[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:
- 50-YEAR DISCHARGE
- The discharge for a recurrence interval of 50 years. It is the flood which may be exceeded on the average once in 50 years.
- DETENTION BASIN
- 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.
- DIVERTED FLOW
- The stormwater runoff flow from the land which is not a part of the natural watershed area at a specific location.
- DRAINAGE AREA
- An area from which surface runoff is carried away by a single drainage system; also called "watershed" or "drainage basin."
- EXCESS STORMWATER
- That portion of stormwater runoff which exceeds the conveyance capacity of existing storm sewers or natural drainage channels serving a specific watershed.
- FLOOD BYPASS CHANNEL
- A channel cut in the ground surface to carry stormwater runoff through a specific area.
- FLOODPLAIN 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.)
- RECURRENCE INTERVAL
- The average interval of time within which a given flood will be exceeded once; also called "return period."
- SAFE STORM DRAINAGE CAPACITY
- 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.
- STORMWATER RUNOFF RELEASE RATE
- The rate at which stormwater runoff is released from dominant to servient land.
- STORMWATER STORAGE AREA
- 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:
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.
Permitted construction shall satisfy the following conditions:
Stormwater detention shall be provided to store the excess stormwater runoff from the developed property.
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.
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.
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.
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.
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.
No sanitary sewage effluent from septic tanks or objectionable industrial waste shall be discharged into the City's drainage system.
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]
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.
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 USGS quadrangle or equivalent map showing proposed site to approximate scale.
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.
A plan showing proposed land development and proposed drainage facilities.
Computations showing the rate of flow for existing and proposed conditions for ten-, fifty-, and one-hundred-year storms.
Calculations for allowable release rate and required storage volume for fifty-year frequency.
Design of detention basin with following data:
If a natural waterway is preserved through the proposed development: