding structures that are six feet or greater in height and that create a maximum impounding capacity of 50 acre-feet or greater and is not otherwise exempt from regulation by the Code of Virginia. |
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Hazard Potential Class of Dam |
Spillway Design Flood (SDF)B for New ConstructionF |
Spillway Design Flood (SDF)B for Existing Impounding StructuresF, G |
Minimum Threshold for Incremental Damage Analysis |
High |
PMFC |
0.9 PMPH |
100-YRD |
Significant |
.50 PMF |
.50 PMF |
100-YRD |
Low |
100-YRD |
100-YRD |
50-YRE |
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)