§ 8.2.2. Storm water detention required.

a.

Whenever a storm water management report indicates that an adverse impact from storm water runoff related impact is expected to result from the development of a property, that project shall be required to provide storm water detention facilities. The meaning of "adverse impact" shall apply when pre-developed flows did not cause difficulties and post-development flows do. Difficulties shall include but not be limited to situations where 25-year velocities exceed the non-erosive velocity of the stream, habitable structures are shown to be subject to increased depth of flooding for any frequency up to and including the regulatory flood and storm water facilities that cannot carry the design storm in accordance with these regulations.

b.

Stormwater detention facilities required in section 8.2.2.a shall be provided, unless the registered professional engineer currently registered in the State of Georgia certifies and provides certified documentation supporting the conclusion to the director of planning and development or his/her designee that at least one of the following is true and correct as applicable.

(1)

The non-detained, post-development runoff will leave the project site as sheet flow, and will not have an adverse impact upon downstream properties. The increase for a 25-year storm should not exceed one cfs over a length perpendicular to the flow of 100 feet.

(2)

The effect of detention would be to concentrate flows where sheet flow had occurred under pre-developed conditions, and any impact of increase sheet flows upon downstream properties would be less adverse than that which would result from the concentrated flows from a detention facility even if energy dissipation devices were employed.

(3)

The undetained flow will pass through downstream properties, in drainage easements obtained by the developer, to an existing detention facility that has been designed to manage the upstream property's runoff or to the point in the downstream analysis (see 8.2.1.c) which shows that detention is not required.

(4)

Where the site runoff will flow directly into a stream or lake without crossing off-site properties:

(a)

24-hour detention of the one-year storm is required if water quality protection is required for the project.

(b)

Only peak detention for the two-year though the 25-year storm is not required if the downstream analysis using timing of the hydrographs shows no adverse impact from the exit of the site to the point immediately downstream from the project in the drainage basin where the project area is 10 percent of the total basin area.

c.

Should the professional engineer conclude that storm water detention might not be necessary because of anticipated compliance with section 8.2.2.b., rigid compliance with all of the following criteria is mandatory:

(1)

A storm water management report shall always be required whether or not storm water detention is required.

(2)

If the applicant proposes to show that the detention requirement may be eliminated for all or a portion of a project, then a pre-submittal conference with the Department staff is required prior to preparation and submittal of construction plans for the project.

(3)

At the pre-submittal conference with the staff, the consultant shall be prepared to discuss the downstream analysis findings as follows:

(a)

The affected stream must be analyzed downstream from the project to a point where the project area is ten percent of the total drainage basin. The analysis must include all culverts, obstructions, existing and potential erosion problems, elevations of existing improvements, and any other existing modifications to natural conditions; and,

(b)

If the existing downstream conditions are overburdened by the pre-developed flows in the stream, then detention shall be required unless the developer elects to eliminate the downstream overburdened conditions at his or her expense when the development occurs;

(c)

If there are any existing drainage complaints downstream, then detention shall be required unless the developer elects to minimize the conditions causing the complaint at his or her expense when the development occurs.

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