§ 14-40. Stormwater management.
(a)
The governing authority believes the county's stream systems are a valuable natural resource that requires joint and cooperative action by the county and the development industry to resolve existing stormwater management and flooding problems, prevention of their worsening or recurrence while utilizing this resource for the good of the entire county.
The development industry and the county shall cooperate to control water quality and maintain the county's drainage and stream systems from stormwater runoff resulting from development activities.
(b)
Standards.
(1)
DeKalb County shall require all land development to comply with the criteria, technical specifications, and standards of the Georgia Stormwater Management Manual, as may be hereafter amended. The rainfall intensities used in hydrologic and hydraulic computations shall be those published in the Georgia Stormwater Management Manual.
(2)
Applicability. A combination of storage and controlled release of stormwater runoff shall be required for all development and construction for the entire site which meets one (1) or more of the following criteria:
(A)
Increases the peak rate of runoff from the site by more than one (1) cubic foot per second for a ten-year frequency storm;
(B)
Involves the creation of five thousand (5,000) square feet or more of impervious cover, or that involves other land development activities of one (1) acre or more;
(C)
Includes the creation, addition or replacement in redevelopment of five thousand (5,000) square feet or more of impervious cover, or that involves other land development activity of one (1) acre or more;
(D)
Any new development or redevelopment, regardless of size, that meets the definition of a stormwater hotspot; or
(E)
Land development activities that are smaller than the minimum applicability criteria set forth in items (A) and (B) above if such activities are part of a larger common plan of development, even though multiple, separate and distinct land development activities may take place at different times on different schedules.
(3)
Exemptions. The provisions of this article shall not apply to the following criteria:
(A)
Individual single-family or duplex residential lots that are not part of a subdivision or phased development project unless they meet one (1) of the criteria listed above in (b)(2);
(B)
Additions or modifications to existing single-family or duplex residential structures unless it meets one (1) of the criteria listed above in (b)(2);
(C)
Agricultural or silvicultural land management activities within areas zoned for these activities; and,
(D)
Repairs to any stormwater management facility or practice deemed necessary by the director.
(E)
The director may exempt the owner from those provisions of this article where complete compliance with those specific provisions is physically impossible.
(F)
If the installation of a stormwater management facility would reduce downstream flood peaks by less than one (1) percent.
(G)
The requirements, or portions thereof, of subsections (2) and (3) above shall not be waived if the director determines that such waiver would increase known flooding problems, or exceed the capacity of the downstream drainage system.
(H)
A waiver of these minimum runoff quantity control requirements may be granted only after a written request is submitted by the applicant containing descriptions, drawings, and any other information that is necessary to evaluate the proposed land disturbing activity. A separate written waiver request shall be required if there are subsequent additions, extensions, or modifications, to the development which would alter the approved stormwater runoff characteristics of a land disturbing activity receiving a waiver.
(I)
Appeals from said waiver decisions may be taken to the zoning board of appeals pursuant to the provisions of section 14-34(c).
(4)
If forty (40) percent of a site is to be redeveloped, all stormwater requirements must be met for the redeveloped area only and the non-disturbed area will be treated as predeveloped prior to the redevelopment. But if more than forty (40) percent of the site is to be redeveloped, then the entire site must meet all stormwater requirements.
(5)
A downstream peak flow analysis will include 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 (10) 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 shall be in accordance with the Georgia Stormwater Management Manual.
(6)
Detention designs may be rejected by the director if they incorporate structures and facilities that will demand considerable maintenance or will be difficult to maintain or will utilize numerous small structures if other alternatives are physically possible.
(7)
Discharge velocities from detention facilities shall be reduced to provide a non-erosive velocity flow from a structure, channel, or other control measure as set forth in the approved Georgia Stormwater Management Manual.
(8)
Stormwater management and flood control facilities may include both structural and nonstructural components. Natural or planted vegetation as well as other natural runoff conduits are examples of these non-structural components and shall be retained where practicable. In addition, these components must provide for or enhance stormwater quantity and/or quality control or other stormwater benefits.
(9)
The drainage system being developed shall have adequate capacity to accommodate the flow from all upstream areas for a one hundred-year storm event.
(10)
The drainage system from a proposed development must discharge into an outfall that has adequate capacity to accommodate the runoff from the development. If the connecting downstream system is not able to accommodate the allowable design flows from the site, then the design engineer must design drainage facilities with the capacity to over-detain flows so they can be accommodated by the existing downstream conveyance structures whereby allowing the existing downstream system to operate correctly. If downstream easements are needed to extend the drainage system to an adequate outfall, the developer shall obtain these easements.
(11)
A landscaping plan shall be submitted for all detention and other storage facilities as part of the overall drainage plan.
(12)
Land uses within the intermediate regional floodplain shall not diminish or restrict the capacity of the channels or floodplains of the stream, its tributaries, drainage ditches or any other stormwater management facilities or systems and shall not increase the IRF elevation or velocity or concentration of flow in downstream areas. The development permit shall be denied if the required hydrologic studies reveal that a request for filling or grading within the intermediate regional floodplain would overload the capacity of the channel downstream or increase flood stages upstream, unless equivalent flow and storage capacity is replaced and maintained by the owner within the intermediate regional floodplain. Altered sections of the intermediate regional floodplain shall have a positive slope so as to provide positive drainage back to the stream flow line and this section must be maintained by the owners in perpetuity so as to prevent or remove silt buildup.
(13)
Excavation within floodplain areas shall not be permitted unless the excavation can be accomplished in such a manner that the existing low level drainage pattern through the floodplain shall be maintained. The area of compensation within the floodplain shall be considered as ineffective flow area for the purpose of calculating floodplain elevations to meet no-rise certification requirements. The amount of compensation shall be limited to one hundred fifty (150) cubic yards per acre of floodplain area.
(14)
The live detention storage to be provided shall be calculated on the basis of the one hundred-year frequency rainfall as published in the Georgia Stormwater Management Manual. The detention system required shall be necessary to handle the runoff of a one hundred-year rainfall, for any and all durations from the post-development, with a release rate that does not exceed the pre-development release rate during the same duration storm. Detention control structures and other drainage improvements shall be located and designed to prevent erosion damage to adjacent property owners.
(15)
When the applicant requests and the director determines that development and construction projects are too small, or that engineering and economic factors make combined detention or other stormwater management facilities more practical, the county may authorize the joint construction of these facilities to serve two (2) or more properties by two (2) or more applicants. This authorization shall be granted by the zoning board of appeals upon application for approval being submitted through the director. Where joint detention facilities serving two (2) or more properties are approved for construction, no use of land or occupancy of buildings within the properties served by these facilities shall be permitted until completion of the detention facility, except upon approval of the zoning board of appeals.
(16)
The following requirements shall apply to the installation, development and maintenance of all stormwater and sedimentation control facilities designed for temporary storage of stormwater runoff:
(A)
Permanent fencing at least four (4) feet in height shall be required around all facilities having a temporary water storage depth of greater than four (4) feet or those designated by the board of health as constituting a public health hazard.
(B)
This fencing shall be designed, installed and maintained to allow the free flow of runoff and sediment into the facility. Fencing shall be established on the outside edge of a facility. The fence shall include a gate of sufficient size to permit entrance of equipment necessary to allow periodic maintenance activities. The gate shall be placed in a manner such that the gate does not obstruct reasonable access or become obstructive. The director may waive fencing in nonresidential areas where a pond is more than five hundred (500) feet from a residential district and in residential districts when detention is provided in natural areas such as stream channels and fencing in the opinion of the director would damage the environment or affect stream flow.
(C)
The access easement to the facility shall not have a profile slope steeper than thirty-three (33) percent and a cross slope of no more than ten (10) percent. The elevation of the maintenance easement around the facility shall be established at the top of the dam or wall elevation and be constructed with a cross slope of no more than ten (10) percent to the drainage facility. Fencing that complies with the requirement of section 14-40(b)(16)(B) shall be constructed on the outside edge of the maintenance easement. Gates that comply with the requirements of section 14-40(b)(16)(B) shall be constructed on each maintenance easement.
(D)
Detention and sedimentation control facilities shall not be placed in any of the following:
(i)
Transitional buffer zones as defined by Chapter 27.
(ii)
Floodplains.
(iii)
Wetlands.
(iv)
Stream buffer zones.
(v)
State buffer zones.
(E)
Perforated standpipes or a French drain, in accordance with published design standards available from the director, or other methods which will achieve equal performance to prevent standing water and inadequate drainage shall be installed within all the detention and sedimentation control facilities.
(F)
Except as provided in subparagraph (F) of this paragraph, the commercial and/or multifamily residential property owner shall be responsible for the maintenance of the stormwater management facilities during grading, construction, and following final approval of the completed project. This maintenance and certification obligation shall be binding on all future owners, successors and assigns of the property.
(G)
In the case of single-family residential subdivisions approved after the date of adoption of this provision, and in accordance with the requirements of Article III of this chapter, the county shall assume maintenance responsibility one (1) year after the release of the performance bond for subdivision streets. A special drainage district as authorized by the Georgia Constitution, Article IX, Section II, Paragraph VI is established for property in a single-family residential subdivision at the time the plat is finally recorded, and will be so noted on the plat. Upon completion of developer maintenance, all detention ponds shall have a positive slope to the outlet in order to facilitate complete drainage.
(H)
Stormwater management facilities in single-family residential subdivisions constructed under permits issued prior to the amendment of the county ordinance assigning maintenance responsibility will not be accepted for county maintenance unless individually approved by and at the discretion of the zoning board of appeals and suitable access easements are provided. At the time the director accepts a detention facility for county maintenance, a special drainage district will be established that includes all properties for which the detention facility is designed to compensate for increased peak runoff rates due to development.
(17)
In residential districts, not less than seventy (70) percent of the minimum lot area, as established by applicable zoning district development standards, shall be above the intermediate regional flood elevation contours with the exception that lots in the R-150 district shall conform to requirements of the R-100 district.
(18)
All buildings located adjacent to the intermediate regional floodplain shall be constructed so that all portions of the structure, including the basement floor or crawl areas, shall be not less than three (3) feet above the intermediate regional flood elevations; however, structural support units may be located within the intermediate regional floodplain, provided they do not conflict with the hydrologic design characteristics of the approved plans and do not conflict with other requirements of this article. Any structure or manufactured home so erected must be designed (or modified) and adequately anchored to prevent flotation, collapse or lateral movement of the structure, and be constructed with flood-resistant materials and utility equipment resistant to flood damage. Electrical, heating, ventilation, plumbing, air-conditioning equipment and other service facilities shall be designed and/or located so as to prevent water from entering or accumulating within the components during conditions of flooding. Floodproofing of residential construction will be in accordance with section 14-40(b)(10) of the Code of DeKalb County. When floodproofing is utilized for a nonresidential structure, the owner of the property shall obtain written certification from a registered professional engineer or architect and shall provide such certification to the director before the director approves such activity.
(19)
The profile elevation of the centerline of all public streets shall be constructed a minimum of one (1) foot above the intermediate regional flood elevation contours. The director may grant exceptions to this provision in cases where construction of the street elevation is below the intermediate regional flood elevation and elevation contours would improve drainage or reduce the effects of flooding.
(20)
Special drainage system maintenance requirements are as follows:
(A)
Pursuant to Chapter 22.5 of the Code of DeKalb County, as amended, trash, garbage, construction materials, construction by-products or other debris shall not be deposited in any part of the drainage system.
(B)
No restriction or barriers, including fences, may be placed in the drainage system or intermediate regional floodplain without first obtaining a development permit. When on-site or off-site debris has accumulated within an intermediate regional floodplain in such a manner as to interfere with the free flow of water so as to increase the risk of hazardous inundation of upstream properties adjacent to the intermediate regional floodplain, the director shall require the owner of the property where this debris was generated, if its source can be identified, to clear and remove the debris so as to permit the free flow of water.
(C)
No impoundment of water which retains in excess of five-tenths (0.5) acre (in feet) of runoff shall be removed without first obtaining a development permit, which shall only be issued after competent engineering studies provided by the applicant show that this removal will not adversely affect downstream properties.
(D)
No permanent structures or additions shall be constructed within the intermediate regional floodplain other than those non-building facilities required or authorized by the director which will not conflict with the hydrologic design characteristics of the approved development and construction plans. Land within the intermediate regional floodplain may be used to meet setback, yard, open space and buffer requirements in accordance with applicable provisions of Chapter 27 and the buffer requirements of this chapter.
(21)
Any proposal for development in a regulatory floodway as identified on the flood boundary-floodway map must be accompanied by engineering certifications assuring that no increase in the flood levels of the base flood would be caused by the proposed development. Such proposals include culverts and bridges.
(22)
New construction or substantial improvements of elevated buildings that include fully enclosed areas formed by foundations and other exterior walls below the base flood elevation shall be designed to preclude finished living space and designed to allow for the entry and exit of floodwaters to automatically equalize hydrostatic flood forces on exterior walls, as follows:
(A)
Designs for complying with this requirement must either be certified by a currently state-registered professional engineer or currently state-registered professional architect and meet the following minimum criteria:
(i)
Provide a minimum of two (2) openings having a total net area of not less than one (1) square inch for every square foot of enclosed area subject to flooding.
(ii)
The bottom of all openings shall be no higher than one (1) foot above grade.
(iii)
Openings may be equipped with screens, louvers, valves or other coverings or devices provided they permit the automatic flow of floodwaters in both directions.
(B)
Electrical, plumbing and other utility connections are prohibited below the base flood elevation.
(C)
Access to the enclosed area shall be the minimum necessary to allow for parking of vehicles (garage door) or limited storage of maintenance equipment used in connection with the premises (standard exterior door) or entry to the living area (stairway or elevator).
(D)
The interior portion of such enclosed area shall not be partitioned or finished into separate rooms.
(Ord. No. 85-00, Pt. I, 12-28-00; Mo. of 1-23-01; Ord. No. 24-03, Pt. I, 10-14-03; Ord. No. 05-17, Pt. I, 11-8-05)