2-Stormwater Management Report for Sheriffs Training Site.pdf

STORMWATER MANAGEMENT REPORT

FOR THE

FORSYTH COUNTY SHERIFF’S TRAINING SITE

9470 OLD FEDERAL RD

LL 71, District 3, 1ST Section

Parcel 001 014

Forsyth County, Georgia

February 13, 2026

Owner/Developer: Forsyth County

514 West Maple St.

Cumming, GA 30040

(770) 205-4550

Engineer: Hussey Gay Bell

3100 Breckinridge Blvd

Building 300

Duluth, GA 30096

(770) 923-1600

HJA Job # 25-037

Project Manager: Ryan Remillard, P.E.

Engineer: Ruth Hansen, P.E.

Certification: Stephen Mark Bond, P.E.

TABLE OF CONTENTS

Location Map & Objective 3

Project Description 4

Pre-Developed Conditions 4

2019 Post-Developed Condition 5

2026-Post-Developed Conditions 6

Stormwater Management 7

Runoff Reduction 7 & A-2

Water Quality 7 & A-12

Channel Protection 8

Detention 8

Appendix A –

Runoff Reduction Infeasibility form A-2

TSS 2015 & 2022 Spreadsheets A-12

Appendix B -Excerpts from, “Hydrology Study and Design Calculations Outdoor

Firing Range”, dated December 2019 by Hodges, Harbin, Newberry & Tribble, Inc.

Current SSD Table B-19

Current OCS & Outlet Pipe B-20

Current & Pre-Developed CN values B-24

WQ Calcs B-27

CP Calcs B-30

Appendix C – Hydrological Data

Summaries C-2

2019 Sediment Pond 2 SSD Data C-10

New Sediment Pond 2 SSD Data C-11

Revised OCS Detail C-13

Hydrographs 1,2,10,25 & 100-year storms C-15

Appendix D- Maps

Pre-Developed Drainage Map – 2019 Study D-1

Post-Developed Drainage Map – 2019 Study D-2

Pre-Developed Drainage Map – 2026 Study D-3

Post-Developed Drainage Map – 2026 Study D-4

WQ Map D-5

LOCATION MAP

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User Polygon

User Callout

SITE

User Text Box OBJECTIVE: The purpose of this study is to verify that the revisions to the Sheriff's Training Facility are in compliance with Forsyth County Requirements.

BACKGROUND

a) Project Description

This report is a revision to the “Hydrology Study and Design Calculations – Outdoor

Firing Range”, dated December 2019 by Hodges, Harbin, Newberry & Tribble, Inc

Consulting engineers. Areas previously graveled in the 2019 study are being paved.

b) 2019 study by HHNT

The study by HHNT was reproduced for the Pre-Developed Condition and Proposed condition. Water Quality was provided in the permanent pool and Channel Protection was not provided. Detention is provided in “Sediment Pond 2”, a large wet pond located adjacent to the eastern portion of the site.

c) 2026 Proposed Condition

Data (Drainage Area, % Impervious, Time of Concentrations, pond Stage Storage and

OCS configuration) was obtained from the 2019 previous study. Graveled areas were changed to pavement. The existing OCS was modified to provide Channel Protection.

c) Model

Hydraflow Hydrographs 2022 (Hydraflow) was used to model the pre-developed and post-developed drainage conditions. The program used the SCS Curve Number Method to calculate the drainage flows for the 1, 2, 10, 25, and 100-year frequency storms. The

Hydraflow reports are included in Appendix C for the pre- and post-developed conditions. To remain consistent with the previous study, the rainfall chart and predeveloped hydrographs were obtained from the 2019 study. All Times of

Concentration resulting in less than 5 minutes will be studied at 5 minutes. CN values for the proposed condition were studied and adjusted if applicable.

PRE-DEVELOPED CONDITIONS

d) Overview

According to the 2019 study, all areas had been disturbed per permit (LDP1308

SD140099). The predeveloped site consisted of the used soil borrow area. All ponds and overflow structures were constructed under the Eagle Point Landfill permit.

The proposed 2019 development of the shooting range placed fill material in one existing sediment ponds (Sediment Pond #1) and all stormwater was directed to existing Sediment Pond #2. The report provided calculations for retrofitting the existing overflow structure to handle the additional flow. Forsyth County was contacted regarding the existence of an As-Built Report for Sediment Pond #2. No as-built was available; however, the county agreed to proceed assuming that the pond and

OCS were constructed per the 2019 design.

e) Pre-Developed Drainage Basins

The following table obtained from, “Hydrology Study and Design Calculations –

Outdoor Firing Range”, dated December 2019 by Hodges, Harbin, Newberry &

Tribble, Inc Consulting engineers summarizes the information used to obtain pre-developed flows. This study relates to Pre-DA2 (HSP2) only.

3) POST-DEVELOPED CONDITIONS 2019 STUDY

a) Overview

The proposed 2019 site to Sediment Pond 2 includes paving the graveled area used in the 2019 study. Additional impervious = 1.0 acres. The OCS was also adjusted to provide Channel Protection Volume.

b) Post-Developed Drainage Basins 2019 Study-The table below shows the proposed basins as documented in the 2019 study.

c)

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4) Post-Developed Drainage Basins 2026 Study

a) Overview

The proposed 2019 site to Sediment Pond 2 includes paving the graveled area used in the 2019 study. Additional impervious = 1.0 acres. The OCS was also adjusted to provide Channel Protection Volume.

b) Post-Developed Drainage Basins 2026 Study-The table below shows the proposed basins as documented in the 2019 study.

Post-DA2 To-Sed Pond 2

The Firing range and training facility reside in this basin. This basin discharges directly to Sediment Pond #2. It is the only basin that had a change within it from the

2019 study.

o Basin Area = 27.56 acres, CN = 76 o Wooded = 1.05 acres, CN = 60 o Poor Grassland = 22.77 acres, CN = 74 o Gravel Parking = .8 acres, CN = 89 o Perm Pool & Buildings = 1.08 acres, CN = 98 o Artificial Turf = .86 acres, CN = 98 o Pavement = 1.0 acres, CN = 98

* 1 acre of Gravel/grass was converted to pavement.

Rainfall 3.15” 3.7” 5.75” 6.75” 7.9”

Basin Area CN TC 1 yr 2 yr 10 yr 25 yr 100 yr

Post DA-1 101.15 66 84.4 20.87 33.79 96.02 130.93 173.21

Post DA-2 (BP) 16.47 62 29.9 4.51 8.21 27.09 37.96 51.26

DA-Pond 2 27.56 76 25.5 28.94 40.02 85.74 109.36 136.97

Rout Sed 2 1.54 1.77 3.84 4.50 6.10

DA-Pond 3 9.31 78 12.8 16.11 21.69 44.05 55.37 68.5

Rout Sed 3 .45 .60 1.0 1.15 1.30

Post-DA3 6.04 71 35.3 3.71 5.49 13.23 17.36 22.28

Note that Basins Post DA-1, DA-Pond 3 and Post-DA3 were not altered from the 2019 report

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c) Stormwater Management

(1) Runoff Reduction

This practice is intended to reduce post-construction runoff rates, volumes, and pollutant loads. It should retain the first 1.0 inch of rainfall onsite to the maximum extent practicable. However, the soil for this site is infeasible due to a severely limited infiltration rate. The soil report from the NRCS as well as the Runoff

Reduction Infeasibility form can be found in Appendix A. In addition to poor infiltration rates, site constraints such as an existing landfill reduces the applicability of infiltration practices.

(2) Water Quality

2019 study information below

Previous impervious = .034 x 27.56=.937 acres

2026 added 1 acre of impervious area. New Impervious area = 1.937 acres.

I = 1.94/27.56=7.03%

WQV required =

Rv = .05+.009(7.03) = .11

WQVreq’d = (1.2 x .11 x 27.56/12) x 43560 = 13,596 cft = .31 ac-ft

WQ provided = 78,874cft=1.8061 ac-ft >.31 ac-ft (13,596 cft)

100% WQv is provided in permanent pool per 2019 study.

The TSS spreadsheet for Forsyth County was not provided in the previous study.

In order to proceed, the amount of existing impervious on the site was obtained by using the 3.4% impervious cover and multiplying by 27.56 acres. This resulted in

.94 acres. An additional 1 acre (for this project) was added resulting in 1.94 acres of impervious surface. The pervious area was obtained by subtracting the impervious from the total to get 25.62 acres pervious. This was input into the 2015

Forsyth County spreadsheet and the new Georgia Stormwater Site Development

Review Tool Version 2.2. The BMP used for the site was a stormwater pond.

The site met the 80% TSS removal rate without requiring additional BMPs.

Refer to Appendix A for the TSS spreadsheets and Runoff Reduction Infeasibility

Form.

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(3) Channel Protection

Stormwater management systems are required to provide 24-hour extended detention storage for the 1-year 24-hour event if they discharge greater than 2.0 cfs in the 1-year storm. The current OCS was routed and the 1-year discharge was 2.11 cfs. Because this is greater than 2 cfs, the existing rectangular orifice will be blocked, and a CP orifice will be installed at a higher elevation. The current CP volume calculations are shown in Appendix B of this report. There appears to be an error in the previous report as they are assuming the CPV occurs in the permanent pool. The permanent pool can’t provide channel protection. New calculations are below.

Area = 27.56 ac, CN = 76, Ia = .63, P=3.15”, S=3.26”, Q = 1.12”

Ia/P = .2, Tc = 25.5 min, qu = 550 csm/in, q0/qi = .03, Vs/Vr = .64

VS = CPV = 71,603cft

Perm Pool = 78,674 cft @ 1066.72

Required CPelev = 78,674+71,603 = 150,277 cft @ 1068.3

(1-year routed from Hydraflow: elev = 1068.15) <1068.72 so CPV provided

Compute orifice size

Storage = 150,277 cft

C = .6

Q ave = 150277/86400 = 1.74 cfs

H ave = 1068.72-1066.72/2 = 1 ft

Area = Qave/(.6x64.4xHave)^.5 = .36 sft

Area = 3.1416 x r^2 = .36sft

Currently 4”x12” orifice = .33 x 1 = .34 sft <.36sft so existing orifice is okay.

Install new 4”x2” orifice @ 1068.7 for stormwater control

(4) Detention

Post-development peak discharge rates must be reduced by 10% from pre-development conditions for the 1-, 2-, 5-, 10-, and 25-year rainfall events while controlling/safely conveying the 100-year rainfall event. Detention for the new construction is provided in Sediment Basin #2. A summary is shown of the revised sediment pond #2 performance in Tables 3 and 4.

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Table 3: Design Considerations

Water Quality Channel Protection

(1-Yr Event) Extended Detention

Pond

ID

WQV

Required /

Provided

Water

Quality

Orifice &

Invert Elev.

Channel

Protection

Required /

Provided

Channel

Protection

Orifice &

Invert Elev.

Rect. Weir

Riser Crest

Size / Elev. /

Vol.

100-Year

Ponding

Elev. / Vol.

cft ft cft ft ft / cft ft / cft ft / cft

A

13,596 cft @

~1064.5

*78,674cft @

1066.72

*N/A

150,277 cft @

1068.3 /

194,111cft @

1068.72

4"x12” @

1066.72

4"x12” @

1068.72

15.2’ @

1070.7 /

408,979cft

1070.71 /

409,810 cft

*100% WQV is provided in the permanent pool

Table 4: Post-Developed Summary Frequency QPre QPost

(to pond)

QPost

(route)

QPost

(bypass)

QSed 3

QPost

(POST-HSP2)

Sed

Pond#2

Elevation

Sed Pond#2

Volume

Hydrograph 2 8 9 7 13 16 - -

1-year 7.05 28.94 1.54 4.52 .45 5.84 1068.15 143,431

2-year 13.82 40.02 1.77 8.22 .60 9.99 1068.48 172,424

10-year 50.65 85.74 3.84 27.09 1.00 29.79 1069.71 282,633

25-year 72.49 109.36 4.50 37.96 1.15 41.52 1070.25 344,640

100-year 99.45 136.97 6.10 51.26 1.30 55.91 1070.74 414,838

5) Outfall Pipe – 30” no change.

6) Existing Downstream System

The discharge from the detention pond was designed to flow through a level spreader prior to entering downstream channels per design plans for Outdoor Firing Range for the Forsyth County Sheriff’s Department by HHNT, dated 5//8/2020.

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(5) SUMMARY OF FLOWS

Summaries of flows are shown in Table 4 for the 1, 2, 10, 25 and 100-year frequency storms. The proposed conditions flow represents Sediment Pond #2 outfall combined with the bypass basin and the discharge from Existing Sediment Pasin #3. The table demonstrates that the proposed post-developed peak discharge rates are at least 10% lower than the pre-developed condition at Study Point POST-HSP2. It should be noted that the peak discharges are not necessarily a numerical summation of individual hydrograph peak flow rates.

The previous study did not require that the post developed flows discharge at 90% of the Existing Conditions flow. Table 5 is provided to show that this condition is met.

Table 5: Summary of Flows for Study Point POST HSP2

HGB - Study Point C (Existing & Proposed Conditions) Pass?

Storm

Event

Existing

Conditions

90% of Allowable

Flow

Proposed Conditions Flow

Flow (cfs) (cfs) (cfs)

1-year 7.05 6.34 5.84 YES

2-year 13.82 12.44 9.99 YES

10-year 50.65 45.59 29.79 YES

25-year 72.49 65.24 41.52 YES

100-year 99.45 89.51 55.91 YES

5) DOWNSTREAM ANALYSIS

The study of this basin is intended to examine the bigger picture of the impact of the proposed development at a point where the site comprises up to 10% of the total drainage area.

The total basin area at Study Point HSP2 is comprised of approximately ±154.5 acres. The approximate relevant onsite area to this study point is ± 10 acres, or 6.5% of the basin;

therefore, Study Point HSP2 is the 10 % study point for this development.

Table 6: 10% Analysis Summary at HSP2

Storm

Event

Existing Conditions Proposed Conditions

Flow

Flow (cfs) (cfs)

1-year 7.05 5.84

2-year 13.82 9.99

10-year 50.65 29.79

25-year 72.49 41.52

100-year 99.45 55.91

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Appendix A Runoff Reduction Infeasibility (RRI) Form for

Determination of Infeasibility

A-1

Appendix A. Runoff Reduction Infeasibility (RRI) Form for Determination of Infeasibility

A-1

Date Submitted:

Forsyth County

Runoff Reduction Infeasibility (RRI) Form for

Determination of Infeasibility

DESIGN PROFESSIONAL CONTACT INFORMATION

Name: ________________________________________________________________________________

Email: ________________________________________________________________________________

Phone: _______________________________________________________________________________

DESCRIPTION OF SITE

Land Development Application Number: ____________________________________________________

Site Address: ___________________________________________________________________________

PROPOSED CONDITIONS OF SITE

Disturbed Area (acres): __________________________________________________________________

Impervious Area (acres): _________________________________________________________________

RUNOFF REDUCTION AND WATER QUALITY VOLUME SUMMARY

Maximum Practicable Runoff Reduction Volume* (cubic feet):

Volume for Water Quality Treatment* (cubic feet):

*If any of the stormwater runoff volume generated by the first 1.0 inches of rainfall cannot be reduced or retained on the site, due to site characteristics or constraints, the remaining volume shall be increased by a multiplier of 1.2 and shall be intercepted and treated in one or more best management practices that provide at least an 80 percent reduction in total suspended solids.

GENERAL SUPPORTING DOCUMENTATION

All General Supporting Documentation must be included with this RRI Form for the submittal for a

Determination of Infeasibility to be considered complete. Please check each item below to confirm it has been included in the submittal package.

Stormwater Management Plan

Forsyth County Stormwater Quality Site Development Review Tool v2.2

Written justification that the site cannot accommodate runoff reduction practices that rely on evapotranspiration and reuse such as rainwater harvesting or green roofs

A-2 rhansen Text Box Mark Bond rhansen Text Box Mbond@HusseyGayBell.com rhansen Text Box 770-923-1600 ext 301 rhansen Text Box rhansen Text Box 9470 Old Federal Rd rhansen Text Box rhansen Text Box

3.3 acres rhansen Text Box 1 acres - new rhansen PolyLine rhansen Text Box 11340 cft (27.56 ac) rhansen Text Box 13,608 cft (27.56 ac) rhansen

A-2

SITE CONDITION APPLICABILITY

(descriptions are in the Forsyth County Policy on Practicability Analysis for Runoff Reduction)

Please check each applicable item below and confirm the supporting documentation has been included in the submittal for a Determination of Infeasibility.

* Note: Economic Hardship must be present with at least one other condition to warrant a Determination of Infeasibility. Additionally, a Determination of Infeasibility for economic hardship is applicable to a maximum 50 percent of the volume required for meeting the runoff reduction stormwater management standard.

Site Condition Supporting Documentation

Soil Infiltration Rate Infiltration test(s), soil boring log(s), and report of results as interpreted by a Professional Engineer, Professional Geologist, or Soil Scientist licensed in Georgia

Water Table Soil boring log(s) and report with results of the seasonal highwater table assessment as interpreted by a Professional Engineer, Professional Geologist, or Soil Scientist licensed in Georgia

Bedrock Soil boring log(s) and report with results of the shallow bedrock assessment as interpreted by a Professional Engineer, Professional Geologist, or Soil Scientist licensed in Georgia

Extreme Topography Site survey showing 50 percent of the contributing drainage area is steeper than 3:1 (H:V) slopes as interpreted by a Professional Engineer or Land Surveyor licensed in Georgia AND Stormwater Management Plan showing the post-development surface grades will reflect the same condition as the site survey

Karst Topography Report developed by a Professional Engineer, Professional Geologist, or Soil Scientist licensed in Georgia

Hotspots/ Contamination

Phase I Environmental Assessment Report

Historic Resources Documentation of the Georgia's Natural, Archaeological, and Historic Resources GIS listing

OR

Report of assessment from a Preservation Professional (including Archaeologist, Architectural Historian, Historian, Historic Preservationist, or Historic Preservation Planner)

Site Constraints Site plan identifying all development requirements (zoning side/front setbacks, build-to-lines, stream buffers, floodplains, septic fields, etc.) that are creating irreconcilable conflicts with onsite runoff reduction

Economic Hardship* An estimated construction cost comparison of proposed runoff reduction practices compared to proposed water quality practices must be included to show the cost of runoff reduction practices is equal to or greater than three times the cost of providing water quality practices

A-3 rhansen PolyLine

PolyLine

A-3

GEORGIA PROFESSIONAL ENGINEER CERTIFICATION

Printed Name: __________________________________________________________________________________

Signature: _____________________________________________________________________________________

Date: _________________________________________________________________________________________

FOR FORSYTH COUNTY INTERNAL USE ONLY

APPROVED

APPROVED with the following requirements:

DENIED

A-4

Stephen Mark Bond

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Infiltration Systems, Shallow

Map unit symbol

Map unit name Rating Component name (percent)

Rating reasons (numeric values)

Acres in AOI Percent of AOI

LcF Louisa fine sandy loam, steep phase

Severely limited Louisa (100%) Slope (1.00) 8.4 99.4%

Water movement (0.45)

Vegetation establishment (0.32)

Adsorptive capacity (0.25)

MbB2 Madison fine sandy loam, eroded very gently sloping phase

Somewhat limited

Madison (100%) Water movement (0.45)

0.1 0.6%

Vegetation establishment (0.32)

Adsorptive capacity (0.25)

Slope (0.04)

Totals for Area of Interest 8.4 100.0%

Rating Acres in AOI Percent of AOI

Severely limited 8.4 99.4%

Somewhat limited 0.1 0.6%

Totals for Area of Interest 8.4 100.0%

Infiltration Systems, Shallow—Forsyth County, Georgia

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

2/12/2026

A-7

Rectangle

Description

Shallow infiltration systems are stormwater management practices that are placed 1 to 3 feet in the ground, depending on the application. These systems include pervious pavement, buffer strips, filter strips, and vegetated swales. They slow the movement of stormwater to surface waters and also filter a significant portion of pollutants from the stormwater. The fundamental function of these systems is to hold the runoff generated by an area, such as a parking lot, from the first 1 inch of rainfall during a 24-hour storm preceded by 48 hours of no measurable precipitation. There should be little or no ponding at the surface. The water should infiltrate into the surrounding soil in 24 to 48 hours. Only that part of the soil between depths of 24 and 80 inches is evaluated.

The ratings are based on the soil properties that affect infiltration of the stormwater, construction and maintenance of the system, and public safety and health. Saturated hydraulic conductivity (Ksat), depth to a water table, ponding, depth to bedrock or a cemented pan, and flooding affect the transmission of rainwater. Stones and boulders, ice, and bedrock or a cemented pan interfere with installation. Subsidence interferes with installation and maintenance.

Excessive slope may cause lateral seepage and surfacing of the water in downslope areas. Some slopes may become unstable and move upon addition of water.

Soils underlain by loose sand and gravel or fractured bedrock at a depth of less than 4 feet below the bottom of the system may adversely affect water quality and public health. In these soils the shallow infiltration system may not adequately filter the stormwater, particularly if the adsorptive capacity of the soil below the system is low. As a result, the ground water may become contaminated. In areas underlain by limestone, solution channels and subsequent subsidence may damage adjacent infrastructure. Also, areas underlain by limestone may be subject to ground-water contamination.

The ratings are both verbal and numerical. Rating class terms indicate the extent to which the soils are limited by all of the soil features that affect the specified infiltration system. "Not limited" indicates that the soil has features that are very favorable for the specified system. Good performance and very low maintenance can be expected. "Somewhat limited" indicates that the soil has features that are moderately favorable for the specified system. The limitations can be overcome or minimized by special planning, design, or installation. Fair performance and moderate maintenance can be expected. "Very limited" indicates that the soil has one or more features that are unfavorable for the specified system. The limitations generally cannot be overcome without major soil reclamation, special design, or expensive installation procedures. Poor performance and high maintenance can be expected.

Numerical ratings indicate the severity of individual limitations. The ratings are shown as decimal fractions ranging from 0.01 to 1.00. They indicate gradations between the point at which a soil feature has the greatest negative impact on the specified system (1.00) and the point at which the soil feature is not a limitation (0.00).

Infiltration Systems, Shallow—Forsyth County, Georgia

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

2/12/2026

A-8

The accompanying Summary by Map Unit table in Web Soil Survey or the Aggregation Report in Soil Data Viewer lists the map unit components. These components are determined by the aggregation method chosen. An aggregated rating class is shown for each map unit. The components listed for each map unit are only those that have the same rating class as the one listed for the map unit.

The percent composition of each component in a particular map unit is shown to help the user better understand the percentage of each map unit that has the rating indicated. Other components with different ratings may occur in each map unit.

The complete ratings list for all components, regardless of the map unit aggregated rating, can be viewed by generating the equivalent report from the Soil Reports tab in Web Soil Survey or from the Soil Data Mart site. Onsite investigation may be needed to validate these interpretations and to confirm the identity of the soil on a given site.

Rating Options

Aggregation Method: Dominant Condition

Component Percent Cutoff: None Specified

Tie-break Rule: Higher

Infiltration Systems, Shallow—Forsyth County, Georgia

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

2/12/2026

A-9

Policy on Practicability Analysis for Runoff Reduction Forsyth County Department of Engineering

Introduction

Runoff reduction practices are stormwater best management practices (BMPs) used to disconnect impervious and disturbed pervious surfaces from the storm drainage system. The purpose is to reduce post-construction stormwater runoff rates, volumes, and pollutant loads. Runoff reduction is more than simple infiltration; the runoff reduction volume (RRV) is the retention volume calculated to infiltrate, evapotranspirate, harvest and use, or otherwise remove runoff from a post-developed condition to more closely mimic natural hydrologic conditions.

Certain conditions, such as soils with very low infiltration rates, high groundwater, or shallow bedrock, may lead the Forsyth County Department of Engineering (Department) to waive or reduce the runoff reduction requirement for proposed site development on a case-by-case basis. If any of the stormwater runoff volume generated by the first 1.0 inches of rainfall cannot be reduced or retained on the site because of site characteristics or constraints, the remaining volume shall be increased by a multiplier of 1.2 and shall be intercepted and treated in one or more BMPs that provide at least an 80 percent reduction in total suspended solids.

The Forsyth County Policy on Practicability Analysis for Runoff Reduction (practicability policy) was developed to provide guidance about the site conditions and supporting documentation that could justify a “Determination of Infeasibility” for the runoff reduction stormwater management standard. This policy does not address stormwater management standards infeasibility for linear transportation projects; refer to the Forsyth County Policy on Practicability Analysis for Linear Transportation Projects for additional information.

The practicability policy is based on the following principles:

▪ Designed to help County administrators implement a process for granting a Determination of Infeasibility that supports review of land development applications.

▪ Applies to new development and redevelopment projects for public and private post-construction stormwater BMPs. It is referenced in the Forsyth County Stormwater Management Ordinance and Forsyth County Addendum to the Georgia Stormwater Management Manual.

▪ Aligns with requirements for runoff reduction in the Georgia Environmental Protection Division’s permit to discharge from the municipal separate storm sewer system (MS4) permit. The MS4 permit states that the stormwater management system shall be designed to retain the first 1.0 inch of rainfall on the site to the maximum extent practicable. Many Georgia Stormwater Management Manual (GSMM) BMPs include a runoff reduction component.

▪ Focused on the site conditions and regulatory environment in the Metro Water District.

▪ Requires ensuring all attempts to provide 100 percent RRv onsite have been exhausted when pursuing a Determination of Infeasibility.

Conditions that may Warrant a Determination of Infeasibility

The GSMM and the Forsyth County Addendum to the GSMM provide broad guidance about conditions that may lead the Department to waive or reduce the runoff reduction stormwater management standard. The following conditions may warrant a Determination of Infeasibility.

▪ Soil Infiltration Rate: The soil infiltration rate is less than 0.5 inch per hour as measured over a meaningful portion of the site. Consideration should be given to infiltration rates throughout the soil profile.

▪ Water Table: The water table seasonal high elevation is measured less than 2 feet from the subgrade of a proposed infiltration practice.

A-10

Policy on Practicability Analysis for Runoff Reduction

▪ Shallow Bedrock: Bedrock that cannot be excavated by mechanical means AND is less than 2 feet from the subgrade of an infiltration practice.

▪ Extreme Topography: Proposed conditions reflect surface grades steeper than 3:1 (H:V) slope for more than 50 percent of the contributing drainage area.

▪ Karst Topography: Any of the existing conditions onsite exhibit karst topography.

▪ Hotspots/ Contamination: Reasonable suspicion that previous land uses have resulted in soil contamination onsite.

▪ Historic Resources: Buildings, structures, or historic sites included in the Georgia Historic Preservation Division’s Historic Resources Survey or listed in the National Register of Historic Places or that has been recommended as a historic resource by a Preservation Professional.

▪ Site Constraints: Sites where the density or nature of the proposed redevelopment would create irreconcilable conflicts for compliance between the runoff reduction stormwater management standard and other requirements such as zoning, floodplains, stream buffers, or septic fields.

▪ Economic Hardship: The cost of retaining the first 1.0 inch of rainfall onsite using runoff reduction practices is equal to or greater than three times the cost of providing water quality practices to meet the stormwater management standards. This condition must be present with at least one other condition to warrant a Determination of Infeasibility. Additionally, a Determination of Infeasibility for economic hardship is applicable to a maximum 50 percent of the volume required for meeting the runoff reduction stormwater management standard.

A-11

N a m e o f

D e v e lo p e r:

D a te S u b m it te d

D e v e lo p m e n t

N a m e

P e rm it N u m b e r:

S it e L o c a ti o n

A d d re s s

D e v e lo p e r

C o n ta c t:

P h o n e N u m b e r:

N a m e o f

E n g in e e r( s

D e v e lo p m e n t

T y p e

M a in te n a n c e

R e s p o n s ib il it y

.5

.5

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.0

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P a c

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A a c

D ra in a g e B a s in

D

B

.9

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D ra in a g e B a s in

D

B

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D ra in a g e B a s in

D

B

.0

.0

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D ra in a g e B a s in

D

B

.0

.0

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D ra in a g e B a s in

D

B

.0

.0

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D ra in a g e B a s in

D

B

.0

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.0

D ra in a g e B a s in

D

B

.0

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D ra in a g e B a s in

D

B

.0

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D ra in a g e B a s in

D

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D ra in a g e B a s in

D B

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.9

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N o

Y e s

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T ra c k in g

C o n d it io n s o f

A p p ro v a l:

R e v ie w e d B y

D a te A p p ro v e d

G e o rg ia S to rm w a te r

M a n a g e m e n t

M a n u l

S rm w a te r Q u a li ty

S it e D e v e lo p m e n t R e v ie w

T o o l

V e rs io n

.2

G e n e ra l

In fo rm a ti o n

J a m e s S k in n e r

H u s s e y G a y B e ll

F o rs y th

C o u n ty

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O n ly

T o ta l

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-D e v e lo p m e n t

A re a a c

I

I m p e rv io u s A re a P

P e rv io u s A re a C

A

C o n s e rv a ti o n A re a

T a rg e t R u n o ff

R e d u c ti o n

V o lu m e A c h ie v e d

T a rg e t T

S S

R e m o v a l A c h ie v e d

T o ta l T a rg e t

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Q u a lit y V o lu m e c f)

T

S S

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R u n o ff R e d u c ti o n V o lu m e

A c h ie v e d c f)

T o ta l

T re a te d A re a a c

T o ta l

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T O

T A

L

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C o u n ty S h e ri ff s T ra in in g

S it e

In s ti tu ti o n a l, P u b lic

S e m i P u b lic

F o rs y th

C o u n ty

80%

0%

0%

0%

0%

0%

0%

0%

0%

0%

D B

D

B

D B

D

B

D B

D

B

D B

D

B

D B

D

B

TSS Reduction

T o ta l

S u sp e n d e d

S o li d s (T

S S

R e m o v a l

0%

0%

0%

0%

0%

0%

0%

0%

0%

0%

D B

D

B

D B

D

B

D B

D

B

D B

D

B

D B

D

B

% RR Target Met

R u n o ff R e d u ct io n

R

R

A-12

Development Name: data input cells

Drainage Basin Name: calculation cells constant values

Indicate Pre-Development Land Cover and Runoff Curve Numbers in the Site's Disturbed Area

HSG* A

(acres) CN HSG B (acres) CN

HSG C

(acres)

CN

HSG D

(acres) CN Total % Cover

0.00 39 27.56 61 74 80 27.56 100%

0 0 0 0 0.00 0%

0 0 0 0 0.00 0%

0 0 0 0 0.00 0%

0 0 0 0 0.00 0%

0.00 0%

0.00 0%

0.00 27.56 0.00 0.00 27.56 100%

0.00

6.39

Indicate Post-Development Land Cover and Runoff Curve Numbers in the Site's Disturbed Area

HSG A

(acres) CN HSG B (acres) CN

HSG C

(acres)

CN

HSG D

(acres) CN Total % Cover

0.00 98 1.94 98 98 98 1.94 7%

0.00 39 25.62 61 74 80 25.62 93%

0 0 0 0 0.00 0%

0 0 0 0 0.00 0%

0 0 0 0 0.00 0%

0.00 0%

0.00 0%

0.00 27.56 0.00 0.00 27.56 100%

1.94

0.11

5.72

Scenario 1: Natural Conservation Area *See the GSMM Volume 2, Section 2.3.3.3 for more information. Scenario 3: Soil Restoration *See the GSMM Volume 2, Section 4.23 for more information.

Scenario 2: Site Reforestation/Revegetation *See the GSMM Volume 2, Section 4.22 for more information. Scenario 4: Site Reforestation/Revegetation & Soil Restoration

Total Conservation Area Credit (acres) 0.00

Potential Max Soil Retention, Spre (in)

Potential Max Soil Retention, Spost (in)

Open space - Good condition (grass cover > 75%)

Select a land cover type…

Other

Total

*HSG = hydrologic soil group

Cover Type

Impervious

Select a land cover type…

Local Jurisdiction Input

Other

Cover Type

Open space - Good condition (grass cover > 75%)

Select a land cover type…

Select a land cover type…

Select a land cover type…

Check the box if a portion of the post-developed area employs site reforestation/revegetation and is protected by a conservation easement or equivalent form of protection.

Area (ac) of development reforested/revegetated and protected by a conservation easement or equivalent form of protection.

Check the box if a portion of the post-developed area employs soil restoration and is protected by a conservation easement or equivalent form of protection.

Area (ac) with restored soils in a reforested & revegetated area and protected by a conservation easement or equivalent form of protection.

Note: The green cell will unlock if the Scenario 1 box above is checked

Note: The green cell will unlock if the Scenario 2 box above is checked

Area (ac) of development with restored soils and protected by a conservation easement or equivalent form of protection.

Check the box if the same portion of the post-developed area employs site reforestation/revegetation and soil restoration, and is protected by a conservation easement or equivalent form of protection.

Note: The green cell will unlock if the Scenario

4 box above is checked

*See the GSMM Volume 2, Section 4.22 and 4.23 for more information.

Note: The green cell will unlock if the Scenario

3 box above is checked

Site Data

Georgia Stormwater Management Manual

Stormwater Quality Site Development Review Tool, v2.2

Area (ac) of development protected by a conservation easement or equivalent form of protection.

Check the box if a portion of the post-developed area is protected by a conservation easement or equivalent form of protection.

Forsyth County Sheriffs Training Site

Impervious (ac)

Weighted CN

Impervious (ac)

Rv

Weighted CN

Select a land cover type…

Local Jurisdiction Input

Total

Conservation Area Credits

Select a land cover type…

A-13 constant values

Site Data

Georgia Stormwater Management Manual

Stormwater Quality Site Development Review Tool, v2.2 Forsyth County Sheriffs Training Site

Target Runoff Reduction Storm (in) 1.00 Total Site Area for Water Quality Volume (acres) 27.56

Target Runoff Reduction Volume (cf) 11,340

Target Water Quality Volume (cf) 13,608

On-site

Pervious Area

(acres)

On-site

Impervious

Area

(acres)

Offsite Area

(acres)

RR Volume from Direct

Drainage (cf)

RR Volume from

Upstream

Practices (cf)

Total RR

Volume

Received by

BMP (cf)

Runoff

Reduction %

RR Achieved

(cf)

Remaining

RR Volume

(cf)

WQv from

Direct

Drainage (cf)

Effective

TSS

Removal %

BMP 1 Stormwater Pond 25.62 1.94 11,340 0 11,340 0% 0 11,340 13,608 80%

BMP 2 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 3 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 4 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 5 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 6 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 7 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 8 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 9 Select a BMP… 0 0 0 N/A 0 0 0 N/A

BMP 10 Select a BMP… 0 0 0 N/A 0 0 0 N/A

TOTAL 25.62 1.94 0.00 11,340 0 13,608

UNTREATED AREA (acres) 0.00 0.00

Target Runoff Reduction Volume (cf) 11,340

Target Achieved? No

Remaining Runoff Reduction Volume (cf) 11,340

Target Water Quality Volume (cf) 13,608

% TSS Removal Achieved 80%

Target Achieved? Yes!

Remaining TSS Removal % 0%

Select BMPs for Runoff Reduction and Water Quality

Water Quality Goals

Storage Volume

Provided by

BMP

(cf)

Area Draining to Each BMP RR Conveyance

Volume

Provided by

BMP

(cf)

WQ CalculationsRunoff Reduction Calculations

Down-stream

BMP

A-14 constant values

Site Data

Georgia Stormwater Management Manual

Stormwater Quality Site Development Review Tool, v2.2 Forsyth County Sheriffs Training Site

1-yr, 24-hr storm

2-yr, 24-hr storm

25-yr, 24-hr storm

100-yr, 24-hr storm

Target Rainfall Event (in)

1-yr, 24-hr storm

2-yr, 24-hr storm

25-yr, 24-hr storm

100-yr, 24-hr storm

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

Adjusted CN 0 0 0 0

*See Stormwater Management Standards to Determine Detention Requirements.

Comments

Post-Development Runoff Volume (in) with BMPs

Post Development Runoff Volume (in) with no BMPs

Pre-Development Runoff Volume (in)

Channel and Flood Protection Calculations

A-15

N m e o f D e v e lo p e r:

D e v e lo p m e n t

N a m e

D e v e lo p m e n t

T y p e

A re a o f D e v e lo p m e n t

(a c

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P o ll u ta n t

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(l b

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.0

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T S

S L o a d in g

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B M

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T S

S L o a d in g

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B M

P s lb /a c /y r)

T S

S C ri te ri o n f o r

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D e v e lo p m e n t( lb

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T o ta l o f S tr u c tu ra l

C o n tr o ls

U s e d

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F ilt e r

S tr ip

R e v ie w e d B y

S to rm w a te r W e tl a n d

G ra s s C h a n n e l

D a te

A p p ro v e d

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O rg a n ic

F ilt e r

S a n d F ilt e r

U n d e rg ro u n d S a n d F ilt e r

C o n d it io n s o f A p p ro v a l:

In fi lt ra ti o n T re n c h

S u b m e rg e d G ra v e l W e tl a n d

E n h a n c e d S w a le s

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S y s te m

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T re a tm e n t

S y s te m

U n d e rg ro u n d D e te n ti o n

P ro p ri e ta ry S tr u c tu ra l C o n tr o l*

R e v ie w C o m m e n ts

L im it e d

A p p li c a ti o n

S tr u c tu ra l

S to rm w a te r C o n tr o ls

D e te n ti o n

S tr u c tu ra l

S to rm w a te r C o n tr o ls

G e n e ra l A p p li c a ti o n

S tr u c tu ra l

S to rm w a te r C o n tr o ls

F o rs y th C o u ty rm w a te r Q u a li ty P e rf o rm a n c e R e v ie w F o rm

N a m e o f E n g in e e r:

T ra c k in g

D a te

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F o rs y th

C o u ty rm w a te r Q u a lit y P e rf o rm a n c e R e v ie w F o rm fo rs y th

S h e ri ff

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H u s s e y G a y B e ll

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P S

W

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TSS (lb/ac/yr)

T S

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M P s

T S

S C ri te ri o n f o r

N e w

D e v e lo p m e n t

T S

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P E ff ic ie n c ie s B

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D is tr ib u ti o n

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D

A F

T

A-16

B

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D is tr ib u ti

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R e v ie w F o rm

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D ra in a g e A re a

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.9

E n te r

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.6

E n te r

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U p la n d A re a

E n te r

U n d is tu rb e d

S tr e a m B u ff e rs

T o ta l

A re a .5

P o ll u ta n t L o a d s

T S

S L o a d w

/o u t B

M P lb s

,4

T o ta l A re a r e c e iv in g C re d it s a c re s

.0

T S

S L o a d in g R a te w /o u t

B M

P lb /a c /y r)

T S

S L o a d w

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T S

S L o a d in g R a te w

B M

P lb /a c /y r)

.3

.3

D ra in a g e A re a

.0

A re a a c re s

E n te r Im p e rv io u s A re a

E n te r

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E n te r

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E n te r

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A re a

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P o ll u ta n t L o a d s

T S

S L o a d w

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T o ta l A re a r e c e iv in g C re d it s a c re s

.0

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T S

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.0

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N o n -S tr u c tu ra l

C o n tr o ls

S it e D e s ig n C re d it s

N a tu ra l C o n s e rv a ti o n A re a a c re s

E n te r

A re a a c re s

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S S

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P

B M

P

B M

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IX

P

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O v e ra ll T

S S

R e d u c ti o n

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B M

P

B M

P

B M

P M

A T

IX

P

B M

P

B M

P

B M

P

B M

P

N o n -S tr u c tu ra l

C o n tr o ls

S it e D e s ig n C re d it s

W a te r Q u a li ty V o lu m e

W Q

V

N a tu ra l C o n s e rv a ti o n A re a a c re s v a c -f

t) w

/o C re d it s

W Q v w

N o n

-S tr u c

C re d it s

E n te r

A re a a c re s

T re a te d b y if a p p li c a b le

U n d is tu rb e d

S tr e a m B u ff e g e ta te d C h a n n e ls v e rl a n d

F lo w F ilt ra ti o n

R e c h a rg

A

IN

F

R M

B M

P E ff ic ie n c ie s

.0

.0

.1

.1

.2

.2

.3

.3 v a c

-f t) w /o

C re d it s W

Q v w

N o n

-S tr u c

C re d it s

WQv (acre-feet)

D A -2

D A -7

D A -3

D A -8

D A -4

D A -9

D A -5

D A -1

D A -6

D A -1

D A -7

D A -3

D A -8

D A -4

D A -9

D A -5

D A -1

D A -6

.0

.2

.4

.6

.8

.0 v a c

-f t) w /o

C re d it s W

Q v w

N o n

-S tr u c

C re d it s

WQv (acre-feet)

S to rm w a te r Q u a lit y P e rf o rm a n c e

R e v ie w T o o l_

.x ls

.F

R M o f

/1

/2

:5

P

A-17

B-1

Text Box

APPENDIX B

EXCERPTS FROM 2019 HYDROLOGY STUDY

rhansen

The entire study shall be provided upon request.

B-2

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