Storm Water Design Manual

STORM WATER DESIGN MANUAL

INDEX OF REVISIONS

NUMBER /

DATE

CHAPTER

DESCRIPTION

1 / August 2, 1999 / 1 – State Waters Buffer Variance / Clarified that buffer is measured from point where vegetation has been wrested by normal stream flow or wave action. Gwinnett assumes that point to be where the drainage area equals 20 acres or greater. The design professional is responsible to determine and identify any buffer.
2 / December 15, 1999 / 1 - Floodplain Management Ordinance
1 -Development Regulations
1 -Development Regulations
2 – Hydrology
3 - Storm Drainage Systems
8 – Water Quality Best Management Practices / Included copy revised Ordinance that added regulatory floodplain and definition of substantial damage.
Included copy of DRA001 that added Storm Water Manual to regulations. Revised articles 7, 8, 9 and 10.
Included copy of DRA002 that revised articles 8 and 9.
Replace page 2-6; typographical error
Replace pages 3-4, 3-12, 3-34 and 3-36; typographical errors
Replace entire chapter and appendices due to page numbering and revisions.
Specific pages revised are 8-8, 8-10, 8-13, 8-24, 8-34, 8-35, 8-A1, 8-A3, 8-A4, and 8-B5 through 8-B12. Replacements are to correct typographical errors, make clarifications, add a procedure to approve alternate structural BMPs and revise the Maintenance Agreement template to handle commercial and residential as well as different types of property owners.
3 / March 31, 2000 / Cover Sheet
1 - Regulations
Revision Index / Replace
Replace Table of Contents. Manual Owner advised to update to latest published regulations.
Add Revision Index
NUMBER /

DATE

CHAPTER

/ DESCRIPTION
4 / January 1, 20001 / 2 /

Updated the rainfall intensity table (Table 2-3) for the Atlanta Metropolitan Area

Revised the Simplified SCS methodology to reflect the Water Quality Storm

Incorporated and revised the Water Quality Storm – Volume and Peak Discharge methodology

Updated the lagtime equation using the most recent USGS publication

8 /

Incorporated water quality (TSS) performance criteria and Storm Water Quality Performance Review Form for storm water BMPs.

Removed anti-seep collar requirements.

Updated the undisturbed riparian buffer definition

Revised structural BMP specifications and figures to reflect the Water Quality Storm

-Dry Extended Detention Ponds
-Wet Extended Detention Ponds
-Sand Filters
-Constructed Wetlands
-Infiltration Trenches
-Vegetated Filter Strips and Flow Spreaders
-Grassed Swales
-Porous Pavement
-Oil/Grit Separator

Revised example problems to reflect the Water Quality Storm
Updated Gwinnett County Forms

Developed Bio-retention BMP specifications using information from the ARC GA Stormwater Management Manual

NUMBER /

DATE

CHAPTER

/ DESCRIPTION
5 / October 1,2001 / 1
2
3
8 /

Removed regulation indexes
Page 2-3 Changed Table 2-1 to reflect that SCS method may not be used for sizing water quality BMPs.

Page 2-5Corrected 10-year, 6-hour rainfall intensity from 0.88 to 0.68. This data was taken from the Georgia Stormwater Management Manual, Table A-12 (Roswell)
Page 2-27Changed example problem pre-developed areas from “Forest land – poor cover” to Forest land – good cover” to reflect appropriate CN values in example problem.

Page 3-4Deleted provision that allowed gutter spread to be 10 feet on one side of the local street so long as the combined spread on both sides of the street did not exceed 16 feet. This was done to be consistent with DRA2001-002, a regulation amendment that was passed in combination with the conservation subdivision overlay.
Page 3-12 Correct bare to “bars”.
Page 3-30Corrected equation 3.9. Equation should read Sf=(Q2/K2 ), not Sf=(Q2)/K
Page 8-13Corrected removal efficiency for BMP3 (Grassed Swales) to read 15% instead of 50%.
Page 8-31 & 32changed required specifications for vegetated filter strips and flow spreaders to clarify the concepts of filter strip length and width.
Page 8B-1 Revise access easement requirements.
Pages 8B-8, 9, &10 Change date from 19__ to 20__.
Page 8B-24 Change column heading from Zoning to Land Use and added Bond Exp. Date

Memorandum

To:Development Community

CC:David Evans, Director Storm Water Management

From:David Chastant, Chief Engineer Storm Water Management

Date:February 20, 2002

Re:Downstream Analysis

The current regulations require that a downstream analysis be performed. One critical part of the analysis that is not being addressed uniformly in the analysis is the receiving channel immediately below the development. The following information provides guidance on what information is needed in a downstream analysis to cover this portion of the analysis.

A downstream analysis shall include, at a minimum, a detail written description of the first 500 feet off site. At least one photograph looking downstream with some object included in the photograph for scale shall be included. The potential impacts of the storm water discharge shall be discussed.

Please contact the Storm Water Management Development Review office with any questions at 770-822-7366.

Memorandum

To:Development Community

CC:David Evans, Director Storm Water Management

From:David Chastant, Chief Engineer Storm Water Management

Date:February 20, 2002

Re:Storm Water Outfall Analysis

When the property downstream is developed, special information and requirements apply for outfalls that meet two or more of the following criteria.

Receiving channel/ stream has a drainage area of less than 100 acres.
Receiving channel/ stream is not in a recorded drainage easement.
Receiving channel/ stream does not have defined channel with a depth of at least one foot.
Receiving channel/ stream is not located along downstream property line.
Receiving channel/ stream has a pipe in the channel within 500 feet that is smaller than the proposed outfall pipe.

In cases where the outfall meets the criteria above, an on-site meeting with the County is recommended. Potential mitigation alternatives include, but are not limited to, the following.

Relocating outlet channel to property line and constructing a channel or installing a pipe.
Piping the outfall to a recorded drainage easement.
Upgrading receiving channel/ undersized pipe.
Designing and constructing the pond such that the principal spillway handles the routed 25-year storm and an emergency spillway is constructed to handle flows exceeding this flow.
Obtaining a drainage easement from the downstream property owner.
Overdetaining the flow so the outfall pipe can be reduced in size.

Mitigation may not be required if the design professional demonstrates that a suitable alternative does not exist. In this case, the problem, the alternatives considered, and the reason they are not acceptable shall be documented.

Please contact the Storm Water Management Development Review office with any questions at 770-822-7366.

Memorandum

To:Engineers and Developers in Gwinnett County

CC:David Evans, P.E.

From: David Chastant, P.E.

Chief Engineer, Storm Water Management

Date:May 6, 2002

Re:Gwinnett County Storm Water Design Manual, Correction

The following changes should be corrected in the Design Manual with a revision date of October 1, 2001. They will be corrected in next edition. The BMP Tracking Form has been revised to add Wet and Dry Water Quality Basins and Stream Buffers. A Surety Information Form has been added. The last revision is to remove the requirement for a legal description from the easement agreements. These corrections should be used in all development submittals to the County effective June 1, 2002.

Memorandum

To:Engineers and Developers in Gwinnett County

CC:David Evans, P.E.

From: David Chastant, P.E.

Chief Engineer, Storm Water Management

Date: August 6, 2003

Re:Gwinnett County Storm Water Design Manual, Correction

The following error should be corrected in the Design Manual with a revision date of October 1, 2001. It will be corrected in next edition. This correction should be used in all development submittals to the County.

Page 4-8, Table 4-3 Manning’s n Values-

Delete reference to Corrugated Metal Pipes, Helical with a Manning’s n of 0.012.

Add “and Helical” between “Annular Corrugations”.

The design references were checked, and a Manning’s n of 0.012 can only be achieved for full flow in a 12” culvert that achieves a spiral flow. Spiral flow cannot be achieved with partial flow, or in a system with catch basins and sediment in the bottom of the pipes.

A Manning’s n of 0.024 should be used for CMP with annular or helical corrugations.

Memorandum

To:Don Jascomb, A.I.C.P.

Director of Development

CC:David Evans, P.E.

Director of Storm Water Management

From: David Chastant, P.E.

Chief Engineer, Storm Water Management

Date: August 11, 2003

Re:Changes in Storm Water Review and Inspection Procedures

I have attached four items regarding changes to Storm Water Procedures that I would like to add to the agenda of the next Development Advisory Committee (DAC) meeting for discussion. The first two items have already been implemented and I would like for the DAC to be aware of them.

1.)Correction to the Storm Water Design Manual, Change the Manning’s “n” value for helical corrugated metal pipe from 0.012 to 0.024.

2.)BMP Facilities Inspection and Enforcement Policy, During the maintenance bond period, when deficiencies are found, the developer will receive a Notice to Comply and 14 days to correct the problem. If the problem is not corrected, the developer will receive a Notice of Violation (NOV) and 14 days to correct the problem. If the problem is not corrected, the bond will be called.

The next two items are proposed to be effective September 1, 2003.

3.)Retaining Wall Permitting and Inspection, Individual permits for site walls will be required along with site inspection and certification by a geotechnical firm and the engineer.

4.)Change in Storm Water Review Procedure, Plans will not be reviewed unless the all the information is submitted. Once the plans are submitted, no revised information to the submittal package will be accepted. When plans are reviewed, the reviewer will indicate on the review sheet how the revised documents should be submitted back to the Storm Water Division. Based on how much time the reviewer feels will be needed to verify that the comments have been addressed, the reviewer will ask that the revised plans be walked through, dropped off, or resubmitted. Plans that are considered of poor quality by the reviewer must be resubmitted and will have the lowest review priority.

Please contact me at 770-822-7149 if you have any questions.

Memorandum

To:Engineers, Landscape Architects, Builders and Developers

From: David Chastant

Date: November 5, 2003

Re:Residential Drainage Study or Plan Approval Process

The following will be added to the Gwinnett County Design Manual in the next revision. This procedure will be effective after the regulations revisions have been passed.

1.5 Development Regulations

Any land disturbance activity or any development activity must first comply with the requirements of the Gwinnett County Development Regulations.

The following articles provide additional procedural information not contained in the Regulation.

1.5.1 Residential Drainage Study (RDS) or Residential Drainage Plan (RDP) Approval Process. Reference: DR 8.1.7 and DR 10.5

All lots noted with a Residential Drainage Study (RDS) or Residential Drainage Plan (RDP) designation on the final plat will have a hold placed on the Certificate of Occupancy (CO). The CO will not be released until the following is received from an engineer or a landscape architect with their seal, signature and date affixed.

Lot ______Block______Subdivision ______

Address______

I have inspected the above referenced lot and the following has been verified in the field: The lot has adequate drainage to insure that the house will not flood in the 100-year storm, the drainage patterns on the lot conform to the hydrology study for the subdivision, the grading plan for the lot if provided in the preliminary plans or Residential Drainage Plan, and any improvements needed to bring the drainage system adjacent to or within the limits of the lot in compliance with Gwinnett County regulations have been made.

______

Engineer / Landscape Architect Typed / Printed Name

______

Signature DateSeal

Procedure:

Designer will provide grading plans of the subdivision as required for approval of the preliminary plans of the subdivision (see DR 8.1.7).
Storm Water Management (SWM) will review and approve the grading and hydrology study during normal subdivision preliminary plan approval.
The developer has three choices regarding grading.
If the developer does not want to grade the lots during preliminary plat stage, then the grading plan will be marked as “For Information Only”, but will be included in the set of approved construction plans. The lots will not be graded prior to approval of the Final Plat.
If the developer wants to grade the lots during the preliminary plat stage, then the grading plan will be included in the set of approved construction plans. The lots will be graded prior to approval of the Final Plat
If the developer decides to grade or not grade some lots during the preliminary plat stage, then the designer must revise the construction drawings to show what grading will be done. The “Information Only” grading plan must also be revised. At a minimum a note should be added on this plan indicating which lots will be graded prior to Final Plat Approval.
SWM will require lots not graded prior to Final Plat approval be identified on the Final Plat as needing a Residential Drainage Study (RDS) or Residential Drainage Plan (RDP). SWM may require a RDS or RDP be identified and noted on the plans during the Preliminary Plat and Final Plat approval process for other drainage concerns.
SWM will also require lots needing an Elevation Certificate be identified and noted as “RDS-E” or “RDP-E” on the plans during the approval process.
Planning and Development (P&D) will use the Final Plat to add a Parcel Tag on each lot in Permit Plan (PP) that a RDS or RDP is required and P&D will add a Parcel Tag in PP on each lot that an Elevation Certificate is required as appropriate.
P&D will require prior to issuance of a building permit:
On a lot with an RDP or RDP-E, the builder shall obtain an approved Residential Drainage Plan from Storm Water Management (see DR 10.5) prior to receiving a building permit.

On a lot with an RDS or RDP, the builder shall sign an affidavit stating the following. On a lot with an RDS-E or RDP-E, the words in brackets shall be added.

Lot ______Block______Subdivision______

Address______

I understand that prior to receiving a Certificate of Occupancy, the above referenced lot must be inspected by an engineer or landscape architect and the following must be certified by the engineer or landscape architect: The lot has adequate drainage to insure that the house will not flood in the 100-year storm, the drainage patterns on the lot conform to the hydrology study for the subdivision, the grading plan for the lot if provided in the preliminary plans, and any improvements needed to bring the drainage system adjacent to or within the limits of the lot in compliance with Gwinnett County regulations have been made.

[In addition, no framing inspection will be approved until an Elevation Certificate is approved by Storm Water Management.

______

Builder Typed/ Printed Name

______

Signature Date

On lots that are not part of subdivision and have no designation of RDS, RDP, RDS-E, or RDP-E, SWM may require a RDS or RDP with an Elevation Certificate as appropriate. The requirements for an RDP per 10.5.2 shall be as applicable. The certification by an engineer or landscape architect may be waived as appropriate.

P&D will require after the signed affidavit for the RDS or RDP has been received,
For lots with a requirement for an elevation certificate, the Building Permit will be issued and a condition will be placed on the Permit that SWM must approved an Elevation Certificate after the foundation is finished but before a framing inspection can be approved. Once the Elevation Certificate is approved by SWM, the procedure in 7.b shall be followed.
For lots with no requirement for an elevation certificate or with an approved Elevation Certificate, the Building Permit will be issued and a condition will be placed on the Permit that the Certificate of Occupancy will not be issued until the following certification has been received. The certification shall be turned into the Building Permits Section of Planning and Development.

Lot ______Block______Subdivision ______

Address______

______

Engineer / Landscape Architect Typed / Printed Name

______

Signature DateSeal

9.P&D will issue a Certificate of Occupancy, once the certification from the designer has been received, the condition of the Permit will be signed off and if all other conditions for the lot have been met.

Memorandum

To:Engineers and Developers in Gwinnett County

CC:David Evans

From: David Chastant

Date: November 10, 2003

Re:Design Manual Revisions

In conjunction with the proposed Development Regulations revisions, the following revisions to the Gwinnett Storm Water Design Manual dated October 1, 2002 have been identified to improve and clarify the manual and to more closely follow the Georgia Stormwater Management Manual. The following revisions will be added to the Gwinnett County Design Manual in the next revision. These changes will be effective after the regulation revisions have been passed.

Design Manual Revisions

1)Floodplain Study Submittals - Section 1.3.1 – Submittals must be in digital format and the details of that submittal are specified.

2)Hydrology Chapter 2 Typographical omissions – Add titles to tables and include Table 2-11.

3)Storm Drainage Systems Chapter 3 – Limit maximum structure spacing to 300’.

4)Water Quality Performance Criteria - Section 8.2 – Water Quality model is updated to print out required BMP tracking from and method to calculate the efficiency of BMPs in series is revised.