[Amended 6-25-2024 by Ord. No. 24-19]
A. Comprehensive stormwater management plan.
(1)
The Director of Public Works shall develop a comprehensive stormwater
management facilities plan to determine the necessary structures,
easements and costs to provide ultimate drainage facilities to serve
City drainage sheds at full development of those sheds. The facilities
plan shall be adopted by the City Council.
(2)
The policy and criteria for determination of pro rata share
of total cost, the financial and implementation procedures, and other
related matters shall be as set forth within the Comprehensive Stormwater
Management Facilities Plan adopted by City Council.
(3)
A subdivider or developer of land shall pay a pro rata share
of the cost of providing reasonable and necessary drainage facilities,
located outside the property limits of the land owned or controlled
by the subdivider or developer but necessitated or required, in whole
or in part, by the construction or improvement of the subdivision
or development.
B. Storm drainage requirements.
(1)
All storm drainage facilities shall be constructed in accordance
with the VDOT Requirements, except as otherwise provided below.
(2)
All pipe for the construction of storm drainage systems shall
be, at a minimum, concrete of ASTM CT6 class III standards. Other
equivalent pipe materials may be permitted upon the approval of the
Public Works Director.
(3)
The minimum size of storm drainage culverts shall be 15 inches
in diameter. Manholes shall be constructed at every stall change of
grade line or pipe size. There shall be a minimum 0.25-foot drop in
inverts at every structure.
(4)
Storm sewer street and yard inlet structures shall be provided
and the spread of water in the street, measured from the curb, shall
not exceed eight feet. Flow and spread computations shall be based
on computed increments of the ten-year frequency rainfall.
(5)
Where the contributing drainage area does not exceed 500 acres,
storm sewer culverts shall be designed using the ten-year frequency
rainfall curve. Where the contribution drainage area exceeds 500 acres,
the fifty-year rainfall frequency curve shall be used.
(6)
All open channels where the quantity of water exceeds the equivalent
capacity of a seventy-two-inch concrete pipe shall be designed using
the twenty-five-year rainfall frequency curve.
(7)
Major stormwater culverts (box culverts, or pipes exceeding
72 inches in diameter) for use on the primary street system or any
bridges shall be designed using the fifty-year rainfall frequency
curve.
(8)
Every development shall provide for stormwater management that
meets the minimum technical and design criteria set forth within the
stormwater regulations enacted by the Commonwealth of Virginia.
(9)
The post-development stormwater runoff rate of flow and characteristics
shall replicate, as nearly as practicable the existing pre-development
runoff characteristics and site hydrology of the development site.
The basic design criteria for stormwater management facilities employs
the ten-year, twenty-four-hour duration storm to determine pre- and
post-development flows. Required storage shall be computed using unit
hydrograph methods. Emergency spillways shall be designed to pass
the one-hundred-year frequency (also known as the 1% annual exceedance
probability), with a two-foot freeboard easement added to the computed
water level. Hydrographs, spillway design, embankment design and flow
computations shall be submitted with the preliminary plan.
(10)
Plans and profiles detailing the provisions for the adequate
deposition of natural water and stormwater in accordance with the
Virginia Department of Environmental Quality standards shall be submitted,
indicating the location, size, type and grade of ditches, catch basins,
and pipes and connections to existing drainage systems, and on-site
stormwater retention where deemed appropriate and necessary by the
City, with supporting contributing area and design computations as
may be required. Data, computations, and calculations shall be submitted
detailing the provision for implementation of stormwater volume management.
(11)
Within the Hazel Run, Rappahannock Canal, Kenmore Flume, and
Deep Run watersheds ("the volume control area"), for new development
or redevelopment, the first one-half inch of runoff from all new impervious
surfaces shall be removed from the runoff flow. A one-year, twenty-four-hour
extended detention may be used to satisfy up to 70% of this volume.
The remaining 30% shall be removed from runoff flow. At least 50%
of the parking lot surface shall drain to a filtration practice. Table
72-54.3B, Runoff Standards, summarizes this requirement.
(12)
For new or redeveloped parking lots outside the Hazel Run, Rappahannock
Canal, Kenmore Flume, and Deep Run watersheds, volume control of stormwater
is not required. However, at least 50% of the parking lot surface
(or in the case of redevelopment, at least 50% of the additional parking
lot area) shall drain to a filtration practice that is sized to treat
the first one-half inch of runoff. Table 72-54.3B, Runoff Standards,
summarizes these requirements.
Table 72-54.3B: Runoff Standards
|
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Development Type
|
Location
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Volume Control Requirement
|
Volume Control Method
|
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New development
|
Within the volume control area
|
1/2" from all impervious surface
|
Infiltration, evaporation, or reuse. Alternatively, up to 70%
of required volume may be satisfied via 1-year extended detention
|
Outside the volume control area
|
None
|
N/A
|
Redevelopment
|
Within the volume control area
|
1/2" from new impervious area
|
Infiltration, evaporation, or reuse. Alternatively, up to 70%
of required volume may be satisfied via 1-year extended detention
|
Outside the volume control area
|
None
|
N/A
|
NOTES:
|
1.
|
At least 50% of new parking lot surface area shall drain to
a filtration practice.
|
2.
|
The volume controls and filtration practices required by Subsection B(9) and (10) can be used to satisfy the stormwater management quantity and quality control requirements for the site.
|
(13)
If, after implementing on-site volume controls to the maximum
extent practicable, the stormwater volume reductions required by this
subsection cannot be fully achieved on-site, the remainder may be
achieved through retrofit of off-site impervious areas within the
same watershed. In the alternative, compliance may be achieved through
the payment to the City of an in-lieu fee per cubic foot of volume
reduction not attained. The City shall apply all funds so collected
to stormwater control projects.
(14)
To assure long-term responsibility for and maintenance of stormwater management control devices and other techniques specified to manage the quality and quantity of runoff, recorded stormwater management and nonpoint source pollution control facilities maintenance agreements and deeds of easement shall be recorded with subdivision plats or before land disturbance or building permits are issued for improvements on site plans. Such documents shall include a drawing or plat showing the facilities, and boundary location of the easement, a maintenance agreement obligating the owner to provide and maintain appropriate landscaping and to provide regular or periodic maintenance of the facilities and the best management practices used therein, and such other provisions as the Stormwater Administrator (see §
38-412) may deem necessary in order to meet the requirements of this section and to ensure the public health and safety.
(15)
Floodplain studies shall be prepared for drainage areas exceeding
100 acres. The one-hundred-year rainfall curve shall be used, with
an eighteen-inch freeboard added to the computed water level. Floodplain
computations shall be performed using the stand-step method or an
equivalent method to achieve a balance of energy. Cross sections,
stream profiles and support calculations shall be submitted with the
preliminary plan.
C. Erosion and sediment control. For the purpose of alleviating the
adverse effects of on- and off-site erosion, siltation and sedimentation
before and after development, adequate controls of both a temporary
and permanent nature shall be provided. No permit shall be issued
by the City authorizing any land disturbing activity, until an erosion
and sediment control plan has been submitted to the Stormwater Administrator
and approved in accordance with the requirements of the City's Virginia
Erosion and Stormwater Management Program (VESCP).
D. Flood damage prevention. All development and redevelopment within a flood hazard area shall comply with the standards set forth in §
72-34.3, Floodplain Overlay District.
E. Wetlands. All development and redevelopment within the Wetlands shall be subject to and shall comply with the standards set forth in §
72-34.4, Wetlands Overlay District.
F. Chesapeake Bay protection standards. All development and redevelopment within the CBPO District shall be subject to and shall comply with the standards set forth in §
72-34.5, Chesapeake Bay Preservation Overlay District.