§ 14-42. Water quality control.
The following provisions are included in the land development regulations in order to control the water quality of storm runoff from all development and construction activities and all developed sites:
(a)
Standards.
(1)
New development and redevelopments in DeKalb County must use the stormwater quality site development review tool to facilitate the evaluation of the project in accordance with recommendations of this ordinance and the Georgia Stormwater Management Manual.
(2)
At a minimum, new projects shall include stormwater quality provisions and the BMPs necessary to accomplish each of the following.
(A)
Water quality BMPs must be installed on all developments to improve the water quality of the storm runoff from the development site. Stormwater management systems (which can include both structural stormwater controls and better site design practices) must be designed to remove eighty (80) percent of the average annual post-development total suspended solids load. As a minimum, the runoff from the first one and two-tenths (1.2) inches of rainfall must be treated. Acceptable BMPs for treating stormwater runoff are set forth in the Georgia Stormwater Management Manual.
(B)
BMPs shall be used to control discharges into the local drainage system of any organic or inorganic matter that cause or tend to cause pollution of such waters.
(i)
Minimization of impervious areas within developments and minimization of impervious areas directly connected to the local drainage system is encouraged as a non-structural BMP for water quality and quantity control.
(ii)
A plan shall be provided to protect all existing wetland/floodplain areas within the development site.
(b)
Drainage plans. The following performance criteria shall be used by the county in evaluating all drainage plans, unless otherwise provided for in this section:
(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, and maintained according to the specific criteria in the Georgia Stormwater Management Manual; and,
(C)
Runoff from a stormwater hotspot and activities identified by DeKalb County are adequately treated and addressed through the use of appropriate structural stormwater controls, nonstructural 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 three (3) approaches:
(1)
Preservation, restoration and/or reforestation (with native vegetation) of the applicable stream buffer;
(2)
Twenty-four-hour extended detention storage of the one-year, twenty-four-hour return frequency storm event;
(3)
Erosion prevention measures such as energy dissipation and velocity control.
(d)
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 twenty-five-year, twenty-four-hour return frequency storm event. If control of the one-year, twenty-four-hour storm is exempted, then peak discharge rate attenuation of the two-year through the twenty-five-year return frequency storm event must be provided.
(e)
Extreme flooding protection. Extreme flood and public safety protection shall be provided by controlling and safely conveying the one hundred-year, twenty-four-hour return frequency storm event such that flooding is not exacerbated.
(f)
Structural stormwater controls. All structural stormwater management facilities shall be 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 director before being included in the design of a stormwater management system.
(g)
Stormwater credits for nonstructural measures. The use of one (1) or more site design measures by the applicant may allow for a reduction in the water quality treatment volume required. The applicant may, if approved by the director, 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 Georgia Stormwater Management Manual.
(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 right-of-ways. Stormwater conveyance facilities that are designed to carry runoff from more than one (1) 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 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.
(i)
Reserved.
(Ord. No. 85-00, Pt. I, 12-28-00; Mo. of 1-23-01; Ord. No. 05-17, Pt. I, 11-8-05; Ord. No. 09-05, Pt. I, 2-24-09)