§ 1-13. 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 the criteria in subdivision 1, 2, or 3 of this subsection, where applicable, from the point of discharge to the limits of analysis defined 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 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 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:
Q Developed ≤ I.F. *(Q Pre-Developed *RV Pre-Developed )/RV Developed
Under no condition shall Q Developed be greater than Q Pre-Developed nor shall Q Developed be required to be less than that calculated in the equation (Q Forest * RV Forest )/RV Developed ; where
I.F. (Improvement Factor) equals 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 the pre-developed condition.
RV Pre-Developed = The volume of runoff from the site in pre-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; or
b.
In accordance with another methodology that is demonstrated by the City to achieve equivalent results and is approved by the State Board.
4.
Limits of analysis. Stormwater conveyance systems shall be analyzed for compliance with channel protection criteria to the point of adequacy as determined in subsection C.
C.
Flood protection. Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet the following criteria as demonstrated by use of acceptable hydrologic and hydraulic methodologies. Adequacy of the downstream system must be demonstrated for all the following:
1.
The downstream system must adequately convey the design storm to the point of adequacy, using freeboard heights and headwater depths stipulated in the City of Virginia Beach Public Works Design Standards Manual and the referenced documents.
2.
The post-development design year Hydraulic Grade Line shall not increase over the predevelopment design year Hydraulic Grade Line in all receiving channels and water bodies and upstream facilities.
3.
Limits of analysis. Stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to the point of adequacy.
D.
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.
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. Onsite areas which provide depression storage must be accounted for in all calculations. Predevelopment runoff calculations utilizing other hydrologic conditions may be utilized provided that it is demonstrated to and approved by the VSMP authority that actual site conditions warrant such considerations.
F.
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 on the Virginia Stormwater BMP Clearinghouse website shall be considered appropriate practices.
G.
The entire drainage area must be considered when determining the design storm and hydrologic methods. This includes both onsite and offsite contributing drainage areas. For drainage areas less than three hundred (300) acres, a ten-year, twenty-four-hour design storm event for capacity design shall be used; for drainage areas equal to or greater than three hundred (300) acres, but less than five hundred (500) acres, a twenty-five-year, twenty-four-hour design storm event shall be used; for drainage areas equal to or greater than five hundred (500) acres, a fifty-year, twenty-four-hour design storm event shall be used.
(Ord. No. 3337, 4-22-14, eff. 7-1-14; Ord. No. 3618, 6-16-20)
(Ord. No. 3337, 4-22-14, eff. 7-1-14; Ord. No. 3618, 6-16-20)