[HISTORY: Adopted by the City Council of the City of Mendota
by Ord. No. 10-15-79. Amendments noted where applicable.]
This chapter shall be known and may be designated as the "City
of Mendota Stormwater Management Ordinance."
The purpose of this chapter is to provide for the orderly growth,
and harmonious development of the City by promoting the health, safety,
comfort, convenience and general welfare of City residents by establishing
uniform procedures and regulations for accommodating private drainage
outlets into the natural waterways and storm sewer systems within
the corporate limits of Mendota. The City has the responsibility to
maintain and preserve the integrity, operational safety and function
of the drainage facilities and systems within the City limits. The
property improvements of land frequently involve the outletting of
storm drainage into the City system affecting the appearance, safe
operation and maintenance of the City facilities. The acceptance of
such drainage must be regulated.
Any person, firm or corporation performing work within the City
limits must have permission from the City. Since work performed by
permittees could create serious problems, great care must be exercised
in specifying the drainage requirements in order that the City facilities
and drainage rights are not jeopardized. The City's objective in requiring a
permit prior to the beginning of such work is to regulate it from
the standpoint of public safety. The work to be permitted must conform
to City policy and procedures, design and construction standards.
All permits shall be carefully scrutinized to ensure that the City
is not required to expend funds for drainage corrections caused by
the permittee or become involved in possible court action.
The requirements of this chapter shall be limited to any construction
on any lot 1/2 acre or more in size, or any construction on any lot
whereby the total area of covered floor space and pavement shall exceed
4,000 square feet.
A City permit shall be required for all work involving stormwater management which is not covered under Chapter 265, Subdivision of Land, of the City Code. Two sets of drainage calculations shall be submitted with the permit application.
The following definitions shall apply in the interpretation
of this chapter:
The discharge for a recurrence interval of 50 years. It is
the flood which may be exceeded on the average once in 50 years.
A dry-bottom stormwater storage area which is designed to
be dry normally and which contains water only when excess stormwater
runoff occurs.
The rate of flow of water in a stream at a given place and
within a given period of time.
The stormwater runoff flow from the land which is not a part
of the natural watershed area at a specific location.
An area from which surface runoff is carried away by a single
drainage system; also called "watershed" or "drainage basin."
That portion of stormwater runoff which exceeds the conveyance
capacity of existing storm sewers or natural drainage channels serving
a specific watershed.
A channel cut in the ground surface to carry stormwater runoff
through a specific area.
The low lands that border a stream, usually dry but subject
to flooding.
The number of occurrences of a certain phenomenon in a given
period of time.
Areas within City limits plus 1 1/2 miles contiguous to the City of Mendota. (See Chapter 340, Zoning, Appendix III.)
The average interval of time within which a given flood will
be exceeded once; also called "return period."
The quantity of stormwater runoff that can be transported
by an existing channel or conduit without having the water surface
rise above the level of ground surface over the conduit or adjacent
to the waterway.
The rate at which stormwater runoff is released from dominant
to servient land.
Areas designated to store excess stormwater.
To ensure that the permitted work will not hinder the performance
of existing or future proposed City drainage facilities, create public
hazards, unnecessary maintenance responsibilities or result in litigation
against the City, it is essential that the following requirements
shall be met:
A.
Drainage structures shall be designed and constructed in accordance
with the Standard Specifications for Road and Bridge Construction
of the State of Illinois and any other approved City standards.
B.
Permitted construction shall satisfy the following conditions:
(1)
Stormwater detention shall be provided to store the excess stormwater
runoff from the developed property.
(2)
The stormwater runoff from the developed land which enters the City's
drainage system and/or the natural waterway through private land within
the corporate limits of the City shall not exceed the runoff rate
in its natural undeveloped state for ten-, fifty-, and one-hundred-year
frequencies, provided that the City's drainage system has the capacity
to accommodate stated frequency runoffs.
(3)
The maximum allowable release rate of stormwater from the developed
property shall not exceed the existing safe storm drainage capacity
of the natural downstream outlet channel and/or storm sewer system.
(4)
The outletting of diverted flow into the City's drainage system and/or
the natural waterway through private land within the corporate limits
of the City shall not be permitted. If a section of the land to be
developed is part of the natural watershed area of the City's drainage
system and the natural flow from this section eventually enters the
City's system at a downstream location, the developer may store the
flow from this section in a detention basin designed for the one-hundred-year
flood with an allowable release rate based only on that portion of
the drainage area which is naturally tributary.
(5)
Flow originating on developed property shall not be allowed to discharge
directly onto the street pavement by flowing over curbs or along entrance
ways. All flow shall be collected and discharged through an approved
system.
(6)
Connection to City sewers shall be made at a manhole or a catch basin
nearest to the developed site. Where no such structure exists, a structure
conforming to the City standards shall be built at the point of connection.
(7)
No sanitary sewage effluent from septic tanks or objectionable industrial
waste shall be discharged into the City's drainage system.
A.
Release rate. The release rate shall be based on the design discharge
of five-year storm frequency event for natural conditions and it shall
be an average value of the direct ratio of the developed land area
within the natural watershed to the total drainage area at the developed
property location, unless otherwise approved differently by the City
Engineer of the City of Mendota.
[Amended by Ord. No. 12-20-10B]
B.
Stormwater detention. The required volume of stormwater detention
shall be calculated on the basis of runoff from a fifty-year-frequency
rainfall. The capacity of outlet structures shall not exceed the allowable
release rate from the storage area and they shall be designed as simple
as possible and shall require little or no maintenance and/or attention
for proper operation. Each stormwater storage area shall be provided
with a method of emergency overflow in the event that a storm in excess
of the fifty-year frequency occurs. This emergency overflow facility
shall be designed to function without maintenance. A minimum of one
foot freeboard shall be provided above required capacity water surface
elevation to the top of the banks. Maximum depths of stormwater storage
shall not exceed 30 feet unless the existing natural ground contours
and other conditions lend themselves to greater depth, which shall
be approved by the City Engineer. The storage area bottom shall have
at least a slope of 3% toward the outlet structures. The sides of
the storage area shall have 4:1 or flatter slopes. Stormwater runoff
velocities shall be kept within allowable limits, and turbulent conditions
at an outfall control structure will not be permitted. The outlet
structures shall function with a positive gravity whenever possible.
When conditions necessitate use of pumps, an auxiliary pump in adequate
capacity shall be installed in addition to the main units, designed
to function automatically in case of failure of the main pumps. The
capacity of the pumps shall be based on the allowable release rate
of the storage area.
C.
Channel systems. A natural or surface channel system shall be designed
with adequate capacity to convey the stormwater runoff from all tributary
upstream areas through the development. This bypass channel shall
be designed to carry the peak rate of runoff from a fifty-year storm.
If the proposed development contains an existing natural waterway,
this land configuration shall be preserved as part of the bypass channel
system.
The following data shall be provided by the developer to enable
the City to properly specify the drainage requirements of the proposed
development:
A.
A USGS quadrangle or equivalent map showing proposed site to approximate
scale.
B.
A plan showing existing topography with two-foot-interval contours.
Existing drainage facilities and the general pattern of drainage adjacent
to the site shall also be shown.
C.
A plan showing proposed land development and proposed drainage facilities.
D.
Hydraulic calculations:
(1)
Computations showing the rate of flow for existing and proposed conditions
for ten-, fifty-, and one-hundred-year storms.
(2)
Calculations for allowable release rate and required storage volume
for fifty-year frequency.
(3)
Design of detention basin with following data:
