Gwinnett County › Unified Development Ordinance

Title 3 — Development and Permitting, Chapter 810 — Stormwater Conveyance Systems

current as of 2025-10-28currency checked manuallyOfficialofficial source ↗3 sections · full chapter

§ 810-10

General Requirements

810-10.1 Drainage Improvements Required.

Stormwater conveyance systems, which may include but are not limited to culverts, storm drainage pipes, catch basins, drop inlets, junction boxes, headwalls, gutter, swales, channels, and ditches, shall be provided for the protection of public right-of-way and private properties adjoining project sites and/or public rights-of-way. Stormwater conveyance systems that are designed to carry runoff from more than one parcel, existing or proposed, shall meet the requirements of this UDO.

810-10.2 Design Criteria—General.

All stormwater conveyance systems design calculations shall be certified by the authorized registered professional (refer to Section 800-20):

A.Methods to calculate stormwater flows shall be in accordance with the Gwinnett County Stormwater Management Manual.

B.All portions of a stormwater conveyance system shall be analyzed using the same methodology.

C.Run-off coefficients used for the Rational Method and runoff Curve Numbers used for the SCS Method shall be consistent with those shown in the Gwinnett County Stormwater Management Manual.

810-10.3 Design Criteria—Conveyance Systems.

Conveyance system design is to be in accordance with the methods contained in the Gwinnett County Stormwater Management Manual. The 100-year ponding limits at and upstream of the culvert shall be shown on the Development Plans and on the Final Plat (if applicable).

§ 810-20

Collection System Guidelines

810-20.1 Outlet Location — Conveyance Systems.

A.Outlet structures (such as headwalls) shall not be located closer to the project site's property line with an adjoining property than the greater of the distance necessary to construct velocity protection or a flow distance equal to six pipe diameters. For non-circular conduits, this distance shall be six times the rise dimension of the conduit.

B.The invert elevation of a piped conveyance system outlet shall be no more than two feet above the elevation of the bottom of the receiving watercourse.

810-20.2 Energy Dissipation.

The maximum post-developed flow velocity at the project site's downstream property line with an adjoining tract shall not exceed the maximum pre-developed flow velocity. Calculations shall be required to support this velocity standard.

810-20.3 Concentrated Flows.

A.The discharge of concentrated flows of stormwater into public roadways shall be avoided as practicable. In no case shall such concentrated flows, including flows from swales, ditches, draws, driveways, or piped systems, exceed the allowable peak flow rates in Table 810-1 below

Table 810-1. Maximum Flows into Streets.

Street Classification	Allowable Peak Flow Rate
	For a 2-Year Storm
Local	2.0 cfs
Minor Collector	1.0 cfs
Other	0.5 cfs

contained within a piped drainage system or maintained in a natural watercourse. The stormwater conveyance shall be in a drainage easement.

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§ 810-30

Walls

810-30.1 When permanent grades are proposed with a resulting slope steeper than one foot vertical for

every three feet of horizontal, resulting wall height (measured from top of footing to top of wall) will exceed four feet, or surcharge loading will be applied to the back side of the wall, then an appropriate earth retaining structure shall be designed and permitted. The wall may be constructed of concrete, masonry, segmental block or wire basket units, or other appropriate structural materials. Walls that do not exceed six feet in height may also be constructed of timber. Retaining Wall design shall be in compliance with the current Georgia State Minimum Standard Building Code- and ASCE/SEI 7-16. As applicable, wall design shall also comply with ACI 318-18 (Concrete Wall), TMS 402-16 (Masonry Wall), NCMA Design Manual for Segmental Walls (Modular Wall), and/or ANSI/AWC NDS-2018 (Wood Wall). Walls exceeding six feet in height shall be designed and drawings shall be prepared by a Georgia Registered Professional Engineer.

810-30.2 Walls and wall foundations or supports shall be located in such a fashion as to not encroach upon

existing or proposed drainage easements, drainage courses, or floodplains. Walls shall be constructed so that the zone of influence from the wall shall not encroach upon the area of potential excavation to maintain, service or replace a utility facility such as stormwater, sanitary sewer, or water facility. Grading on each side of the wall shall be designed so that the wall will not encumber the natural flow of stormwater surface runoff. Walls and wall supports shall be located at a distance from easements to allow for anticipated future maintenance of the easement while still maintaining the structural integrity of the wall.

810-30.3 Retaining/detention pond walls of any height that are proposed for the purpose of stormwater

management must be designed by a Georgia Registered Professional Engineer and must be permitted. Walls shall be designed for hydrostatic loading under full pond conditions and retained backfill soil under dry pond conditions.

The hydrological design must allow for a freeboard dimension and emergency overflow capacity that meets the requirements outlined in the Gwinnett County Stormwater Management Manual. The overflow device should be placed above the projected 100-year flood elevation within the detention area. Detention area and wall calculations are required submittals to Gwinnett County Planning and Development.

810-30.4 Retaining walls constructed in or adjacent to a residential subdivision shall comply with the

following:

A.For proposed developments with a residential component, no greater than 50% of a stormwater management facility perimeter may be walled without approval from the Director of Planning & Development.

B.Retaining Walls along the perimeter of a stormwater management facility where the top of wall elevation exceeds the surface elevation and which face a habitable structure and/or a public right-of-way, private access easement or other means of access, shall be screened. The screening shall include native vegetation supplemented as necessary via a preserved landscape strip at least 20-ft in width, a planted landscape strip at least 15-ft in width and the wall may be faced with brick or stone. The strip shall be located a minimum of 10-ft from the face of wall. Vegetation shall be maintained by the property owner on non-residential properties, or a qualified homeowner's association on residential properties or a property owner's association.

C.Retaining walls which face the property line of a residential property or properties shall be located a minimum of 1.5 times the height of the retaining wall from the property line or within the building setbacks of the property/lot. The wall setback area shall be landscaped with native vegetation to screen the wall from the adjacent property.

810-30.5 Any construction that may impact or be within the right-of-way of an existing or proposed water or

sanitary sewer easement must be approved by the utility providing service.