§ 9.6.8. Stormwater Management Plan

A.

The stormwater management plan shall detail how post-development stormwater runoff will be controlled or managed and how the proposed project will meet the requirements of this Division, including the performance criteria set forth in Sec. 9.6.12.

B.

This plan is in accordance with the criteria established in this section and must be submitted with the stamp and signature of a design professional licensed in the state, who must verify that the design of all stormwater management facilities and practices meet the submittal requirements outlined in the current Georgia Stormwater Management Manual 2016 Edition (here and henceforth all references to this manual assume the 2016 edition, including all amendments as may be forthcoming from time to time) and the City's submittal requirements for commercial and single-family residential development.

C.

The stormwater management plan must ensure that the requirements and criteria in this Division are being complied with and that opportunities are being taken to minimize adverse post-development stormwater runoff impacts from the development. The plan shall consist of maps, narrative, and supporting design calculations (hydrologic and hydraulic) for the proposed stormwater management system. The plan shall include all of the applicable design requirements and forms found in the Georgia Stormwater Management Manual and the City's submittal requirement for commercial and single-family residential development. This includes but is not limited to:

1.

The common address and legal description of the site.

2.

Vicinity map.

3.

Existing conditions and proposed site plans. Existing conditions and proposed site layout plans which illustrate at a minimum: existing and proposed topography; perennial and intermittent streams; mapping of predominant soils from soil surveys; boundaries of existing predominant vegetation and proposed limits of clearing and grading; and location of existing and proposed roads, building parking area and other impervious surfaces.

4.

Infiltration rates. Infiltration rates shall be determined by soil surveys, on-site soil analysis or a percolation test. If the site has been previously developed or graded or contains urban soil types, a percolation test is required.

5.

Natural resources inventory. A written or graphic inventory of the natural resources in existence prior to the commencement of the project. This inventory shall address resources both on the site and in the surrounding area that are or may be impacted by the project. This inventory shall also include a description of the soil conditions, forest cover, topography, wetlands, and other native vegetative areas on the site, as well as the location and boundaries of other natural features protection and conservation areas such as wetlands, lakes, ponds, floodplains, stream buffers and other setbacks, including but not limited to drinking water well setbacks and septic setbacks. Particular attention should be paid to environmentally sensitive features that present constraints for development.

6.

Existing conditions hydrologic analysis. The existing condition hydrologic analysis for stormwater runoff rates, volumes, and velocities in accordance with the current Georgia Stormwater Management Manual, which shall include: a topographic map of existing site conditions with the drainage basin boundaries indicated; acreage, soil types and land cover of areas for each subbasin affected by the project; all perennial and intermittent streams and other surface water features; all existing stormwater conveyances and structural control facilities; direction of flow and exits from the site; analysis of runoff provided by off-site areas upstream of the project site; and methodologies, assumptions, site parameters and supporting design calculations used in analyzing the existing conditions site hydrology. For redevelopment sites, pre-development conditions are modeled using guidelines established by the Director for the portion of the site undergoing land development activities.

7.

Postdevelopment hydrologic analysis. The post-development hydrologic analysis for stormwater runoff rates, volumes, and velocities shall be calculated in accordance with the Georgia Stormwater Management Manual and include: a topographic map of developed site conditions with the post-development drainage basin boundaries indicated; total area of postdevelopment impervious surfaces and other land cover areas for each subbasin affected by the project; calculations for determining the runoff volumes that need to be addressed for each subbasin for the development project to meet the post-development stormwater management performance criteria in Sec. 9.6.12; location and boundaries of proposed natural feature protection and conservation areas; documentation and calculations for any applicable site design credits that are being utilized; methodologies, assumptions, site parameters and supporting design calculations used in analyzing the existing conditions site hydrology. If the land development activity on a redevelopment site constitutes more than 50 percent of the site area for the entire site, then the performance criteria in Sec. 9.6.12 must be met for the stormwater runoff from the entire site. For a subdivision of land or planned development, post-development runoff volumes, rates, and velocities shall be calculated based on the built-out conditions of the entire parcel to be subdivided, regardless of future ownership of individual lots. Estimates of impervious surfaces shall be made based on maximum allowable lot coverage in accordance with the City's Development Code when meeting the performance criteria. The developer of said subdivided parcel may provide runoff reduction and water quality measures for individual lots, which must be reflected accordingly on the final plat.

8.

Stormwater management system. The description, scaled drawings and design calculations for the proposed post-development stormwater management system, which shall include: A map and/or drawing or sketch of the stormwater management facilities, including the location of nonstructural site design features and the placement of existing and proposed structural stormwater controls, including design water surface elevations, storage volumes available from zero to maximum head, location of inlet and outlets, location of bypass and discharge systems, and all orifice/restrictor sizes; a narrative describing how the selected structural stormwater controls will be appropriate and effective; cross section and profile drawings and design details for each of the structural stormwater controls in the system, including supporting calculations to show that the facility is designed according to the applicable design criteria; a hydrologic and hydraulic analysis of the stormwater management system for all applicable design storms (including stage-storage or outlet rating curves, and inflow and outflow hydrographs); documentation and supporting calculations to show that the stormwater management system adequately meets the post-development stormwater management performance criteria in section 109-195; drawings, design calculations, elevations and hydraulic grade lines for all existing and proposed stormwater conveyance elements including stormwater drains, pipes, culverts, catchbasins, channels, swales and areas of overland flow; and where applicable, a narrative describing how the stormwater management system corresponds with any watershed protection plans and/or local greenspace protection plan.

9.

Post development downstream analysis. A downstream peak flow analysis that includes the assumptions, results and supporting calculations to show safe passage of post-development design flows downstream. The analysis of downstream conditions in the report shall address each and every point or area along the project site's boundaries at which runoff will exit the property. The analysis shall focus on the portion of the drainage channel or watercourse immediately downstream from the project. This area shall extend downstream from the project to a point in the drainage basin where the project area is ten percent of the total basin area. In calculating runoff volumes and discharge rates, consideration may need to be given to any planned future upstream land use changes. The analysis is in accordance with the stormwater design manual.

10.

Construction-phase erosion and sedimentation control plan. An erosion and sedimentation control plan in accordance with the Georgia Erosion and Sedimentation Control Act of 1975 (O.C.G.A. § 12-7-1 et seq.) or NPDES permit for construction activities. The plan shall also include information on the sequence/phasing of construction and temporary stabilization measures and temporary structures that will be converted into permanent stormwater controls. Prior to the approval of the stormwater management plan, the applicant or responsible party shall submit a proposed staged construction and inspection control schedule for approval; otherwise, the construction and inspection control schedule will be for the entire drainage system. No stage work related to the construction of stormwater management facilities or BMPs shall proceed until the next proceeding stage of work, according to the sequence specified in the approved stage construction and inspection control schedule, as inspected and approved. Runoff reduction and water quality measures shall be installed in the final phase of construction to prevent clogging.

11.

Landscaping and open space plan. A detailed landscaping and vegetation plan describing the woody and herbaceous vegetation that will be used within and adjacent to stormwater management facilities and practices. The landscaping plan must also include: the arrangement of planted areas, natural and greenspace areas and other landscaped features on the site plan; information necessary to construct the landscaping elements shown on the plan drawings; descriptions and standards for the methods, materials and vegetation that are to be used in the construction; density of plantings; descriptions of the stabilization and management techniques used to establish vegetation; and a description of who will be responsible for ongoing maintenance of vegetation for the stormwater management facility and what practices will be employed to ensure that adequate vegetative cover is preserved.

12.

Operations and maintenance plan. Detailed description of ongoing operations and maintenance procedures for stormwater management facilities and practices to ensure their continued function as designed and constructed or preserved. These plans will identify the parts or components of a stormwater management facility or practice that need to be regularly or periodically inspected and maintained, and the equipment and skills or training necessary. The plan shall include a narrative describing how the stormwater management system is designed to function, including capture, runoff control, water quality treatment, channel and flood protection, and ongoing operations and maintenance procedures for all stormwater management facilities and practices shown on the stormwater management site plan. The plan shall include an inspection and maintenance schedule, maintenance tasks, responsible parties for maintenance, funding, access and safety issues. Provisions for the periodic review and evaluation of the effectiveness of the maintenance program and the need for revisions or additional maintenance procedures are included in the plan.

13.

Maintenance access easements. The applicant must ensure access from right-of-way to stormwater management facilities and practices requiring regular maintenance at the site for the purpose of inspection and repair by securing all the maintenance access easements needed on a permanent basis. Such access is sufficient for all necessary equipment for maintenance activities. Upon final inspection and approval, a plat or document indicating that such easements exist is recorded and shall remain in effect even with the transfer of title of the property.

14.

Inspection and maintenance agreements. Unless an on-site stormwater management facility or practice is dedicated to and accepted by the Department as provided in Sec. 9.6.9, the applicant must execute an easement and an inspection and maintenance agreement binding on all subsequent owners of land served by an on-site stormwater management facility or practice in accordance with Sec. 9.6.9.

15.

Evidence of acquisition of applicable local and nonlocal permits. The applicant shall certify and provide documentation to the Department that all other applicable environmental permits have been acquired for the site prior to approval of the stormwater management plan.

( Ord. of 4-17-2018(7) , §§ 9-68—9-71)