Chapter 148 SUBDIVISION AND LAND DEVELOPMENT. (updated 7-28-08)
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.