§ 8.2.1. Storm water management report required.

a.

Every project shall provide a storm water management report prepared by a professional engineer currently registered in the State of Georgia. The purpose of this report shall be to formulate a plan to manage storm water runoff so that storm water runoff hazards are not created and existing runoff-related problems are not exacerbated, either upstream or downstream from or within the boundaries of the property being developed. The engineer shall be responsible for obtaining all information necessary for the report. Hydrologic analysis and detention pond hydraulics (excluding dams as defined in section 8.6 of these regulations), pipe and open channel hydraulics, culvert hydraulics and water quality best management practices shall be certified by a professional engineer registered in the State of Georgia. Flood studies for any floodplain or flood prone areas, and hydrologic and hydraulic analysis and design calculations which are performed for the design of a dam as defined in section 8.6 of these regulations, shall be certified by a professional engineer registered in the State of Georgia.

b.

The storm water management report shall identify the locations and quantities of storm water runoff entering and exiting the site for both pre- and post-developed conditions. Analysis of the off-site properties shall anticipate future development in addition to addressing existing conditions. It shall contain drainage area delineation maps and other exhibits at satisfactory scale and sufficient in quantity and scope to define the boundaries of the site relative to watercourses, drainage divides, drainage structures, and other pertinent features.

All culverts, pipe systems and open channel flow systems shall be sized based on all on-site upstream areas being developed per the development plans and the off-site upstream areas being fully developed per the land use plan with no detention. Upstream detention may be included when determining flows, provided the engineer calculates the reduced flows by routing the developed flows through any storm water facility included in the analysis rather than assuming the reduction will occur. The engineer shall show that detention facilities used in the analysis will remain, be properly maintained and the storage volume and outlet structure is based on current conditions.

Detention facilities shall be designed using pre-development flows based on existing conditions for all upstream areas including existing on-site lakes and detention. Post-development flows, except the 100-year flow, shall be based on on-site upstream areas being developed per the development plans and existing conditions for off-site upstream areas. The 100-year flow shall be based on on-site upstream areas being developed per the development plans and the off-site upstream areas being developed per the land use plan with no detention. Upstream detention may be included if it meets the conditions as described for culverts and pipe systems. Existing conditions shall be defined as the conditions of the site at the time of application for a land disturbance permit. The existing condition includes on-site lakes and ponds. Pre-development flows shall be determined by routing the pre-development flows through these storm water facilities. Flows used to size the outlet structures for detention facilities that exceed the 25-year design flow, shall be sized as described for culverts and pipe systems.

When more than 50 percent of the property of a developed project site is disturbed for either redevelopment or improvement, the storm water management report shall be prepared for the entire site and existing impervious areas shall be treated as forest in the pre-developed analysis. When 50 percent or less of the property is disturbed, detention shall be provided as required by these regulations for the disturbed area and existing impervious areas which are disturbed shall be treated as forest in the pre-developed analysis.

The report shall contain drainage area delineation maps and other exhibits at satisfactory scale and sufficient in quantity and scope to define the boundaries of the site and off-site areas relative to the watercourses, drainage divides, drainage structures and other pertinent features. The Gwinnett County Geographical Information System (GIS) mapping shall be used where appropriate.

c.

For the purposes of these regulations, the words "downstream" and "analysis" shall have the following meanings. 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 drainageway "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.

(1)

The report shall examine the conditions downstream of the project to a point where the project area is ten percent of the total drainage basin.

(a)

The analysis must include all culverts, obstructions, existing and potential erosion problems, elevations of existing improvements, existing drainage complaints and any other existing modifications to natural conditions. The downstream watercourses and receiving conveyance shall be analyzed to ensure that the channel velocities do not exceed values recommended in the Design Manual nor does the pipe system exceed current design criteria of these regulations, and,

(b)

If the existing downstream conditions are overburdened by the pre-developed flows in the stream, then the developer shall work with the city to resolve the problem. The meaning of "overburdened" 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 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, and,

(c)

If there are any problems identified downstream that are a result of the development, then the developer shall eliminate the conditions causing the problem.

(2)

Hydrographs shall be analyzed at least at two points. One study point shall be at the downstream property line where the watercourse crosses the project site's downstream property line. The second study point shall be downstream of the project at the point where the project area is ten percent of the total drainage basin.

(a)

The study will compare pre-developed hydrographs with post-developed hydrographs for the two, five, ten, 25, 50 and 100-year flood frequencies, and,

(b)

Comparison of peak flows shall include the timing of the hydrographs, and,

(c)

Hydrographs shall be based on a 24-hour storm.

(d)

The analysis shall be in accordance with the Gwinnett County Storm Water Design Manual.

d.

The following criteria shall be evaluated by the engineer preparing the storm water management report and in determining whether or not detention should be required for any portion of any site:

(1)

Existing land uses downstream,

(2)

Anticipated future land uses downstream,

(3)

Magnitude of increase in peak flows due to development,

(4)

Presence of existing drainage problems,

(5)

Capacity of existing and anticipated drainage systems,

(6)

Creation of concentrated flows where none had occurred previously,

(7)

Availability of feasible locations for detention facilities,

(8)

Existing flows generated off-site which pass through the project site, and,

(9)

The nature of the receiving watercourse.

e.

Where detention for a proposed project is provided in a regional detention facility that was permitted prior to January 1, 2001, the developer shall provide a copy of the original study that met the regulations at the time the facility was permitted. If the approved study cannot be found, then the engineer shall provide a recreated study. The project shall be exempt from restudy and any modifications required to meet regulations effective after January 1, 2001, provided the proposed project is in keeping with the intent of the original detention study and the detention facility is maintained.

f.

When a development uses an existing facility where the last approved certification and record drawing of the facility was over 18 months prior to the new development's submittal, the engineer shall provide one of the following.

(1)

A new survey, drawing and certification showing that the outlet structure is constructed as approved and the flood storage and water quality volume of the facility is equal to or greater than the volume required when the facility was approved.

(2)

Construction plans and calculations showing that the outlet structure will function as designed and the flood storage and water quality volume of the facility will be equal to or greater than the volume required when the facility was approved once the proposed maintenance has been performed.

(3)

A new record survey, drawing, study and certification showing that the facility meets the development requirements when the facility was approved.

f.

Design Criteria—General. All design related to storm water shall be in accordance with the Gwinnett County Storm Water Design Manual.

g.

Evidence of acquisition of applicable non-local permits. The applicant shall certify and provide documentation that all other applicable environmental permits have been acquired for the site prior to approval of the storm water management report.

(Ord. of 6-28-2004(5))