§ 24-211. Off-site and regional detention concepts.

(1)

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.

a.

Fees for offsite stormwater detention, for use of the city-owned detention facility, located in the Northwest corner of Lot 1, Block 2, Myers Park Subdivision #3, shall be paid to the city within ninety (90) days after issuance of the building permit by the engineering department. Use of said facility shall be subject to available capacity. Said fees shall be based on cubic feet of storage required for detention, as follows:

Number of cubic feet required	Fee per cubic foot
1 cu. ft.—9999 cu. ft.	$0.90 per cu. ft.
10,000 cu. ft.—49,999 cu. ft.	0.75 per cu. ft.
50,000 cu. ft.—above	0.65 per cu. ft.

 

(2)

Regional detention. Detention facilities designed and located to provide detention on major drainage channels will be considered as a regional detention facility. The drainage area considered for a regional detention facility must be five hundred (500) acres or greater. The facility must provide a detention volume for a 100-year storm for the entire drainage area, and must be designed with a variable control outlet structure that has a one (1) year maximum outlet 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 Carthage will accept the responsibility for the maintenance of the regional facility. Drainage and access easements will be required giving the City of Carthage the authority to gain vehicular access to the facility from a public street.

As additional development occurs upstream of the regional facility, onsite detention requirements may be waived, 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 100-year storm for the entire basin upstream from the regional facility.

Construction Requirements

1.

Minimum time of concentration (tc) shall be five (5) minutes.

2.

Volume of detention shall be that computed amount between the 100-year developed and 100-year undeveloped conditions.

3.

Discharge structure shall be designed for one (1) year design storm with minimum size outlet being not less than a four (4) inch diameter pipe.

4.

All outlet structures and detention walls shall be concrete.

5.

Detention for any revision, upgrade or modification to an existing property shall be designed to accommodate any increase in stormwater runoff resulting from the modification being made.

6.

Designers not wishing to show calculations may design detention at a rate of three thousand two hundred and forty (3,240) cubic feet per acre of total site.

7.

Temporary retention during construction shall be provided.

8.

Minimum detention required = five hundred (500) cubic feet.

(Ord. No. 6628, § I (K), 7-26-94; Ord. No. 00-76, § I, 10-24-2000)