§ 50-132. Post-development stormwater management performance criteria (mandatory).

The following performance criteria shall be applicable to all stormwater management plans, unless otherwise provided for in this ordinance.

(1)

Water quality. All stormwater runoff generated from a site shall be adequately treated before discharge. It will be presumed that a stormwater management system complies with this requirement if:

a.

It is sized to treat the prescribed water quality treatment volume from the site, as defined in the Georgia Stormwater Management Manual;

b.

Appropriate structural stormwater controls or nonstructural practices are selected, designed, constructed or preserved, maintained according to the specific criteria in the Georgia Stormwater Management Manual; and,

c.

Runoff from hotspot land uses and activities identified by the manager are adequately treated and addressed through the use of appropriate structural stormwater controls, nonstructural practices and pollution prevention practices.

(2)

Stream channel protection. Protection of stream channels from bank and bed erosion and degradation shall be provided by using all of the following three approaches:

a.

Preservation, restoration and/or reforestation (with native vegetation) of the applicable stream buffer;

b.

24-hour extended detention storage of the one-year, 24-hour return frequency storm event. This requirement may be adjusted or waived by the manager for sites that discharge directly into larger streams, rivers, wetlands, or lakes, or to a man-made channel or conveyance system where the reduction in these flows will not have an impact on upstream or downstream streambank or channel integrity.

c.

Erosion prevention measures such as energy dissipation and velocity control.

(3)

Overbank flooding protection. Downstream overbank flood and property protection shall be provided by controlling (attenuating) the post-development peak discharge rate to the pre-development rate for the two-year through 25-year, 24-hour return frequency storm event (type 2 or balanced storm distribution). This requirement may be adjusted or waived by the manager for sites where the post-development downstream analysis shows that uncontrolled post-development conditions will not increase downstream peak flows, or that meeting the requirement will cause greater peak flow downstream impacts than the uncontrolled post-development conditions.

(4)

Extreme flooding protection. Extreme flood and public safety protection shall be provided by attenuating peak flow rates to undeveloped conditions and safely conveying the 100-year, 24-hour return frequency storm event (SCS type 2 or balanced storm distribution) such that flooding is not exacerbated. This requirement may be adjusted or waived by the manager for sites where the post-development downstream analysis shows that uncontrolled post-development conditions will not increase downstream peak flows, or that meeting the requirement will cause greater peak flow downstream impacts than the uncontrolled post-development conditions.

(Ord. of 4-13-04; Ord. of 2-27-07)