City of Parkville, MO
Platte County
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Table of Contents
Table of Contents
Editor's Note — Ord. no. 2069 adopted April 15, 2003, repealed ch. 507 and enacted the new provision set out herein. Former ch. 507 derived from ord. no. 1929 §1, 1-16-2001 and ord. no. 2032 §1, 8-6-2002.

Section 507.005 Applicability.

[Ord. No. 2069 §2, 4-15-2003]
Whenever homes, buildings or other structures are flooded downstream from a proposed development in a 100-year or smaller flood, or where flood damage areas have been identified, or an engineering study indicates the proposed development would cause or increase such flooding, stormwater detention facilities shall be constructed. Exceptions are when such flooding occurs within the limits of the 100-year flood on lower reaches of the main channel of major streams as established by the current federal Flood Insurance Study and when such development is a single family or multi-family residential lot of one-half (½) acre or less and not part of a larger development.

Section 507.010 Definitions.

[Ord. No. 2069 §3, 4-15-2003]
For the purpose of this Chapter, the following terms, phrases, words, and their derivations shall have the meanings given herein:
DAM
An artificial barrier or embankment having greater than a six (6) foot difference in elevation between the crest of the emergency spillway and the lowest point of the cross section along the centerline of the dam, which does or may impound water.
DETENTION BASIN
Any manmade area or structure which serves as a means of temporarily storing storm water runoff.
DETENTION STORAGE
The temporary detaining or storage of storm water on or beneath the ground surface, on rooftops, parking lots, or by other means under predetermined or controlled conditions.
DEVELOPMENT
Any change in land use or improvement on any parcel of land.
DISCHARGE
The rate of outflow of water from detention storage.
DRAINAGE AREA
The geographical area contributing stormwater runoff to a point under consideration, i.e., a watershed, tributary area or catchment area.
DRY BOTTOM BASIN
A detention basin or facility not intended to have a permanent pool.
2-YEAR FLOOD
A flood having a fifty percent (50%) chance of occurrence in any given year. The 2-year flood is assumed to be caused by a 2-year storm.
2-YEAR STORM
A rainstorm of a given duration and depth of precipitation having a fifty percent (50%) chance of occurrence in any given year.
5-YEAR FLOOD
A flood having a twenty percent (20%) chance of occurrence in any given year. The 5-year flood is assumed to be caused by a 5-year storm.
5-YEAR STORM
A rainstorm of a given duration and depth of precipitation having a twenty percent (20%) chance of occurrence in any given year.
10-YEAR FLOOD
A flood having a ten percent (10%) chance of occurrence in any given year. The 10-year flood is assumed to be caused by a 10-year storm.
10-YEAR STORM
A rainstorm of a given duration and depth of precipitation having a ten percent (10%) chance of occurrence in any given year.
25-YEAR FLOOD
A flood having a four percent (4%) chance of occurrence in any given year. The 25-year flood is assumed to be caused by a 25-year storm.
25-YEAR STORM
A rainstorm of a given duration and depth of precipitation having a four percent (4%) chance of occurrence in any given year.
50-YEAR FLOOD
A flood having a two percent (2%) chance of occurrence in any given year. The 50-year flood is assumed to be cause by a 50-year storm.
50-YEAR STORM
A rainstorm of a given duration and depth of precipitation having a two percent (2%) chance of occurrence in any given year.
100-YEAR FLOOD
A flood having a one percent (1%) chance of occurrence in any given year. The 100- year flood is assumed to be cause by a 100-year storm.
100-YEAR STORM
A rainstorm of a given duration and depth of precipitation having a one percent (1%) chance of occurrence in any given year.
FREEBOARD
The difference in elevation between design water surface in the detention facility and the elevation at which uncontrolled overtopping of the facility begins.
HYETOGRAPH
Intensity distribution of a rainfall event with respect to time.
PROJECT
Any development involving the construction, reconstruction or improvement of structures and/or grounds.
STORMWATER RUNOFF
The waters derived from precipitation falling within a drainage area, flowing over the surface of the ground or collected in channels or conduits.
WET BOTTOM BASIN
A detention basin intended to have a permanent pool.

Section 507.020 Maximum Release Rate.

[Ord. No. 2069 §3, 4-15-2003]
In general the maximum allowable release rate of stormwater runoff originating within the proposed development shall be limited to an amount that will not cause or increase downstream flooding within that tributary in a 100-year or smaller flood. Such allowable release rate shall be defined as the rate of runoff occurring from the proposed development in its undeveloped condition and shall be determined mathematically as the runoff resulting from the 2-, 5-, 10-, 25-, 50- and 100-year storm return frequency rainfall calculated using the rational formula. The difference between the pre- and post-construction values is the required detention and should be released through a weir or orifice design at each respective storm level. Deviations from this rainfall frequency in design calculations shall be only where approved by the Director of Public Works. Actual rainfall intensity shall be determined for the time of concentration of the tributary area in its undeveloped and natural state. The release rate as identified in the current edition of the American Public Works Association (APWA) Standard Specifications and Design Criteria may be used on large developments over ten (10) acres only if it determines a more restrictive release rate. In no case shall the release rate exceed the existing "safe" storm drainage capacity of the downstream drainage system or watercourse as determined by the Director of Public Works based on a study of the tributary area and the downstream drainage system. Where no study exists but a downstream flooding problem has been identified, the Director of Public Works may require a study of the downstream drainage system and/or watercourse to determine the developments maximum release rate. Detention Basin outflows shall discharge directly into the downstream drainage system.

Section 507.030 Detention Storage Volume.

[Ord. No. 2069 §3, 4-15-2003]
The volume of storage provided in detention basins shall be sufficient to store the storm water runoff generated by the development during a 100-year flood, less the discharge that is permitted in this Chapter.

Section 507.040 Emergency Spillways.

[Ord. No. 2069 §3, 4-15-2003]
A. 
Emergency spillways shall be sized to carry an additional 100-year flood assuming the detention basin is already filled to design storage capacity.
B. 
Freeboard for earthen detention basins shall be one (1) foot difference in elevation between the top of the settled embankment and the water surface, with the emergency spillway flowing at design depth.

Section 507.050 Design Criteria.

[Ord. No. 2069 §3, 4-15-2003]
A. 
A minimum of the following detention design criteria will be required on plans, however graphs and charts may be submitted separately from the plans to the Director of Public Works for a developments proposed detention basin:
1. 
Detention calculation summary table and other graphs containing the following information:
a. 
Total site area (acres).
b. 
Design storm frequency as required in this Chapter.
c. 
Runoff coefficient (both pre- and post-development).
d. 
Allowable release rate as permitted in this Chapter (cubic feet per second (CFS)).
e. 
Required storage volume as required in this Chapter (cubic feet).
f. 
Storage volume at design pool elevation as required in this Chapter.
g. 
Stage storage curves for proposed detention facilities plotted in units of detention facility water surface elevation (and depth) as ordinates and cumulative volume of storage as abscissas.
h. 
Stage outflow curves for outlet works plotted in units of detention facility water surface elevation (and depth) as ordinates and discharge rate (cubic feet per second) as abscissas.
i. 
Inflow and outflow hydrographs plotted in units of cubic feet per second of inflow and outflow as ordinates and time from start of storm as abscissas.
j. 
Inflow and outflow mass curves plotted in units of cumulative volume as ordinates and time from start of storm as abscissas. Curves shall be so arranged that the vertical distance between the cumulative storage and the cumulative discharge will indicate the net volume of storage at any time. These curves may be omitted if the inflow and outflow hydrographs are so plotted and labeled as to show total storage requirements and time to empty the detention facility.
k. 
Emergency spillway flowline elevation, width and flow when passing the 100-year storm (design depth) as required in this Chapter.
l. 
Elevation of top of settled embankment-must have one (1) foot freeboard.
m. 
Provide details of the detention basin emergency spillway and controlled outlet pipe along with a section through the detention basin.
2. 
If the design engineer determines that detention will not be necessary for a particular development, he shall submit a stormwater study and report summarizing the details of the study and may request a waiver of the detention requirement to the Director of Public Works.

Section 507.060 Other Requirements For The Project Site Information.

[Ord. No. 2069 §3, 4-15-2003]
A. 
A topographic map of the project site and immediately adjacent areas of suitable scale and contour interval which shall define the location of any streams, extent of floodplains and calculated high water elevations, and shorelines of lakes and ponds or swamps.
B. 
The size, location and flowline elevations of all existing sewage treatment facilities which fall within the project limits and within the distance of five hundred (500) feet beyond the boundaries of said project.
C. 
A proposed grading plan and/or site plan showing existing and proposed contours, buildings, parking lots, platted lots and other development features.
D. 
Drainage area map showing upstream drainage areas tributary to the development and to each proposed detention facility.
E. 
Special study to determine effects of development if required by the Director of Public Works.
F. 
Indicate on the plan proposed areas and/or methods to be used for detention facilities. The developer or builder shall coordinate with the Director of Public Works on the location, size, shape or other desired design features of proposed detention basins.

Section 507.070 Detention Methods.

[Ord. No. 2069 §3, 4-15-2003]
A. 
In addition to the foregoing criteria, the following shall be applicable depending on the detention alternative(s) selected.
1. 
Wet bottom basins-for basins with permanent pools.
a. 
Minimum depths. The minimum depth of water before introduction of excess stormwater shall be four (4) feet. The side slopes of the basin shall conform as closely as possible to regraded or natural land contours.
b. 
Multi-purpose features. These shall be designed to serve secondary purposes for recreation, open space, or other types of use which will not be adversely affected by occasional or intermittent flooding, if possible.
2. 
Rooftop storage. Detention may be met in total or in part by detention on roofs. Details of such designs shall include the depth and volume of storage, details of outlet devices and down-drains, elevations and details of overflow scuppers and emergency overflow provisions. Connection of roof drains to sanitary sewers is prohibited. Design loadings and special building and structural details shall be subject to approval by the City Building Inspector.
3. 
Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of stormwater on a portion of their surfaces. Generally such detention areas shall be in the more removed portions of such parking lots. Depths of storage shall be limited to a maximum depth of seven (7) inches in parking areas. Where storage is to be deeper than seven (7) inches, such areas shall be precluded from parking use and shall be located so that access to and from parking areas is not impaired.
4. 
Other storage. All or a portion of the detention storage may also be provided in underground or surface detention areas including, but not limited to, oversized storm sewers, vaults, tanks, swales, etc.

Section 507.080 Plans, Specifications and Calculations.

[Ord. No. 2069 §3, 4-15-2003]
Based on design criteria in this Chapter, plans and specifications shall be submitted to the Director of Public Works for approval prior to any building permit being issued, grading or land disturbance permit, or the beginning of any construction.

Section 507.090 Storm Water Runoff Not Detained.

[Ord. No. 2069 §3, 4-15-2003]
The rate of discharge from a detention facility and the rate of discharge of stormwater runoff from areas of the development not controlled by the detention facility shall not collectively exceed the maximum release rate established pursuant to the provisions of Section 507.020.

Section 507.100 Construction.

[Ord. No. 2069 §3, 4-15-2003]
A. 
Standards for construction of inlets, pipes, manholes, paved ditches, and other detention appurtenances shall be in accordance with the most current edition of APWA standards, unless otherwise designated by the Director of Public Works.
B. 
If slopes for wet or dry detention basins exceed twenty percent (20%), both erosion control and safety measures shall be provided. In no case shall earthen slopes exceed thirty-three percent (33%). If vertical walls are used, the basin must be fenced with steps, ramps or other means of egress provided.
C. 
Wet detention facilities shall have an outlet works consisting of valves, gates, pipes or other devices necessary to completely drain the facility in seventy-two (72) hours or less when required for maintenance.

Section 507.110 Easements and Maintenance.

[Ord. No. 2069 §3, 4-15-2003]
A. 
Maintenance of private detention facilities shall be the responsibility of the property owner and shall include debris removal and cleaning, cutting of vegetation, repair of erosion, removal of silt and maintenance of structural facilities.
B. 
Maintenance of public detention facilities shall be the responsibility of the City of Parkville, and permanent easements shall be dedicated to the City and sized according to the latest edition of APWA.

Section 507.120 Violations.

[Ord. No. 2069 §3, 4-15-2003]
Violation of any provision of this Chapter shall be a misdemeanor punishable by a fine of not more than five hundred dollars ($500.00) or by imprisonment of not more than thirty (30) days, or by both such fine and imprisonment. Each day of violation shall constitute a separate offense.