26-Big Creek Greenway Bethelview Trailhead - Hydrology Report.pdf

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Drainage Report

Big Creek Greenway Design Services – On Call

Task 1 – Bethelview Trailhead Parking Lot Design

H&L # 1023020.02

For

Forsyth County Capital Projects

Prepared By

May 2024

BCC Engineering, LLC dba

Heath & Lineback

2390 Canton Road Bldg. 200

Marietta, Georgia 30066

(770) 424-1668 * Fax: (770) 424-2907

June 2024

Professional Engineering Certificate

I hereby certify that I am a registered professional engineer in the State of Georgia practicing with BCC Engineering, LLC dba Heath & Lineback, a Florida Limited Liability Company, authorized to operate as an engineering business with the State of Georgia, and that I have reviewed or approved the evaluation, findings, opinions, conclusions, or technical advice of this

Conceptual Drainage Report.

Project: Bethelview Trailhead Parking Lot Design

Prepared for: Forsyth County Capital Projects

I acknowledge that the procedures and references used to develop the results contained in this

Conceptual Drainage Report are standard to the professional practice of engineering and planning as applied through professional judgment and experience.

Name: Victor H. Herrera

Digital Signature: ___________________________________

BCC Engineering, LLC dba

Heath & Lineback

The official record of this document has been electronically signed and sealed by

Victor H. Herrera, State of Georgia, PE No. 47786, on 06/07/2024 using a digital signature as required by Rule 61G 15-23.004, F.A.C. Printed copies of this document are not considered signed and sealed. The signature must be verified on the electronic documents.

STATE OF

LICENSE

PROFESSIONAL ENGIN

EE

R

GEORGIA

No.047786VI

CTOR H. HERRERA

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1 Table of Contents

1.0 INTRODUCTION

a) Site Location, acreage, and current and proposed land use ………………………………………………

b) Offsite Drainage Areas ………………………………………………………………………………………………………

c) Project Methodology …………………………………………………………………………………………………………

2.0 SUMMARY TABLES

a) Storm event ……………………………………………………………………..………………………………………………..

b) Pre-developed flow rate ………..…………………………………………………………………………………………..

c) Developed flow rate ……………………………………………………………………………………………………………

d) Allowable flow rate ……………………………………………………………………………………………………………..

e) Peak elevation in the stormwater facility …………………………………………………………………………….

f) Developed velocity ………………………………………………………………………………………………………………

Appendix A - Pre-developed Drainage Map Appendix B - Pre-developed Weighted Curve Number Calculations Appendix C - Pre and Post Time of Concentration Calculations Appendix D - Developed Site Map Appendix E - Developed Weighted Curve Number Calculations Appendix F - Details of Water Quality facilities Appendix G - Water Quality Calculations Appendix H - Water Quality Performance Review Tool Appendix I - Pre-developed hydrographs and routings for the 1, 2, 10, 25 & 100 Year Storms Appendix J - Developed hydrographs and routings for the 1, 2, 10, 25 & 100 Year Storms Appendix K - Storage Calculations Appendix L - Control Structures Details

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1.0 INTRODUCCION

a) Forsyth Bethelview Trail – Site Description

The jobsite, nestled in Cummings, Georgia, just east of Bethelview Road, spans approximately 1.91 acres and resides within a suburban landscape. Adjacent to the Bethelview Road Trailhead, with a gravel parking surface that currently serves the purpose of a parking lot, trees surrounding the area, and overlooking a pond.

The utilities on the jobsite include a 1.5” waterline, an existing culvert pipe structure that channels under the Big Greek Greenway trailhead, and a septic tank. Notably, upon entry, there's a 185-foot stretch of asphalt leading to the site, and electrical lines that run over Bethelview Road. Currently, three (3) designated ADA parking spots, restroom facilities and a warm-up area reside on the site.

Pre-Developed land use

Basin Name

Impervious

Area

(Ac.)

Pervious

Area

(Ac.)

Total

Area

(Ac.)

Area 1 (NW) 0.0000 0.4371 0.4371

Area 2 (NE) 0.0280 0.2631 0.2911

Area 3 (SE) 0.5413 0.0100 0.5513

Area 4 (S) 0.1703 0.0100 0.1803

Area 5 (SW) 0.4407 0.0100 0.4507

Total (Ac.) 1.1803 0.7302 1.9105

b) Offsite drainage areas

The Offsite Drainage Area of 0.44 Ac. has been identified in Drainage Maps as Basin 1, please refer to

Appendix A (Pre-developed) and Appendix D (Developed).

c) Project Methodology

The proposed drainage system has been divided into two main basins: Basin 1 and Basin 2. Basin 1 includes Area 1 (NW) with a total area of 0.44 Ac which runoff will be intercepted by Swale 1 (BMP 1), and

Area 2 (NE) with a total area of 0.73 Ac which runoff will be intercepted by Swale 2 (BMP 2). Basin 2 includes Area 3 (SE) with a total area of 0.55 Ac which runoff will be intercepted by Swale 3 (BMP 3); Area

4 (S) with a total area of 0.18 Ac which runoff will be intercepted by Swale 4 (BMP 4), and Area 5 (SW) with a total area of 0.45 Ac which runoff will be intercepted by Swale 5 (BMP 5).

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a) BMP 1 & BMP 2

To eliminate contaminants through vegetative filtration, infiltration and biofiltration, Swale 1 (BMP 1) has been designed as an Enhanced Dry Swale that collects runoff from Area 1 (NW), which is considered an offsite area and therefore, it does not require treatment. However, it will be connected through an 18-inch pipe with Swale 2 (BMP 2), intended as a grass channel, part of the treatment from this Swale will be credited to Swale 1 (BMP 1).

Please refer to Appendix G for details and dimensions of the Enhanced Dry Swale (BMP 1). Calculations for this Enhanced Dry Swale are detailed in Appendix G.

b) Outlet Control Structure CS-1

The control structure CS-1 (please refer to Appendix L) is a square box of 3.5 ft x 3.5 ft and 6.17 ft high.

On its northeast face, there is a 6-inch high by 3.5 feet wide opening functioning as a vertical weir. The weir control elevation has been set at elevation 1056.55 ft (NAVD). The peak stage for 25 year and 24-hour storm is 1056.00 ft NAVD (refer to the Node Maximum Conditions Report, Swale 1 (NW), Appendix

J) and therefore, the maximum water level maintains a freeboard greater than 6 inches, 1057.55-1056.60=

0.95 ft > 0.5 ft.

Inside the box at the bottom, there is an underdrain covered for maximum infiltration. The discharge from the box is through a 36-ft 18-inch pipe, with an invert elevation of 1051.38 ft NAVD discharging into the lake at elevation of 1043.00 ft NADV, please refer to Appendix L for Control Structure details. Riprap has been placed at the outlet of the discharge for erosion control purposes.

c) BMP 3

The runoff coming from the 0.48 Ac impervious area of Area 3 (NE) will be treated in Swale 3. This treatment is planned as a Bioretention Area which reduces stormwater pollution through filtration, biological absorption, and activity microbial, using vegetation, an engineered soil mix and bottom drainage.

Please refer to Appendix G for details and dimensions of the Bioretention Area (BMP 3). Calculations for the Bioretention Area (BMP 3) are detailed in Appendix G.

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d) Outlet Control Structure CS-3

The control structure CS-3 (please refer to Appendix L) is a square box of 3.5 ft x 3.5 ft and 5.58 ft high.

On its east face, there is a 6-inch high by 3.5 feet wide opening functioning as a vertical weir. The weir control elevation has been set at elevation 1050.03 ft (NAVD). The peak stage for 25 year and 24-hour storm is 1050.27 ft NAVD (refer to the Node Maximum Conditions Report, Swale 3 (SE) Appendix J) and therefore, the maximum water level maintains a freeboard greater than 6 inches, 1051.03-1050.27=0.76 ft > 0.5 ft.

Inside the box at the bottom, there is an underdrain covered for maximum infiltration. The discharge from the box is through a 7-ft 18-inch pipe, with an invert elevation of 1045.45 ft NAVD discharging into the

Bioretention Area BMP 3 at elevation of 1042.00 ft NADV, please refer to Appendix L for Control Structure details. Riprap has been placed at the outlet of the discharge for erosion control purposes.

e) BMP 4

The runoff coming from the 0.16 Ac impervious area of Area 4 (S) will be treated in Swale 4 (BMP 4). This treatment is intended to reduce stormwater pollution through filtration, biological absorption, and microbial activity, using vegetation, an engineered soil mix and underdrainage.

Please refer to Appendix G for details and dimensions of the Bioretention Area (BMP 4). Calculations for the Bioretention Area (BMP 4) are detailed in Appendix G.

f) Outlet Control Structure CS-4

The control structure CS-4 (please refer to Appendix L) is a square box of 3.5 ft x 3.5 ft and 5.58 ft high.

On its North face, there is a 6-inch high by 3.5 feet wide opening functioning as a vertical weir. The weir control elevation has been set at elevation 1048.05 ft (NAVD). The peak stage for 25 year and 24-hour storm is 1048.17 ft NAVD (refer to the Node Maximum Conditions Report, Swale 4 (S) Appendix J) and therefore, the maximum water level maintains a freeboard greater than 6 inches, 1049.05-1048.17=0.88 ft > 0.5 ft.

Inside the box at the bottom, there is an underdrain covered for maximum infiltration. The discharge from the box is through a 7-ft 24-inch pipe, with an invert elevation of 1043.47 ft NAVD discharging into the lake at elevation of 1042.00 ft NADV, please refer to Appendix L for Control Structure details. Riprap has been placed at the outlet of the discharge for erosion control purposes.

This structure is connected to the control structure CS-5 (invert elev. 1044.23 ft NADV).

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g) BMP 5

Area 5 (NW) is located in the northwest section of the parking area, and it has an impervious area of 0.37

Ac. and the runoff coming from the impervious area will be treated in Swale 5 (BMP 5). This treatment is intended to reduce stormwater pollution through filtration, biological absorption, and microbial activity, using vegetation, an engineered soil mix and underdrainage.

Please refer to Appendix G for details and dimensions of the Bioretention Area (BMP 5). Calculations for the Bioretention Area (BMP 5) are detailed in Appendix G.

h) Outlet Control Structure CS-5

The control structure CS-5 (please refer to Appendix L) is a square box of 3.5 ft x 3.5 ft and 5.58 ft high.

On its North face, there is a 6-inch high by 3.5 feet wide opening functioning as a vertical weir. The weir control elevation has been set at elevation 1049.01 ft (NAVD). The peak stage for 25 year and 24-hour storm is 1049.22 ft NAVD (refer to the Node Maximum Conditions Report, Swale 5 (NW) Appendix J) and therefore, the maximum water level maintains a freeboard greater than 6 inches, 1050.01-1049.22=0.79 ft > 0.5 ft.

Inside the box at the bottom, there is an underdrain covered for maximum infiltration. The discharge from the box is through a 21-ft 18-inch pipe, with an invert elevation of 1044.43 ft NAVD discharging into the

CS-4 at elevation of 1044.23 ft NADV, please refer to Appendix L for Control Structure details.

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2.0 SUMMARY TABLES

Summary table for LAKE (SWALE 1) discharge point

Storm

Event

Pre-Developed

Flow rate

Developed

Flow rate

Allowable

Flow Rate

Peak Elev.

SWALE 1

(NW)

Peak Elev.

SWALE 2

(NE)

Developed

Velocity cfs cfs cfs ft, (NAVD) ft, (NAVD) ft/seg

01Y24H 0.81 0.51 46.58 1053.02 1061.62 8.70

02Y24H 0.96 0.76 46.58 1054.04 1061.64 9.81

25Y24H 1.86 1.34 46.58 1054.22 1061.75 11.64

100Y24H 0.71 0.58 46.58 1054.25 1061.68 9.05

Summary table for LAKE (SWALE 4) discharge point

Storm

Event

Pre-Developed

Flow rate

Developed

Flow rate

Allowable

Flow Rate

Peak Elev.

SWALE 4

(S)

Peak Elev.

SWALE 5

(SW)

Developed

Velocity cfs cfs cfs ft, (NAVD) ft, (NAVD) ft/seg

01Y24H 0.81 0.51 48.26 1047.45 1049.20 5.02

02Y24H 0.96 0.76 48.26 1047.47 1049.23 5.66

25Y24H 1.86 1.34 48.26 1047.51 1049.30 6.71

100Y24H 0.71 0.58 48.26 1047.45 1049.19 5.22

Summary table for BIG CREEK (SWALE 3) discharge point

Storm

Event

Pre-Developed

Flow rate

Developed

Flow rate

Allowable

Flow Rate

Peak Elev.

SWALE 3

(SE)

Developed

Velocity cfs cfs cfs ft, (NAVD) ft/seg

01Y24H 0.69 0.57 47.05 1048.77 9.06

02Y24H 0.79 0.70 47.05 1048.79 9.64

25Y24H 1.26 1.18 47.05 1048.86 11.29

100Y24H 0.41 0.40 47.05 1048.74 8.14

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Appendix A

Pre-developed Drainage Map

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Appendix B

Weighted Curve Number Calculations

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Appendix C

Time of Concentration Calculations

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Appendix D

Summary of pre-developed flow rate

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Appendix E

Developed Site Map

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Appendix F

Weighted Curve Number Calculations

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Appendix G

Time of Concentration Calculations

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Appendix H

Water Quality facilities

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Appendix I

Proposed Stormwater Ponds

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Appendix J

Pre-developed hydrographs and routings for the 1, 2, 10, 25 100 Year Storms

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Appendix K

Developed hydrographs and routings for the 1, 2, 10, 25

100 Year Storms

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Appendix L

Stage/Storage/Discharge Calculations

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Appendix M

Control Structures Details

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Appendix N

Water Quality Calculations