Article 4. Water Quantity and Water Quality Technical Criteria for Grandfathered Projects and Time Limits of Applicability Projects
9VAC25-875-670. Definitions.
For the purposes of this article only, the following words and terms have the following meanings unless the context clearly indicates otherwise:
"Aquatic bench" means a 10-foot to 15-foot wide bench around the inside perimeter of a permanent pool that ranges in depth from zero to 12 inches. Vegetated with emergent plants, the bench augments pollutant removal, provides habitats, conceals trash and water level fluctuations, and enhances safety.
"Average land cover condition" means a measure of the average amount of impervious surfaces within a watershed, assumed to be 16% or a calculated watershed-specific value for the average land cover condition as approved by the Chesapeake Bay Local Assistance Board prior to September 13, 2011.
"Bioretention basin" means a water quality BMP engineered to filter the water quality volume (i) through an engineered planting bed consisting of a vegetated surface layer (vegetation, mulch, ground cover), planting soil, and sand bed and (ii) into the in-situ material.
"Bioretention filter" means a bioretention basin with the addition of a sand filter collector pipe system beneath the planting bed.
"Constructed wetlands" means areas intentionally designed and created to emulate the water quality improvement function of wetlands for the primary purpose of removing pollutants from stormwater.
"Development" means a tract of land developed or to be developed as a unit under single ownership or unified control that is to be used for any business or industrial purpose or is to contain three or more residential dwelling units.
"Grassed swale" means an earthen conveyance system that is broad and shallow with erosion resistant grasses and check dams, engineered to remove pollutants from stormwater runoff by filtration through grass and infiltration into the soil.
"Infiltration facility" means a stormwater management facility that temporarily impounds runoff and discharges it via infiltration through the surrounding soil. While an infiltration facility may also be equipped with an outlet structure to discharge impounded runoff, such discharge is normally reserved for overflow and other emergency conditions. Since an infiltration facility impounds runoff only temporarily, it is normally dry during nonrainfall periods. Infiltration basin, infiltration trench, infiltration dry well, and porous pavement shall be considered infiltration facilities.
"Layout" means a conceptual drawing sufficient to provide for the specified stormwater management facilities required at the time of approval.
"Nonpoint source pollutant runoff load" or "pollutant discharge" means the average amount of a particular pollutant measured in pounds per year, delivered in a diffuse manner by stormwater runoff.
"Planning area" means a designated portion of the parcel on which the land development project is located. Planning areas shall be established by delineation on a master plan. Once established, planning areas shall be applied consistently for all future projects.
"Sand filter" means a contained bed of sand that acts to filter the first flush of runoff. The runoff is then collected beneath the sand bed and conveyed to an adequate discharge point or infiltrated into the in-situ soils.
"Shallow marsh" means a zone within a stormwater extended detention basin that exists from the surface of the normal pool to a depth of six to 18 inches, and has a large surface area and, therefore, requires a reliable source of baseflow, groundwater supply, or a sizeable drainage area to maintain the desired water surface elevations to support emergent vegetation.
"Stormwater detention basin" or "detention basin" means a stormwater management facility that temporarily impounds runoff and discharges it through a hydraulic outlet structure to a downstream conveyance system. While a certain amount of outflow may also occur via infiltration through the surrounding soil, such amounts are negligible when compared to the outlet structure discharge rates and are, therefore, not considered in the facility's design. Since a detention facility impounds runoff only temporarily, it is normally dry during nonrainfall periods.
"Stormwater extended detention basin" or "extended detention basin" means a stormwater management facility that temporarily impounds runoff and discharges it through a hydraulic outlet structure over a specified period of time to a downstream conveyance system for the purpose of water quality enhancement or stream channel erosion control. While a certain amount of outflow may also occur via infiltration through the surrounding soil, such amounts are negligible when compared to the outlet structure discharge rates and, therefore, are not considered in the facility's design. Since an extended detention basin impounds runoff only temporarily, it is normally dry during nonrainfall periods.
"Stormwater extended detention basin-enhanced" or "extended detention basin-enhanced" means an extended detention basin modified to increase pollutant removal by providing a shallow marsh in the lower stage of the basin.
"Stormwater retention basin" or "retention basin" means a stormwater management facility that includes a permanent impoundment, or normal pool of water, for the purpose of enhancing water quality and, therefore, is normally wet even during nonrainfall periods. Storm runoff inflows may be temporarily stored above this permanent impoundment for the purpose of reducing flooding or stream channel erosion.
"Stormwater retention basin I" or "retention basin I" means a retention basin with the volume of the permanent pool equal to three times the water quality volume.
"Stormwater retention basin II" or "retention basin II" means a retention basin with the volume of the permanent pool equal to four times the water quality volume.
"Stormwater retention basin III" or "retention basin III" means a retention basin with the volume of the permanent pool equal to four times the water quality volume with the addition of an aquatic bench.
"Vegetated filter strip" means a densely vegetated section of land engineered to accept runoff as overland sheet flow from upstream development. It shall adopt any natural vegetated form, from grassy meadow to small forest. The vegetative cover facilitates pollutant removal through filtration, sediment deposition, infiltration, and absorption and is dedicated for that purpose.
"Water quality volume" means the volume equal to the first 1/2-inch of runoff multiplied by the impervious surface of the land development project.
Statutory Authority
§ 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 40, Issue 8, eff. July 1, 2024.
9VAC25-875-680. Applicability.
This part specifies the technical criteria for regulated land-disturbing activities that are not subject to the technical criteria of Article 3 (9VAC25-875-570 et seq.) of this part in accordance with 9VAC25-875-490.
Statutory Authority
§ 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 40, Issue 8, eff. July 1, 2024.
9VAC25-875-690. General.
A. Determination of flooding and channel erosion impacts to receiving streams due to land-disturbing activities shall be measured at each point of discharge from the land disturbance and such determination shall include any runoff from the balance of the watershed that also contributes to that point of discharge.
B. The specified design storms shall be defined as either a 24-hour storm using the rainfall distribution recommended by the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) when using NRCS methods or as the storm of critical duration that produces the greatest required storage volume at the site when using a design method such as the Modified Rational Method.
C. For purposes of computing runoff, all pervious lands in the site shall be assumed prior to development to be in good condition (if the lands are pastures, lawns, or parks), with good cover (if the lands are woods), or with conservation treatment (if the lands are cultivated); regardless of conditions existing at the time of computation.
D. Construction of stormwater management facilities or modifications to channels shall comply with all applicable laws, regulations, and ordinances. Evidence of approval of all necessary permits shall be presented.
E. Impounding structures that are not covered by the Impounding Structure Regulations (4VAC50-20) shall be engineered for structural integrity during the 100-year storm event.
F. Predevelopment and post-development runoff rates shall be verified by calculations that are consistent with good engineering practices.
G. Outflows from a stormwater management facility or stormwater conveyance system shall be discharged to an adequate channel.
H. Proposed residential, commercial, or industrial subdivisions shall apply these stormwater management criteria to the land disturbance as a whole. Individual lots in new subdivisions shall not be considered separate land-disturbing activities, but rather the entire subdivision shall be considered a single land development project. Hydrologic parameters shall reflect the ultimate land disturbance and shall be used in all engineering calculations.
I. All stormwater management facilities shall have an inspection and maintenance plan that identifies the owner and the responsible party for carrying out the inspection and maintenance plan.
J. Construction of stormwater management impoundment structures within a Federal Emergency Management Agency (FEMA) designated 100-year floodplain shall be avoided whenever possible. When this is unavoidable, all stormwater management facility construction shall be in compliance with all applicable regulations under the National Flood Insurance Program (44 CFR Part 59).
K. Natural channel characteristics shall be preserved to the maximum extent practicable.
L. Land-disturbing activities shall comply with the ESCL or VESMA, as applicable, and attendant regulations.
M. Flood control and stormwater management facilities that drain or treat water from multiple development projects or from a significant portion of a watershed may be allowed in Resource Protection Areas defined in the Chesapeake Bay Preservation Act provided such facilities are allowed and constructed in accordance with the VESMA and this chapter, and provided that (i) the local government has conclusively established that the location of the facility within the Resource Protection Area is the optimum location; (ii) the size of the facility is the minimum necessary to provide necessary flood control, stormwater treatment, or both; (iii) the facility must be consistent with a comprehensive stormwater management plan developed and approved in accordance with 9VAC25-875-660 or with a stormwater management plan that has been approved prior to July 1, 2012, by the department, the Chesapeake Bay Local Assistance Board prior to its abolishment on July 1, 2012, or the Board of Conservation and Recreation; (iv) all applicable permits for construction in state or federal waters must be obtained from the appropriate state and federal agencies; (v) approval must be received from the local government prior to construction; and (vi) routine maintenance is allowed to be performed on such facilities to ensure that they continue to function as designed. It is not the intent of this subsection to allow a best management practice that collects and treats runoff from only an individual lot or some portion of the lot to be located within a Resource Protection Area.
Statutory Authority
§ 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 40, Issue 8, eff. July 1, 2024.
9VAC25-875-700. Water quality.
A. Compliance with the water quality criteria may be achieved by applying the performance-based criteria or the technology-based criteria to either the site or a planning area.
B. Performance-based criteria. For land-disturbing activities, the calculated post-development nonpoint source pollutant runoff load shall be compared to the calculated predevelopment load based upon the average land cover condition or the existing site condition. A BMP shall be located, designed, and maintained to achieve the target pollutant removal efficiencies specified in Table 1 of this section to effectively reduce the pollutant load to the required level based upon the following four applicable land development situations for which the performance criteria apply:
1. Situation 1 consists of land-disturbing activities where the existing percent impervious cover is less than or equal to the average land cover condition and the proposed improvements will create a total percent impervious cover that is less than the average land cover condition.
Requirement: No reduction in the after disturbance pollutant discharge is required.
2. Situation 2 consists of land-disturbing activities where the existing percent impervious cover is less than or equal to the average land cover condition and the proposed improvements will create a total percent impervious cover that is greater than the average land cover condition.
Requirement: The pollutant discharge after disturbance shall not exceed the existing pollutant discharge based on the average land cover condition.
3. Situation 3 consists of land-disturbing activities where the existing percent impervious cover is greater than the average land cover condition.
Requirement: The pollutant discharge after disturbance shall not exceed (i) the pollutant discharge based on existing conditions less 10% or (ii) the pollutant discharge based on the average land cover condition, whichever is greater.
4. Situation 4 consists of land-disturbing activities where the existing percent impervious cover is served by an existing stormwater management BMP that addresses water quality.
Requirement: The pollutant discharge after disturbance shall not exceed the existing pollutant discharge based on the existing percent impervious cover while served by the existing BMP. The existing BMP shall be shown to have been designed and constructed in accordance with proper design standards and specifications and to be in proper functioning condition.
C. Technology-based criteria. For land-disturbing activities, the post-developed stormwater runoff from the impervious cover shall be treated by an appropriate BMP as required by the post-developed condition percent impervious cover as specified in Table 1 of this section. The selected BMP shall be located, designed, and maintained to perform at the target pollutant removal efficiency specified in Table 1 or those found in 9VAC25-875-590.
D. Design standards and specifications for the BMPs in Table 1 of this section that meet the required target pollutant removal efficiency are available in the Virginia Stormwater Management Handbook. Other approved BMPs available through the Virginia Stormwater BMP Clearinghouse may also be utilized.
Table 1 | ||
Water Quality BMP* | Target Phosphorus Removal Efficiency | Percent Impervious Cover |
Vegetated filter strip Grassed swale | 10% 15% | 16-21% |
Constructed wetlands Extended detention (2 x WQ Vol) Retention basin I (3 x WQ Vol) | 20% 35% 40% | 22-37% |
Bioretention basin Bioretention filter Extended detention basin-enhanced Retention basin II (4 x WQ Vol) Infiltration (1 x WQ Vol) | 50% 50% 50% 50% 50% | 38-66% |
Sand filter Infiltration (2 x WQ Vol) Retention basin III (4 x WQ Vol with aquatic bench) | 65% 65% 65% | |
*Innovative or alternate BMPs not included in this table may be allowed at the discretion of the local program administrator or the department. Innovative or alternate BMPs not included in this table that target appropriate nonpoint source pollution other than phosphorous may be allowed at the discretion of the local program administrator or the department | ||
E. The VESMP authority or department as the VSMP authority may allow the use of offsite nutrient credits under Article 4 (9VAC25-875-670 et seq.) of this part in accordance with 9VAC25-875-610.
Statutory Authority
§ 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 40, Issue 8, eff. July 1, 2024.
9VAC25-875-710. Stream channel erosion.
A. Properties and receiving waterways downstream of any land-disturbing activity shall be protected from erosion and damage due to changes in runoff rate of flow and hydrologic characteristics, including changes in volume, velocity, frequency, duration, and peak flow rate of stormwater runoff in accordance with the minimum design standards set out in this section.
B. The VESMP authority or department as the VSMP authority shall require compliance with subdivision 19 of 9VAC25-875-560.
C. The locality's VESMP authority may determine that some watersheds or receiving stream systems require enhanced criteria in order to address the increased frequency of bankfull flow conditions (top of bank) brought on by land-disturbing activities or where more stringent requirements are necessary to address total maximum daily load requirements or to protect exceptional waters. Therefore, in lieu of the reduction of the two-year post-developed peak rate of runoff as required in subsection B of this section, the land development project being considered shall provide 24-hour extended detention of the runoff generated by the one-year, 24-hour duration storm.
D. In addition to subsections B and C of this section, a locality's VESMP authority by local ordinance may in accordance with § 62.1-44.15:33 of the Code of Virginia, or the board by state regulation may adopt more stringent channel analysis criteria or design standards to ensure that the natural level of channel erosion, to the maximum extent practicable, will not increase due to the land-disturbing activities. These criteria may include the following:
1. Criteria and procedures for channel analysis and classification.
2. Procedures for channel data collection.
3. Criteria and procedures for the determination of the magnitude and frequency of natural sediment transport loads.
4. Criteria for the selection of proposed natural or manmade channel linings.
Statutory Authority
§ 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 40, Issue 8, eff. July 1, 2024.
9VAC25-875-720. Flooding.
A. Downstream properties and waterways shall be protected from damages from localized flooding due to changes in runoff rate of flow and hydrologic characteristics, including changes in volume, velocity, frequency, duration, and peak flow rate of stormwater runoff in accordance with the minimum design standards set out in this section.
B. The 10-year post-developed peak rate of runoff from the development site shall not exceed the 10-year predeveloped peak rate of runoff.
C. In lieu of subsection B of this section, localities may, by ordinance in accordance with § 62.1-44.15:33 of the Code of Virginia, adopt alternate design criteria based upon geographic, land use, topographic, geologic factors, or other downstream conveyance factors as appropriate.
D. Linear development projects shall not be required to control post-developed stormwater runoff for flooding, except in accordance with a watershed or regional stormwater management plan.
Statutory Authority
§ 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 40, Issue 8, eff. July 1, 2024.
9VAC25-875-730. Regional (watershed-wide) stormwater management plans.
Water quality requirements and where allowed, water quantity requirements, may be achieved in accordance with sections 9VAC25-875-610 and 9VAC25-875-660.
Statutory Authority
§ 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Derived from Virginia Register Volume 40, Issue 8, eff. July 1, 2024.