Article 3. Water Quantity and Water Quality Technical Criteria
9VAC25-875-570. Applicability.
In accordance with the board's authority and except as provided in 9VAC25-875-490, this article establishes the minimum technical criteria that shall be employed to protect the quality and quantity of state waters from the potential harm of unmanaged stormwater runoff resulting from land-disturbing activities.
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.
9VAC25-875-580. Water quality design criteria requirements.
A. In order to protect the quality of state waters and to control the discharge of stormwater pollutants from regulated activities, the following minimum design criteria and statewide standards for stormwater management shall be applied to the site.
1. New development. The total phosphorus load of new development projects shall not exceed 0.41 pounds per acre per year, as calculated pursuant to 9VAC25-875-590.
2. Development on prior developed lands.
a. For land-disturbing activities disturbing greater than or equal to one acre that result in no net increase in impervious cover from the predevelopment condition, the total phosphorus load shall be reduced at least 20% below the predevelopment total phosphorus load.
b. For regulated land-disturbing activities disturbing less than one acre that result in no net increase in impervious cover from the predevelopment condition, the total phosphorus load shall be reduced at least 10% below the predevelopment total phosphorus load.
c. For land-disturbing activities that result in a net increase in impervious cover over the predevelopment condition, the design criteria for new development shall be applied to the increased impervious area. Depending on the area of disturbance, the criteria of subdivision 2 a or 2 b of this subsection shall be applied to the remainder of the site.
d. In lieu of subdivision 2 c of this subsection, the total phosphorus load of a linear development project occurring on prior developed lands shall be reduced 20% below the predevelopment total phosphorus load.
e. The total phosphorus load shall not be required to be reduced to below the applicable standard for new development unless a more stringent standard has been established by a locality.
B. Compliance with subsection A of this section shall be determined in accordance with 9VAC25-875-590.
C. Nothing in this section shall prohibit a VESMP authority from establishing more stringent water quality design criteria requirements in accordance with § 62.1-44.15:33 of the Code of Virginia.
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.
9VAC25-875-590. Water quality compliance.
A. Compliance with the water quality design criteria set out in subdivisions A 1 and A 2 of 9VAC25-875-580 shall be determined by utilizing the Virginia Runoff Reduction Method or another equivalent methodology that is approved by the department.
B. The BMPs listed in this subsection are approved for use as necessary to effectively reduce the phosphorus load and runoff volume in accordance with the Virginia Runoff Reduction Method. Other approved BMPs found through the Virginia Stormwater BMP Clearinghouse may also be utilized. Design specifications and the pollutant removal efficiencies for all approved BMPs are found through the Virginia Stormwater BMP Clearinghouse.
1. Vegetated Roof (Version 2.3, March 1, 2011);
2. Rooftop Disconnection (Version 1.9, March 1, 2011);
3. Rainwater Harvesting (Version 1.9.5, March 1, 2011);
4. Soil Amendments (Version 1.8, March 1, 2011);
5. Permeable Pavement (Version 1.8, March 1, 2011);
6. Grass Channel (Version 1.9, March 1, 2011);
7. Bioretention (Version 1.9, March 1, 2011);
8. Infiltration (Version 1.9, March 1, 2011);
9. Dry Swale (Version 1.9, March 1, 2011);
10. Wet Swale (Version 1.9, March 1, 2011);
11. Sheet Flow to Filter/Open Space (Version 1.9, March 1, 2011);
12. Extended Detention Pond (Version 1.9, March 1, 2011);
13. Filtering Practice (Version 1.8, March 1, 2011);
14. Constructed Wetland (Version 1.9, March 1, 2011); and
15. Wet Pond (Version 1.9, March 1, 2011).
C. Nonproprietary BMPs differing from those listed in subsection B of this section shall be reviewed and approved by the director in accordance with procedures established by the department.
D. Proprietary BMPs listed through the Virginia Stormwater BMP Clearinghouse are approved for use in accordance with the Virginia Runoff Reduction Method. Any proprietary BMP approved for use after July 1, 2020, must meet the requirements of § 62.1-44.15:28 A 9 of the Code of Virginia.
E. A VESMP authority may establish limitations on the use of specific BMPs in accordance with § 62.1-44.15:33 of the Code of Virginia.
F. The VESMP authority or department as the VSMP authority shall have the discretion to allow for application of the design criteria to each drainage area of the site. However, where a site drains to more than one HUC, the pollutant load reduction requirements shall be applied independently within each HUC unless reductions are achieved in accordance with a comprehensive watershed stormwater management plan in accordance with 9VAC25-875-660.
G. Offsite alternatives where allowed in accordance with 9VAC25-875-610 may be utilized to meet the design criteria of subsection A of 9VAC25-875-580.
H. Any publicly owned treatment works that is permitted under the watershed general VPDES permit pursuant to § 62.1-44.19:14 of the Code of Virginia and is constructing or expanding the treatment works, wastewater collection system, or other facility used for public wastewater utility operations may, in accordance with § 62.1-44.19:21.2 C of the Code of Virginia, permanently retire a portion of the publicly owned treatment works' wasteload allocation to meet the design criteria of subsection A of 9VAC25-875-580. Notice shall be given by such applicant to the VESMP authority and to the department.
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.
9VAC25-875-600. Water quantity.
A. Channel protection and flood protection shall be addressed in accordance with the minimum standards set out in this section, which are established pursuant to the requirements of § 62.1-44.15:28 of the Code of Virginia or as permitted in accordance with § 62.1-44.15:27.2 of the Code of Virginia. Nothing in this section shall prohibit a locality's VESMP authority from establishing a more stringent standard in accordance with § 62.1- 44.15:33 of the Code of Virginia especially where more stringent requirements are necessary to address total maximum daily load requirements or to protect exceptional state waters. Compliance with the minimum standards set out in this section shall be deemed to satisfy the requirements of subdivision 19 of 9VAC25-875-560.
B. Channel protection. Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet the criteria in subdivision 1, 2, or 3 of this subsection, where applicable, from the point of discharge to a point to the limits of analysis in subdivision 4 of this subsection.
1. Manmade stormwater conveyance systems. When stormwater from a development is discharged to a manmade stormwater conveyance system, following the land-disturbing activity, either:
a. The manmade stormwater conveyance system shall convey the post-development peak flow rate from the two-year 24-hour storm event without causing erosion of the system. Detention of stormwater or downstream improvements may be incorporated into the approved land-disturbing activity to meet this criterion, at the discretion of the VESMP authority or department as the VSMP authority; or
b. The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance systems in subdivision 3 of this subsection shall be met.
2. Restored stormwater conveyance systems. When stormwater from a development is discharged to a restored stormwater conveyance system that has been restored using natural design concepts, following the land-disturbing activity, either:
a. The development shall be consistent, in combination with other stormwater runoff, with the design parameters of the restored stormwater conveyance system that is functioning in accordance with the design objectives; or
b. The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance systems in subdivision 3 of this subsection shall be met.
3. Natural stormwater conveyance systems. When stormwater from a development is discharged to a natural stormwater conveyance system, the maximum peak flow rate from the one-year 24-hour storm following the land-disturbing activity shall be calculated either:
a. In accordance with the following methodology:
QDeveloped ≤ I.F.*(QPre-developed* RVPre-Developed)/RVDeveloped
Under no condition shall QDeveloped be greater than QPre-Developed nor shall QDeveloped be required to be less than that calculated in the equation (QForest * RVForest)/RVDeveloped; where
I.F. (Improvement Factor) equals 0.8 for sites > 1 acre or 0.9 for sites ≤ 1 acre.
QDeveloped = The allowable peak flow rate of runoff from the developed site.
RVDeveloped = The volume of runoff from the site in the developed condition.
QPre-Developed = The peak flow rate of runoff from the site in the pre-developed condition.
RVPre-Developed = The volume of runoff from the site in pre-developed condition.
QForest = The peak flow rate of runoff from the site in a forested condition.
RVForest = The volume of runoff from the site in a forested condition; or
b. In accordance with another methodology that is demonstrated by the VESMP authority to achieve equivalent results and is approved by the department.
4. Limits of analysis. Unless subdivision 3 of this subsection is utilized to show compliance with the channel protection criteria, stormwater conveyance systems shall be analyzed for compliance with channel protection criteria to a point where either:
a. Based on land area, the site's contributing drainage area is less than or equal to 1.0% of the total watershed area; or
b. Based on peak flow rate, the site's peak flow rate from the one-year 24-hour storm is less than or equal to 1.0% of the existing peak flow rate from the one-year 24-hour storm prior to the implementation of any stormwater quantity control measures.
C. Flood protection. Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet one of the following criteria as demonstrated by use of acceptable hydrologic and hydraulic methodologies:
1. Concentrated stormwater flow to stormwater conveyance systems that currently do not experience localized flooding during the 10-year 24-hour storm event: The point of discharge releases stormwater into a stormwater conveyance system that, following the land-disturbing activity, confines the post-development peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system. Detention of stormwater or downstream improvements may be incorporated into the approved land-disturbing activity to meet this criterion, at the discretion of the VESMP authority.
2. Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding during the 10-year 24-hour storm event: The point of discharge either:
a. Confines the post-development peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system to avoid the localized flooding. Detention of stormwater or downstream improvements may be incorporated into the approved land-disturbing activity to meet this criterion at the discretion of the VESMP authority or department as the VSMP authority; or
b. Releases a post-development peak flow rate for the 10-year 24-hour storm event that is less than the predevelopment peak flow rate from the 10-year 24-hour storm event. Downstream stormwater conveyance systems do not require any additional analysis to show compliance with flood protection criteria if this option is utilized.
3. Limits of analysis. Unless subdivision 2 b of this subsection is utilized to comply with the flood protection criteria, stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where:
a. The site's contributing drainage area is less than or equal to 1.0% of the total watershed area draining to a point of analysis in the downstream stormwater conveyance system;
b. Based on peak flow rate, the site's peak flow rate from the 10-year 24-hour storm event is less than or equal to 1.0% of the existing peak flow rate from the 10-year 24-hour storm event prior to the implementation of any stormwater quantity control measures; or
c. The stormwater conveyance system enters a mapped floodplain or other flood-prone area, adopted by ordinance, of any locality.
D. Increased volumes of sheet flow resulting from pervious or disconnected impervious areas or from physical spreading of concentrated flow through level spreaders shall be identified and evaluated for potential impacts on down-gradient properties or resources. Increased volumes of sheet flow that will cause or contribute to erosion, sedimentation, or flooding of down gradient properties or resources shall be diverted to a stormwater management facility or a stormwater conveyance system that conveys the runoff without causing down-gradient erosion, sedimentation, or flooding. If all runoff from the site is sheet flow and the conditions of this subsection are met, no further water quantity controls are required.
E. For purposes of computing predevelopment runoff, all pervious lands on the site shall be assumed to be in good hydrologic condition in accordance with the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) standards, regardless of conditions existing at the time of computation. Predevelopment runoff calculations utilizing other hydrologic conditions may be utilized provided that it is demonstrated to and approved by the VESMP authority that actual site conditions warrant such considerations.
F. Predevelopment and post-development runoff characteristics and site hydrology shall be verified by site inspections, topographic surveys, available soil mapping or studies, and calculations consistent with good engineering practices. Guidance provided in the Virginia Stormwater Management Handbook and the Virginia Stormwater BMP Clearinghouse shall be considered appropriate practices.
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.
9VAC25-875-610. Offsite compliance options.
A. No offsite option shall be used in contravention of local water quality-based limitations (i) determined pursuant to subsection B of § 62.1-44.19:14 of the Code of Virginia, (ii) adopted pursuant to § 62.1-44.15:33 of the Code of Virginia or other applicable authority, (iii) deemed necessary to protect public water supplies from demonstrated adverse nutrient impacts, or (iv) as otherwise may be established or approved by the department. Where such a limitation exists, offsite options may be used provided that such options do not preclude or impair compliance with the local limitation.
B. Unless prohibited by subsection A of this section, a VESMP authority or the department as the VSMP authority:
1. May allow the use of offsite options for compliance with water quality and water quantity technical criteria established pursuant to § 62.1-44.15:28 of the Code of Virginia, in whole or in part; and
2. Shall allow the use of nutrient credits for compliance with the water quality technical criteria when:
a. Less than five acres of land will be disturbed;
b. The phosphorous water quality reduction requirement is less than 10 pounds per year; or
c. It is demonstrated to the satisfaction of the VESMP authority or department as the VSMP authority that (i) alternative site designs have been considered that may accommodate onsite best management practices, (ii) onsite best management practices have been considered in alternative site designs to the maximum extent practicable, (iii) appropriate onsite best management practices will be implemented, and (iv) compliance with quality technical criteria cannot practicably be met onsite. The requirements of clauses (i) through (iv) of this subdivision shall be deemed to have been met if it is demonstrated that onsite control of at least 75% of the required phosphorous water quality reduction will be achieved.
C. The VESMP authority or department as the VSMP authority shall require that offsite options approved by the department or applicable state board achieve the necessary phosphorous water quality reductions prior to the commencement of the land-disturbing activity. In the case of a phased project, the land disturber may acquire or achieve the offsite nutrient reductions prior to the commencement of each phase of the land-disturbing activity in an amount sufficient for each such phase.
D. Nutrient credits shall not be used to address water quantity technical criteria.
E. Nutrient credits shall be generated in the same or adjacent fourth order subbasin, as defined by the hydrologic unit boundaries of the National Watershed Boundary Dataset, as the land-disturbing activity. If no credits are available within these subbasins when the VESMP or department as the VSMP authority accepts the final site design, credits available within the same tributary may be used. The following requirements apply to the use of nutrient credits:
1. Documentation of the acquisition of nutrient credits shall be provided to the VESMP authority and the department or the department as the VSMP authority in a certification from the credit provider documenting the number of phosphorus nutrient credits acquired and the associated ratio of nitrogen nutrient credits at the credit-generating entity.
2. Application fees are provided in Certification of Nonpoint Source Nutrient Credits (9VAC25-900). Fees shall be deposited into the Virginia Stormwater Management Fund established by § 62.1-44.15:29 of the Code of Virginia.
3. For that portion of a site's compliance with water quality technical criteria being obtained through nutrient credits, the land disturber shall (i) comply with a one-to-one ratio of the nutrient credits to the site's remaining post-development nonpoint nutrient runoff compliance requirement being met by credit use and (ii) use credits certified as perpetual credits pursuant to Article 4.02 (§ 62.1-44.19:12 et seq.) of Chapter 3.1 of Title 62.1 of the Code of Virginia.
4. A VESMP or the department as the VSMP authority shall allow the full or partial substitution of perpetual nutrient credits for existing onsite nutrient controls when (i) the nutrient credits will compensate for 10 or fewer pounds of the annual phosphorous requirement associated with the original land-disturbing activity or (ii) existing onsite controls are not functioning as anticipated after reasonable attempts to comply with applicable maintenance agreements or requirements and the use of nutrient credits will account for the deficiency. Upon determination by the VESMP or department that the conditions established by clause (i) or (ii) of this subdivision have been met, the party responsible for maintenance shall be released from maintenance obligations related to the onsite phosphorous controls for which the nutrient credits are substituted.
F. Exchange of a credit released by the department is subject to the provisions of § 62.1-44.15:35, 62.1-44.19:15, or 62.1-44.19:21 of the Code of Virginia. Where necessary to ensure compliance with local water quality requirements, the exchange of a credit released by the department is conditioned by 9VAC25-900-91 B and C.
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.
9VAC25-875-620. Design storms and hydrologic methods.
A. Unless otherwise specified, the prescribed design storms are the one-year, two-year, and 10-year 24-hour storms using the site-specific rainfall precipitation frequency data recommended by the U.S. National Oceanic and Atmospheric Administration (NOAA) Atlas 14. Partial duration time series shall be used for the precipitation data.
B. Unless otherwise specified, all hydrologic analyses shall be based on the existing watershed characteristics and how the ultimate development condition of the subject project will be addressed.
C. The U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) synthetic 24-hour rainfall distribution and models, including TR-55 and TR-20; hydrologic and hydraulic methods developed by the U.S. Army Corps of Engineers; or other standard hydrologic and hydraulic methods shall be used to conduct the analyses described in this part.
D. For drainage areas of 200 acres or less, the VESMP authority or department as the VSMP authority may allow for the use of the Rational Method for evaluating peak discharges.
E. For drainage areas of 200 acres or less, the VESMP authority or department as the VSMP authority may allow for the use of the Modified Rational Method for evaluating volumetric flows to stormwater conveyances.
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.
9VAC25-875-630. Stormwater harvesting.
In accordance with § 62.1-44.15:28 of the Code of Virginia, stormwater harvesting is encouraged for the purposes of landscape irrigation systems, fire protection systems, flushing water closets and urinals, and other water handling systems to the extent such systems are consistent with federal, state, and local regulations.
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.
9VAC25-875-640. Linear development projects.
Linear development projects shall control post-development stormwater runoff in accordance with a site-specific stormwater management plan or a comprehensive watershed stormwater management plan developed in accordance with this chapter.
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.
9VAC25-875-650. Stormwater management impoundment structures or facilities.
A. Stormwater management wet ponds and extended detention ponds that are not covered by the Impounding Structure Regulations (4VAC50-20) shall, at a minimum, be engineered for structural integrity for the 100-year storm event.
B. Construction of stormwater management impoundment structures or facilities may occur in karst areas only after a study of the geology and hydrology of the area has been conducted to determine the presence or absence of karst features that may be impacted by stormwater runoff and BMP placement.
C. Discharge of stormwater runoff to a karst feature shall meet the water quality criteria set out in 9VAC25-875-580 and the water quantity criteria set out in 9VAC25-875-600. Permanent stormwater management impoundment structures or facilities shall only be constructed in karst features after completion of a geotechnical investigation that identifies any necessary modifications to the BMP to ensure its structural integrity and maintain its water quality and quantity efficiencies. The person responsible for the land-disturbing activity is encouraged to screen for known existence of heritage resources in the karst features.
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.
9VAC25-875-660. Comprehensive stormwater management plans.
A VESMP authority may develop comprehensive stormwater management plans to be approved by the department that meet the water quality objectives, quantity objectives, or both of this part:
1. Such plans shall ensure that offsite reductions equal to or greater than those that would be required on each contributing site are achieved within the same HUC or within another locally designated watershed. Pertaining to water quantity objectives, the plan may provide for implementation of a combination of channel improvement, stormwater detention, or other measures that are satisfactory to the locality's VESMP authority to prevent downstream erosion and flooding.
2. If the land use assumptions upon which the plan was based change or if any other amendments are deemed necessary by the locality's VESMP authority, such authority shall provide plan amendments to the department for review and approval.
3. During the plan's implementation, the locality's VESMP authority shall document nutrient reductions credited to the BMPs specified in the plan.
4. State agencies and federal entities may develop comprehensive stormwater management plans and may participate in locality-developed comprehensive stormwater management plans where practicable and permitted by the locality's VESMP authority.
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.