21A-73 Detention or retention | St. John Town Code

Sec. 21A-73. Detention or retention.

(a) Detention or Retention Information. In developed and developing urban and suburban areas, several means for controlling stormwater runoff could be utilized. These usually involve storing runoff on or below the ground surface. The following types of storage facilities may be considered for detention: parking lots, underground tanks or chambers, and surface basins or ponds:

(1) Parking Lot Storage. Parking lot storage using shallow ponding (six (6) inches maximum) designed to flood specifically graded areas of the parking lot may be used. Controlled release features must be incorporated into the surface drainage system of the parking lot. This method is intended to control the runoff directly from the parking area and is usually not appropriate for storing large runoff volumes.

(2) Tank Storage. Tank storage in an underground tank or chamber, either pre-fabricated or constructed in place, with a gravity controlled release will be considered for detention.

a. Wet ponds are permanent ponds where additional storage capacity is provided above the normal water level and special features for controlled release are included. Because of large land requirements and the necessity of maintaining a permanent pool of water, wet ponds have a broader application for in-stream control where large watershed areas are involved compared to their use as on-site facilities for small urban areas.

b. Dry basins are surface storage areas created by constructing a typical excavated or embankment basin. There is no normal pool level and a specific controlled release feature is included to control the rate of discharge. The detention flow structure is usually a multi-stage device and the retention flow control structure is usually a single-stage device.

(1) Design Frequency. One hundred (100) year storm: based on Bulletin 71 or latest NOAA report for the Chicagoland Area.

a. Area under two hundred (200) acres use Rational Method (Q = CiA) or industry recognized computer programs.

b. Areas between two hundred (200) and three hundred (300) acres transition between Rational Method and Technical Release No. 55 or industry recognized computer programs.

c. Areas over three hundred (300) acres use Technical Release No. 55 or industry recognized computer programs.

(3) Runoff Coefficient. Based on Table No. 1 (see elsewhere in this article).

(4) Release Rates. The release rate for all developments requiring detention shall not exceed the stormwater runoff rate from the tributary areas of greater than that calculated from a storm of two (2) year frequency, with a runoff coefficient of 0.15 or the ability of the downstream sewers or stream channel to handle the flow satisfactorily.

a. The surface of a detention area should be constructed with sufficient slopes (minimum of 0.5 percent) to drain properly so that all of the runoff is removed following a storm.

b. A ditch, or ditches, shall be constructed from the pipe, pipes, or turn-outs, outletting into the basin to the outlet structure.

c. Seeding and other erosion control methods will be used to protect all slopes: sod, jute matting, rock protection or concrete.

d. Side slopes for a retention facility shall be four to one (4:1) maximum below permanent storage elevation. Minimum permanent water depth shall be five (5) feet to the normal water level. Side slopes for a retention facility shall be six to one (6:1) maximum above the permanent storage elevation.

e. A ten (10) foot wide safety ledge at an elevation no more than two (2) feet below the permanent storage elevation is required at the perimeter of the permanent storage water line.

(6) Time of Concentration. Based on Chart No. 2 (see elsewhere in this article).

(7) Debris-control structures may be required in the detention methods and should be considered as an essential part of design.

a. An emergency spillway shall be provided for flows in excess of the design storm.

b. Fifty (50) year storm frequency shall be used to design size of emergency spillway.

c. The emergency spillway shall be made of concrete or other material as approved by the Town Engineer.

(9) Freeboard. The freeboard shall be twelve (12) inches from the high water level to top of bank of basin.

(10) Aerators and Fountains. Aerators and/or fountains shall be provided and installed by the subdivider for all retention basins. The developer shall be responsible to operate and maintain the aeration and/or fountains for a period of two (2) years. Plans are to be submitted to the Town Engineer for approval.