[Ord. No. 95-12 §4.1, 3-7-1995; Ord. No. 484-2011 §1, 7-5-2011]
A.
The
following rules shall govern the design of 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 Building
Inspector. 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 Building
Inspector 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 Building Inspector the most recent Design Criteria
for Storm Sewers and Appurtenances, Revised, 1973 prepared by the
Kansas City Metropolitan Chapter of the American Public Works Association,
Sections 5100 to 5112, inclusive, as or amended, which is by reference
made a part hereof as though expressly rewritten and incorporated
in the ordinance, shall govern the design of enclosed systems and
open channels within the City of Neosho.
4.
Methods of controlling downstream flooding. The
City Building Inspector shall determine whether the proposed plan
will cause or increase downstream flooding conditions. This determination
shall be made on the basis of existing downstream development and
an analysis of stormwater runoff with and without the proposed development
as set forth in this Article. When the City Building Inspector determines
that the proposed development will cause or increase downstream flooding
conditions during the one hundred (100) 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 Building Inspector and 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. Development 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 volume of storage provided in
detention basins shall be sufficient to control the differential runoff
from the fifty (50) year storm frequency of twenty-four (24) hour
duration. The differential runoff is that volume and rate of flow
of stormwater runoff discharged from a parcel of land or drainage
area which is or will be greater than that volume and rate which pertained
prior to proposed development or redevelopment.
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 fifty (50) 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 fifty (50) 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 Building
Inspector 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 the 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 Building Inspector and the developers' 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 (¼) of the area of the
permanent pool must have a minimum depth of eight (8) 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 roof 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 of surface detention facilities,
to include basins, tanks, or swales, etc.
[Ord. No. 95-12 §4.2, 3-7-1995]
A.
Stormwater Channel Location. Generally acceptable locations
of stormwater runoff channels in the design or any development including
subdivision, may include, but not be limited to, the following:
1.
In a depressed median of a double roadway, street or parkway provided
the median is wide enough to permit maximum three (3) to one (1) side
slopes.
2.
Centered on lot lines or entirely within the rear yards of a single
row of lots or parcels.
3.
In each of the foregoing cases, a drainage easement to facilitate
maintenance and design flow shall be provided and shown on the plat.
No structures shall be construed within or across stormwater channels.
4.
The minimum channel shall be designed to handle a twenty-five (25)
year, twenty-four (24) year storm with a maximum velocity of five
(5) feet/second.
B.
Storm Sewer Outfall. The storm sewer outfall shall be designed
so as to provide adequate protection against downstream erosion and
scouring.
C.
Lot Lines. Whenever the plans call for the passage and/or
storage of floodwater, surface runoff or stormwater 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 may be erected
which will obstruct the flow of stormwater, no fences, shrubbery,
or trees planted, or changes made to the prescribed grades and contours
of the specified floodwater or stormwater runoff channels.
D.
Manholes. All sanitary sewers manholes constructed in a
floodplain or in an area designed for the storage or passage of flood
or stormwater, 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 base flood or the high
water elevation of the design storm, whichever is applicable to the
specific area.
E.
Easements. Permanent easements for the detention and conveyance
of stormwater, including easements of access to structures and facilities,
shall be dedicated to the City of Neosho.
F.
Maintenance. Provisions acceptable to the City of Neosho for perpetual maintenance of detention facilities, outlet works, and appurtenances shall be made, as provided, in Section 430.188(B) of these regulations.
G.
Drainage Permits. A drainage permit for projects including detention facilities may be granted by the City Building Inspector only after the Stormwater Management Plan has been approved and all easements have been dedicated, accepted, and recorded, and all required maintenance assurances and required bonds have been executed, as required in Section 430.184 of these regulations.
