(a)
Developments
which increase the amount of impermeable area, such as the construction
of roof structures, paved areas or compacted areas, shall be subject
to the terms of this article. The following, however, shall not be
included:
(b)
The
maximum controlled stormwater runoff release rate shall not exceed
the natural safe stormwater drainage capacity of the downstream system,
which has been determined by Storm Drainage Evaluation, dated July,
1982, by Owen Ayers & Associates, Inc., which is on file in the
office of the city secretary. Pipe outlets of less than 12 inches
in diameter shall not be allowed. Multiple outlets from a stormwater
storage area shall be avoided if they are designed to be less than
12 inches in diameter. Removable orifice plates shall be employed
when these pipe size requirements cannot be met.
(c)
When
the maximum controlled stormwater runoff release rate shall be exceeded,
any or all of the following stormwater storage methods shall be provided
and constructed:
(1)
Dry bottom stormwater storage.
a.
Dry bottom stormwater storage areas must be designed to serve a secondary
purpose for recreation, open space, or similar type of use, which
will not be adversely affected by occasional intermittent flooding.
b.
The combination of storage of major floodwater runoff from a 100-year
return frequency storm and the allowable release rate shall not result
in a storage duration in excess of 48 hours.
c.
Minimum grades for turf areas shall be one-half percent (200 units
horizontal to one vertical) and maximum side slopes shall be 30 percent
(three units horizontal to one unit vertical). Storage area side slopes
shall follow the natural land contours as closely as practicable,
and a minimum of earth excavation shall be used to create the storage
facility.
d.
Temporary seeding or other soil stabilization measures shall be established
in the stormwater storage area and major floodwater passageway immediately
following the construction or reconstruction of these areas. During
the construction of the overall development, it is recognized that
a limited amount of sediment buildup may occur in the stormwater storage
area due to erosion. In no case shall the volume of the storage area
be reduced to less than three-quarters of the required volume during
the construction phase of the development.
e.
Permanent erosion control measures such as mulching, hydroseeding,
conventional seeding, nurse crops, fertilizing or sod installation
shall be utilized to control soil movement and erosion within the
storage area and major flood water passageway. These measures shall
meet or exceed the standards established by the soil and water conservation
district. The installation of these permanent measures shall take
place only after the majority of construction and other silt and sediment
producing activities have been completed. Prior to the establishment
of the permanent erosion control measures, the required capacity of
the stormwater storage area and the excess stormwater passageway shall
be restored.
f.
The control structure shall be provided with an interceptor for trash
and debris and it shall be designed and constructed to prevent soil
erosion and not to require manual adjustments for its proper operation.
An inlet design that will produce turbulent flow conditions during
any portion of the stormwater storage cycle will not be acceptable.
g.
Adequate impact stilling basins shall be provided to ensure that
downstream soil erosion is alleviated and the regime of the downstream
drainage facility is not disturbed.
h.
Each stormwater storage area shall be provided with a method of overflow
in the event a storm in excess of the design capacity occurs. This
overflow facility shall be constructed to function without specific
attention and can become a part of the excess stormwater passageway
described in this subsection.
i.
The entire stormwater storage area shall be designed and constructed
to fully protect the public health, safety and welfare. If a condition
occurs in the stormwater storage area which is hazardous to the public
health, safety or welfare, the person responsible for the condition
will be required to provide approved corrective measures. In the event
these corrective measures are not provided, the city may eliminate
the hazard at the expense of the person responsible.
j.
Low flow conduits or channels shall be provided in stormwater storage
areas; these conduits or channels shall be so constructed that they
will not interfere with the secondary usage of the storage areas and
will reduce the frequency of time that the storage area will be covered
with water.
(2)
Wet bottom stormwater storage.
Wet bottom stormwater
storage areas shall be designed in compliance with all of the regulations
which are applicable and govern the construction of dry bottom stormwater
storage areas. The following additional regulations shall apply:
a.
The water surface area of the permanent pool shall not exceed one-tenth
of the area of the tributary watershed.
b.
Protection of the shoreline must be provided to alleviate soil erosion
due to wave action.
c.
Minimum normal water depth shall be four feet.
d.
Facilities shall be provided to lower the pond elevation by gravity
flow for cleaning purposes and shoreline maintenance.
e.
The control structure for stormwater release shall be designed to
operate at full design release rate with only a minor increase in
the water depth in order to minimize the land surface wetted by frequent
minor stormwater runoff conditions.
f.
Measures shall be included in the design to prevent pond stagnation.
This may be accomplished by fountain aeration or some other method
used to ensure aerobic pond conditions.
g.
The volume of water permanently stored shall not be considered to
be part of the required excess stormwater storage volume.
(3)
Paved stormwater storage.
Design and construction of
the pavement base must ensure that there is not pavement damage due
to flooding. Control structures in paved areas must be readily accessible
for maintenance and cleaning. Vortex control devices will be required.
(4)
Rooftop stormwater storage.
Rooftop storage of excess
water shall be designed and constructed to provide permanent control
inlets and parapet walls to contain excess stormwater. Adequate structural
roof design must be provided to ensure that roof deflection does not
occur which could cause the roofing material to fail and result in
leakage. Overflow areas must be provided to ensure that the weight
of stored stormwater will never exceed the structural capacity of
the roof.
(5)
Automobile parking stormwater storage areas.
Automobile
parking facilities used to store excess water must be constructed
having a maximum depth of stored stormwater of 1½ feet and
these areas shall be located in the most remote, least used areas
of the parking facility.
(6)
Underground stormwater storage.
Underground stormwater
storage facilities must be designed for easy access in order to remove
accumulated sediment and debris. These facilities must be provided
with a positive gravity outlet.
(d)
Stormwater detention facilities shall be designed and their construction supervised by a registered professional engineer. Furthermore, compliance with this section shall be as provided for in subsection 42-133(a). In addition, upon completion of construction, a set of as-built drawings certified by a registered professional engineer and an estimated schedule of development phases shall be required by the city.
[Code 1984, ch. 3, § 7, art. II(D)]
(a)
The
volume of required stormwater storage shall be calculated on the basis
of the maximum value achieved from the runoff of a 100-year return
frequency storm less the volume of water released through the outlet
structure. Any generally recognized and substantiated method acceptable
to the city may be used for these calculations. The release rate of
the outlet structure when only half of the storage area is filled
may be used in lieu of routing techniques in small drainage areas.
The control structure shall be designed to maintain as uniform a flow
as possible, independent of the stormwater storage volume. Where the
proposed structure, project or land development forms only a portion
of a watershed or contains portions of several watersheds, the storage
volume calculations shall be based upon the area of the entire project,
development or land use change. The maximum release rate shall not
exceed that computed in the Drainage System Evaluation Report, July,
1982, on file in the office of the city secretary.
(b)
Stormwater
storage areas which will be filled to capacity by high frequency storms
shall be designed in a manner that will protect immediate downstream
properties, and all overflow structures shall be designed to function
properly and effectively without the necessity of making manual adjustments.
A larger outlet for a stormwater storage area may be permitted by
the city for the orderly management of stormwater runoff where large
tributary areas are developed without detention.
(c)
If
the orderly management of the stormwater runoff cannot be achieved
by passing the entire tributary area runoff through the stormwater
storage area, then the stormwater storage area shall be constructed
to exclude the runoff from the tributary area originating outside
the area to be developed.
[Code 1984, ch. 3, § 7, art. II(E)]
