[R.O. 1993 § 545.130; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Stormwater runoff and the velocity of discharge are considerably increased through development and growth of the City.
Prior to the development of the land, surface conditions provide a higher percentage of permeability and longer time of concentration. With the construction of buildings, parking lots, etc., permeability and the time of concentrations are significantly decreased, resulting in an increase in both the rate and volume of runoff.
These modifications may create harmful effects on properties downstream. Therefore, to minimize these effects, the following minimum stormwater detention requirements have been established.
[R.O. 1993 § 545.140; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
A complete set of stormwater detention plans and calculations shall be provided for all construction projects that increase stormwater runoff.
Plans for the construction of public improvements shall be submitted to and approved by the Building Official.
The required stormwater detention plans for private improvements shall be submitted, along with the building plans, to the Building Official for review and approval. An escrow for stormwater detention will not be accepted.
[R.O. 1993 § 545.150; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Differential runoff evaluation consists of the determination of the rates of runoff, before and after development, determination of required volume of detention and verification of adequacy of discharge and control structures. The one-hundred-year (frequency) runoff coefficients shall be used. Differential runoff rates shall be evaluated by equation:
[R.O. 1993 § 545.160; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Volume of detention for areas less than twenty (20) acres shall be evaluated according to the "Simplified Volume Formula." For areas larger than twenty (20) acres, other methods may be used upon the approval of the City Manager and/or Planning and Zoning Board. The "Rational Formula" method will generally be accepted for areas greater than twenty (20) acres.
[R.O. 1993 § 545.170; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
The following conditions and limitations shall be observed in the selection and use of method of detention:
General Location. Detention facilities shall be located within the parcel limits of the project under consideration, with the following exceptions:
No detention or ponding will be permitted within public road rights-of-way without specific written approval of the Superintendent of Utilities.
Location of detention facilities immediately downstream of the project will be considered by special request if proper documentation is submitted with reference to practicality, feasibility, proof of ownership or right-of-use of the area proposed, and provisions are made for perpetual maintenance.
Dry Reservoirs. Wet-weather ponds or dry reservoirs shall be designed with proper safety, stability and ease of maintenance features. Maximum side slopes for grassed reservoirs shall not exceed one (1) foot vertical for three (3) horizontal (3:1). In no case shall the limits of maximum ponding elevation be less than two (2) feet vertically below the lowest sill elevation, nor should the maximum limits of ponding be designed closer than ten (10) feet from a building unless waterproofing of the building and pedestrian accessibility are properly mulched, sodded or paved. A minimum of one (1) foot of freeboard is required above the spillway. The outlet structure shall be concrete or other equivalent material. Spillway areas shall be paved with a minimum of six (6) inches of concrete.
Normally permitted open channels may be used as detention areas, provided that the limits of the maximum ponding elevation are not closer than thirty (30) feet horizontally from any buildings with habitable areas below ground level and less than two (2) feet below the lowest sill elevation of any building. In no case should the maximum limits of ponding be designed closer than ten (10) feet from a building unless waterproofing of the building and pedestrian accessibility are properly documented. No ponding will be permitted within public rights-of-way without specific written approval of the Building Official. Maximum depth of detention in open channels shall be four (4) feet. Minimum flow-line grade shall be one-half of one percent (0.5%).
For trapezoidal sections, the maximum side slopes of the detention area of the channel shall not exceed one-foot vertical for three (3) horizontal (3:1). For design of other typical channel sections, the features of safety, stability and ease of maintenance shall be observed.
The entire reservoir area of the open channel shall be seeded, fertilized and mulched, sodded or paved.
The hydraulic elevations resulting from channel detention shall not adversely affect adjoining properties.
Permanent lakes with fluctuating volume controls may be used as detention areas, provided that the limits of maximum ponding elevations are no closer than thirty (30) feet horizontally from any building and less than two (2) feet below the lowest sill elevation of any building.
Maximum side slopes for the fluctuating area of permanent lakes shall be one (1) foot vertical to three (3) feet horizontal (3:1) unless proper provisions are included for safety, stability, and ease of maintenance.
Maximum fluctuation from permanent pool elevation to maximum ponding elevation shall be three (3) feet.
The entire fluctuating area of the permanent reservoir shall be seeded and fertilized and mulched, or sodded or concrete paved. Any area susceptible to or designed as overflow shall be paved with concrete.
Detention will not be permitted in primary parking lots. The primary parking lot will be considered to be the most accessible eighty percent (80%) of total parking for a facility.
In non-primary parking lots, detention will be permitted to a maximum depth of twelve (12) inches.
In no case should the maximum limits of ponding be designed closer than ten (10) feet from a building unless waterproofing of the building and pedestrian accessibility are properly documented.
When detention is being effected on parking lots by means of retaining walls or curbs, these retaining walls and curbs must be constructed of reinforced concrete.
The minimum freeboard from the maximum ponding elevation to the lowest sill elevation shall be one (1) foot.
Other Methods. Other methods of detention, such as seepage pits, french drains, etc., will not be approved.
Retention Areas. In drainage basins where sinkholes provide the only outlet for storm sewer runoff, and those basins are greater than three hundred twenty (320) acres in area, a retention area capable of storing the first inch of runoff is required for and development within the basin. It shall have a four-inch outlet pipe to insure eventual discharge.
[R.O. 1993 § 545.180; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Analysis of all elements of design shall be performed by a professional engineer licensed in the State of Missouri. The following outline is provided to ascertain that certain critical elements of design are in workable compliance to the aims of design.
Routing calculations shall be submitted in legible tabulated form. Proof of adequacy of the volume of detention and sizing computations for low-flow structure shall also be submitted. Features of stability and safety will also need to be documented if the scope of the project requires special attention in this area of design.
Spot elevations shall be included in sufficient detail on the site plan so that the final direction of water flow can be determined, and so that the volume of detention can be ascertained. Projects over two hundred (200) acres in area shall provide documented verification of adequacy according to scope and complexity of design.
[R.O. 1993 § 545.190; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Detention facilities shall be provided with obvious and effective outlet control structures. These outlet structures may include v-notch weirs or rectangular weirs, as well as pipe. Plan view and sections of the structure with adequate detail shall be included in plans.
The design discharge (Q) for the low-flow outlet shall not exceed the existing runoff for the one-year storm. The maximum discharge shall be designed to take place under total anticipated design-head conditions. The design-head storage volume is not to be considered as part of the volume of detention required.
Sizing of a low-flow pipe shall be by inlet control.
Low-flow pipes shall not be smaller than four (4) inches in diameter to minimize maintenance and operating problems, except in parking lot and roof detention, where minimum size and configuration of opening shall be designed specifically for each condition. The low-flow pipe shall be provided with a bar-screen on a minimum 2:1 slope to reduce blockage by debris.
Overflow spillways will be required on all detention facilities that have storage volumes of one thousand (1,000) or more cubic feet.
The overflow opening or spillway shall be designed so that the combination flow of the low flow outlet and the flow over the spillway will not exceed the total peak runoff for the improved area. The total peak runoff is to be determined from a twenty-five-year frequency rain for drainage areas less than one (1.0) square mile and from a one-hundred-year frequency rain for drainage areas one (1.0) square mile or greater.
The overflow spillway shall exit into a natural or improved drainageway. If the drainageway does not provide for public access, then topographic detail, along with a profile of the center line of the drainageway, shall be provided from the overflow spillway to the point of public access. This detail shall show all topography within ten (10) feet of the center line of the drainageway, center line profile, typical cross section and capacity of the drainageway.
If the capacity of the existing drainageway is inadequate to carry the total peak runoff, necessary improvements to the drainageway may be required to provide for the total peak runoff.
[R.O. 1993 § 545.200; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Two (2) types of easements shall be provided for stormwater detention.
Access Easement. All detention reservoirs, with the exception of parking lot and roof detention, shall be enclosed by an access easement. The limits of the easement shall extend ten (10) feet beyond the maximum anticipated ponding area. The limits and designation of detention facilities shall be shown on the project plans of final plat.
Drainage Easement. A minimum ten (10) feet wide drainage easement shall be provided within the reservoir area, connecting the tributary pipes and the discharge system, along the most direct possible routing of a piping system for possible future elimination of detention. The limits of the drainage easement shall be shown on the project plans of final plat.
[R.O. 1993 § 545.210; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Detention facilities are to be built in conjunction with the storm sewer installation and/or grading. Since these facilities are intended to control increased runoff, they must be fully operational soon after the clearing of the vegetation. Silt and debris connected with early construction shall be removed periodically from the detention area and control structure in order to maintain full storage capacity.
[R.O. 1993 § 545.220; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
It may be advantageous in some situations to delay the building of the permanent detention facilities until after the completion of the other improvements. In these situations, temporary detention facilities must be provided. The permanent or temporary detention facilities must be constructed and be functional before proceeding with any other construction.
[R.O. 1993 § 545.230; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Off-Site Detention. Stormwater detention facilities designed and constructed off site or outside the limits of the proposed development will be considered for approval. This approval is contingent upon documentation being furnished to verify that drainage easements have been obtained for the channel area from the proposed development to the detention facility and including the detention area. The drainage easements must clearly set out provisions for maintenance.
Detention facilities designed and located to provide detention on major drainage channels will be considered as a regional detention facility and must be one hundred sixty (160) acres or greater. The facility must provide a detention volume for a one-hundred-year storm for the entire drainage area and must be designed with a variable control outlet structure that has one (1) year maximum outlet to opening. The regional detention facility must be designed with a low flow concrete channel through the limits of the basin. Upon conceptual approval of the location and final approval of the design and construction, the City of Aurora may accept the responsibility for the maintenance of the regional facility. Drainage and access easements will be required giving the City of Aurora the authority to gain vehicular access to the facility from the public street.
As additional development occurs upstream of the regional facility, payment in lieu of constructing detention may be allowed, provided the regional detention basin has been designed for full development of the basin, or if modifications are made to the regional facility by the developer to provide for the additional volume of detention required for the new development. Easements must be provided along the drainage channel from the new development to the regional facility and the channel must be constructed to carry the peak rate of runoff from the one-hundred-year storm for the entire basin upstream from the regional facility.
[R.O. 1993 § 545.235; Ord. No. 2006-2722 § 1, 5-9-2006]
Notwithstanding any other provisions of Article III, if development upon a site requires stormwater detention and if during the design of the required stormwater improvements a determination is made that channelization or other improvements can be shown to be a viable alternative to detention, then the developer may make application to the City to participate in an alternative program consisting of channelization and payment in lieu of a detection facility.
If payment in lieu of detention is approved, it is still the developer's responsibility to obtain design approval from the Planning and Zoning Director, construct any required drainage improvements to collect and convey additional or concentrated stormwater flow from the site to an existing watercourse, drainage easement, storm sewer and to procure any necessary easements.
The City will evaluate each written request for payment in lieu of detention based on the following criteria.
Size of the site in relation to the stormwater generated.
Size of the site in relation to the size of the drainage area and location in the drainage area.
Impact on the properties downstream of the site.
Location of the site with respect to floodplains, streams and large watercourses.
Location of the site with respect to environmentally sensitive areas such as sinkholes.
Approval of a payment in lieu of detention is not automatic and will be considered on a case-by-case basis upon written request to the Planning and Zoning Director. The request shall include a letter signed by the developer and submittals prepared by a registered professional engineer showing calculations of the normally required stormwater detention basin volume and proposed stormwater drainage conveyance from the site to an existing watercourse, drainage easement or storm sewer. Evaluation of the request will be made by the Planning and Zoning Director and by the City Engineer.
The amount of payment in lieu of detention shall be based upon the agreed upon volume calculated for detention resulting from the proposed development improvements, in the case of non-agreement the calculations of the City Engineer shall prevail. When the development is a vacant land subdivision and is zoned residential and consists of two (2) or more lots, then the calculations of the impervious area per lot shall use three thousand five hundred (3,500) square feet for single-family zoned lots and four thousand five hundred (4,500) square feet for duplex and patio home zoned lots. When the development is a vacant land subdivision or site zoned for multi-family, commercial, office or industrial use, the calculations of the impervious area shall be determined based upon the maximum impervious areas allowed by zoning for the development. The actual dollar value of the payment in lieu of detention shall be determined using three dollars ($3.00) per cubic foot of calculated detention volume for the first one thousand (1,000) cubic feet plus two dollars ($2.00) per cubic foot of detention volume required above one thousand (1,000) cubic feet.
Payment in lieu of detention funds may be used by the City for any drainage improvements at any location within the corporate limits of the City.
An agreement for payment in lieu of detention shall be fully executed by the City and developer and the payment shall be received by the City before the pertinent building permit can be issued or before the pertinent final plat is recorded.
The order of appeal from this decision by the Planning and Zoning Director as stated in this Section 545.250 and the applicable parts contained therein shall be from the decision of the Planning and Zoning Director directly into the Circuit Court of Lawrence County, Missouri. Administrative rules of the State of Missouri shall apply.