§ 9.6.12. Performance Criteria

A.

For new developments, the following performance criteria shall be applied to the area of the site impacted by the proposed work. For redevelopment, the following performance criteria shall be applied to the area of the site impacted by the proposed work, provided that the impacted area does not exceed 35 percent of the previously developed area. If the impacted area exceeds 35 percent of the previously developed area, the following performance criteria shall be applied to the entire development, including previously developed area:

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 it satisfies the stormwater reduction criteria in this section. However if any of the stormwater runoff volume generated by the first 1.2 inches of rainfall cannot be reduced or retained on site due to constraints such as a high water table, rock, low infiltration rates or the presence of a hotspot, the remaining volume shall be increased by a multiplier of 1.2 and shall be intercepted and treated in one or more stormwater management practices that provide at least an 80 percent reduction in total suspended solids loads in accordance with the following criteria:

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, and maintained according to the specific criteria in the Georgia Stormwater Management Manual or constitutes an alternative practice responsibly designed and documented by the design professional to reproduce the intent of the Georgia Stormwater Management Manual; and

c.

Runoff from hotspot land uses and activities identified by the Department 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 is provided by using all of the following three approaches:

a.

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

b.

Twenty-four-hour extended detention storage of the one-year, 24-hour return frequency storm event;

c.

Erosion prevention measures such as energy dissipation and velocity control.

d.

For redevelopment projects that create, add, or demolish and replace less than 5,000 square feet of impervious surface and meet the performance criteria of this section, stream channel protection is not required.

3.

Overbank Flooding Protection  

a.

Downstream overbank flood and property protection is provided by controlling (attenuating) the post-development peak discharge rate to the pre-development rate for the 25-year, 24-hour return frequency storm event. If control of the one-year, 24-hour storm under subsection (a) of this section is exempted, then peak discharge rate attenuation of the two-year through the 25-year return frequency storm event must be provided. For redevelopment projects overbank flood and property protection shall be provided by reducing the peak discharge rate up to the 25-year, 24-hour storm event in accordance with the following formula:

b.

%PIC/2 = %PDRR

c.

PIC = Predevelopment Impervious Cover

i.

PDRR = Peak Discharge Rate Reduction

d.

For sites where previous demolition has removed impervious surfaces, pre-development peak discharge rate calculations and percentage of impervious coverage shall be calculated based on pre-demolition conditions. For sites that have been demolished and have remained fallow and stabilized with vegetation for a minimum of five years, they shall be considered as having pre-development conditions of 20 percent impervious cover for purposes of calculating peak discharge rate reduction.

e.

For land development permitted after 2005 and served by appropriate stormwater management facilities, subsequent redevelopment of the same area is not required to further reduce the peak discharge rate, provided that the site continues to meet the reduction previously achieved.

f.

For redevelopment projects that create, add, or demolish and replace less than 5,000 square feet of impervious surface and meet the performance criteria of this section, overbank flooding protection is not required.

4.

Extreme Flooding Protection  

a.

Extreme flood and public safety protection is provided by controlling and safely conveying the 100-year, 24-hour return frequency storm event such that flooding is not exacerbated.

b.

For redevelopment projects that create, add, or demolish and replace less than 5,000 square feet of impervious surface and meet the performance criteria of this section, extreme flooding protection is not required.

5.

Structural Stormwater Controls All structural stormwater management facilities are selected and designed using the appropriate criteria from the Georgia Stormwater Management Manual. All structural stormwater controls must be designed appropriately to meet their intended function. For other structural stormwater controls not included in the Georgia Stormwater Management Manual, or for which pollutant removal rates have not been provided, the effectiveness and pollutant removal of the structural control must be documented through prior studies, literature reviews, or other means and receive approval from the Department before being included in the design of a stormwater management system. In addition, if hydrologic or topographic conditions, or land use activities warrant greater control than that provided by the minimum control requirements, the City may impose additional requirements deemed necessary to protect upstream and downstream properties and aquatic resources from damage due to increased volume, frequency, and rate of stormwater runoff or increased non-point source pollution loads created on the site in question. Applicants shall consult the Georgia Stormwater Management Manual for guidance on the factors that determine site design feasibility when selecting and locating a structural stormwater control.

6.

Stormwater Credits for Nonstructural Measures The use of one or more site design measures by the applicant may allow for a reduction in the water quality treatment volume required under Sec. 9.6.12.A.1. The applicant may, if approved by the Department, take credit for the use of stormwater better site design practices and reduce the water quality volume requirement. For each potential credit, there is a minimum set of criteria and requirements that identifies the conditions or circumstances under which the credit may be applied. The site design practices that qualify for this credit and the criteria and procedures for applying and calculating the credits are included in the Georgia Stormwater Management Manual.

7.

Drainage System Guidelines Stormwater conveyance facilities, which may include but are not limited to culverts, stormwater drainage pipes, catchbasins, drop inlets, junction boxes, headwalls, gutters, swales, channels, ditches, and energy dissipaters, are provided when necessary for the protection of right-of-way and private properties adjoining project sites and/or rights-of-way. Stormwater conveyance facilities that are designed to carry runoff from more than one parcel, existing or proposed, shall meet the following requirements:

a.

Methods to calculate stormwater flows are in accordance with the stormwater design manual;

b.

All culverts, pipe systems and open channel flow systems are sized in accordance with the stormwater management plan using the methods included in the Georgia Stormwater Management Manual; and

c.

Design and construction of stormwater conveyance facilities are in accordance with the criteria and specifications found in the Georgia Stormwater Management Manual.

8.

Dam Design Guidelines Any land disturbing activity that involves a site that proposes a dam shall comply with the Georgia Safe Dams Act of 1978 (O.C.G.A. § 12-5-370 et seq.) and rules for dam safety as applicable.

( Ord. of 4-17-2018(7) , §§ 9-73, 9-73.1)