§ VII. Detention storage.
Frequently, undeveloped upper reaches of watersheds can use detention facilities to correct runoff problems or to restrict runoff from development to that which existed prior to the change in land use. Because of the proximity of future development in the city to receiving streams and the general lack of suitable sites, on-site detention storage will probably find limited use.
When desired by the developer or as required by the city to control runoff, detention storage facilities shall be designed by a registered professional engineer experienced in such design. Prior to design of detention facilities, the developer and his engineer shall consult with the city engineer who will stipulate parameters to be used in establishing the allowable release rate and review siting of the facility.
The list of references in this manual contains several references covering design of detention facilities. These and other references covering state-of-the-art design should be used as appropriate.
Where appropriate, state and federal laws pertaining to dams shall control and shall take precedence to the extent that the detention facilities may be classified as "dams" thereunder.
Requirements covering submittal of design information on detention facilities are included in the next section of this manual titled Design Review.
A.
General provisions.
1.
Detention/retention facilities shall have 1,000 acres or less area tributary to the facility.
2.
Dams which are greater than ten feet in height but do not fall into State or Federal requirement categories shall be designed in accordance with the latest edition of SCS Technical Release No. 60, "Earth Dams and Reservoirs," as Class "C" Structures.
3.
All lake and pond developments must conform to local, state, and federal regulations. Legal definitions and regulations for dams and reservoirs can be found in the Missouri Code of State Regulations, Division 22.
B.
Release rate. The maximum release rate from any development shall be the existing undeveloped rate for the 100-year storm.
C.
Detention basin size. Owners/engineers may utilize methodology outlined in (SCS, 1986). A Type II rainfall distribution shall be the required storm hyetograph. Maximum detention storage shall be based upon the allowable release rate and upon the developed condition for the site. Basin volume shall be sized for the 100-year storm.
D.
Primary outlet works. The primary outlet shall be designed to meet the following requirements:
1.
The outlet shall be designed to function without requiring attendance or operation of any kind or requiring use of equipment or tools, or any mechanical devices.
2.
All discharge from the detention facility when inflow is equal to or less than the 100-year inflow shall be via the primary outlet.
3.
The design discharge rate via the outlet shall continuously increase with increasing head and shall have hydraulic characteristics similar to weirs, orifices or pipes.
4.
For dry detention basins, the design shall allow for discharge of at least 80 percent of the detention storage volume within 24 hours after the peak or center of mass of the inflow has entered the detention basis.
5.
Ponds shall be designed with a nonclogging outlet such as a reverse-slope pipe, or a weir outlet. A reverse-slope pipe draws from below the permanent pool extending in a reverse angle up to the riser and establishes the water elevation of the permanent pool. Because these outlets draw water from below the level of the permanent pool, they are less likely to be clogged by floating debris.
6.
No orifice shall be less than three inches in diameter. (Smaller orifices are more susceptible to clogging.)
E.
Emergency spillways. The emergency spillway may either be combined with the outlet works or be a separate structure or channel meeting the following criteria:
1.
Elevation: Emergency spillways shall be designed so that their crest elevation is 0.5 feet or more above the maximum water surface elevation in the detention facility attained by the 100-year storm.
2.
Capacity: In cases where the impoundment/emergency spillway is not regulated by either state or federal agencies, the emergency spillway shall be designed to pass the 100-year storm with one foot of freeboard from the design state to the top of dam, assuming zero available storage in the basin and zero flow through the primary outlet. This design provides an added level of protection in the event of a clogged primary outlet or a subsequent 100-year storm event that occurs before the flood pool from the initial storm event recedes to the principal outlet elevation.
F.
Wet bottom basins/retention facility. For basins designed with permanent pools:
1.
Minimum depth: The minimum normal depth of water before the introduction of excess stormwater shall be four feet plus a sedimentation allowance of not less than five years accumulation.
2.
Depth for fish: If the pond is to contain fish, at least one-quarter of the area of the permanent pool must have a minimum depth of ten feet plus sedimentation allowance.
3.
Side slopes: The side slopes shall conform as closely as possible to regarded or natural land contours, and should not exceed three horizontal to one vertical. Slopes exceeding this limit shall require erosion control and safety measures and a geotechnical analysis.
G.
Dry bottom basins/detention facility. For basins designed to be normally dry:
1.
Interior drainage: Provisions must be incorporated to facilitate interior drainage to outlet structures. Grades for drainage facilities shall not be less than two percent on turf. Concrete swales, with a minimum gradient of one percent, may be used as needed to conduct stormwater from turfed bottom areas to the outlet structure.
2.
Side slopes: The side slopes of dry ponds should be relatively flat to reduce safety risks and to help to lengthen the effective flow path. Slopes shall not be steeper than three horizontal to one vertical.
H.
Other storage. All or a portion of the detention storage may also be provided in underground or surface detention areas, including, but not limited to, oversized storm sewers, vaults, tanks, swales, etc.
I.
Access. Provisions shall be made to permit access and use of auxiliary equipment to facilitate emptying, cleaning, maintenance, or for emergency purposes.
J.
Underground storage. Underground detention facilities shall be designed with adequate access for maintenance (cleaning and sediment removal). Such facilities shall be provided with positive gravity outlets. Venting shall be sufficient to prevent accumulation of toxic or explosive gases.
(Ord. No. 4119, § 1(Exh. A), 6-16-08)