The purpose of these regulations is to diminish threats to public health, safety, and welfare caused by runoff of excessive storm water from new development and redevelopment. This excessive storm water could result in the flooding of damageable properties. The cause of increase in storm water runoff quantity is the development and improvement of land and, as such, this section regulates these activities to reduce adverse impacts.
No building permit shall be issued for the construction, reconstruction or structural alteration of a building, and/or its parking area nor shall a certificate of occupancy be granted for a use nor shall a preliminary plat, final plat or site plan be approved without conformity with the provisions of the storm water detention regulations.
C. Storm water Detention Criteria:
1. All storm water storage areas must be designed to contain and safely pass storm water runoff. When platting, the land extending to the centerline of any adjacent right-of-way shall be considered part of the development for purposes of determining the runoff rate. Adequate spillway provisions shall be provided to transport peak runoff from a 100-year storm assuming the outlet to be plugged. All storm water facilities shall be designed in accordance with the design standards established by the city.
2. All areas within the City of Eldridge shall comply with these regulations.
a. The required volume of storm water detention shall be that necessary to handle the runoff of a one hundred-year rainfall, for the critical duration from the drainage area tributary to the storm water storage area based on full development of said tributary area, less the volume discharged during the same duration at the approved release rate. In addition, the detention basin shall be designed to restrict the peak rate of discharge from the drainage area during a two-year, 24-hour rainfall event to be at or below the predevelopment levels.
b. For sites less than twenty acres, the required detention storage (RDS) will be that found to be the most critical resulting from the inflow from the run-off of a fully developed tributary area from a one hundred-year storm and the outflow of the five-year storm on a predevelopment basis with the same area. This can be expressed by the following equation:
c. For sites greater than twenty acres, the method of sizing of the detention storage volume must be approved by the city engineer.
3. Detention storage may be provided as a dry bottom or wet bottom storage area.
a. Dry bottom storm water storage areas may be designed to serve a secondary purpose for recreation, open space parking, or other types of uses that will not be adversely affected by intermittent flooding.
1) A method of carrying the low flow through these areas shall be provided in addition to a system of drains to prevent soggy areas.
2) Outlet control structures shall be designed as simply as possible and shall require little or no attention for proper operation. Each storm water storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the one hundred-year frequency storm occurs. This emergency overflow facility shall be designed to function without attention and shall become part of the excess storm water passageway. Hydraulic calculations shall be submitted to the city engineer to substantiate all design features.
3) Both outlet control structures and emergency overflow facilities shall be designed and constructed to fully protect the public health, safety and welfare. Storm water runoff velocities shall be kept at a minimum and turbulent conditions at an outlet control structure will not be permitted without complete protection for the public safety. The use of fences shall be kept to a minimum and used only as a last resort when no other method of protection is feasible.
4) Paved surfaces that are to serve as storm water storage shall be designed with permanent-type control outlets. Emergency overflow areas shall be provided. The maximum storm water ponding depth in any paved area shall not exceed six (6) inches.
b. Wet bottom storm water storage areas shall be designed with all of the items required for dry bottom storm water storage areas, except that the provisions of subsection 3.a.1. of this section shall not be required. However, the following additional conditions shall be complied with:
1) Water surface areas shall not exceed one-fifteenth of the tributary drainage area.
2) Facilities shall be provided to lower the pond elevation by gravity flow for cleaning purposes and shoreline maintenance. Shoreline protection shall be provided to prevent erosion from wave action.
3) Minimum normal water depth shall be four feet. If fish are to be maintained, some portion of the pond area should be a minimum of nine feet deep.
4) Control structures for storm water release shall be designed to operate with only a minor increase in the water surface level when the design storage capacity has been exceed. Hydraulic calculations shall be submitted to the city engineer to substantiate all design features.
4. Infiltration Practices. The city may in lieu of the detention requirements detailed in the ordinance accept infiltration practices designed in accordance with the Iowa Storm Water Management Manual. Engineering calculation demonstrating infiltration rates shall be included with the application.
The following shall be exempt from these regulations:
1. Any final plat approved by January 1, 2003, and based upon a preliminary plat that was approved prior to January 1, 2000, subject to the development not being required to provide detention facilities by any other ordinances of the city.
2. A preliminary or final plat of single-family development consisting of lots, all of which are one acre or larger.
3. Any single-family and two-family developed and undeveloped lots which existed on January 1, 2000.
E. Storm water Detention Plan Review:
Storm water detention plans shall be submitted to the City Engineer for review.
1. For new or expanding commercial, industrial, and multi-family developments, a storm water detention plan shall be submitted as part of the site plan review.
2. For new single-family and two-family residential developments, a storm water detention plan shall be submitted as part of the plat review.
3. If no plat or site plan review is required, a storm water detention plan must be approved by the city engineer before a building permit is issued.
F. Storm water Detention Plan Requirements:
Each applicant shall submit all calculations and other such information as deemed necessary to demonstrate to the City Engineer the storm water runoff rate and capacity of storm water storage facilities. Required information may vary according to the size of development, but shall include, but not be limited to, the following:
1. Map of the property and immediate vicinity showing elevation of contours (based on city datum).
2. Mapping and description of existing and proposed drainage system features of the property and immediate vicinity.
3. Physical features of the property and immediate vicinity including, but not limited to the state and type of development in the area, public infrastructure and other similar characteristics.
4. If the development is to be staged, a plan of how the storm water detention requirement will be met for each stage.
G. Common Detention Facilities:
The city may, in lieu of on-site detention facilities, accept the furnishing of off-site facilities which, when added to on-site detention facilities shall provide sufficient storage capacity to provide the storm water detention required to meet this regulation. Until such time as the permanent off-site storm water detention facilities are available, temporary on-site detention shall be provided. An acceptable legal contract between the developer and the off-site land owner shall be approved by the city council.
H. Completion of Facilities:
The storm water detention facilities shall be constructed and in use according to the following criteria:
1. For commercial, industrial and multi-family developments, the facilities shall be constructed concurrent with site grading. A certificate of completion shall be furnished by the developer’s engineer certifying that they have been constructed according to the approved plans before a certificate of occupancy is issued by the city.
2. For single-family and two-family residential building, the developer’s engineer shall certify that the structural improvements have been substantially complete and the detention facility rough graded before a building permit is issued. A certificate of completion shall be furnished by the engineer certifying that the facilities into which the lot(s) drain have been constructed according to the approved plans before a certificate of occupancy permit is issued by the city.
1. The developer shall be responsible for providing a plan of ownership of the detention facilities involving a single owner, an association, or another plan of ownership approved by the city.
2. The owner shall be responsible to insure the continuing functioning of the facility as originally designed and intended. All plans submitted for storm water detention systems shall describe an adequate procedure for normal maintenance for the detention system. Any failure of the storm water detention system due to inadequate normal or capital maintenance shall be the responsibility of the owner of the property on which the detention system is located. It shall also be the property owner’s responsibility to remedy any negligence in maintenance that resulted in the failure of the system. The submittal of plans for such a system or the purchase of property on which such system is located shall be deemed as acceptance of responsibility for normal and capital maintenance of the system.
3. The developer shall dedicate an easement to the city reserving the land for use as a storm water detention facility providing the city with the right to inspect the facility, and the necessary easements for ingress and egress.
a. The detention basin owner shall be responsible for the following items:
1) An annual report on the detention basin condition, using a checklist developed by the City, shall be submitted to the director of public works.
2) At five (5) year intervals, the basin shall be inspected by a professional engineer registered in the state of Iowa, using a checklist developed by the City . A report of this inspection shall be submitted to the director of public works within sixty (60) days of the inspection. An annual report is not required the year when the five (5) year report is due.
3) Detention basin owners shall notify subsequent owners of their maintenance responsibilities and transfer basin maintenance records to the party with active maintenance responsibility.
b. At any time the director of public works, or his designee, may provide inspection of the detention facilities as deemed appropriate. If the director finds any maintenance work is necessary, the director shall serve a written order to the owner of the facilities, specifying therein the work necessary to be done and providing for a reasonable time for its completion. Any property owner to whom an order is directed shall have the right, within three (3) days from service of the order, to appeal to the city administrator who shall review the order within five (5) working days and file his decision. If the city administrator fails to respond within five (5) working days, it shall be assumed that the appeal is denied. Unless the order is revoked or modified, it shall remain in full force and be obeyed by the owner as directed within the time established by the director's written order or at the time as modified by the city administrator's decision. When an owner to whom an order has been issued fails to comply within the time specified, the director shall remedy the condition, or contract with others for such purpose, and charge all costs, including administration, to whom the order is directed. If the cost of remedying a condition is not paid within thirty (30) days after the mailing of a statement from the director of public works, the cost shall be assessed against the property for collection in the same manner as a property tax. In the event the association fails or is unable to pay the costs associated with detention facility maintenance, these costs shall be assessed against those users of the facility based upon their percentage of use.
The city council may, in the resolution approving the preliminary plat of any subdivision and upon appeal of the developer, or the recommendation of the city engineer, waive or change the requirements of this ordinance for a particular development whenever the developer demonstrates either:
1. That, due to the location of the property, strict compliance would provide no discernible benefit for reduction of flooding on any plat adjacent or downstream property, or
2. That, due to the nature of the property, strict compliance would create unusual hardship out of proportion to any foreseeable benefit and that the requested change or reduction in requirements will not increase the risk of flooding of any building or the buildable portion of any lot on any adjacent or downstream property, or
3. That the developer, in lieu of a detention facility, provide storm water management sufficient to achieve the purpose of this ordinance by keeping storm water runoff from the property in accordance with paragraph C entitled, “Storm water Detention Criteria.”
Developers of new or expanding commercial, industrial and multi-family developments shall make appeals as per the provisions of the site plan review regulations in city code.