[CC 1985 §5-60; Ord. No. 2963 §1(Art. II §2.1), 5-14-1998]
A.
This
Article shall apply as follows:
1.
Any person, entity or government within the City shall apply to the administrative officer for approval of a stormwater management plan and obtain a permit before commencing any development or development activity which would involve application of any ordinance listed Section 515.010.
2.
A stormwater
management plan shall be required of any type of development and/or
any regulated excavation, grading or filling for which no improvement
plans have been submitted at the time this Article takes effect or
which no stormwater management plan has been filed and approved by
the City.
3.
Stormwater
requirements of a development activity are directly related to permitted
land use in the zoning ordinance of the City. The permitted densities
and minimum lot area are important factors in the anticipated runoff.
Table 1 below lists the zoning districts with the major permitted
use category, minimum lot area and the corresponding percent of imperviousness.
4.
Development
activities involving streets, alleys and lots to be paved, repaved,
expanded or otherwise improved which are over three thousand (3,000)
square feet in area will require compliance with this Article.
5.
For
conditional land use in any zoning district, the same ratio determination
criteria in Table 1 shall apply.
6.
For
permitted land uses other than those listed in Table 1, such as schools,
churches, fire stations, etc., the rate of imperviousness shall be
determined by the same method as for commercial uses listed in Table
1.
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TABLE 1
STORMWATER DATA BY ZONING DISTRICTS
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|---|---|---|---|---|
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R-1
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Residential
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3 acres
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10%
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R-2
|
Residential
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10,000 square feet
|
35%
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|
R-2 & MH
|
Residential
|
7,500 square feet
|
45%
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|
C-1
|
Commercial*
|
(Determined)
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|
C-2
|
Commercial*
|
(percentage of impervious)
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|
I-1
|
Industrial*
|
(area by ratio of impervious)
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|
I-2
|
Industrial*
|
(surface area to total)
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|
*
|
Unimproved (natural) areas shall be five percent (5%) impervious.
Impervious areas, i.e., pavements, sidewalks, building roofs, etc.,
shall be one hundred percent (100%) impervious.
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[CC 1985 §5-61; Ord. No. 2963 §1(Art. II §2.1.1), 5-14-1998]
A.
The following
categories are exempt from enforcement of this Article:
1.
If an
off-site stormwater management system has been either constructed
or programmed or identified for construction by the City and the applicant
has agreed to and entered into a formal written agreement with the
City to contribute to or participate in the construction thereof.
2.
If the
increased amount of velocity or stormwater generated by the development
will have minor detrimental effect on the receiving watercourse and
the applicant has agreed to:
a.
Contribute
to the City fund an amount equal to the cost of the otherwise required
on-site stormwater management.
b.
Set
aside the property necessary for construction of a stormwater detention
facility should the need become evident subsequent to development.
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The developer with approval of the City Engineer shall prepare
an estimate of construction cost for a detention basin, which shall
be reviewed and approved by the City for the purpose of establishing
an amount to be contributed.
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3.
Development
on isolated lots of record for single-family dwelling purposes shall
not require approval of a stormwater management plan nor a grading
permit, unless development involves altering, rerouting, deepening,
widening, obstructing or changing in any way existing drainage facilities,
degrades the quality of water, adversely affects any wetland or adversely
affects any sinkhole, watercourse or waterbody.
[CC 1985 §5-62; Ord. No. 2963 §1(Art. II §2.2), 5-14-1998]
A.
The required
stormwater management plan shall identify means for controlling the
stormwater runoff release rate from the development and providing
storage potential for the excess stormwater runoff. All computations,
plans and specifications related to the implementation of this Article
must be prepared and sealed by a professional engineer registered
in Missouri. The entire acreage to be developed shall be studied to
determine the runoff impact within the catchment(s).
B.
The stormwater
management plan shall contain, but not be limited to, the following
information unless specifically excluded by the City.
1.
A topographic
map of the project site and adjacent areas on a scale of one (1) inch
is less than or equal to two hundred (200) feet with a maximum five
(5) foot contour intervals, except in floodplains or other areas of
low relief where a smaller interval may be required, which shall define
the location of streams, the extent of floodplains and calculated
high water elevations, the shoreline of lakes, ponds and detention
basins including their inflow and outflow structures, if any, within
the designated catchment.
2.
The
location and flowline elevation of all existing sanitary, storm or
combined sewers and other waterways within the proposed development
and adjacent area including the location of streams and other floodwater
runoff channels, their normal channels and the extent of the floodplains
at the established high water elevations and the limits of the floodway.
3.
Detailed
determination of runoff anticipated for the entire project site following
development indicating design volumes and rates of proposed runoff
for each portion of the watershed tributary to the storm drainage
system, the calculations used to determine said runoff volumes and
rates and restatement of the criteria which have been used by the
project engineer throughout his/her calculations.
4.
A layout
of the proposed stormwater management system including the location
and size of all drainage structures, storm sewers, channels and channel
sections, detention basins and analysis regarding the effect said
improvements will have upon the receiving channel and its high water
elevation.
5.
For
all detention basins, a plot or tabulation of storage volumes with
corresponding water surface elevations and of the basin outflow rates
for those water surface elevations.
6.
A profile
and one (1) or more cross sections of all existing and proposed channels
or other open drainage facilities showing existing conditions and
the proposed changes thereto, together with the high water elevations
expected from stormwater runoff under the controlled conditions called
for by these regulations and the relationship of structures, streets
and other utilities to such channels.
7.
Where
percolation or exfiltration systems are proposed, information as to
the location and type of vegetative cover and soil types and characteristics
representative of the design condition.
8.
Identification,
description and location of farm drains, inlets and outfalls, storm
sanitary and combined sewers and outfalls, sinkholes, septic tank
systems and outlets, if any, and seeps, springs and flowing or other
wells.
[CC 1985 §5-63; Ord. No. 2963 §1(Art. II §2.3), 5-14-1998]
A.
The following
rules shall govern the design of improvements with respect to managing
stormwater runoff:
1.
Point of runoff.
a.
Rooftop storage. Although rooftops are a significant portion
of impervious area, they can be designed to release stormwater slowly
to effect detention by installation of controlled outlets. By designing
drain outlets to release water at one (1.0) cfs per acre of rooftop
at a storage depth of two (2) inches, the roof area may be considered
to be pervious ground for purposes of this Section.
b.
Structural
and material aspects of ponding must be taken into account by the
design architect or engineer.
c.
To
avoid the possibility of an overload, the roof should be designed
with multiple outlets and free overflows, the latter providing a rapid
release rate should any controlled outlet become clogged or the storage
is exceeded by a storm of greater than design intensity.
2.
Upstream temporary storage.
a.
Parking lot storage. Temporary storage on parking lots may
be effectively utilized for commercial, industrial, institutional
and multi-family projects. To ensure proper function of system, property
owner shall check system periodically, clean as necessary. There may
be some inconvenience to the users of the parking lot, but usually
it will be no more serious than a temporary delay.
To prevent possible damage to vehicles, the depth of ponding
must be limited. A maximum depth of six (6) inches will be permitted
for automobiles, whereas eighteen (18) inches will be permitted for
lots serving only over-the-road tractors and trailers.
A parking lot storage facility design should include overflow
arrangements to prevent damage to downstream properties. Grated inlets
shall not be used to provide outlet control because of the possibility
of clogging. Open-throated curb or area inlets, pipe orifices or pipe
friction may be used for outlet control.
b.
Swale and channel storage. Temporary storage of stormwater
in a swale or channel is an effective and relatively maintenance free
technique which may be applicable if an acceptable restricted outlet
and overflow device are provided.
3.
On-stream detention.
a.
For
many on-site and most off-site stormwater management facilities, on-stream
techniques will be most feasible. These techniques are suitable for
larger watersheds and may serve any kind of development. They can
be developed on a watershed basis as joint effort between two (2)
or more developers.
b.
Temporary
storage is achieved by damming streams or by diverting stormwater
flows from the stream to storage areas, which would not otherwise
be available for the design flow. The amount of storage provided is
dependent on the inflow and the allowable release rate. In all cases,
a suitable emergency spillway or other device is made available for
the safe passage or by-pass of runoff in excess of that which can
be handled by the principal release structure.
c.
On-stream
impoundments or storage areas may be designed to drain after the cessation
of the storm and be normally dry in which case they are called detention
ponds. If a permanent pool is provided, only the storage provided
above the normal pool will be considered as available for stormwater
management.
4.
Criteria.
a.
The
minimum hydrologic criteria for projects subject to stormwater management
shall be that the two (2) year and the fifteen (15) year design discharges
of stormwater shall be no greater than those which would occur prior
to any development.
b.
Runoff
coefficients and times of concentrations for pre- and post-development
conditions shall be computed on the basis of the rational method.
Other methods may be used with approval from the City Engineer. The
durations analyzed shall be the following:
c.
The
maximum storage capacity and permitted discharged rate shall be based
upon the maximum calculated volume and peak flow of stormwater runoff,
respectively The maximum calculated volume and peak flow may result
from storms of different durations.
5.
Design.
a.
Sufficient
volume must be provided in a detention facility to store the amount
of runoff in excess of the discharge rate.
b.
Several
analytical routing methods exist in practice for the determination
of detention volumes based upon routing inflow hydrographs through
storage basins. The most widely used is the storage-indication method,
which is based on an equation, which is solved repeatedly for each
time period for the duration of the hydrograph:
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I1 + I2
2
|
(t2 - t1)
|
-
|
O1 - O2
2
|
(t2 - t2)
|
=
|
S2 - S1
|
|
|
where:
|
t1 and t2
|
=
|
time at beginning and end of time period, respectively
|
|
|
I1 and I2
|
=
|
flow rates at beginning and end of time period
| |
|
|
O1 and O2
|
=
|
outflow rates at beginning and end of time period
| |
|
|
S1 and S2
|
=
|
storage volume at beginning and end of time period
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c.
The
equation expressed above states that the average inflow for the time
period, minus the average difference in outflow rates for the time
period, equals the net storage increase or decrease for the time period.
d.
Tables
and graphs shall be provided for storage versus water surface elevation
and outflow versus water surface elevation.
e.
Design
hydrographs of inflow and outflow shall be provided for the two (2)
and fifteen (15) year differential runoff rate from the site under
proposed development conditions.
f.
Detailed
instructions and examples are given in National Engineering Handbook,
Section 4, Hydrology, U.S. Soil Conservation Service (1972).
g.
Flows
from upstream areas outside the site should be based upon the assumption
that those areas are fully developed under forecast use patterns.
The required storage volume will be based upon the site only, with
flows from upstream areas being by-passed or discharged via overflow
spillways or other devices.
h.
Construction
timing of runoff control measures shall be integrated in the construction
schedule during the course of development as directed by the City
Engineer.
i.
If
detention storage is provided within a floodplain, only the net increase
in storage volume above that which naturally existed on the floodplain
shall be credited to the development. No credit will be given for
volumes below the elevation of the base flood at that location unless
compensatory storage is also provided.
j.
The
City Engineer shall submit all calculations and plans for the proposed
management system to the City for review and approval.
6.
General design features.
a.
Dry bottom 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 or less than before development. The following
features shall be incorporated into the design of any detention basin:
(1)
Storage volume. The volume of storage provided shall
be sufficient to provide enough storage to control the differential
runoff from the 2-year or 15-year frequency storm, whichever is greater.
(2)
Freeboard. Detention storage areas shall have adequate
capacity to contain the storage volume of tributary stormwater runoff
with at least two (2) feet of freeboard above the water surface.
(3)
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, unless specifically approved by the City Engineer
(4)
Spillway. Emergency spillways shall be provided
to permit the safe passage of runoff generated from a 100-year storm
after detention of the design frequency storm.
(5)
Maximum depth. The maximum planned depth of stormwater
stored shall not normally exceed five (5) feet.
(6)
Side slopes. The maximum side slopes for grassed
basins shall not exceed one (1) foot vertical for three (3) feet horizontal.
(7)
Limits of ponding. In no case shall the limits of
maximum ponding be closer than thirty (30) feet horizontally from
any building and less than two (2) feet vertically below the lowest
sill elevation.
(8)
Interior drainage. The basin bottom should be designed
to drain expeditiously. If the bottom is to be grass, it shall have
a minimum slope of two percent (2%).
(9)
Low flow channel. Small flows through the detention
basin should be handled by paved ditches from inflow structure to
outflow structure to minimize erosion.
(10)
Multi-purpose 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.
(11)
Aesthetics. Designs should result in aesthetically
pleasing configurations which will enhance public acceptability.
b.
Wet bottom basins. Wet bottom basins may also be used to
temporarily detain the differential runoff from the development. In
addition to the general design features enumerated above for the dry
bottom basins, the following features shall be incorporated into the
design of any wet bottom basin:
(1)
Normal pool depth. In order to minimize weed growth,
the normal pool depth should be four (4) feet minimum.
(2)
Depth for fish. If fish are to be kept in the pond,
at least one-quarter (¼) of the area of the permanent pool
should have a minimum depth of ten (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)
Bank stabilization. In order to minimize the effects
of waves or ice, some type of bank stabilization such as riprap or
concrete may be placed along the normal pool shoreline.
(5)
Slide lopes below normal pool. The side slopes below
the normal pool elevation may exceed the maximum side slope permitted
above normal pool (3:1 slope). The design shall, however, include
provisions for a safety ledge having a depth of water not greater
than three (3) feet immediately adjacent to the shoreline. A qualified
geotechnical engineer shall determine the maximum permissible side
slope below the normal pool elevation.
(6)
Earthen basins. To minimize the effects of erosion
due to transient stormwater flows, earthen basins shall be sodded
up to a minimum of one (1) foot above the maximum anticipated high
water elevation within the basin.
c.
Maintenance. Designs of stormwater systems shall incorporate
features which facilitate their inspection and maintenance. The growth
of obnoxious weeds, the creation of conditions which support the growth
of mosquitoes and other insects and the decrease in available storage
by accumulated sediments must be controlled. The cleanup of accumulated
debris, flotsam and other materials after runoff events have subsided
must be assured. Assignment of responsibility for maintaining facilities
servicing more than one (1) lot or holding shall be documented by
appropriate covenants to property deeds unless responsibility is formally
assigned to the City.
d.
Safety. Designs of stormwater systems shall incorporate
safety features, particularly at outlets, on steep slopes and at any
attractive nuisances to include, as necessary, fencing, handrails,
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. Any
detention basin in which water surface elevations fluctuate more than
three (3) feet shall be fenced.
