§ 8.2.3. Detention design criteria—General.

a.

All storm water detention design calculations shall be certified by a professional engineer currently registered in the State of Georgia.

b.

All storm water detention facilities shall be designed to detain the one-year storm runoff, for the area draining to the pond, for 24 hours. For the project, this volume called the channel protection volume, shall be equal to or greater than the one-year storm runoff volume from the project. In addition, these facilities shall control the peak flow rates associated with storms having two-year, five-year, ten-year, and 25-year return frequencies so that flows from the developed site do not exceed those associated with pre-development conditions at the project boundary nor increase the peak flows downstream from the project to the point in the drainage basin where the project area is ten percent of the total basin. Where adverse impacts, as defined in section 8.2.2.a, occur during the 100-year storm, the 100-year storm shall also be regulated.

c.

A variety of methods of achieving storm water management goals shall be acceptable in providing detention facilities. The type of facility provided shall be based on the following criteria:

(1)

The type of development which the detention facility is being provided,

(2)

The type of development which the detention facility is intended to protect,

(3)

Volume of storm water to be stored,

(4)

Origin and magnitude of the flows to be managed,

(5)

Topographic opportunities and limitations,

(6)

Safety considerations,

(7)

Maintenance requirements,

(8)

Aesthetic considerations,

(9)

Likelihood of facility operation interfering with access to public or private facilities,

(10)

Proximity of facility to property lines, utilities, buffers, etc., and;

(11)

Similar site-specific constraints.

d.

Detention facilities may be of any of the following types, and two or more types may be used in combination with one another:

(1)

Normally-dry basins, whether excavated or created by damming a natural drainage feature, or a combination of both methods,

(2)

Lakes and ponds, whether excavated or created by damming a natural drainage feature, or a combination of both methods,

(3)

Parking lot facilities,

(4)

Underground facilities, and

(5)

Roof top facilities.

e.

Reservoir routing methods shall be used for all detention facility design. The size of the orifice to detain the one-year storm for the facility shall be computed using the following orifice equation with a 24-hour draw down time from the elevation of the total channel protection volume (CPV) and an orifice coefficient of 0.60. The minimum elevation of the two-year control shall be at the maximum routed pool elevation of the one-year storm and not the pool elevation of the total one-year storm volume.

h	=	head measured in feet from the elevation needed to store the total one-year runoff volume (CPV) to the centroid of the orifice;
Qa	=	average CPV outflow rate in cfs;
Qa	=	CPV/3600 × 24;
A	=	required orifice area in square feet;
A	=	Qa/(0.6 × (64.4 × h/2) 0.5 ).

 

f.

The detention methodology used for any given project shall conform to the Gwinnett County Storm Water Design Manual.

g.

Runoff coefficients and runoff curve numbers used for pre- and post-development conditions shall be consistent with those shown in the Gwinnett County Storm Water Design Manual. The USGS method shall be used where applicable to check the magnitude of peak flows when other hydrologic methods recommended in the manual are used.

h.

Calculations shall be provided showing how all times of concentration or lag times were computed, both for pre- and post-developed conditions. Likewise, adequate support must be provided for all composite runoff coefficients or curve numbers used.

i.

If a computer program is used for hydrologic and hydraulic analysis and design, including generating and routing hydrographs, the output from the program shall be summarized in the storm water management report, and the name and version of the program shall be indicated. Computer output sheets shall be attached to the report as required to substantiate the input parameters used in the computer evaluation.

j.

The design of every detention facility of any type shall consider the effects both of inflows in excess of those the facility is designed to accommodate and of malfunctioning of the primary outlet system. A safe path for overflow condition flows shall be provided.

k.

Weirs shaped like a "V" (V-notch weirs) shall be used where practical, considering structural or hydrological concerns.

l.

Prior to beginning the design of the storm water detention facility, the design engineer shall submit a concept plan outlining the proposed system which will include as a minimum the following items:

(1)

Pre-development and post-development drainage basin maps,

(2)

Proposed site plan sketch (to scale) showing major site items, contours and all impervious areas,

(3)

Locations, approximate size and type of all BMP and detention facilities.

The concept plan will be reviewed and agreed upon between the director of planning and development or his/her designee and the design engineer and will form the basis for the final storm water facility design.

m.

In residential subdivisions, no more than 50 percent of the basin perimeter may be a wall of any type.

(Ord. of 6-28-2004(5); Ord. of 4-25-2005(1))