§ 124-4-4. Water Quantity.

A.

Channel protection and flood protection shall be addressed in accordance with the minimum standards set out in this section.

B.

Channel protection. Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet criteria 1, 2 or 3 of this subsection, where applicable, from the point of discharge to a point to the limits of analysis in § 124-4-4.B.5 as demonstrated by use of acceptable hydrologic and hydraulic methodologies.

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 postdevelopment 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 Director; or

b.

The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance systems in § 124-4-4.B.3(a) or 3(b) 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 channel 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 § 124-4-4.B.3(a) or 3(b) 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 by one of the following:

a.

In accordance with the following methodology:

Q _Developed < (Q _Forest * RV _Forest )/RV _Developed

Where:

Q _Developed = The allowable peak flow rate of runoff from the developed site.

RV _Developed = The volume of runoff from the site in the developed condition.

Q _Forest = The peak flow rate of runoff from the site in a forested condition.

RV _Forest = The volume of runoff from the site in a forested condition.

b.

In accordance with another methodology that is determined by the Director to achieve equivalent results and is approved by the State Water Control Board.

c.

If the maximum peak flow rate for the 2-year 24-hour storm can be conveyed within the system from the point of discharge to the limit of analysis in § 124-4-4.B.6 without causing erosion, the maximum peak flow rate from the one-year 24-hour storm following the land-disturbing activity may be calculated in accordance with the following methodology:

Q _Developed ≤ I.F. * (Q _{Pre-Developed} * RV _{Pre-Developed} )/RV _Developed

Under no condition shall Q _Developed be greater than Q _{Pre-Developed}

Where:

I.F. (Improvement Factor) = 0.8 for sites > 1 acre or 0.9 for sites < 1 acre.

Q _Developed = The allowable peak flow rate of runoff from the developed site.

RV _Developed = The volume of runoff from the site in the developed condition.

Q _{Pre-Developed} = The peak flow rate of runoff from the site in a pre-developed condition.

RV _{Pre-Developed} = The volume of runoff from the site in a pre-developed condition.

4.

If § 124-4-4.B.3(a) or 3(b) is used to show compliance with the channel protection criteria, the downstream review shall be limited to providing cross-sections to show a defined channel, which may include sections of natural streams with braided channels or wetlands as determined by the Director, or man-made drainage facility for the extent of review described in § 124-4-4.B.6.

5.

Limits of analysis. Unless § 124-4-4.B.3 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.

6.

Alternative limits of analysis. If § 124-4-4.B.3 is used to show compliance with the channel protection criteria above the downstream limit of analysis may extend to a point where:

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; or

c.

To a point that is at least 150 feet downstream of a point where the receiving pipe or channel is joined by another that has a drainage area that is at least 90 percent of the size of the first drainage area at the point of confluence; or

d.

To a point that is at least 150 feet downstream of a point where the drainage area is 360 acres or greater.

C.

Flood protection. Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet criteria 1, 2, or 3 below, where applicable, from the point of discharge to a point to the limits of analysis in § 124-4-4.C.5 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 postdevelopment 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 Director.

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 releases stormwater into a stormwater conveyance system that, following the land-disturbing activity, confines the postdevelopment 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 Director.

3.

Localized flooding is defined as follows:

a.

For manmade stormwater conveyance systems, localized flooding occurs when: the capacity of the channel, pipe system, or culvert is exceeded for the 10-year 24-hour storm event or the design storm, whichever is greater; or existing dwellings or buildings constructed under an approved building permit are flooded by the 100-year storm event.

b.

For natural stormwater conveyance systems, localized flooding occurs when: the capacity of the channel, or a system of braided channels or wetlands as determined by the Director, is exceeded for the 2-year 24-hour storm event; or existing dwellings or buildings constructed under an approved building permit are flooded by the 100-year storm event.

c.

For a restored stormwater conveyance system that has been designed using natural channel design concepts, localized flooding occurs when: the capacity of the channel and overbanks are exceeded for the stated design storm; or existing dwellings or buildings constructed under an approved building permit are flooded by the 100-year storm event.

4.

As an alternative to criteria 1 or 2 above, detention of stormwater may be provided that releases the postdevelopment peak flows for the 2-year 24-hour storm event and the 10-year 24-hour storm event at rates that are determined utilizing the method in § 124-4-4.B.3(a) or 3(b). If this method is used, the downstream review analysis shall be limited to providing cross-sections to show a defined channel, which may include sections of streams with braided channels or wetlands as determined by the Director, or man-made drainage facility, and checking for flooding of existing dwellings or buildings constructed under an approved building permit from the 100-year storm event for the extent of review described in § 124-4-4.C.6.

5.

Limits of analysis. Unless § 124-4-4.C.4 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; or

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. ¹

6.

Alternative limits of analysis. If § 124-4-4.C.4 is utilized to comply with the flood protection criteria, or the detention requirements of § 124-4-4.D are met and the receiving conveyance system complies with the flood protection criteria from the point of discharge to the limits of analysis in this subdivision, the downstream limit of analysis may extend 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; or

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; or

d.

To a point that is at least 150 feet downstream of a point where the receiving pipe or channel is joined by another that has a drainage area that is at least 90 percent of the size of the first drainage area at the point of confluence; or

e.

To a point that is at least 150 feet downstream of a point where the drainage area is 360 acres or greater.

7.

If an existing dwelling or a building constructed under an approved building permit, which is located within the extent of review described in § 124-4-4.C.6, is flooded by the 100-year storm, the peak flow of the 100-year storm at the development site shall be reduced to a level below the pre-development based on the methodology in § 124-4-4.B.3(a) or 3(b).

D.

Detention. Unless waived by the Director, the postdevelopment peak flow for the 2-year 24-hour storm event shall be released at a rate that is equal to or less than the predevelopment peak flow rate from the 2-year 24-hour storm event and the postdevelopment peak flow for the 10-year 24-hour storm event shall be released at a rate that is less than or equal to the predevelopment peak flow rate from the 10-year 24-hour storm event. In the Four Mile Run watershed, the postdevelopment peak flow for the 100-year storm event shall be released at a rate that is equal to or less than the predevelopment peak flow rate from the 100-year storm unless it is contraindicated by the watershed model developed for the Four Mile Run Watershed Management Program.

E.

Increased volumes of sheet flow resulting from pervious or disconnected impervious areas, or from physical spreading of concentrated flow through level spreaders, must 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.

F.

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 Director that actual site conditions warrant such considerations.

G.

Predevelopment and postdevelopment 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 by the Virginia Stormwater BMP Clearinghouse shall be considered appropriate practices as modified and supplemented by the Public Facilities Manual. (08-14-124; 06-15-124.)