STORMWATER

MANAGEMENT REPORT FOR

DOWNTOWN NORCROSS

PARKING IMPROVEMENTS

Certifying seal or stamp:

07/20/26

Date:

06/05/2026 Revised:

07/20/2026

Project Name:

Downtown Norcross Parking Improvements

CES #6170.05

Project Address:

65 Lawrenceville St Norcross, GA 30071

Owner:

City of Norcross

PREPARED BY:

Columbia Engineering and Services, Inc.

2862 Buford Highway, Suite 200 Duluth, GA 30096

770.925.0357

Table of Contents

Introduction

Stormwater Quantity Management

Pre-Development Conditions Hydrologic Analysis

Post-Development Conditions Hydrologic Analysis

Methodology

Results

Stormwater Quality Management

Stormwater Conveyance Calculations

Analysis of Downstream Conditions

Exhibits:

Exhibit A Location Map

Exhibit B Pre-Developed Drainage Map

Exhibit C Post-Developed Drainage Map

Exhibit D BMP Map

Exhibit E1 Dry Pond A OCS Detail

Exhibit E2 Dry Pond A Cross-Section

Exhibit E3 Orifice End-Cap Detail

Exhibit F Storm Pipe Table

Exhibit G Downstream Drainage Map

Appendices:

Appendix A Site Data Information

Pre- and Post- Developed Curve Number Calculations

Time of Concentration Calculations

Pond Stage Storage Calculations

Stream Channel Protection Calculations

Permeable Pavers Runoff Reduction Calculations

Bioretention Area Runoff Reduction Calculations

Appendix B Hydraflow Hydrographs Return Period Recap and Summary Report

Hydraflow Hydrographs 1 through 100-Year Report with Pond Report

Appendix C Contech CDS® Documents and Details

Appendix D Stormwater Quality Site Development Review Tool

Appendix E City of Norcross Runoff Reduction Infeasibility Form

Appendix F United Consulting Soil Infiltration Testing Results

Appendix G Downstream Analysis Calculations

Downstream Analysis Hydraflow Hydrographs Return Period

Downstream Analysis Hydraflow Hydrographs Summary Report

Appendix H NOAA Point Precipitation Frequency Estimates for Norcross, Georgia

Appendix I Operations and Maintenance Plans for Bioretention Area

Operations and Maintenance Plans for Permeable Bricks

Inspection and Maintenance Guide for Contech CDS®

Operations and Maintenance Plans for Dry Extended Detention Basin

Page | 1

Introduction

The subject site is located at 65 Lawrenceville Street in Norcross, Gwinnett County, Georgia (see

Exhibit A, Location Map). The approximately 6.11-acre site is currently developed as part of downtown Norcross and contains the existing City Hall building, associated parking areas, and supporting infrastructure. The proposed project consists of improvements to the existing parking facilities, including the addition of new parking spaces, and redevelopment of portions of the site to create a public plaza area with enhanced landscaping. The project also incorporates several stormwater best management practices (BMPs), including the construction of a stormwater detention pond and other measures designed to improve stormwater management and water quality.

The proposed stormwater management facilities will be analyzed in this report addressing

Stormwater Quantity Management, Stormwater Quality Management and Analysis of

Downstream Conditions for the development per City of Norcross requirements.

Stormwater Quantity Management

Pre-Development Conditions Hydrologic Analysis

The subject site consists of an existing City Hall building with associated parking areas and an adjacent wooded tract. The combined property encompasses approximately 6.11 acres.

For purpose of this analysis, the overall project site has been separated into six drainage basins:

On-site Drainage Basin A1, On-site Drainage Basin A1 Bypass, Off-site Drainage Basin A1, On-site

Drainage Basin A2, Off-site Drainage Basin A2 and Off-site Drainage Basin A3. See Exhibit B, Pre-

Developed Drainage Map. All drainage basins within the study area ultimately discharge to Beaver

Ruin Creek.

On-Site Drainage Basin A1 consists of 2.93 acres of the site of which 1.59 acres are impervious from this existing parking lot and portion of City Hall. The basin drains in a northeasterly direction to Study Point A. On-Site Drainage Basin A1 has a curve number of 77 and a time of concentration of 13.33 minutes.

On-site Drainage Basin A1 Bypass consists of 1.58 acres of the site of which 1.43 acres are impervious. This basin is the Historic Norcross Cotton Gin that will essentially remain unchanged.

The basin drains in a northeasterly direction to Study Point A. On-site Drainage Basin A1 Bypass has a curve number of 96 and a minimum time of concentration of 5.0 minutes.

Off-site Drainage Basin A1 consists of 2.21 acres of which 0.94 acres are impervious. This basin consists of the off-site area coming from the railroad right of way and Wingo Street onto the site.

The basin drains in a northeasterly direction to Study Point A. Off-site Drainage Basin A1 Bypass has a curve number of 82 and a minimum time of concentration of 11.54 minutes.

On-site Drainage Basin A2 consists of 1.61 acres of the site of which 1.11 acres are impervious.

The basin drains in a northeasterly direction to Study Point A. On-Site Drainage Basin A2 has a curve number of 92 and a time of concentration of 9.08 minutes.

Page | 2

Off-site Drainage Basin A2 consists of 0.05 acres of the site of which 0.04 acres are impervious.

The basin drains in a northeasterly direction to Study Point A along Lawrenceville Street. Off-Site

Drainage Basin A2 has a curve number of 93 and a minimum time of concentration of 5.0 minutes.

Off-site Drainage Basin A3 consists of 0.14 acres of the site of which 0.13 acres are impervious.

The basin drains in a northeasterly direction to Study Point A along Jones Street. Off-Site Drainage

Basin A3 has a curve number of 97 and a minimum time of concentration of 5.0 minutes.

See Appendix A for the pre-developed basin information.

Post-Development Conditions Hydrologic Analysis

The proposed development consists of the redevelopment and expansion of the existing parking facilities, including the addition of new parking spaces and the construction of a decorative paver plaza area that may be utilized for event parking. The total land disturbance associated with the proposed development is approximately 2.18 acres.

For the purposes of this analysis, the post-development project site has been separated into six drainage basins: On-site Drainage Basin A1, On-site Drainage Basin A1 Bypass, Off-site Drainage

Basin A1, On-site Drainage Basin A2, Off-site Drainage Basin A2, and Off-site Drainage Basin A3.

These drainage basin boundaries are shown on Exhibit C, Post-Development Drainage Map.

Similar to the pre-development conditions, runoff from all drainage basins within the study area is conveyed through the existing drainage network and ultimately discharges to Beaver Ruin

Creek.

On-Site Drainage Basin A1 consists of 2.99 acres of the site of which 1.99 acres are impervious.

The increase in impervious area is primarily attributable to the addition of new parking facilities.

The basin drains in a northeasterly direction to Study Point A. On-Site Drainage Basin A1 has a curve number of 86 and a time of concentration of 5.30 minutes via a pipe network. Stormwater runoff from this basin will be treated through multiple BMPs. Although a portion of the parking area within the basin will remain unchanged, the entire drainage basin will be routed through the proposed BMP facilities. By treating the entire basin, including existing impervious surfaces, the proposed improvements provide an overall reduction in post-development runoff rates and volumes from the site.

On-site Drainage Basin A2 consists of 1.54 acres of the site of which 1.01 acres are impervious.

The basin drains in a northeasterly direction to Study Point A. Compared to the pre-development condition, the basin has been slightly reduced in size and contains less impervious area as a result of the proposed parking lot reconfiguration. On-Site Drainage Basin A2 has a curve number of 90 and a time of concentration of 8.50 minutes.

On-Site Drainage Basin A1 Bypass is the only on-site drainage basin that remains unchanged from the pre-development condition. Similarly, Off-Site Drainage Basin A1, Off-Site Drainage Basin A2

Bypass, and Off-Site Drainage Basin A3 are unchanged from the pre-development condition. No modifications to the drainage area, land cover characteristics, or drainage patterns are proposed as part of the development; therefore, the hydrologic characteristics of these basins remain unchanged.

Page | 3

Off-Site Drainage Basin A2 Bypass remains unchanged from the pre-development conditions. No modifications to the drainage area, land cover characteristics, or drainage patterns are proposed as part of the development.

Off-Site Drainage Basin A3 Bypass remains unchanged from the pre-development conditions. No modifications to the drainage area, land cover characteristics, or drainage patterns are proposed as part of the development.

See Appendix A for the post-developed basin information.

Methodology

According to the U.S. Department of Agriculture Soils Maps the onsite areas are comprised of

Group A and D Hydrologic type soil for the site. For the pre- and post-developed conditions, Open

Space/Landscape Area was used in good condition. The CN for impervious was set at 98.

The inflow hydrographs for the detention pond were calculated using the SCS method and generated using the HYDRAFLOW program by Autodesk, Inc. The rainfall intensity was obtained from the Rainfall Tables for Norcross, Georgia in the NOAA Atlas 14, Volume 9, Version 2 as shown in Appendix H and included as the Site Data Information in Appendix A. Pond performance for the 1, 2, 5, 10, 25, 50 and 100-year storm events were analyzed using the Reservoir Routing

Method. The SCS TR-55 method of determining time of concentration was used for all drainage basins in the pre-developed and post-developed conditions. Time of concentration calculation is included in Appendix A.

Results

The results of the computer analysis show that the post-developed peak flows for all Study Points from the pre-developed peak flows will not cause damage to the surrounding properties and are summarized below for the 1, 2, 5, 10, 25, 50 and 100-year storm events. Complete calculations area included in Appendix A and computer print outs are included in Appendix B.

Drainage Basin Summary Table

Drainage

Basin

Pre-

Developed

Area

(acres)

Post-

Developed

Area

(acres)

Pre-

Developed

CN

Post-

Development

CN

Pre-

Developed

Tc (min.)

Post-

Development

Tc (min.)

On-site Drainage

Basin A1

2.93 2.99 77 86 13.33 5.3

On-site Drainage

Basin A1 Bypass

1.58 1.58 96 96 5.0 5.0

Off-site Drainage

Basin A1

2.21 2.21 82 82 11.54 11.54

On-site Drainage

Basin A2

1.61 1.54 92 90 9.08 8.5

Off-site Drainage

Basin A2

0.05 0.05 93 93 5.0 5.0

Off-site Drainage

Basin A3

0.14 0.14 97 97 5.0 5.0

Study Point A Analysis Summary

Page | 4

Storm

Event

(years)

Total Allowable

Pre-Developed

Study Point

Flow

(cfs)

Total

Post-Developed

Study Point Flow with Bypass

(cfs)

Ponding

Elevation for

Pond A

(ft)

1 22.83 14.30 1016.56

2 27.07 18.07 1016.87

5 34.28 25.61 1017.31

10 40.67 32.04 1017.75

25 49.87 39.34 1018.44

50 57.31 43.85 1019.05

100 65.00 48.36 1019.39

As shown in the tables above, there is a decrease for the post-developed flows for the 1- through

100-year storm event for Study Point A. The reduction in post-development runoff is primarily attributable to the proposed stormwater management measures and treatment of the contributing drainage areas. Runoff from the site is piped to an existing downstream detention pond prior to passing beneath Lawrenceville Street through an existing culvert. The flow then discharges to an open drainage channel located along Mitchell Road and is subsequently conveyed through a series of existing storm drainage pipes. The stormwater system continues beneath Buford Highway before ultimately discharging into Beaver Ruin Creek.

Stormwater Quality Management

Runoff reduction practices are stormwater BMPs that reduce the volume of runoff entering the storm drainage system by disconnecting impervious and disturbed pervious surfaces. Any volume not captured by the runoff reduction measures will be treated in a water quality facility.

The first flush runoff from all on-site areas draining to the detention facilities should be treated in a water quality facility. The purpose of channel protection is to provide 24 hours of extended detention of the 1 year, 24-hour rainfall event to protect the downstream channel from scouring during the lower storm frequencies that typically occur more often and have historically not been detained. The required water quality volume and channel protection volume were calculated using the procedure detailed in the Georgia Stormwater Management Manual, 2016 edition.

In order to treat the stormwater runoff for this site, we proposed two bioretention area and seven permeable pavers systems. See the next page for the BMP Chart and Exhibit D for the BMP Map showing each drain basin to the bioretention area and permeable pavers systems. On-site

Drainage Basin A1 then drains to the dry extended pond to reduce the post-developed flows while safely passing the off-site drainage basin. The pond cross sections have been provided in Exhibit

E1, E2 and E3.

This report also includes the drawdown time calculations for the proposed bioretention area and permeable paver system, demonstrating that both stormwater management practices fully drain within the required 72-hour period. Supporting calculations are provided in Appendix A.

Page | 5

BMP Chart

BMP ID BMP Type

Runoff Reduction

Volume Required

(CF)

Runoff Reduction

Volume Provided

(CF)

Bio Area A Bioretention Area 312 612

Bio Area B Bioretention Area 219 566

PPA1 Permeable Paver System 2,156 1,316

PPA2 Permeable Paver System 71 530

PPA3 Permeable Paver System 34 162

PPA4 Permeable Paver System 86 513

PPA5 Permeable Paver System 345 706

PPA6 Permeable Paver System 243 682

PPA7 Permeable Paver System 278 466

The Site Development Review Tool as shown in Appendix D shows that a portion of the Runoff

Reduction volume was not met. The runoff reduction that was not captured via runoff reduction measures will be treated via two water quality devices. The Contech CDS Hydrodynamic

Separator was selected as it separates and traps trash, debris, sediment and hydrocarbons.

Contech CDS Unit detail and documentation including 80% removal efficiency has been provided in Appendix C. Please see the completed City of Norcross Runoff Reduction Infeasibility Form for

Determination of Infeasibility in Appendix E. Please see Appendix F for the soil infiltration testing results completed by United Consulting meeting the minimum 0.5 inches per hour.

The outlet orifices for the channel protection volumes were sized using the orifice equation to provide a detention time for a minimum of 24 hours. The pond is controlled through an Outlet

Control Structure as shown in E1, E2 and E3. Below is a summary of the results and complete calculations can be found in Appendix A.

Facility

CP

Orifice

Elev.

(ft)

CPv

Req’d/

Provided

(cf)

CP

Orifice

Size

(inch)

CPv

Elev.

(ft)

2 thru 25-

Year

Detention

Elevation

(ft)

Dry

Pond

1013.22 4,871 1.25 1015.00 1018.44

Per Georgia Stormwater Management Manual, the water quality BMP provided for a project is measured by determining the removal of total suspended solids (TSS). Onsite Drainage Basin A1

Bypass was not included since the pre- and post-conditions for this area remain unchanged. Per the Stormwater Quality Site Development Review Tool, after treatment with permeable pavers, bioretention basin, and Contech CDS Hydrodynamic Separator(Proprietary System), the TSS removal is 83% for all 4.53 acre of the site. See Appendix D for the Review Tool calculations.

Please see Appendix I for the Operations and Maintenance Plans for Bioretention Area, Permeable

Paver System, Dry Extended Detention Basin and for Contech CDS proprietary structure.

Page | 6

Stormwater Conveyance Calculations

Runoff rates for the storm water collection system were calculated using the Rational Method with a minimum time of concentration of 5 minutes and runoff coefficients of 0.35 for landscaped areas and 0.95 for impervious areas. The storm sewer system was sized for the 25-year storm event and checked for the 100-year storm event to determine if any structures would be flooded.

All pipes were analyzed for inlet and outlet control using the Storm Sewers extension program by

Autodesk Civil 3D. Please see the 25-year storm pipe table as shown in Exhibit F, Storm Sewer

Pipe Table. The proposed A Line has also been analyzed for the 100-year storm event. Hydraulic calculations confirm that all structures and pipes along the A Line convey the design flow within the closed storm drainage system without surcharging.

The existing storm pipe entering the project site from Jones Street conveys runoff from an off-site drainage area of approximately 3.77 acres. Based on discussions with City of Norcross staff, no known flooding or capacity issues have been reported with the existing storm drainage system during rainfall events. Therefore, the existing 18-inch pipe entering the site was assumed to convey a constant flow of 12 cfs for all design storm events. Hydraulic modeling indicates that the existing inline structures along the B Line surcharge and overflow during the 100-year storm event. The proposed design improves the hydraulic capacity of the B Line by increasing the minimum pipe slope from 0.11% to 1.91%, replacing the existing 15-inch pipe with a 24-inch pipe, and reducing the contributing drainage area by diverting approximately 0.07 acre to the A Line.

These improvements significantly increase the conveyance capacity of the B Line.

Analysis of Downstream Conditions

In accordance with City of Norcross requirements an analysis was performed on predicted flows pre- and post-development at a point in the drainage basin where the project’s area is a maximum of 10% of the total basin area. Study Point A drain to the southeast to Beaver Ruin Creek. The

10% downstream study basin is approximately 58.79 acres for Downstream Study Point as shown in Exhibit G. The time of concentration is 20.5 minutes for the basin area with a curve number of

89 for the entire drainage area.

Downstream Study Point Analysis Summary

Return

Frequency

(Years)

Pre-Developed

Flow at

Downstream

Study Point

(CFS)

Post-Developed

Flow at

Downstream

Study Point

Difference

1 80.11 76.75 (3.36)

2 102.27 100.03 (2.24)

5 141.42 141.24 (0.18)

10 177.22 176.50 (0.72)

25 230.10 227.09 (3.01)

50 273.73 268.14 (5.59)

100 319.29 314.72 (4.57)

Page | 7

As shown in the tables on the previous page, there is a decrease for the post-developed flows for the 1- through 100-year storm event for the Downstream Study Point. The project site discharges to the existing MS4 along Mitchell Road through a system of existing storm pipes and roadside ditches. These conveyances ultimately discharge into an existing culvert beneath Buford Highway, which outfalls to Beaver Ruin Creek.

The following photographs of the existing roadside ditch and culvert along Mitchell Road documenting the existing downstream drainage conditions. Refer to Appendix G for the downstream analysis calculations and the 10% Downstream Study hydrograph printout.

Headwall across from the Cotton Gin at the intersection of Lawrenceville St and Mitchell Rd

Page | 8

Existing roadside ditch along Mitchell Rd conveying runoff toward the culvert

The existing culvert before crossing Buford Hwy.

EXHIBITS

COLUMBIA

ENGINEERING

NORTH

LOCATION MAP

65 LAWRENCEVILLE ST

NORCROSS, GEORGIA

GWINNETT COUNTY

SITE

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SHEET NO.

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CIVIL ENGINEERS * LAND PLANNERS LANDSCAPE ARCHITECRTS *SURVEYORS

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2862 BUFORD HIGHWAY SUITE 200 DULUTH, GEORGIA 30096 (770) 925-0357

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PROJECT NO.:

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DATE:

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CLIENT:

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PROJECT LOCATION:

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SHEET TITLE:

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EX A

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CITY OF NORCROSS

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PARKING IMPROVEMENTS

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NORCROSS, GA

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LOCATION MAP

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ENGINEERING

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DATEREVISIONS

DRAWING TITLE

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DRAWN BY:

PROJECT NO.

DATE:

SCALE:CIVIL ENGINEERS * LAND PLANNERS

* LANDSCAPE ARCHITECTS * SURVEYORS

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07/20/26COMMENTS

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POST BASIN A2 ON-SITE

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CN

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POST BASIN A3 OFF-SITE

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CN

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AutoCAD SHX Text

POST BASIN A1 ON-SITE BYPASS

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CN

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1.58 ACRES 96 5.00 MIN

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2.21 ACRES 82 11.54 MIN

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0.05 ACRES 93 5.00 MIN

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1.54 ACRES 90 8.50 MIN

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0.14 ACRES 97 5.00 MIN

AutoCAD SHX Text

AutoCAD SHX Text

%%UBAR SCALE

AutoCAD SHX Text 80'

AutoCAD SHX Text 40'

AutoCAD SHX Text 80'

1" = 80'

S T

ACACACACAC

AC

AC

S T

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S T S T

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BOLBOLBOL BOLBOLBOLBOL BOL

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DULUTH, GEORGIA 30096

2862 BUFORD HIGHWAY

SUITE 200

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ENGINEERING

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DATEREVISIONS

DRAWING TITLE

DRAWING NO.

FILE NAME:

DRAWN BY:

PROJECT NO.

DATE:

SCALE:CIVIL ENGINEERS * LAND PLANNERS

* LANDSCAPE ARCHITECTS * SURVEYORS

6170.05-Hydro

1"=80'

07/20/26COMMENTS

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MAP

BMP

LEGEND

AutoCAD SHX Text

WINGO STREET

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JONES STREET

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LAWRENCEVILLE STREET

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SOIL GROUP: MCF

AutoCAD SHX Text SOIL GROUP: ToA

AutoCAD SHX Text

AutoCAD SHX Text

%%UBAR SCALE

AutoCAD SHX Text 60'

AutoCAD SHX Text 30'

AutoCAD SHX Text 60'

AutoCAD SHX Text 1" = 60'

AutoCAD SHX Text

PPA = PERMEABLE PAVERS SYSTEM

AutoCAD SHX Text

BIO AREA = BIORETENTION AREA

WQ UNIT = CONTECH CDS HYDRODYNAMIC SEPARATOR

AutoCAD SHX Text

SHEET NO.

AutoCAD SHX Text

CIVIL ENGINEERS * LAND PLANNERS LANDSCAPE ARCHITECRTS *SURVEYORS

AutoCAD SHX Text

2862 BUFORD HIGHWAY SUITE 200 DULUTH, GEORGIA 30096 (770) 925-0357

AutoCAD SHX Text

PROJECT NO.:

AutoCAD SHX Text

DATE:

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CLIENT:

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PROJECT NAME:

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PROJECT LOCATION:

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SHEET TITLE:

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EX E1

AutoCAD SHX Text

CITY OF NORCROSS

AutoCAD SHX Text

PARKING IMPROVEMENTS

AutoCAD SHX Text

NORCROSS, GA

AutoCAD SHX Text

DRY POND A OCS DETAIL

AutoCAD SHX Text 6170.05

AutoCAD SHX Text 07/20/26

AutoCAD SHX Text

C-1 DRY POND A OCS DETAIL

AutoCAD SHX Text

NTS

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MANHOLE LADDER PER CITY REQUIREMENTS ON SPACING AND TYPE

AutoCAD SHX Text

6" THICK CONCRETE TOP

AutoCAD SHX Text

6"X6"X6" PEDESTAL (TYPICAL-4 CORNERS)

AutoCAD SHX Text

18" RCP OUTLET PIPE INV=1012.72

AutoCAD SHX Text

5'X5'X8" BASE TOP ELEV=1012.72

AutoCAD SHX Text

4'X4' CONCRETE OCS

AutoCAD SHX Text

24" MANHOLE ACCESS

AutoCAD SHX Text

3' WEIR OPEN 1-SIDE INV=1016.50

AutoCAD SHX Text

3' WEIR OPEN 2-SIDES INV=1017.75

AutoCAD SHX Text

100-YR PONDING ELEV=1019.39

AutoCAD SHX Text

TOP OF STRUCTURE ELEV=1020.50

AutoCAD SHX Text

TOP OF DAM ELEV=1020.00

AutoCAD SHX Text

1.25" ORIFICE (CHANNEL PROTECTION) INV=1013.22

AutoCAD SHX Text

ED 25-YR PONDING ELEV=1018.44

AutoCAD SHX Text

(SEE ORIFICE END-CAP DETAIL)

AutoCAD SHX Text PERMANENT 24" HALF-ROUND PERFORATED CMP RETROFIT WITH HINGED LID OF EXPANDED METAL WITH "X " OPENING34"X " OPENING12" OPENING

1' WEIR OPEN 1-SIDE INV=1015.00

DRY POND A

DRY POND A CROSS-SECTION

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DATEREVISIONS

DRAWING TITLE

DRAWING NO.

FILE NAME:

DRAWN BY:

PROJECT NO.

DATE:

SCALE:CIVIL ENGINEERS * LAND PLANNERS

* LANDSCAPE ARCHITECTS * SURVEYORS

6170.05-Hydro

NTS

07/20/26COMMENTS

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CROSS-SECTION

DRY POND A

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SEE ORIFICE END CAP DETAIL

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CONCRETE BASE

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SEE DETAIL GDOT D-44 FOR HALF ROUND INFO

AutoCAD SHX Text

BOTTOM ELEV.=1013.22

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#57 STONE SHALL COVER ENTIRE FACE OF HALF ROUND CMP

AutoCAD SHX Text

PERMANENT HALF ROUND 24" CMP TO ELEVATION 1015.00

AutoCAD SHX Text

C-1 OCS TOP=1020.50

AutoCAD SHX Text

WALL ELEV.=1020.50

AutoCAD SHX Text NOTE: ATTACH HALF ROUND CMP TO STRUCTURE WHERE CONTACT OCCURS WITH L-BRACKETS 6" OC VERTICAL WELDED TO THE FULL ROUND AND EPOXY-BOLTED INTO THE CONCRETE STRUCTURE

AutoCAD SHX Text

100-YR ELEV.=1019.39

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SCALE:NTS

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18" RCP INV=1012.72

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0.61' FREEBOARD

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EXISTING PIPE

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CHANNEL PROTECTION THREADED END CAP WITH 1.25" ORIFICEINV=1013.22

AutoCAD SHX Text

BOTTOM OF POND ELEV.=1013.22

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CP ELEV.=1015.00

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C-1 OCS TOP=1020.50

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100-YR ELEV.=1019.39

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WALL ELEV.=1020.50

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25-YR ELEV.=1018.44

AutoCAD SHX Text

TOP OF DAM ELEV=1020.00

0.61' FREEBOARD

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SHEET NO.

AutoCAD SHX Text

CIVIL ENGINEERS * LAND PLANNERS LANDSCAPE ARCHITECRTS *SURVEYORS

AutoCAD SHX Text

2862 BUFORD HIGHWAY SUITE 200 DULUTH, GEORGIA 30096 (770) 925-0357

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PROJECT NO.:

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DATE:

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CLIENT:

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PROJECT NAME:

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PROJECT LOCATION:

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SHEET TITLE:

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EX E3

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CITY OF NORCROSS

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PARKING IMPROVEMENTS

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NORCROSS, GA

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ORIFICE END-CAP DETAIL

AutoCAD SHX Text 6170.05

AutoCAD SHX Text 07/20/26

AutoCAD SHX Text

NOTES:

AutoCAD SHX Text #4's @ 3" O.C. MAX WELDED

AutoCAD SHX Text 1.5"

AutoCAD SHX Text 1.5"

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NTS

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TRASH RACK DETAIL

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ANCHOR BOLT DRILLED AND GROUTED IN PLACE (SEE NOTE 1)

AutoCAD SHX Text 6"

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1. " ANCHORED BOLTS (EMBEDDED 3") 34" ANCHORED BOLTS (EMBEDDED 3") 2. TRASH RACK ARE SCHEMATIC. CONTRACTOR SHALL VERIFY ALL TRASH RACK ARE SCHEMATIC. CONTRACTOR SHALL VERIFY ALL DIMENSIONS OF THE WEIR OPENING PRIOR TO FABRICATION. SUBMIT SHOP DRAWINGS FOR REVIEW BY THE ENGINEER PRIOR TO FABRICATION

AutoCAD SHX Text 1.5"

AutoCAD SHX Text

ANCHOR BOLT DRILLED AND GROUTED IN PLACE (SEE NOTE 1)

12"

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DATEREVISIONS

DRAWING TITLE

DRAWING NO.

FILE NAME:

DRAWN BY:

PROJECT NO.

DATE:

SCALE:CIVIL ENGINEERS * LAND PLANNERS

* LANDSCAPE ARCHITECTS * SURVEYORS

6170.05-Hydro

NTS

07/20/26COMMENTS

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EX F

TABLE

STORM PIPE

T

V

V

T

V

V

T V

V

T V

V

T

VV

T

S T

S T

S T

ACACACACAC

AC

AC

S T

S T S T

S TS T

S T

S T S T

S T S T

S T S T

S T

S T

S T

S T S T

S TS T

S TS T

WV

TCWV

WV

WV

FHW

WM

S T

UP

WV

UP

W

WM

ICVICV

W

WM

FH

ICV

ICV

S T

S T

S T

S T

W

WM

W

WM WW

FH

WV

TC

BOLBOLBOL BOLBOLBOLBOL BOL

BOL

PED

W

WM W

WM

W

WM

W

WMWV

W

WM

W

WM W

WM

W

WM

W

WM

W

WM

W

WM

FH

WV

T

PED

WV

WVFH

WV W

WM

WV

WV

FH

WV

W

WM

WV

W

WM

T

T

T

PED

W

WM

W

WMICVICV

ICV

STMH

TOP=1039.64

INV OUT=1036.63

15" R C

P

STMH

TOP=1039.38

INV IN=1036.56

INV OUT=1035.88

BOTTOM BOX=1035.97

18" RCP

STMH

TOP=1034.15

INV IN=1031.03 (A)

INV IN=1031.02 (B)

INV OUT=1030.71 (C)

18" RCP

24" RCP

(A)

(B)

(C)

STMH

TOP=1034.73

INV IN=1031.22

INV OUT=1031.08

18" RCP

STMH

TOP=1035.59

INV IN=1031.62

INV OUT=1031.56

BOTTOM BOX=1031.69

15" RCP

(A)

(B) (C)

DI

TOP=1035.15

INV IN=1032.31 (A)

INV IN=1032.25 (B)

INV OUT=1032.21 (C)

15" RCP

8" D

IP

(A)

(B) (C)

DI

TOP=1035.84

INV IN=1033.30 (A)

INV IN=1033.09 (B)

INV OUT=1032.99 (C)

DI

TOP=1035.73

INV OUT=1034.10

18"R C

P

24" CMP

STMH

TOP=1038.81

INV IN=1035.15

INV OUT=1034.93

DI

TOP=1044.95

INV IN=1041.58 (A)

INV IN=1040.79 (B)

INV OUT=1040.24 (C)

18" RCP

18" RCP

(A) (B)

(C)

18" R C

PSTMH

TOP=1046.64

INV IN=1043.10

INV OUT=1042.90

BOTTOM BOX=1043.03

STMH

TOP=1049.72

INV IN=1046.10

INV OUT=1045.97

BOTTOM BOX=1046.33

STMH

TOP=1046.99

INV IN=1043.79

INV OUT=1042.22

DI

TOP=1045.69

INV IN=1042.26

INV IN=1041.43

BOTTOM BOX=1042.04

18" RCP

24" RCP

STMH

TOP=1032.40

INV IN=1029.40

INV OUT=1029.00

15" R C

P

15" RCP 24" RCP

15" RCP

24" RCP

STMH

TOP=1030.80

INV OUT=1027.99

STMH

TOP=1031.18

INV IN=1028.03 (A)

INV IN=1027.86 (B)

INV IN=1027.60 (C)

INV OUT=1027.44 (D)

BOTTOM BOX=1027.50

(A)

(B)

(C) S T

(D)

STMH

TOP=1026.11

INV OUT=1023.47 STMH

TOP=1026.33

INV IN=1023.37 (A)

INV IN=1023.22 (B)

INV OUT=1023.11 (C)

BOTTOM BOX=1023.11

(A) (B)

(C)

30" RCP

STMH

TOP=1019.40

INV OUT=1016.90

15" RCP 30" RCP

STMH

TOP=1019.75

INV IN=1016.61 (A)

INV IN=1015.89 (B)

INV OUT=1015.38 (C)

(A)

(B)

(C)

30" RCP

DI

TOP=1015.51

INV IN=1010.90

INV OUT=1009.64

(A)

(B)

(C)

(D)

18" PVC

CURB INLET

TOP=1014.27

INV IN=1008.95 (A)

INV IN=1008.10 (B)

INV IN=1005.96 (C)

INV OUT=1005.72 (D)

18" PVC

18" PVC

DI

TOP=1013.85

INV IN=1009.31

INV OUT=1009.17

STMH

TOP=1014.62

INV IN=1011.35

INV OUT=1011.29

DI

TOP=1014.41

INV IN=1010.68

INV OUT=1010.61

18" RCP

15" RCP

STMH

TOP=1013.55

INV IN=1011.01

INV OUT=1008.64

STMH

TOP=1012.33

INV IN=1008.84

INV IN=1004.13

INV OUT=1003.98

DI

TOP=1026.93

INV OUT=1023.03

12" R C

P

18" RCP

STMH

TOP=1024.46

INV IN=1021.56

INV IN=1021.45

INV OUT=1021.43

BOTTOM BOX=1021.49

(A)

(C)

STMH

TOP=1023.95

INV IN=1019.62 (A)

INV IN=1019.55 (B)

INV OUT=1019.43 (C)

DI

TOP=1023.33

INV IN=1019.77

INV OUT=1019.70

18" RCP

15" R C

P

DI

TOP=1023.84

INV IN=1020.88

INV OUT=1020.44

DI

TOP=1023.86

INV IN=1021.51

INV OUT=1021.46

HEADWALL

INV=1011.94

STMHTOP=1025.02

TOP=1025.02

INV IN=1021.88 (A)

INV IN=1021.16 (B)

INV OUT=1020.89 (C)

D O

U

BLE

24" R C

P (E)

STMH

TOP=1030.26

INV OUT=1028.26

SITE DATA

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ENGINEERING

COLUMBIA

DULUTH, GEORGIA 30096

2862 BUFORD HIGHWAY

SUITE 200

(770) 925-0357

ENGINEERING

COLUMBIA

DATEREVISIONS

DRAWING TITLE

DRAWING NO.

FILE NAME:

DRAWN BY:

PROJECT NO.

DATE:

SCALE:CIVIL ENGINEERS * LAND PLANNERS

* LANDSCAPE ARCHITECTS * SURVEYORS

6170.05-Hydro

1"=200'

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EX G

DRAINAGE MAP

DOWNSTREAM

AutoCAD SHX Text

FIRE DEPARTMENT CONNECTION

AutoCAD SHX Text

EXISTING CITY HALL BUILDING

AutoCAD SHX Text

EXISTING FIRE HYDRANT

AutoCAD SHX Text

EXISTING FIRE HYDRANT

AutoCAD SHX Text

EXISTING FIRE HYDRANT

AutoCAD SHX Text

EXISTING FIRE HYDRANT

AutoCAD SHX Text

SOIL GROUP: MCF

AutoCAD SHX Text SOIL GROUP: ToA

AutoCAD SHX Text

SOIL GROUP: WHD

AutoCAD SHX Text

SOIL GROUP: MCF

AutoCAD SHX Text

SOIL GROUP: WHD

AutoCAD SHX Text SOIL GROUP: Ub

AutoCAD SHX Text

SOIL GROUP: WHD

AutoCAD SHX Text

SOIL GROUP: HYB

AutoCAD SHX Text SOIL GROUP: Cfs

AutoCAD SHX Text SOIL GROUP: PgC2

AutoCAD SHX Text SOIL GROUP: PgC2

AutoCAD SHX Text

SOIL GROUP: ARE

AutoCAD SHX Text SOIL GROUP: WkB

AutoCAD SHX Text SOIL GROUP: PgD2

AutoCAD SHX Text SOIL GROUP: Ub

AutoCAD SHX Text SOIL GROUP: PfC2

AutoCAD SHX Text

EXISTING FIRE HYDRANT

AutoCAD SHX Text DOWNSTREAM DRAINAGE BASIN 58.79 AC. CN=74 Tc=20.50 MIN.

AutoCAD SHX Text

ON-SITE DRAINAGE BASIN 6.11 AC

AutoCAD SHX Text

10%%% TOTAL DOWNSTREAM BASIN

AutoCAD SHX Text

%%P64.90 ACRES

AutoCAD SHX Text

TOTAL SITE AREA

AutoCAD SHX Text

%%P6.11 ACRES

AutoCAD SHX Text

AutoCAD SHX Text

%%UBAR SCALE

AutoCAD SHX Text 200'

AutoCAD SHX Text 100'

AutoCAD SHX Text 200'

1" = 200'

APPENDIX A

Project Name: Downtown Norcross Parking Improvements

Project Number: 6170.05

Project Street Address or nearest Intersection: 65 Lawreceville St

City: Norcross

County: Gwinnett

State: Georgia

Zip Code: 30071

Permitting Jurisdiction: City of Norcross

Max Pre-Developed Curve Number Required: 55

LL:

6254 -196, -241, -195, -248, -193, -192, -242, -198, 6254A005, 6254A004B

District: 9

Acreage: 6.11

Tax Parcel(s):

R6254 -196, -241, -195, -248, -193, -192, -242, -198, R6254A005, R6254A004B

Current Land Use: Commercial parking lot use

Proposed Land Use: Commercial parking lot use

Flood Zone X

Firm Panel: 13135C0082G

SCS 24-HR Rainfall (in.)

1-yr 3.28

2-yr 3.70

5-yr 4.40

10-yr 5.01

25-yr 5.88

50-yr 6.58

100-yr 7.30

* Acreage includes sliver of Skin Alley

SITE DATA INFORMATION

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

Site Data Information - 1/26

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 1.59 1 D 98

Open Space: Good Condition (grass cover <75%) 0.56 0 D 80

Open Space: Good Condition (grass cover <75%) 0.21 0 A 39

Wood or forest land: good cover 0.03 0 D 77

Wood or forest land: good cover 0.54 0 A 30

Total Acreage (ac): 2.93

Impervious (ac): 1.59

Weighted CN: 77

Jurisdiction Maximum CN: 55

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 1.43 1 D 98

Open Space: Good Condition (grass cover <75%) 0.15 0 D 80

Total Acreage (ac): 1.58

Impervious (ac): 1.43

Weighted CN: 96

Jurisdiction Maximum CN: 55

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 0.79 1 D 98

Open Space: Good Condition (grass cover <75%) 0.71 0 D 80

Open Space: Good Condition (grass cover <75%) 0.02 0 A 39

Wood or forest land: good cover 0.23 0 D 77

Wood or forest land: good cover 0.21 0 A 30

Street and roads: Gravel (including ROW) 0.25 1 D 91

Total Acreage (ac): 2.21

Impervious (ac): 0.94

Weighted CN: 82

Jurisdiction Maximum CN: N/A

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

PRE-DEVELOPED CURVE NUMBER CALCULATIONS

PRE-DEVELOPED ON-SITE DRAINAGE BASIN A1

Area

(acres)

PRE-DEVELOPED ON-SITE DRAINAGE BASIN A1 BYPASS

Area

(acres)

PRE-DEVELOPED OFF-SITE DRAINAGE BASIN A1

Area

(acres)

Pre-Developed CN Calculations - 2/26

Soil Group

Curve

Number

Impervious areas 1.11 1 D 98

Open Space: Good Condition (grass cover <75%) 0.50 0 D 80

Total Acreage (ac): 1.61

Impervious (ac): 1.11

Weighted CN: 92

Jurisdiction Maximum CN: 55

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 0.04 1 D 98

Open Space: Good Condition (grass cover <75%) 0.02 0 D 80

Total Acreage (ac): 0.05

Impervious (ac): 0.04

Weighted CN: 93

Jurisdiction Maximum CN: N/A

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 0.13 1 D 98

Open Space: Good Condition (grass cover <75%) 0.01 0 D 80

Total Acreage (ac): 0.14

Impervious (ac): 0.13

Weighted CN: 97

Jurisdiction Maximum CN: N/A

Area

(acres)

PRE-DEVELOPED OFF-SITE DRAINAGE BASIN A3

Area

(acres)

PRE-DEVELOPED OFF-SITE DRAINAGE BASIN A2

Area

(acres)

PRE-DEVELOPED ON-SITE DRAINAGE BASIN A2

Pre-Developed CN Calculations - 3/26

Soil Group

Curve

Number

Impervious areas 1.99 1 D 98

Open Space: Good Condition (grass cover <75%) 0.52 0 D 80

Open Space: Good Condition (grass cover <75%) 0.34 0 A 39

Wood or forest land: good cover 0.03 0 D 77

Wood or forest land: good cover 0.02 0 A 30

Pave Drain 0.01 0 D 61

Pave Drain 0.07 0 A 61

A1 Total Acreage (ac): 2.99

A1 Impervious (ac): 1.99

A1 Weighted CN: 86

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 1.43 1 D 98

Open Space: Good Condition (grass cover <75%) 0.15 0 D 80

A1 BYPASS Total Acreage (ac): 1.58

A1 BYPASS Impervious (ac): 1.43

A1 BYPASS Weighted CN: 96

*Hydrologic Soil Group are assumed to be D for this basin

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 0.79 1 D 98

Open Space: Good Condition (grass cover <75%) 0.71 0 D 80

Open Space: Good Condition (grass cover <75%) 0.02 0 A 39

Wood or forest land: good cover 0.23 0 D 77

Wood or forest land: good cover 0.21 0 A 30

Street and roads: Gravel (including ROW) 0.25 1 D 91

N/A Total Acreage (ac): 2.21

N/A Impervious (ac): 0.94

N/A Weighted CN: 82

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

Area

(acres)

POST-DEVELOPED ON-SITE DRAINAGE BASIN A1

POST-DEVELOPED CURVE NUMBER CALCULATIONS

Area

(acres)

POST-DEVELOPED OFF-SITE DRAINAGE BASIN A1

POST-DEVELOPED ON-SITE DRAINAGE BASIN A1 BYPASS

Area

Post-Developed CN Calculations - 4/26

Soil Group

Curve

Number

Impervious areas 0.04 1 D 98

Open Space: Good Condition (grass cover <75%) 0.02 0 D 80

N/A Total Acreage (ac): 0.05

N/A Impervious (ac): 0.04

N/A Weighted CN: 93

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 1.01 1 D 98

Open Space: Good Condition (grass cover <75%) 0.42 0 D 80

Pave Drain 0.11 0 D 61

A2 Total Acreage (ac): 1.54

A2 Impervious (ac): 1.01

A2 Weighted CN: 90

Land Cover Type and Condition Hydrologic

Soil Group

Curve

Number

Impervious areas 0.13 1 D 98

Open Space: Good Condition (grass cover <75%) 0.01 0 D 80

N/A Total Acreage (ac): 0.14

N/A Impervious (ac): 0.13

N/A Weighted CN: 97

POST-DEVELOPED OFF-SITE DRAINAGE BASIN A2

Area

(acres)

POST-DEVELOPED ON-SITE DRAINAGE BASIN A2

Area

(acres)

POST-DEVELOPED OFF-SITE DRAINAGE BASIN A3

Area

Post-Developed CN Calculations - 5/26

SHEET FLOW

Surface description (Table 3.1.5-2) Woods: dense underbrush

Woods: dense underbrush

Grass: bermuda grasses Grass: dense grasses Grass: bermuda grasses

Manning's roughness coefficient, n 0.800 0.800 0.410 0.240 0.410

Flow fall, H (ft) 3.70 6.00 3.50 4.50 4.00

Flow length, L (ft) 50 50 50 50 50

Two-yr 24-hr rainfall, P2 (in) 3.70 3.70 3.70 3.70 3.70

Land slope, S = H/L (ft/ft) 0.074 0.120 0.070 0.090 0.080

(3.1.8) (min) 11.83 9.75 7.09 4.18 6.72

SHALLOW CONCENTRATED FLOW

Surface description Unpaved Paved Unpaved Unpaved Unpaved

Flow fall, H (ft) 11.40 4.00 4.50 0.50 3.20

Flow length, L (ft) 157.66 57.82 134.26 7.51 119.83

Watercourse slope, S = H/L (ft/ft) 0.072 0.069 0.034 0.067 0.027

Average velocity, V (3.1.9 & 3.1.10) (fps) 4.34 5.35 2.95 4.16 2.64

(3.1.7) (min) 0.61 0.18 0.76 0.03 0.76

SHALLOW CONCENTRATED FLOW

Surface description Paved Unpaved Paved Unpaved Paved

Flow fall, H (ft) 1.40 18.50 5.24 3.20 7.54

Flow length, L (ft) 16.6 249.81 225.19 24.48 224.55

Watercourse slope, S = H/L (ft/ft) 0.084 0.074 0.023 0.131 0.034

Average velocity, V (3.1.9 & 3.1.10) (fps) 5.90 4.39 3.10 5.83 3.73

(3.1.7) (min) 0.05 0.95 1.21 0.07 1.00

SHALLOW CONCENTRATED FLOW

Surface description Paved Paved Paved

Flow fall, H (ft) 1.40 3.14 5.20

Flow length, L (ft) 16.6 37.5 91.9

Watercourse slope, S = H/L (ft/ft) 0.084 0.084 0.057

Average velocity, V (3.1.9 & 3.1.10) (fps) 5.90 5.88 4.84

(3.1.7) (min) 0.05 0.11 0.32

PIPE FLOW *Avg. Pipe Diameter Used

Pipe Diameter (in) 21.75 22.7 15 18 15

Cross sectional flow area, A (ft ) 2.58 2.81 1.23 1.77 1.23

Wetted perimeter, Pw (ft) 5.69 5.94 3.93 4.71 3.93

Hydraulic radius, R = A/Pw (ft) 0.45 0.47 0.31 0.38 0.31

Flow fall, H (ft) 15.8 15.5 0.23 23.04 0.23

Flow length, L (ft) 547 437.73 10.67 527.52 10.67

Channel slope, S = H/L (ft/ft) 0.029 0.035 0.022 0.044 0.022

Pipe Material Concrete Concrete Concrete Concrete Concrete

Manning's roughness coefficient, n 0.013 0.013 0.013 0.013 0.013

(3.1.11) (fps) 11.49 13.09 7.75 12.46 7.75

(3.1.7) (min) 0.79 0.56 0.02 0.71 0.02

Total Time of Concentration, Tc (min) 13.33 11.54 9.08 5.30 8.50

TIME OF CONCENTRATION CALCULATIONS

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

DRAINAGE BASIN: Pre Basin A1 on-Site Pre/Post Basin A1 Off-

Site

DRAINAGE BASIN: Pre Basin A1 on-Site Pre/Post Basin A1 Off-

Site

Post Basin A1 on-Site Post Basin A2 On-Site

Post Basin A1 on-Site Post Basin A2 On-SitePre Basin A2 On-Site

Pre Basin A2 On-Site

T� =

0.42 nL

P

.�S

T� = L

60 V

V =

1.49 R

�⁄ S � n

T� = L

60 V

T� = L

60 V

T� = L

60 V

Time of Concentration Calculations - 6/26

Stormwater Pond A

Drainage Basin = A1

Drainage Basin Area = 2.99 Ac. 130,244.40 sf

Drainage Basin CN = 86

Tc = 12.00 minutes

Impervious Area = 1.99 Ac. 66.7 % Impervious

Pervious Area = 1.00 Ac.

Bottom Elevation = 1013.22 Ft

Top Elevation = 1020.00 Ft

Contour Spacing = 1 Ft

Stage Elevation Area Incremental Total

(ft) (ft) (SF) Vol. (CF) Vol. (CF)

0.00 1013.22 0 0 0

0.78 1014.00 4,185 1632 1632

1.78 1015.00 4,430 4308 5940

2.78 1016.00 4,671 4551 10490

3.78 1017.00 4,910 4791 15281

4.78 1018.00 5,146 5028 20309

5.78 1019.00 5,379 5263 25571

6.78 1020.00 5,609 5494 31065

POND A

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

POND STAGE STORAGE CALCULATIONS

Ponds Stage Storage Calculations - 7/26

Channel Protection Volume for Stormwater Pond A

Direct Runoff for 1-Year 24 Hour Storm

(P - 0.2S)

Modified Rainfall Runoff Equation 3.1.5

P + 0.8S with no Runoff Reduction

P = 3.28 inches

1000 - 10 CN = 86

CN

S = 1.58

Qd = 1.93 inches

Ia = 0.32 Ia = (200/CN)-2

Ia/P = 0.10

Tc(hrs) = 0.20 or 12.00 min

850.00 qo/qi= 0.020

Vs/Vr = 0.654

Volume to Detain for 24-hours (Eq 3.3.10)

CPV1yr = 4,871 CF

Area= 46,196 sf = 1.06 AC

1014.75 FT

Elevation value was interpolated from stage storage chart

24-hour Release Calculations

Orifice Equation (3.4.1, 3.4.3.3, and Average Release Rate Equation) t = time to drain pond

CPV1yr = 4871 CF

Ao = Area of the orifice = πr

1.33 inches

Ao = 0.0096 SF

H = top volume elev. - elevation at center of orifice

H = 1014.75 -1013.28

H = 1.48 ft t = 24.00 hours

1.25 inches

Orifice Invert El. = 1013.22 ft Top of WQv Elevation

STREAM CHANNEL PROTECTION CALCULATIONS

qu (cfs/mi /in) =

Qd =

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

Orifice Diameter= Round down

Storage Elevation CPV1Yr= t = WQv

0.6Ao(2gH) 0.5

(3600)

Calculated Orifice Diam. =

(Vs/Vr)(Qd)(A ft

S = from figure 3.1.5-6 from figure 3.3.5-1 for 24 hours

CPV = Vs =

CP Calculations - 8/26

Runoff Reduction Volume for Bioretention Area A

Direct Runoff for Runoff Reduction Area (Eq 3.1.19 and 3.1.20)

Total Area to BMP = 0.10 acres

Total Impervious Area = 0.09 acres

I (%) 90.0

Rv = 0.05 + I(0.009)

I = 90.0 % impervious

Rv = 0.86

RRR = 1.0"(Rv)

RRR = 0.86 inches

Runoff Reduction Volume (RRV)

0.10 ac or 4,356 sf

312 CF

Sizing the Bioretention Volume

VP = PV + VES*(N)

Solve for PV

408 SF x-section area of island

0.5 ft ponding depth

PV = 204 CF ponding volume

Solve for VES

2.5 ft soil/gravel media depth

VES= 1,020 CF volume of engineeered soils

Solve for VP

PV = 204 CF Ponding Vol

VES = 1,020 CF Vol Eng. Soils/gravel

N = 0.4 soil/gravel porosity

Provided RRv: VP = 612 CF Total Volume

RR % Credit = 100%

Adj. Provided RRv: VP = 612 CF must be greater than: 312 CF

RRV Min =

12 in./ft

RR POND Treated Area (ac)=

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

BIORETENTION AREA RUNOFF REDUCTION CALCULATIONS

RRv = RRR x (Drainage Area Acres x 43,560 sf/ac)

RR Bioretention Area Calculations - 9/26

Drawdown Time for Bioretention Area A tf =

Filter Bed Depth (df) = 2.5 ft

Ponding Above Bed (hf) = 0.5 ft

Infiltration Rate (k) = 0.48 in/hr

0.96 ft/day

Required Af = 408 SF

Time to Drain Bioretention (t f) = 16 hours

RRv df k (hf + df) Af

RR Bioretention Area Calculations - 10/26

Runoff Reduction Volume for Bioretention Area B

Direct Runoff for Runoff Reduction Area (Eq 3.1.19 and 3.1.20)

Total Area to BMP = 0.07 acres

Total Impervious Area = 0.06 acres

I (%) 90.0

Rv = 0.05 + I(0.009)

I = 90.0 % impervious

Rv = 0.86

RRR = 1.0"(Rv)

RRR = 0.86 inches

Runoff Reduction Volume (RRV)

0.07 ac or 3,049 sf

219 CF

Sizing the Bioretention Volume

VP = PV + VES*(N)

Solve for PV

377 SF x-section area of island

0.5 ft ponding depth

PV = 189 CF ponding volume

Solve for VES

2.5 ft soil/gravel media depth

VES= 943 CF volume of engineeered soils

Solve for VP

PV = 189 CF Ponding Vol

VES = 943 CF Vol Eng. Soils/gravel

N = 0.4 soil/gravel porosity

Provided RRv: VP = 566 CF Total Volume

RR % Credit = 100%

Adj. Provided RRv: VP = 566 CF must be greater than: 219 CF

RRV Min =

RRv = RRR x (Drainage Area Acres x 43,560 sf/ac)

12 in./ft

RR POND Treated Area (ac)=

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

BIORETENTION AREA RUNOFF REDUCTION CALCULATIONS

RR Bioretention Area Calculations - 11/26

Drawdown Time for Bioretention Area B

Filter Bed Depth (df) = 2.5 ft

Ponding Above Bed (hf) = 0.5 ft

Infiltration Rate (k) = 0.48 in/hr

0.96 ft/day

Required Af = 377 SF

Time to Drain Bioretention (t f) = 12 hours

RRv df

RR Bioretention Area Calculations - 12/26

Runoff Reduction Target Volume for Permeable Pavers

Drainage Basin BMP Name =

Contributing Drainage Basin Area = 0.72 Ac.

Impervious Area = 0.62 Ac.

Runoff Reduction Target Volume

RRv = (P) (Rv) (A) / 12

P = 1.0 inches Target runoff reduction rainfall

Rv = 0.05 + 0.009 (I) Volumetric runoff coefficient

I = 86 % impervious

Rv = 0.825

A = 31,363 SF Area draining to the practice

12 = Unit conversion factor (in/ft)

RRv = 2,156 CF

VPMIN ≥ RRv (target) / RR%

RRV (target) = 2,156 CF

RR% = 100 Runoff Reduction percentage using Table 4.1.3-2

VPMIN = 2,156 CF

Storage Volume Provided

Actual Storage Volume Provided

VP = PV + VES (N)

PV = 0 CF Ponding Volume

VES = AP * d Volume of Engineered Soils

AP = 3,290 SF Area of Permeable Paver System d = 1.00 Ft Depth of gravel

VES = 3289.65 CF

N = 0.40

VP = 1,316 CF

Determine if Minimum Storage Volume was Met

VP ≥ VPMIN

1,316 ≥ 2,156

Runoff Reduction requirements are not met

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

PERMEABLE PAVERS RUNOFF REDUCTION CALCULATIONS

PPA1

RR Permeable Pavers Calculations - 13/26

Drawdown Time for Permeable Pavers PPA1

Filter Bed Depth (df) = 1.00 ft

Ponding Above Pavers (hf) = 0.00 ft

Infiltration Rate (k) = 0.48 in/hr

0.96 ft/day

Required Af = 3,290 SF

Time to Drain Bioretention (t f) = 16 hours

RRv df

RR Permeable Pavers Calculations - 14/26

Contributing Drainage Basin Area = 0.03 Ac.

Impervious Area = 0.02 Ac.

Runoff Reduction Target Volume

RRv = (P) (Rv) (A) / 12

P = 1.0 inches Target runoff reduction rainfall

Rv = 0.05 + 0.009 (I) Volumetric runoff coefficient

I = 67 % impervious

Rv = 0.65

A = 1,307 SF Area draining to the practice

12 = Unit conversion factor (in/ft)

RRv = 71 CF

VPMIN ≥ RRv (target) / RR%

RRV (target) = 71 CF

RR% = 100 Runoff Reduction percentage using Table 4.1.3-2

VPMIN = 71 CF

Storage Volume Provided

Actual Storage Volume Provided

VP = PV + VES (N)

PV = 0 CF Ponding Volume

VES = AP * d Volume of Engineered Soils

AP = 1,325 SF Area of Permeable Paver System d = 1.00 Ft Depth of gravel

VES = 1325.46 CF

N = 0.40

VP = 530 CF

Determine if Minimum Storage Volume was Met

VP ≥ VPMIN

530 ≥ 71

Runoff Reduction requirements are met

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

PERMEABLE PAVERS RUNOFF REDUCTION CALCULATIONS

PPA2

RR Permeable Pavers Calculations - 15/26

Drawdown Time for Permeable Pavers PPA2

Filter Bed Depth (df) = 1.00 ft

Ponding Above Pavers (hf) = 0.00 ft

Infiltration Rate (k) = 0.48 in/hr

0.96 ft/day

Required Af = 1,325 SF

Time to Drain Bioretention (t f) = 1 hours

RRv df

RR Permeable Pavers Calculations - 16/26

Contributing Drainage Basin Area = 0.01 Ac.

Impervious Area = 0.010 Ac.

Runoff Reduction Target Volume

RRv = (P) (Rv) (A) / 12

P = 1.0 inches Target runoff reduction rainfall

Rv = 0.05 + 0.009 (I) Volumetric runoff coefficient

I = 100 % impervious

Rv = 0.95

A = 436 SF Area draining to the practice

12 = Unit conversion factor (in/ft)

RRv = 34 CF

VPMIN ≥ RRv (target) / RR%

RRV (target) = 34 CF

RR% = 100 Runoff Reduction percentage using Table 4.1.3-2

VPMIN = 34 CF

Storage Volume Provided

Actual Storage Volume Provided

VP = PV + VES (N)

PV = 0 CF Ponding Volume

VES = AP * d Volume of Engineered Soils

AP = 406 SF Area of Permeable Paver System d = 1 Ft Depth of gravel

VES = 405.9657 CF

N = 0.40

VP = 162 CF

Determine if Minimum Storage Volume was Met

VP ≥ VPMIN

162 ≥ 34

Runoff Reduction requirements are met

DOWNTOWN NORCROSS PARKING IMPROVEMENTS

PERMEABLE PAVERS RUNOFF REDUCTION CALCULATIONS

PPA3

RR Permeable Pavers Calculations - 17/26

Drawdown Time for Permeable Pavers PPA3

Filter Bed Depth (df) = 1.00 ft

Ponding Above Pavers (hf) = 0.00 ft

Infiltration Rate (k) = 0.48 in/hr

0.96 ft/day

Required Af = 406 SF

Time to Drain Bioretention (t f) = 2 hours

RRv df

RR Permeable Pavers Calculations - 18/26

Contributing Drainage Basin Area = 0.03 Ac.

Impervious Area = 0.02 Ac.

Runoff Reduction Target Volume

RRv = (P) (Rv) (A) / 12

P = 1.0 inches Target runoff reduction rainfall

Rv = 0.05 + 0.