B-12. - STORMWATER MANAGEMENT PLAN.

A Stormwater Management Plan shall be prepared to address the impact a development will have on existing drainage stormwater management facilities and to provide a basis for designing the stormwater management system within the development.

(a)

CONTENTS. A stormwater management plan shall include the following:

(1)

Measurement of flooding and channel erosion impacts to receiving streams due to land development projects at each point of discharge from the development project, including any runoff from the balance of the watershed which also contributes to that point of discharge. The specified design storms shall be defined as either a 24-hour storm using the rainfall distribution recommended by the U.S. Soil Conservation Service when using U.S. Soil Conservation Service 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. 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.

(2)

Evidence of approval of all necessary permits for the construction of stormwater management facilities or modifications to channels to demonstrate compliance with all applicable laws and regulations.

(3)

Impounding structures that are not covered by the Impounding Structure Regulations (4 VAC 50-20-10 et seq.) shall be engineered for structural integrity during the 100-year storm event.

(4)

Pre-development and post-development runoff rates shall be verified by calculations that are consistent with good engineering practices.

(5)

The location, dimensions and capacity of all outflows from a stormwater management, including all channels, velocity dissipaters along the outfall of stormwater management facilities and along the length of outfall channels as necessary to provide a nonerosive velocity of flow from the basin to a channel.

(6)

The hydrologic parameters and engineering calculations, which shall reflect the ultimate land development and shall be applied to the land development as a whole. Individual lots in new subdivisions shall not be considered separate land development projects, but rather the entire subdivision shall be considered a single land development project.

(7)

A maintenance plan for all stormwater management facilities which identifies the owner and the responsible party for carrying out the maintenance plan.

(8)

The location of any Federal Emergency Management Agency (FEMA) designated 100-year floodplain and demonstration of compliance with all applicable regulations under the National Flood Insurance Program, 44 CFR Part 59.

(9)

The location and boundaries (based upon the mean high water line) of all natural channels.

(b)

STORMWATER MANAGEMENT PLAN REQUIRED. A Stormwater Management Plan shall accompany an application for:

(1)

A Final Plat of land for which no Stormwater Management Plan has been prepared;

(2)

The master plan of any multi-phase development requiring only zoning compliance certificate or conditional use approval.

(3)

Any lands not covered by a detailed Stormwater Management Plan.

(4)

Other development requiring a grading plan, unless waived by the Director of Public Works.

(c)

MODIFICATIONS. Modifications to an approved Stormwater Management Plan may be submitted along with an application for a subdivision plat, engineering plan or site plan or the application for approval of any phase of a multi-phase development.

(d)

WAIVER. The requirement for a Stormwater Management Plan may be waived by the Director after consultation with the Director of Public Works where the system of drainage for an area has been defined by development of surrounding property or where a plan for the entire watershed has been prepared. In the case of the review of general development plans, the Director of Public Works may allow a period of grace on the submittal of the required Stormwater Management Plan.

(Ord. No. 19-O-030, § 2(Exh. B), 4-17-2019)

(Ord. No. 19-O-030, § 2(Exh. B), 4-17-2019)