[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.
TABLE 1
STORMWATER DATA BY ZONING DISTRICTS
R-1
Residential
3 acres
10%
R-2
Residential
10,000 square feet
35%
R-2 & MH
Residential
7,500 square feet
45%
C-1
Commercial*
(Determined)
C-2
Commercial*
(percentage of impervious)
I-1
Industrial*
(area by ratio of impervious)
I-2
Industrial*
(surface area to total)
*
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.
[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.
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.
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:
(1) 
The duration equivalent to the time of concentration for the site, but as a minimum, ten (10) minutes.
(2) 
One (1) hour.
(3) 
Twenty-four (24) hours.
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:
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
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.