Section 148-33 STORM DRAINAGE.
A. Storm drainage facilities: adequacy, size and capacity.
1. Stormwater drainage facilities designed and installed for subdivisions and developments shall have the hydraulic characteristics necessary to accommodate the expected flow of stormwater from a given watershed or portion thereof for a specified duration and intensity of rainfall, as stated in Subsection B.
2. Stormwater drainage facilities shall be designed to:
a. Account for both on-site and off-site stormwater;
b. Honor natural drainage divides;
c. Convey stormwater to a stream, water channel, natural drainageway or existing facility;
d. Discharge stormwater into an existing facility of sufficient capacity to receive the same; and
e. Not increase stormwater flow so as to create the potential for an increased hazard to downstream landowners.
In determining the size and capacity of an adequate stormwater drainage system design, the applicant shall take into account the characteristics and location of the subdivision or development in the watershed or affected portions thereof. The design shall not adversely affect adjacent or neighboring properties.
B. Basic design criteria. The basic design criteria for stormwater management facilities employs the ten-year frequency two-hour duration storm to determine predevelopment and postdevelopment flows. Drainage structures and stormwater management facilities shall be designed and constructed in such a manner that they can be maintained at a reasonable cost. All plans and computations shall comply with the Stormwater Control Master Plan for the Town of Front Royal, Virginia dated September 22, 1989, and as may be revised in the future. All drainage structures shall conform to town and Virginia Department of Transportation standards.
C. Off-site stormwater management. The following minimum criteria shall apply to protect waterways and property downstream from erosion due to increases in the volume, velocity and peak flow rate of stormwater runoff resulting from new subdivision and development activities.
1. Concentrated stormwater runoff leaving a subdivision or development site must be discharged directly into a well-defined natural or man-made receiving channel or pipe. If there is no well-defined receiving channel or pipe, one must be constructed to convey stormwater to the nearest adequate channel. Increased volumes of unconcentrated sheet flows which will cause erosive action or sedimentation on adjacent properties shall be diverted to a stable outlet or detention facility.
2. Existing receiving channels shall be considered adequate where:
a. The total contributing drainage area to the receiving channel at the point of access is at least one hundred (100) times greater than the drainage area of the subdivision or development site in question; or
b. The peak rate of runoff from the subdivision or development site generated by a ten-year six-hour storm will not increase after the site is developed. Newly constructed receiving channels shall be capable of conveying the runoff from the subdivision or development for a ten-year six-hour storm without overtopping or eroding the channel banks.
3. Runoff rate and adequacy of channel design shall be verified by the town or its representative.
4. In the event that any existing off-site receiving channel is to be employed for subdivision or development runoff but said channel does not conform to the requirements of this chapter, the applicant shall:
a. Obtain permission from downstream property owners and improve the receiving channel so as to bring it into compliance. Such drainage improvements shall extend downstream until an adequate channel section is reached.
b. Develop a site design that will not cause an increase in the existing predevelopment peak runoff rate for ten-year six-hour storms. Such a design may be accomplished by enhancing the stormwater infiltration capacity of the site or by providing on-site stormwater detention measures. The estimation of predevelopment and postdevelopment peak runoff rates shall be verified by engineering calculations submitted to the town.
c. Provide a combination of channel improvement, stormwater detention or other measures satisfactory to the town designed to prevent overflow and to prevent channel erosion.
5. All channel improvements or modifications shall comply with applicable laws and regulations. Modifications to flowing waterways shall be performed in accordance with Best Management Practices for Hydrologic Modifications, Virginia State Water Control Board Planning Bulletin 319, 1979, or as may be subsequently amended.
D. Drainage easements and fencing.
1. Where a subdivision or development is traversed by a watercourse, drainageway, channel or stream, the town shall be granted a drainage easement, conforming to the line of such watercourse, drainageway, channel or stream, of such width as will be adequate to preserve the unimpeded flow of natural drainage or such additional width as may be necessary to provide for the widening, deepening, relocation, improvement or protection of the watercourse and drainage facilities.
2. All stormwater outlet pipes installed on residential lots shall extend to a point beyond the buildable area of the lot.
3. Dedicated drainage easements shall be granted to the town for all stormwater management facilities. Fencing of stormwater drainage facilities designed to retain or detain runoff may be required by the town.