[Ord. 15-01-03-52, 1-19-2015]
No development shall be authorized in the City unless there
is either an adequate outlet or the City has approved detention facilities.
The applicant shall provide detailed calculations of the outlet capacity
and the City shall determine its adequacy. In the event the City determines
the outlet is not adequate, the developer shall be required to construct
detention facilities. Such facilities shall be based on the design
formulas. At the option of the City the developer may be asked to
pay a fee in lieu of providing detention facilities.
In those locations already served by an existing stormwater
detention facility or where the City is contemplating constructing
a detention facility to serve upstream watersheds not yet fully developed,
or where the developer cannot provide adequate storage capacity in
a detention facility within his/her development, at the option of
the City, the developer shall pay a fee in lieu of providing said
detention facility, or the deficient volume thereof.
Facilities shall be designed so that local storm sewer systems
and drainageways will discharge to proposed or existing detention
facilities, or to an adequate trunk line storm sewer and flood route,
or drainageway feeding same, which discharges into a proposed or existing
stormwater detention facility. Where adequate storm sewer or drainageway
capacity is not available to convey design runoff to an existing downstream
detention facility, an on site detention facility shall be required.
All drainage and detention calculations and designs shall be
provided by a professional engineer licensed in the state of Illinois.
No statement contained within this chapter shall be construed
to interfere with any additional requirements that may be imposed
by the City.
[Ord. 15-01-03-52, 1-19-2015]
A combination of storage and controlled release of stormwater
is required for all:
A.
Nonresidential developments greater than two acres in area or less
than two acres with greater than 50% impervious area.
B.
Residential developments greater than five acres in area or less
than five acres with greater than 50% impervious area.
C.
Where a subdivision plat or a planned unit development involves new
lots that are zoned for commercial or industrial purposes, the developer/owner/applicant
may provide for storage and controlled release of stormwater runoff
for the developed lots by sufficient and enforceable covenants upon
the individual lots. The covenant shall require construction of on
site stormwater detention facilities upon each lot at the time of
subsequent development of the lot. The form of covenant shall be approved
by the City attorney, and the substance of the covenant shall require
construction of on site stormwater runoff storage and controlled release
of stormwater runoff from each lot in a manner approved by the City
and in accordance with the provisions of this chapter.
[Ord. 15-01-03-52, 1-19-2015]
Exceptions to the required stormwater management plan are:
A.
Traditional agricultural land uses;
B.
Improvement of existing roadways;
C.
Modification of single-family residential or two-family residential
structures which will continue to be used as single-family or two-family
dwellings;
D.
Modification of existing structures or appurtenances, other than
a single-family dwelling or two-family dwelling, which do not increase
the amount of impervious area of the lot or parcel upon which it is
constructed;
E.
When runoff can be directed into a storm sewer, channel, or natural
drainageway with sufficient capacity to transport stormwater to an
acceptable location as determined by the City.
[Ord. 15-01-03-52, 1-19-2015]
Unless otherwise approved by the City, formulas to be used in
connection with the calculation of runoff volumes and allowable release
rates reasonably expected from the minimum design storm shall be the
rational method for total contributing areas of 20 acres or less and
the soil conservation hydrograph method for areas greater than 20
acres.
[Ord. 15-01-03-52, 1-19-2015]
For postdeveloped conditions, the amount of pervious and impervious
land shall be determined and prorated using the following coefficients:
pervious (C = 0.30, CN = 61); impervious (C = 0.90, CN = 98). The
overland flow coefficient (n) shall not be greater than 0.15.
For predeveloped conditions the runoff coefficients shall not
be greater than C = 0.30 or CN = 69. The overland flow coefficient
(n) shall not be less than 0.13.
[Ord. 15-01-03-52, 1-19-2015]
A.
Allowable Release Rate: This will be determined by the design formula
using a minimum design storm with an average rainfall intensity associated
with average recurrence interval of five years for the storm period
calculated by the time of concentration as outlined by the latest
technical letters of the Illinois state water survey for rainfall
frequencies. Runoff coefficients or CN numbers used shall reflect
predeveloped condition of the watershed, as previously noted.
B.
Storage Volume When Using The Rational Method: This will be the volume
generated by the difference between the average runoffs of fifty-year
rainfall frequency in postdevelopment condition and five-year rainfall
frequency in predevelopment condition, assuming such difference occurs
for one hour. The average discharge being half of the peak discharge
calculated by the rational method.
C.
Storage Volume When Using the Soil Conservation Hydrograph Method:
This will be determined from inflow hydrographs generated by the design
formula using a minimum design storm with a range of rainfall intensities
associated with an average recurrence interval of 50 years. The storage
volume will be the difference between the inflow hydrograph and a
designed outflow hydrograph that releases no more than the allowable
release rate.
[Ord. 15-01-03-52, 1-19-2015]
A.
Hydrologic Design Calculations:
2.
Hydrologic design calculations shall include contour map(s) clearly
showing the design drainage area(s), stormwater facilities, storm
sewers, swales, ditches, culverts, bridges and receiving stormwater
facilities. Critical spot elevations, invert elevations, pervious
and impervious areas shall be clearly indicated.
3.
Submittals shall include design calculations, drainage basin area
and surface types, and rainfall data used.
4.
Plans shall clearly indicate normal and high water elevations, design
storage volume, and minimum and maximum typical slopes.
5.
Emergency spillways shall be clearly designated for runoff in excess
of the fifty-year storm.
6.
Minimum outlet storm sewer size shall be 12 inches; smaller diameter
restrictions (e.g., orifice plate) are acceptable.
7.
Inlet and outlet pipes shall be provided with end sections and erosion
protection.
8.
Weirs, dams, retaining walls, and specialized outflows shall be designed
by a professional engineer or structural engineer licensed in the
state of Illinois, as required.
9.
Temporary erosion techniques shall be used as required to ensure
a full stand of cover vegetation in minimum time.
10.
Detention basin side slopes shall be 4:1 or flatter and shall be
designated with permanent erosion protection consisting of grass,
nongrass vegetation, or other permanent finish. Permanent erosion
protection shall be aesthetically suitable to the development or existing
surrounding land use.
11.
Rooftop detention is not allowed.
12.
A building's lowest water entry point shall be a minimum of
two feet above the fifty-year design high water elevation.
B.
Earthen Dams:
2.
All earthen dams, whether publicly or privately owned and maintained,
shall meet the following:
a.
Freeboard: All earthen dams shall be constructed with a minimum of
one foot of freeboard over the maximum anticipated water level in
the emergency spillway.
b.
Core: All earthen dams shall include, at a minimum, an impervious
compacted clay core. The core shall be of sufficient width to allow
for mechanical compaction and of sufficient depth to penetrate the
overburden of topsoil, keying into the underlying strata of virgin
clay.
c.
Permits: The developer shall be responsible for obtaining all construction
permits required by governmental agencies.
C.
Emergency Spillway: All detention facilities, whether publicly or
privately owned and maintained, shall include an emergency spillway.
1.
Virgin Ground: Emergency spillways shall, wherever feasible, be constructed
in virgin ground.
2.
Erosion Protection: Emergency spillways shall be protected from erosion
with approved surface paving or vertical cutoff walls appropriate
for the size of the impoundment and the height of the spillway crest.
3.
Capacity: Emergency spillways shall be sized for no less than 75%
of the maximum peak inflow into the basin.
D.
Dry Bottom Basins:
1.
Side slopes shall be a maximum of 4H:1V. If retaining walls are used,
their height is limited to four feet. Retaining wall design and material
type shall be approved by the City engineer.
2.
Dry bottom basins shall have 2% minimum bottom slopes.
3.
Dry bottom basins shall include a low flow channel with some form
of erosion protection, or a bypass sewer.
E.
Wet Bottom Basins:
1.
Above water side slopes shall be a maximum of 4H:1V extending to
two feet below normal water surface elevation. If retaining walls
are used, their height is limited to four feet. Retaining wall design
and material type shall be approved by the City engineer.
2.
At two feet below normal water level there shall be a five foot wide
(minimum) flat shelf. Subsequent underwater slopes shall be 3H:1V
(maximum) in public and residential areas and 2H:1V (maximum) in industrial
areas.
3.
Erosion protection shall be required at the water's edge on
all developments. The protection shall extend from a minimum of one
foot below to one foot above the normal water level.
4.
Wet bottom basin design shall include an evaluation of soil permeability.
A basin liner shall be included in the design if needed to ensure
water retention to normal pool elevation.
[Ord. 15-01-03-52, 1-19-2015]
All construction requirements shall conform to the applicable
sections of the "Standard Specifications For Road And Bridge Construction",
Illinois department of transportation.
The applicant will be responsible for all engineering costs
for design of storm sewers, drainageways, and detention facilities
within his/her development.
Detention basins may be maintained and owned by the applicant,
the lot owner, the developer, a homeowners' association, or by
the City. The decision on ownership will be made by the City of Le
Roy at the time of submission of the preliminary plan.
[Ord. 15-01-03-52, 1-19-2015]
Alternate methods of detention will be allowed pending review
of the City of Le Roy and its City engineer. The alternate methods
may be, but are not limited to, the following:
[Ord. 15-01-03-52, 1-19-2015]
The applicant shall pay to the City of Le Roy:
