§ 6.06.05. Stormwater quantity criteria.

1.

Peak discharge attenuation: All projects shall incorporate stormwater quantity control utilizing retention, detention, infiltration, or other appropriate stormwater management facilities in accordance with the following standards.

a.

Open basin (rate control): For a project or portion of a project located within an open basin, the post-development rate of runoff shall not exceed the pre-development rate of runoff for the critical storm.

b.

Closed basin (volume and rate control): For a project or portion of a project located within a closed drainage basin, the total post-development volume and rate of runoff leaving the site shall not exceed the total pre-development volume and rate of runoff leaving the site for the 100-year storm event for a range of durations through the 24-hour duration (i.e. one, two, four, eight and 24-hour).

c.

Sites within an open basin where the positive outlet conveyance or collection system is deemed to have no capacity for additional flow shall provide rate control attenuation for the 100-year storm event for a range of durations through the 24-hour duration. Capacity of the positive outlet conveyance or collection system will be determined by the County Engineer.

2.

Recovery of attenuation volume: Storage volumes required to meet the volume and rate controls defined above for retention systems must recover within 72 hours following the end of the design storm event.

3.

Methodologies: Quantity and rate control computations shall be performed using the following methodologies:

a.

Hydrologic method: The hydrologic method used for preparing pre-development and post-development hydrographs shall be the SCS Unit Hydrograph Method or the Rational Method.

b.

Hydraulic method: The hydraulic analysis shall consist of flood routing through the proposed stormwater management facility using the critical storm approach defined in this ordinance.

c.

Rainfall data: Rainfall values shall be obtained through a statistical analysis of historical long-term rainfall data or from sources or methods generally accepted as good engineering practice. For purposes of this ordinance, rainfall data presented in the FDOT Drainage Handbook, Hydrology (Latest Edition) will be considered an acceptable source.

(Ord. No. 11-01, § 2, 1-18-11)