§ 18-186. Postdevelopment stormwater management performance criteria.

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

(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 development regulations in combination with the standards and specifications set forth in the Georgia Stormwater Management Manual Volumes 1, 2 and 3;

b.

Appropriate structural stormwater controls or nonstructural practices are selected, designed, constructed or preserved, and maintained according to the specific criteria in the development regulations in combination with the standards and specifications set forth in the Georgia Stormwater Management Manual Volumes 1, 2 and 3; and,

c.

Runoff from hotspot land uses and activities identified by the city community development 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 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.

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.

(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 25-year, 24-hour return frequency storm event. If control of the one-year, 24-hour storm under subsection (1) 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.

(4)

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

(5)

Structural stormwater controls . All structural stormwater management facilities shall be selected and designed using the appropriate criteria from the development regulations in combination with the standards and specifications set forth in the Georgia Stormwater Management Manual Volumes 1, 2 and 3. All structural stormwater controls must be designed appropriately to meet their intended function. For other structural stormwater controls not included in the development regulations in combination with the Georgia Stormwater Management Manual Volumes 1, 2 and 3, 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 city community development 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 nonpoint source pollution loads created on the site in question. Applicants shall consult the regulations 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 subsection (1) of this section. The applicant may, if approved by the city community development 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 identify 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 regulations.

(7)

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 that one parcel, existing or proposed, shall meet the following requirements:

a.

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

b.

All culverts, pipe systems and open channel flow systems shall be sized in accordance with the stormwater management plan using the methods included in the stormwater design manual; and

c.

Design and construction of stormwater conveyance facilities shall be in accordance with the criteria and specifications found in the stormwater design 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. No. 2015-01-40, § 16, 3-17-2015; Ord. No. 2015-09-53, § II, 10-20-2015)