METHOD OF DETENTION

The following conditions and limitations shall be observed in selection and use of the method of detention:

5.4.1 GENERAL LOCATION

Detention facilities shall be located within the parcel limits of the project under consideration. No detention or ponding will be permitted within

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public road right-of-ways. Location of detention facilities immediately upstream or downstream of the project will be considered by special request if proper documentation is submitted with reference to practicality, feasibility, and proof of ownership or right-of-use of the area proposed.

Conditions for general location of detention facilities are identified in the following sections.

5.4.2 DRY RESERVOIRS

Wet weather ponds or dry reservoirs shall be designed with proper safety, stability, and ease of maintenance facilities, and shall not exceed eight (8) feet in depth for the 100-year event. Maximum side slopes for grass reservoirs shall not exceed one (1) foot vertical for four (4) feet horizontal (4:1) unless adequate measures are included to provide for the above noted features. In no case shall the limits of 100-year ponding elevation be closer than twenty (20) feet horizontally and less than two (2) feet vertically below the lowest sill or floor elevation for any structures within 100 feet. Pond bottoms shall slope a minimum of 1% from the pond inlet to the pond outlet. The entire reservoir area shall be sodded as required prior to final plat approval or issuance of certificate of occupancy. The reservoir area shall include bottom, all side slopes (interior and exterior), and top of berm/embankment. Any area susceptible to, or designed as, overflow by higher design intensity rainfall shall be sodded or paved depending upon the outflow velocity. Concrete trickle channel will not be allowed unless specific approval is granted by variance by the City.

5.4.3 DETENTION IN OPEN CHANNELS

Open channels may be used as detention areas provided that the limits of the 100-year ponding elevation are not closer than twenty (20) feet horizontally and not less than two (2) feet below the lowest sill or floor elevation of any building within 100 feet. No ponding will be permitted within public road right-of-way unless approval is given in writing by the City Engineer. Maximum depth of detention in open channels shall be four (4) feet. Minimum flow line grade shall be 1.0 percent for grass or untreated bottoms or 0.5 percent for paved channels.

For trapezoidal sections, the maximum side slopes of the channel used for detention shall not exceed one (1) foot vertical for four (4) feet horizontal (4:1). For design of other typical channel sections, the features of safety, stability, and ease of maintenance shall be observed by the Design Engineer.

Unless concrete lined, the entire reservoir area of the open channel shall be sodded as required in the original design. The hydraulic or water

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surface elevations resulting from channel detention shall not adversely affect adjoining properties.

5.4.4 PERMANENT LAKES OR RETENTION PONDS/WET PONDS

Permanent lakes or (retention ponds) with fluctuating volume controls may be used as detention areas upon specific approval by the Planning Commission and provided that the limits of 100-year ponding elevations are no less than two (2) feet below the lowest sill or floor elevation of any building and, the minimum normal pool depth shall be your (4) feet.

Maximum side slopes for the fluctuating area of permanent lakes shall be one (1) foot vertical to four (4) feet horizontal (4:1) unless provisions are included for safety, stability, and ease of maintenance.

Special consideration such as fencing, shoreline slope, depth of water, and abrupt change of grade in inundated areas of two (2) feet or more depth, etc., shall be given to the safety of children and the general public in design of permanent lakes in residential areas.

The entire fluctuating area of the permanent reservoir shall be sodded.

Also, calculations must be provided to ensure adequate "live storage" is provided for the 100 year storm. Any area susceptible to or designed as overflow by higher design intensity rainfall (100-year frequency) shall be sodded or paved, depending on the design velocities. An analysis shall be furnished of any proposed earthen dam construction soil. A boring of the foundation for the earthen dam may be requested by the City Engineer. Earthen dam structures shall be designed by a Professional Engineer.

Aeration devices are required for lakes/ponds less than 10-acre water surface area.

5.4.5 PARKING LOTS

Detention is permitted in parking lots to maximum depths of 12 inches. In no case should the 100-year of ponding limits be designed closer than twenty (20) feet from a building.

The 100-year ponding elevation shall be 1’ below the lowest sill or floor elevation for the 100 year event.

5.4.6 OTHER METHODS

Underground detention is acceptable if designed conditions warrant.

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Parking lots and associated curb and gutter is to serve as the overflow and/or freeboard area. Freeboard is used to account for issues such as sedimentation, clogged discharge, factor of safety, etc. If a parking lot or overflow area is available on site, there is no need for additional storage.

If a parking lot or overflow area is not available on site, and/or any overflow would immediately discharge to adjacent property, extra storage/volume should be provided (designed for 105%).

Other methods of detention such as seepage pits, French drains, etc., are discouraged. If other methods are proposed, proper documentation of soil data, percolation, geological features, etc., will be needed for review and consideration.

5.4.7 VERIFICATION OF ADEQUACY

All detention facilities shall be approved by the City prior to or at the time of approval of the Final Plat for a subdivision or before issuance of certificate of occupancy for a commercial or industrial development, in accordance with the following procedures.

1. Developer shall have a survey conducted, by a licensed surveyor, of the final as-built condition of the detention pond and outlet structure.

Developer must notify the City 48 hours in advance of when this survey will be conducted, to allow the City to schedule observation. A representative of the City must be present when the survey is being performed. A survey conducted without prior notice to the City, or without the City being present, will not be accepted.

2. Developer shall submit a written request to the City for approval of the detention facility. This request shall include the following:

a. A certification by the Engineer of Record stating that the detention facility has been constructed in conformance with the approved design, and in accordance with the City of Centerton Drainage Design Manual.

b. Copy of the as-built survey, unless submitted under separate cover (indicate if so submitted).

c. Revised detention facility calculations that reflect actual detention pond and outlet structure constructed.

d. A check for any fees required by the City associated with City observing pond survey, review of pond survey, and review of revised detention facility calculations.

e. A statement that the developer understands the developer will be responsible for maintenance of the detention facility until such time as the maintenance will be assumed by the property owners or the POA.

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f. Covenants for subdivisions shall be submitted to and approved by the City, and shall include a statement providing for ownership and maintenance of detention facilities and other private drainage systems.

3. The City will review the as-built survey, and revised detention facility calculations and compare it to the approved design, inspect the detention pond, and inspect the development area that contributes to the detention pond.

4. Final approval of the project will not be granted until City staff concurs with the Engineer of Record’s conclusion that the detention facility is in compliance with the approved design and requirements of this Drainage Manual. The City will issue a written notice that the pond is in compliance with the approved design and this Drainage Manual and noting any variances granted.

5. City reserves the right to use City staff or hire outside consultants to fulfill its responsibilities under this section.

5.4.8 CONTROL STRUCTURES

Detention facilities shall be provided with effective control structures. To the extent possible, designer shall avoid detention basin outlet systems that direct outfall to a single point, and provide outlet systems that

“spread” flow over a wide area, thus minimizing erosion and adverse effects. Control structures shall be located a sufficient distance from adjoining property so that stormwater discharge does not adversely affect property downstream. Plan view and sections of the structure with adequate details shall be included in Plans.

The structure selected shall have documented evidence that it will control the 2, 5, 10, 25, 50, and 100 year storms. Small diameter pipes or orifices that are susceptible to clogging shall be protected with a trash rack or screen.

The overflow opening or spillway shall be designed to accept the total peak runoff of the improved tributary area. Conveyance for any off-site drainage shall also be provided.

Construction Requirements A. Timing of Construction

Normally, the detention pond and outlet structure will be the first item

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constructed on the project. This is recognizing that land disturbing activities result in an increase in storm water run-off that can be detrimental to downstream life and property. The detention pond must be fully or partially completed to control the increased peak run-off from the disturbed area.

B. Relationship to Erosion Control

The detention pond may be used to assist in erosion control. Separate erosion control systems shall be included in the construction to minimize the collection of sediment in the detention pond which would serve to reduce detention capacity.

If sediment does accumulate in the detention pond during construction, the developer or contractor will clean the pond as often as necessary to maintain its required detention capacity.

Erosion control is covered by separate regulations.

C. Progressive Enlargement

A detention pond may be designed to be progressively enlarged as construction progresses and as peak storm water run-off increases.

The drainage report shall indicate if this is intended, and include calculations to demonstrate that a detention pond progressively enlarged will adequately reduce construction phase peak run-off to the level of pre-developed peak run-off. Calculations shall be for the specific pond volume and outlet structure proposed.

D. Miscellaneous Items

The detention pond shall be finished as soon as it is constructed to final lines and grades, either in part or in whole, with sod or hardened surface in accordance with the approved design.

5.4.9 DISCHARGE SYSTEMS

For site-specific runoff, the effectiveness of local detention structures can be acknowledged in the design of any on-site downstream drainage facilities assuming that the detention facilities comply with all criteria and that they are properly constructed and maintained.

In the case of regional detention basins, sizing of the system below the control structure shall be for the total improved peak runoff tributary to the structure with no allowance for detention unless approved in writing by the

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