[Ord. No. 11042013 §II, 11-4-2013]
A.
Unless otherwise provided, the following rules shall govern the design improvements with respect to managing stormwater runoff:
1.
Methods of determining stormwater runoff. Developments where the area contributing runoff is twenty-five (25) acres or less shall be designed using the Rational Method of calculating runoff. Developments where the area contributing runoff is four hundred (400) acres or less shall be designed using either the Rational Method of calculating runoff or other methods as approved by the City Engineer. For developments where the area contributing runoff is four hundred (400) acres or more, the applicant shall submit a proposed method of evaluation for the calculation of runoff to the City Engineer for review and approval. All such development proposals shall be prepared by a licensed professional engineer.
2.
Development design. Streets, blocks, depth of 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 stormwater into the ground, and to preserve and utilize existing and planned streams, channels, and detention basins, and include, whenever possible, streams and floodplains within parks and other public grounds.
3.
Enclosed systems and open channels. Unless otherwise provided by the City Engineer, the Design Criteria for Storm Sewers and Appurtenances, Revised, 1973, prepared by the Kansas City Metropolitan Chapter of the American Public Works Association, Section 5100 to 5112, inclusive, or as amended, which is by reference made part hereof as though expressly rewritten and incorporated in this Chapter, shall govern the design enclosed systems and open channels within the City of Peculiar.
4.
Methods of controlling downstream flooding. The City Engineer shall determine whether the proposed plan will cause or increase downstream development and an analysis of stormwater runoff with and without the proposed development as set forth in the Article. When the City Engineer determines that the proposed development will cause or increase downstream flooding conditions during the one-hundred-year storm, provisions to minimize such flooding conditions shall be included in the design of storm drainage improvements. Such provisions may include downstream improvements and/or the temporary controlled detention of stormwater runoff and its regulated discharge to the downstream storm drainage system.
5.
Downstream improvements. Improvements to minimize downstream flooding conditions may include, but not be limited to, the construction of dams, dikes, levees and floodwalls; culvert enlargements; and channel clearance and modification projects. Such downstream improvements should be discussed between the City Engineer and the developer's engineer so that appropriate provisions with respect to planning and implementing said improvements can be made at the earliest possible date.
6.
Detention. Developments also may include temporary detention of stormwater runoff in order to minimize downstream flooding conditions. The following design criteria shall govern the design of temporary detention facilities:
a.
Storage volume. The storage volume provided in detention basins shall be as stipulated in the Design Criteria for Storm Sewers and Appurtenances, Section 5600, 1990, Kansas City Chapter of American Public Works Association, or as amended.
b.
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 of flow in the emergency spillway in a one-hundred-year storm or as required by State law.
c.
Outlet control works.
(1)
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.
(2)
Outlet works shall not include any mechanical components or devices and shall function without requiring attendance or control during operation.
(3)
Size and hydraulic characteristics shall be such that all water in detention storage is released to the downstream storm sewer systems within twenty-four (24) hours after the end of the design rainfall.
d.
Spillway. Emergency spillways shall be provided to permit the safe passage of runoff generated from a one-hundred-year storm, or greater, if required by law, or if appropriate because of downstream high hazard, such as loss of life or damage to high-value property.
e.
Design data submittal. In addition to complete plans, the following design data shall be submitted to the City Engineer for all projects including temporary detention facilities;
(1)
Rainfall hyetograph plotted in units of inches per hour as ordinates, and time from beginning of the storm as abscissas.
(2)
Runoff hydrograph plotted in units of cubic feet per second runoff rate of the tributary area as ordinates, and time from the start of runoff as abscissas.
(3)
Area. Capacity curve for proposed detention facility plotted in units of datum elevation as ordinates, and cumulative volume of storage as abscissas.
(4)
Discharge characteristics curve or outlet works plotted in units of detention facility water surface elevation as ordinates, and discharge rate for cubic feet per second (CFS) as abscissas.
(5)
Storage capacity. Inflow and outflow curves in units of accumulated volume as ordinates, and time from start of runoff as abscissas. Curves shall be so arranged that the vertical distance between the accumulated storage and accumulated discharge will indicate the net volume in storage at any point in time. Curves shall be extended to the time required for complete discharge of all runoff stored in the detention facility.
f.
Detention methods. In addition to the above criteria, the following shall be applicable, depending on the detention alternative(s) selected by the City Engineer and the developer's engineer.
(1)
Wet-bottom basins. For basins designed with permanent pools.
(a)
Minimum depths. The minimum normal depth of water before the introduction of excess stormwater shall be four (4) feet.
(b)
Depth for fish. If fish are to be used to keep the basin clean, at least one-quarter (1/4) of the area of the permanent pool must have a minimum depth of ten (10) feet.
(c)
Facilities for emptying. For emptying purposes, cleaning or shoreline maintenance, facilities shall be provided or plans prepared for the use of auxiliary equipment to permit emptying and drainage.
(2)
Dry-bottom basins. For basins designed to be normally dry.
(a)
Interior drainage. Provisions must be incorporated to facilitate interior drainage to outlet structures.
(b)
Multipurpose 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.
(3)
Rooftop storage. Detention storage may be met in total or in part by detention on roofs. Details of such designs, which shall be included in the drainage permit applications, shall include the depth and volume of storage, details of outlet devices and downdrains, elevations of overflow scuppers, design loadings for the roof structure and emergency overflow provisions. Direct connection of rooftop drains to sanitary sewers is prohibited.
(4)
Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of stormwater on all or a portion of their surfaces. Outlets will be designed so as to slowly empty the stored waters, and depths of storage must be limited so as to prevent damage to parked vehicles.
(5)
Other storage. All or a portion of the detention storage may also be provided in underground or surface detention facilities, to include basins, tanks, or swales, etc.