[HISTORY: Adopted by the City Council of the City of Columbia
as Ch. 15.52 of the 1997 Code. Amendments noted where applicable.]
The development of land in any zoning district in the City shall
require stormwater management as provided in this chapter.
This chapter shall be known and may be cited as the "Stormwater
Management Ordinance."
Any person, firm, corporation or other entity proposing to construct
buildings or develop land within any zoning district of the City shall
prepare, for approval by the City Engineer, a stormwater management
plan that describes the manner in which erosion, sediment and runoff
resulting from the development will be controlled and managed. No
building or construction permits shall be issued by the City and no
final plat and no improvement plans for a real estate development
in the City shall be approved by the City until the stormwater management
plan has been approved by the City Engineer as meeting the requirements
of this chapter, or the requirement for such stormwater management
plan has been waived by the City Engineer. Downstream property owners,
watercourses, channels or conduits shall not receive stormwater runoff
from proposed upstream developments at a higher peak flow rate than
would have resulted from the same storm event occurring over the site
of the proposed development with the land in its natural, undeveloped
conditions, nor shall stormwater runoff exceed the capacity of the
natural drainage system.
A.
Detention basins. Basins may be constructed to temporarily detain
the stormwater runoff so that the rate at which it is released is
the same rate as before development. The following features shall
be incorporated into the design of any detention basin:
(1)
Storage volume.
(a)
The volume of storage provided shall be sufficient to store
flows both during and immediately after the maximum storm event which
can be expected to occur once every 10 years and 100 years.
(b)
After the storage volume has been determined as required above,
a one-foot freeboard shall be added to the dam height.
(2)
Outlet control works.
(a)
Outlet works shall be designed to limit peak outflow rates from
detention storage areas to or below peak flow rates that would have
occurred prior to the proposed development.
(b)
Outlet works shall not include any mechanical components or
devices and shall function without requiring attendance or control
during operation.
(3)
Spillways. Emergency spillways shall be provided to permit the safe
passage of runoff generated from a one-hundred-year storm.
(4)
Maximum depth. The maximum planned depth of stormwaters stored shall
not normally exceed four feet.
(5)
Side slopes. The maximum side slopes for grassed basins shall not
exceed one foot vertical for two feet horizontal (2:1 slope) for basins
less than or equal to four feet deep; for basins greater than four
feet deep, the maximum side slope shall not exceed 3:1.
(6)
Limits of ponding. In no case shall the limits of maximum ponding
be closer than 30 feet horizontally to any building and less than
two feet vertically below the lowest sill elevation.
(7)
Interior drainage. The basin bottom should be designed to drain expeditiously.
If the bottom is to be grass, it should have a minimum slope of 1%.
(8)
Low-flow channel. Small flows through the detention basin should
be handled by paved ditches from inflow structure to outflow structure
to minimize erosion.
(9)
Multipurpose basins. If the detention basin is to have other uses,
the design of the basin bottom should include underdrains to expedite
drying of the bottom between runoff events.
(10)
Aesthetics. Designs should result in aesthetically pleasing
configurations which will enhance public acceptability.
B.
Detention ponds. Detention ponds may also be used to temporarily
detain the differential runoff from the development. In addition to
the general design features enumerated above for detention basins,
the following features should also be incorporated into the design
of any detention pond:
(1)
Normal pool depth. In order to minimize weed growth, the normal pool
depth should be four feet.
(2)
Depth for fish. If fish are to be kept in the pond, at least 1/4
of the area of the permanent pool should have a minimum depth of 10
feet.
(3)
Facilities for emptying. In order to ease cleaning of the pond or
shoreline maintenance, the pond design should include provisions for
emptying the pond.
(4)
Low-flow bypass. The design of any pond may include a low-flow bypass
channel or pipeline to divert runoff that can be accommodated by downstream
drainageways.
(5)
Bank stabilization. In order to minimize the effects of waves or
ice, some type of bank stabilization such as rip-rap or concrete should
be placed along the normal pool shoreline.
(6)
Side slopes below normal pool. The side slopes below the normal pool
elevation may exceed the maximum side slope permitted above normal
pool. The design shall, however, include provisions for a safety ledge
having a depth of water not greater than three feet immediately adjacent
to the shoreline.
C.
Rooftop storage. Detention storage may be met in total or in part
by detention on roofs. Details of such design, which shall be included
in the building permit application, shall include the depth, the volume
of storage, details of outlet devices and downdrains, elevations of
overflow scuppers, design loadings for the roof structure and emergency
overflow provisions. Direct connection of roof drains to sanitary
sewers is prohibited.
D.
Parking lot storage. Paved parking lots may be designed to provide
temporary detention storage of stormwater on all or a portion of their
surfaces. Outlets will be designed so as to slowly empty the stored
waters, and depths of storage must be limited so as to prevent damage
to parked vehicles.
E.
Other detention methods. All or a portion of the detention storage
may also be provided in underground or surface detention facilities,
to include basins, tanks or swales, etc.
Designs of detention facilities shall incorporate safety features,
particularly at outlets, on steep slopes, and at any attractive nuisances,
to include, as necessary, fencing, hand rails, lighting, steps, grills,
signs and other protective or warning devices so as to restrict access
during critical periods and to afford some measure of safety to both
authorized and unauthorized persons.
The provisions of this chapter shall be applicable in the following
areas:
A.
Any development in a residential or agricultural zoning district
having a gross bulk aggregate area of five acres or more; and
B.
Any development in a residential or agricultural zoning district
in the City of less than five acres with a 50% impervious surface,
including roads, buildings, utility rights-of-way and other improvements;
and
C.
Any development in any other zoning district in the City (including
but not limited to commercial, commercial park, business park, industrial
and historical zoning districts) having a gross bulk aggregate area
of one acre or more.
The stormwater detention facilities shall be built in conjunction
with the storm sewer installation and be fully operational after the
clearing of vegetation.
A.
Silt and debris connected with early construction shall be removed
periodically from the detention area to maintain full storage capacity.
B.
The maintenance responsibility of the detention area shall remain
with the developer and/or contractor until final inspection and applicable
construction performance guarantees are released.
C.
Before final plat approval and before approval of subdivision improvement
plans by the City, the developer shall submit his commitment for future
maintenance responsibility of the detention area. The City may withhold
final plat approval and approval of improvement plans unless adequate
arrangements are made to provide for the future maintenance of the
detention area by the developer or the property owners in the subdivision
benefitted thereby.
When applicable, the provisions of this chapter shall be reviewed
by the City Engineer to assure compliance.
