[R.O. 2009 §23.5-21; Ord. No. 2242 §2.1, 11-13-1991]
A. 
A storm water management plan shall be submitted to the City by the owner of any proposed residential, commercial or industrial development within the City, unless the City waives such requirement for one (1) of the following reasons:
1. 
Off-site facility, two (2) or more developments. If two (2) or more developments, including that of the applicant, have provided for a common system.
2. 
Off-site facility by City. If an off-site storm water management system has been either constructed or programmed or identified for construction by the City and the applicant has agreed to contribute to or participate in the construction thereof.
3. 
If the increased amount of velocity or storm water generated by the development will have minor detrimental effect on the receiving watercourse and the applicant has agreed to contribute to the City fund an amount equal to the cost of the otherwise required on-site storm water management facility. For the purpose of establishing an amount to be contributed, the developer shall prepare an estimate of construction cost for a detention basin which shall be reviewed and approved by the City.
4. 
Other management techniques. Management techniques other than detention facilities may be utilized by the development, provided the techniques proposed meet the intent of this Chapter and provide a benefit to the watershed that equals or exceeds the benefit that a detention facility would provide.
5. 
Non-urban (N-1), Single-Family (R-1 and R-2), One-Family and Two-Family (R-3) and One- and Two-Multi-Family (R-4) development of less than two (2) acres in which the owner can demonstrate to the City that the receiving streams are adequate and that affects to downstream landowners will be minimal. If a project is developed in phases, the total acreage of the conceptual project must be considered.
No final subdivision or development plat shall be approved and no building permits shall be issued until the storm water management plan has either been waived or reviewed and approved by the City.
[R.O. 2009 §23.5-22; Ord. No. 2242 §2.2, 11-13-1991]
A. 
The required storm water management plan shall identify means for controlling the storm water runoff release rate from the development and providing storage potential for the excess storm water runoff. All computations, plans and specifications related to the implementation of this Chapter 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 storm water 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 of suitable scale and contour interval, which shall define the location of streams, the extent of flood plains 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.
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 storm water 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 storm water runoff under the controlled conditions called for by these regulations and the relationship of structures, streets and other utilities to such channels.
[R.O. 2009 §23.5-23; Ord. No. 2242 §2.3, 11-13-1991; Ord. No. 2979 §I(23.5-23), 6-28-2001]
A. 
The following rules shall govern the design of improvements with respect to managing storm water runoff:
1. 
Methods of determining storm water runoff rate and volume. Proposed development project shall be classified in two (2) categories according to acreage:
a. 
Less than twenty (20) acres: Calculations shall be evaluated by the rational formula, considering differential runoff rates between the proposed development and the undeveloped tract, as shown on Table I, for a 15-year (frequency), 20-minute (inlet time) rainfall.
b. 
Greater than twenty (20) acres: Calculation shall be evaluated by the Soil Conservation Survey's "Urban Hydrology for Small Watersheds Technical Release 55" Method, considering differential runoff rates between the proposed development and the undeveloped tract, for a 25-year (frequency), 24-hour (point rainfall) rainfall.
2. 
Release rate. The controlled release rate of storm water runoff from all developments as described in Section 520.090 shall not exceed the pre-development rate. The rate at which storm water runoff is delivered to a designated storm water storage area shall be restricted as to minimize erosion. The pre-development storm water runoff rate calculations shall be based upon the following:
a. 
Classification A — Development less than twenty (20) acres: Rational Formula, considering a 15-year (frequency) 20-minute (inlet time) rainfall, considering five percent (5%) imperviousness (1.7 cfs per acre).
b. 
Classification B — Developments greater than twenty (20) acres: Soil Conservation Survey, considering a 25-year (frequency), 24-hour (point rainfall) rainfall, for the generated pre-development Curve Number (CN).
In the event the natural downstream channel or storm sewer system is inadequate to accommodate the release rate provided above, then the allowable release rate shall be reduced to that rate permitted by the capacity of the downstream channel or storm sewer system.
3. 
Development design. Streets, blocks, lots, parks and other public grounds shall be located and laid out in such a manner as to minimize the velocity of overland flow and allow maximum opportunity for infiltration of storm water into the ground and to preserve and utilize existing and planned streams, channels and detention basins and include, whenever possible, streams and flood plains within parks and other public grounds.
4. 
Detention. The increased storm water runoff resulting from the proposed development may be accommodated by the provision of appropriate detention facilities including wet or dry bottom reservoirs, flat roofs, unrequired parking areas or streets. The following shall govern the design of detention facilities:
a. 
Storage volume. The volume of storage potential provided in detention facilities shall be sufficient to control the excess storm water runoff, as determined to be the difference between the storm water quantity from the site in its developed state.
TABLE I.
STORM WATER MANAGEMENT
Simplified volume formula: Total volume of detention shall be computed for Classification A developments (less than twenty (20) acres) by the following equation:
V = R x A x 30 (min) x 60 (sec/min)
where:
V = Total volume of detention
R = Differential runoff rate (Developed runoff rate — pre-developed runoff rate)
A = Area of project in acres
AVERAGE C.I. (P.I.) FACTORS IN C.F.S. PER ACRE
20-MINUTE INLET TIME
% Impervious
C or P (Factor)
15 Yr.
25 Yr.
50 Yr.
100 Yr.
5
.37
1.7
2.00
2.18
2.37
10
.39
1.79
2.11
2.30
2.50
15
.41
1.89
2.21
2.42
2.62
20
.435
2.00
2.35
2.57
2.78
25
.455
2.09
2.46
2.68
2.91
30
.475
2.19
2.57
2.80
3.04
35
.495
2.28
2.67
2.92
3.17
40
.52
2.39
2.81
3.07
3.33
45
.54
2.48
2.92
3.19
3.46
50
.56
2.58
3.02
3.30
3.58
55
.58
2.67
3.13
3.42
3.71
60
.60
2.76
3.24
3.54
3.84
65
.625
2.88
3.38
3.69
4.00
70
.645
2.97
3.48
3.81
4.13
75
.665
3.06
3.59
3.92
4.26
80
.685
3.15
3.70
4.04
4.38
85
.705
3.24
3.81
4.16
4.51
90
.73
3.36
3.94
4.31
5.67
95
.75
3.45
4.05
4.43
4.80
100
.77
3.54
4.16
4.54
4.93
Rainfall
4.60
5.40
5.90
6.40
From U.S. Weather Bureau Technical Paper No. 40: "Rainfall Frequency Atlas of the United States for duration from thirty (30) minutes to twenty-four (24) hours and Return Periods from 1 to 100 years."
Classification A — Developments less than twenty (20) acres: Facilities shall be sized according to the Rational Formula to accommodate the storm water volume for a thirty (30) minute duration of a 15-year (frequency), 20-minute (inlet time) rainfall, less the allowable release rate as set forth in Section 520.110(2).
Classification B — Developments greater than twenty (20) acres: Facilities shall be sized according to the Soil Conservation Survey, TR-55 Method to accommodate a storm water volume for a 25-year (frequency), 24-hour (point rainfall) rainfall, less the allowable release rate as set forth in Section 520.110(2).
b. 
Release rate. At no time during the design storm shall the storm water runoff release rate exceed the allowable release rate as set forth in Section 520.110(2).
c. 
Release velocity. Detention facilities shall release storm water at a non-erosive velocity. Protected channels receiving detention discharge shall incorporate features to reduce velocity to non-erosive levels at the point where such discharge enters the unprotected channel. If release is into a subsurface conduit, the energy gradient in the receiving facility shall not be increased beyond the slope of the conduit.
d. 
Construction standards for detention basins. Detention facilities shall be designed and constructed with proper safety stability and ease of maintenance. Minimum side slopes for grassed reservoirs shall not exceed one (1) foot vertical for three (3) feet horizontal (3:1). In no case shall the limits of maximum ponding elevation be closer than thirty (30) feet horizontally from any building and less than two (2) feet vertically below the lowest sill elevation. The entire reservoir shall be seeded, fertilized and mulched, sodded or paved. Any area susceptible to, or designed as, overflow by a higher design intensity rainfall (15-year, 20-minute rainfall for Classification A projects or 25-year, 24-hour point rainfalls for Classification B projects) shall be sodded or paved.
e. 
Parking lot detention. Detention is permitted in parking lots to a maximum depth of twelve (12) inches for unrequired parking areas only. In no case shall the maximum limits of ponding be designed closer than twenty (20) feet from a building unless waterproofing of the building and pedestrian accessibility are properly documented.
f. 
Spillway. Emergency spillways shall be provided as a part of the detention basin to permit the safe passage of runoff generated from a high frequency rainfall then designated herein. Spillways shall be either seeded, fertilized and mulched, sodded or paved. Considerations of a 50-year storm frequency is recommended.
g. 
Freeboard. Detention facilities shall have adequate capacity to contain the storage volume required, with at least one (1) foot of vertical freeboard above the maximum spillway water surface elevation. The freeboard shall also be at an elevation of at least one (1) foot above the FEMA designated 100-year flood elevation.
h. 
Control structures.
(1) 
Detention facilities shall be provided with obvious and effective control structures. Plan view and sections of the structure with adequate details shall be included in the plans. The design discharge shall not exceed the stipulated non-developed runoff rate (15-year, 20-minute rainfall for Classification A developments or 25-year, 24-hour point rainfall for Classification B developments). Runoff discharge shall be calculated as five percent (5%) impervious (1.7) cfs per acre) for Classification A developments and the generated pre-development curve number (CN) for Classification B developments.
(2) 
Sizing of the discharge pipe shall be by inlet control or hydraulic gradient requirements, as published by the U.S. Department of Transportation, Federal Highway Administration's "Hydraulic Charts for the Selection of Highway Culverts", Hydraulic Engineering Circular No. 5, December 1965 or other approved design standards.
(3) 
The discharge pipe shall not be less than eight (8) inches in diameter for minimizing operation and maintenance requirements. A bar screen on a minimum 2:1 slope shall be provided to reduce blockage by debris.
i. 
Liability. This design criteria and guidelines establish minimum elements of design which must be implemented with sound engineering and good workmanship. Use of the information contained herein for placement of any structure or use of land shall not constitute a representation, guarantee or warranty of any kind by the City of Festus, its officials or employees of the practicability, adequacy or safety and shall not create liability upon or cause action against any such public body, official or employee for any damages that may result pursuant thereto.
[R.O. 2009 §23.5-24; Ord. No. 2242 §2.4, 11-13-1991]
A. 
Storm Water Channel Location. Generally acceptable locations of storm water channels in the design of a subdivision may include, but are not limited to, the following:
1. 
Adjacent to roadways.
2. 
In a depressed median of a double roadway, street or parkway provided the median is wide enough to permit slopes of one (1) foot drop in six (6) feet horizontal or flatter.
3. 
Centered on lot lines or entirely within the rear yards of a single row of lots or parcels.
4. 
In each of the foregoing cases, a drainage easement with sufficient width to facilitate maintenance and design flow shall be provided and shown on the plat.
B. 
Storm Sewer Outfall. The storm sewer outfall shall be designed to provide adequate protection against downstream erosion and scouring.
C. 
Lot Lines. Whenever the plans call for the passage and/or storage of storm water runoff along lot lines, the grading of all such lots shall be prescribed and established for the passage and/or storage of waters and no structure or vegetation which would obstruct the flow of storm water shall be allowed, nor shall any change be made to the prescribed grades and contours of the specified storm water channels.
D. 
Manholes. All utility sewer manholes constructed in an area designed for the storage or passage of storm water shall be provided with either a water-tight manhole cover or be constructed with a rim elevation of a minimum of one (1) foot above the high water elevation of the design storm.
E. 
Easements. Permanent easements for the conveyance of storm water, including easements of access to structures and facilities, shall be denoted within the recorded plat as denoted in Section 520.140 of this Chapter.
F. 
Obstruction Of Drainage. The keeping or disposal of grass clippings, trash, debris, obstructions or unwanted materials into the storm sewers or within or along storm water channels or in adjacent flood plain areas which may wash into sewers and channels is prohibited.
G. 
Maintenance. Maintenance of the detention facilities shall be the responsibility of the owner or as assigned to the homeowners' association, subdivision board, etc., as designated in the recorded plat at the Recorder of Deeds office. All open channels and closed conduit storm water facilities and appurtenances shall be maintained by the owner, assigned homeowner association, subdivision board, etc., excepting any storm water conveyance facilities as approved by the City of Festus and subsequently constructed on public rights-of-way and public easements.