§ 74-513. Performance criteria for stormwater management.

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 the previously developed area:

(a)

Stormwater runoff reduction. The stormwater runoff volume generated by the first one inch of rainfall shall be retained on-site in order to help maintain pre-development site hydrology and help protect the local watershed from several indirect impacts of the land development process, including decreased groundwater recharge, decreased baseflow and degraded water quality. A stormwater management system is presumed to comply with this criteria if:

(1)

It includes green infrastructure practices that provide for the interception and evapotranspiration, infiltration, or capture and reuse of stormwater runoff, that have been selected, designed, constructed and maintained in accordance with the information presented in the latest edition of the Georgia Stormwater Management Manual and applicable sections of the Coastal Stormwater Supplement; or constitutes an alternative practice responsibly designed and documented by the engineer to reproduce the intent of the Georgia Stormwater Management Manual and applicable sections of the Coastal Stormwater Supplement; and

(2)

It is designed to provide the amount of stormwater runoff reduction from the runoff generated by the first one inch of rainfall as specified in the latest edition of the Georgia Stormwater Management Manual and applicable sections of the Coastal Stormwater Supplement.

(b)

Water quality protection. 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 runoff reduction criteria in section 74-513(a). However, if any of the stormwater runoff volume generated by the first one inch of rainfall cannot be reduced or retained on the development site, due to site characteristics or constraints, such as, a high water table, surface bedrock, contaminated soils, low soil infiltration rates, or the presence of a high risk operation or 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:

(1)

The practice shall be sized to treat the prescribed water quality treatment volume from the site, as defined in the Georgia Stormwater Management Manual;

(2)

Appropriate structural stormwater controls or nonstructural management practices are selected, designed, constructed or preserved, and maintained according to the specific criteria in the Georgia Stormwater Management Manual and applicable sections of the Coastal Stormwater Supplement; or constitutes an alternative practice responsibly designed and documented by the engineer to reproduce the intent of the Georgia Stormwater Management Manual and applicable sections of the Coastal Stormwater Supplement; and

(3)

Runoff from a high risk operation or hotspot land use activity identified by the City of Atlanta is adequately treated and addressed through the use of appropriate structural stormwater controls, nonstructural management practices and pollution prevention practices.

(c)

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

(1)

Preservation, restoration and/or reforestation with native vegetation of the applicable stream buffer pursuant to Chapter 74, Article VII: Riparian Buffer Requirements;

(2)

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

(3)

Erosion prevention measures such as energy dissipation and velocity control.

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 Section 74-513(a) and (b), stream channel protection is not required.

(d)

Overbank flooding protection. For new development (greenfield), overbank flood and property protection shall be provided by attenuating the post-development peak discharge rate to the pre-development rate up to the 25-year, 24-hour return frequency storm event.

For redevelopment, 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:

PIC = Pre-development Impervious Cover

PDRR = Peak Discharge Rate Reduction

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.

For land development permitted post-2004 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.

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 section 74-513(a) and (b), overbank flooding protection is not required.

(e)

Extreme flooding protection. Extreme flood and public safety protection shall be provided by attenuating and safely conveying the 100-year, 24-hour return frequency storm event without any increase in peak discharge rate, such that flooding is not exacerbated.

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 section 74-513(a) and (b), extreme flooding protection is not required.

(f)

Structural stormwater controls.

(1)

All structural stormwater management facilities shall be selected and designed using the appropriate criteria from the Georgia Stormwater Management Manual and applicable sections of the Coastal Stormwater Supplement. 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 City of Atlanta 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 of Atlanta 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 nonpoint source pollution loads created on the site in question.

(2)

Applicant or responsible party 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.

(g)

Stormwater credits for nonstructural measures. The applicant or responsible party may take credit for the use of stormwater better site design practices and reduce the water quality volume and runoff reduction requirements. 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 and applicable sections of the Coastal Stormwater Supplement.

(h)

Drainage system guidelines. Stormwater conveyance facilities, which may include but are not limited to culverts, stormwater drainage pipes, catch basins, drop inlets, junction boxes, headwalls, gutter, swales, channels, ditches, and energy dissipaters shall be provided when necessary for the protection of public right-of-way and private properties adjoining project sites and/or public 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:

(1)

Methods to calculate stormwater flows shall be in accordance with the Georgia Stormwater Management Manual;

(2)

All culverts, pipe systems and open channel flow systems shall be sized in accordance with the stormwater management plan, pursuant to section 74-511, using the methods included in the Georgia Stormwater Management Manual; and

(3)

Design and construction of stormwater conveyance facilities shall be in accordance with the criteria and specifications found in the Georgia Stormwater Management Manual and applicable sections of the Coastal Stormwater Supplement.

(i)

Rules for dam safety and any other applicable federal, state, or local requirement. Any land development activity that involves a site which proposes a dam shall comply with the Georgia Safe Dams Act and Rules for Dam Safety as applicable.

(Ord. No. 2004-56, §§ 2—4, 9-15-04; Ord. No. 2013-05(12-O-1761), § 11, 2-13-13)