RFP_ 26_02_010 Hydrology Report.pdf

STORMWATER MANAGEMENT

REPORT

FOR

OCONEE VETERANS PARK TENNIS COURT EXPANSION

3500 HOG MOUNTAIN ROAD

Watkinsville, Georgia

Initial Submittal Date: December 10, 2024 Revised: April 16, 2025

W&A PROJECT NUMBER: 240044

355 ONETA STREET SUITE D100 – ATHENS, GEORGIA – 30601

PHONE: 706.310.0400 FAX: 706.310.0411

hwilson Snapshot

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 2

TABLE OF CONTENTS PAGE

DESIGN PROFESSIONAL’S CERTIFICATE STATEMENT

PROJECT SUMMARY

METHODOLOGY

Stormwater Runoff

Hydraulic Routing

EXISTING SITE CONDITIONS

PROPOSED SITE CONDITIONS

WATER QUALITY

CHANNEL PROTECTION

STORMWATER MANAGEMENT

EXECUTIVE SUMMARY

DOWNSTREAM ANALYSIS

CONCLUSION

APPENDICES

Appendix A

NOAA Precipitation Frequency Estimates (Depth)

NOAA Precipitation Frequency Estimates (Intensity) GSMM Curve Number Table 3.1.5-1

Appendix B Pre-Developed Map, Drainage Areas, Time Flow Path Analysis, and Composite CN Calculations

Post-Developed Map, Drainage Areas, Time Flow Path Analysis, and Composite CN Calculations Appendix C

Water Quality Calculations

Channel Protection Calculations

GSMM Stormwater Quality Site Development Review Tool Appendix D

Pond Layout & Outlet Control Details

Pond Report Appendix E

Hydrograph Summary Reports for 1 through 100-Year Storms

Time of Concentration Calculations (TR55 Worksheets) Appendix F

Operations and Maintenance Plan Appendix G

Pages from the Original Hydrology Study for the Existing Site Development Plans (ca 2005)

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 3

DESIGN PROFESSIONAL’S CERTIFICATE STATEMENT

I, Headley E. Wilson, PE, hereby certify that this report dated December 10, 2024 has been developed in conformance with acceptable standards and practices in the field of hydrology.

Any person(s) who makes use of this report in future design calculations or assumptions shall assume all risk regarding their design.

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 4

PROJECT SUMMARY

The report presents a hydraulic and hydrologic analysis for the proposed development known as Oconee Veterans Park Tennis Court Expansion. The purpose of this report is to evaluate the proposed improvements and their effects on the storm water runoff as it relates to the surrounding properties that will meet the requirements laid out in the Georgia Stormwater Management Manual (GSMM), Volumes I and II. This will be accomplished by using Stormwater Best Management Practices (BMP’s) including:

Wet Extended Detention Pond with Water Quality and Channel Protection

The BMP follows the guidelines set forth in the Manual for Erosion and Sediment Control in Georgia and the Georgia Stormwater Management Manual. Prior to construction of this development, all applicable environmental permits will be obtained, and a copy will be provided in the appendices.

Project Location: 3500 Hog Mountain Road City/County: Watkinsville State: Georgia

GPS Coordinates: 33.873526°N, -83.482281°W

FEMA Map Number: 13219C0130E, 13219C0065E Date: 09/15/2022 FEMA Map Designation: X

Existing Site Description & Adjacent Properties: The site is currently a park with sports courts & fields, drive aisles, parking bays, a paved trail, buildings, detention ponds, and utilities. This project pertains to the area (22 acres) surrounding existing tennis courts that drains to an existing detention pond (“West” pond in the plans for the existing development). The area in question is a mixture of grassed areas, impervious areas (tennis courts, a paved trail, and a few small buildings and concrete areas), and a small wooded area. The area in question includes an existing storm drainage network to collect & route water to an existing detention pond. In addition, there is an offsite meadow area that drains towards the site (Offsite 1” basin in the plans for the existing development). The existing site has slopes from 1 to 33 percent with several draws described in this report. The site is zoned AG and is bordered by residential lots to the Northeast, lots zoned AG to the West, lots zoned AG and R-2-MD to the East, and Hog Mountain Road to the South. Relevant information from the Hydrology Study prepared by W.K. Dickson & Co. (Original Hydrology Study) and used for the existing development was used as a reference for this hydrology analysis. See pages from that Report in Appendix G. For the purposes of this study, the pre-development conditions were assumed to be the designed post-development conditions of the current infrastructure as established in the Original Hydrology Study. This assumption ensures that the proposed improvements to this portion of the site will meet the allowable release rates from the West Pond in the original design and therefore will meet the reduction to the true pre-development peak flows as established in Original Hydrology Study.

Proposed Project Description: The project consists of the addition of tennis courts and a small building within the site and modifications & additions to the existing trail, storm drain network, utility services, drive aisles & parking bays, and detention pond. The proposed project disturbs 12 acres. A storm drain network is proposed to collect the runoff from the proposed development and be treated and managed by the modified existing stormwater management facility. The stormwater runoff generated by this project will be managed in such a way as to ensure runoff from the disturbed areas exits the stormwater management pond at, or below the rate of runoff present before the current infrastructure was implemented. This will ensure that the surrounding properties will

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 5 continue to not be adversely affected by the development. The facility will provide the extended detention of the water quality volume and channel protection volume detention and meet the detention regulations for the development.

Water Quality Requirements: In accordance with the GSMM, the proposed development is required to provide water quality treatment for the proposed site to reduce the total suspended solids (TSS) loadings by 80%. All impervious areas that can reasonably be captured & treated fall in Basin A. Water Quality treatment for the proposed development is treated completely through the wet extended detention pond.

Channel Protection Requirements: The project is required to provide 24-hour detention of the 1 year 24-hour rainfall event (Channel Protection Volume). This volume is calculated using the procedure presented in the Georgia Stormwater Management Manual. The required volume for this development is summarized in the Channel Protection portion of this report. Extended (24-hour) detention of these volumes will be provided in the modified wet extended detention pond.

An ESPC plan meeting the requirements of the current applicable NDPES permit will be provided.

Detention Regulations: The project is required to provide the Overbank Flood Protection and Extreme Flood Protection for this site. Overbank Flood Protection requires that the existing 25-year, 24-hour runoff peak flow rates be detained to or below exiting rates. Detention of the 1, 2, 5, & 10-year, 24-hour peak flow rate to existing rates is required in basins where extended detention of the Channel Protection Volume is not provided. Extreme flood protection requires that peaks flow rates for the 50- & 100-year storm event be safely conveyed through project & downstream conveyances. See the summary of site flows in Table 1.

Wetlands: Wetlands are not delineated within the boundary of the subject property.

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Site Location Map

VICINITY MAP

NTS

Figure 1

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 7

Aerial Vicinity Map

AERIAL MAP

NTS

Figure 2

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 8

METHODOLOGY

Stormwater Runoff

Stormwater peak flows and runoff volumes for the site were determined using the Tabular Hydrograph Method presented in the Soil Conservation Service’s Urban Hydrology for Small Watersheds (TR 55). The Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2020 was utilized to generate the tabular hydrographs and route them through the proposed detention facility. The following inputs are required in this method: drainage area, SCS curve number, Time of Concentration, Time Interval, Storm Distribution, Storm Duration, and cumulative Precipitation for each return period.

Area & Curve Number: Survey Maps, Site & Drainage Plans, GIS data, USGS maps, and other available data are consulted to establish existing and proposed drainage patterns and soil cover conditions. A point of analysis is then chosen such that the project falls within the basin draining to this point. Where this point is not close to the project boundary it is necessary to choose multiple points of analysis. Once the overall basins are chosen the basins may be divided into sub-basins draining to specific points of interest within the overall basin (e.g.

hydraulic features such as pond and channels). These sub-basins are then divided into areas of relatively homogeneous surface coverage and SCS curve numbers as assigned based on tables 2-2a through 2-2d in Urban Hydrology for Small Watersheds. Select cover types from these tables are reproduced as table 2.1.5-1 in the Georgia Stormwater Management Manual. From these homogeneous cover areas an area-weighted average curve number is calculated and assigned to each sub-basin.

Time of Concentration: The time of concentration is calculated by dividing the flow path from the hydraulically most distant point to the point of analysis into segments with the same flow regime and reasonably uniform parameters. The travel time of each segment is calculated based on the formulas provided in Urban Hydrology for Small Watersheds. The sum of the individual travel times is the time of concentration for the basin. Since it is not possible to calculate the travel time from every point within the basin the selection of the hydraulically most distant point is a matter of engineering judgment and experience. Where there are several points which the engineer believes may be the hydraulically most distant point the travel time for each point is calculated and the longest is chosen as the time of concentration. Where the travel time is less than five minutes a time of concentration of five minutes is assigned to the basin.

Time Interval: The software default time interval of 2 minutes meets the validity criteria of the tabular method and is accepted.

Storm Distribution: The storm distribution is selected based on figure B-2 in Urban Hydrology for Small Watersheds. For this study a Type II storm was used.

Storm Duration: Storm Durations are selected based on engineering judgment and regulatory requirements.

For this study 24-hour storms were used.

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 9

Cumulative Precipitation: Cumulative precipitation values are calculated based on the data provided by NOAA Atlas 14, Volume 9, Version 2 for Watkinsville, GA. For this study the following values were calculated.

FREQUENCY 24-HOUR RAINFALL

1-year 3.26 in.

2-year 3.72 in.

5-year 4.54 in.

10-year 5.26 in.

25-year 6.31 in.

50-year 7.18 in.

100-year 8.08 in.

The NOAA printouts are provided in Appendix A of this report. Once these parameters are selected, they are entered into the software which generates hydrographs and peak flow calculations based on the procedure specified in Urban Hydrology for Small Watersheds.

The Original Hydrology Report used the rainfall data from Table A-3 (Athens) of the GSMM. The use of this data produces different peak flows that a model using the current NOAA data for Watkinsville.

Hydraulic Routing

Hydraulic routing is conducted utilizing the Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2020.

In order to generate a discharge hydrograph from a hydraulic structure Hydraflow requires three inputs: a stage-storage relationship, a stage-discharge relationship, and an inflow hydrograph.

The stage storage-relationship is determined from existing or design contours. The stage-discharge relationship is calculated from the configuration of the outlet control structure. The appropriate sub-basin hydrograph is assigned to the structure as its inflow. Based on these inputs the software solves the continuity equation, I - O = S/t for each time step until the inflow and outflow of the structure are 0. In this equation I and O are the average inflow and outflow rates for the time period, t, and S is the change in storage during the time period. A more convenient form is obtained by assuming that the average flow rates for the time period, t, is equal to the average of the flows at the beginning and end of the time period.

(I + I

)/2 - (O

1 + O )/2 = (S

- S

)/t

The routing period, t, must be selected which does not violate this concept. This routing period is the same as the time interval selected for the hydrographs above. In the equation above, I

, I

, O and S are known. For the equation to be accurate the value of I must be essentially a straight line between I and I

. O and S must be determined. Rearranging the continuity equation above yields:

I

+ I

+ 2S

/t - O

= 2S

/t + O

From the stage-storage and stage-discharge curves for the hydraulic structure a 2S2/t + O versus O curve can be prepared. After a value for 2S

+ O is computed, the value for O can be obtained directly from this curve. The

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 10 computation is then repeated for succeeding routing periods. Although the techniques provide a direct solution to reservoir routing the design of a detention basin requires a trial-and-error approach. A detention basin must be sized, and an outlet structure selected before the stage-discharge, stage-storage, and 2S2/t + O2 versus O curves can be prepared. The design storm must then be routed through the tentative detention pond to determine its ability to produce an acceptable discharge rate. If the peak discharge rate is too high or the detention volume is excessive, the design must be modified, and the routing calculations repeated for the new design.

Once the sub-basin and routing hydrographs have been generated the software combines the appropriate hydrographs by summing the values of each corresponding time step for each sub-basin draining to the point of analysis. The resulting composite hydrograph can be used to compare existing and proposed peak discharge rates.

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 11

EXISTING SITE CONDITIONS

For the purposes of this study, Pre-Development Basin A is the same as the proposed “West Detention Pond Basin” from the Original Hydrology Study. This is a 21.40-acre basin on-site that drains to the location of the current detention pond. The limits of this basin were delineated using the Original Hydrology Study (Appendix G) from the existing development. The weighted curve number of 75.9 and a time of concentration 26.8 minutes from the Original Hydrology Study were used for the basin. The point at which these flows are released from the current pond location is labeled as “Point of Analysis A”. See Appendix G for more information on the previous hydrology study & stormwater management plan.

For the purposes of this study, Pre-Development Basin A1 - Offsite is the same as the proposed “Offsite 1” basin from the Original Hydrology Study. This is a 15.80-acre basin composed of an offsite area that drains towards the site. The limits of this basin were delineated using the Original Hydrology Study (Appendix G) from the existing development. The basin is a meadowland in good condition. In the existing development and associated hydrology analysis, this basin is allowed to flow to the location of the existing pond, and the outlet control structure was designed (according to their Hydrology Study) to allow the flows from this basin to pass through it. Per the Original Hydrology Study, a curve number of 58 was used for the entire basin (meadow in good condition). A time of concentration of 38.7 minutes was used for the analysis. See Appendix G for more information on the previous hydrology study & stormwater management plan.

Please refer to the pre-developed map, calculated pre-developed times of concentrations, and composite CN calculations table included in Appendix B of this report.

Hydrograph Pre-Development Total to West Pond was produced by combining the hydrographs of Pre- Development Basin A and Pre-Development Basin A1 – Offsite. This hydrograph was routed through the existing West Pond as designed in the Original Hydrology Study to determine the determine the flows to Point of Analysis A.

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 12

PROPOSED SITE CONDITIONS

The project consists of the addition of tennis courts and a small building. The development includes the installation of drive aisles, parking bays, sidewalks & paved trail modifications, stormwater management modifications, and utility services. The proposed project disturbs 12 acres. A storm drain network is proposed to collect the runoff from the proposed improvements and be treated and managed by the existing stormwater management facility. The existing facility will be modified to provide water quality volume detention, channel protection volume detention, and stormwater detention for the planned improvements and to ensure that it is able to handle the increased runoff from the proposed site improvements.

Post-Development Basin A is a 24.07-acre basin composed of the area that drains to the modified stormwater detention pond after site improvements. Run-off from the basin will be managed in accordance with the requirements set forth by the Georgia Stormwater Management Manual. Point of Analysis A is where the basin exits the modified stormwater detention facility.

Post-Development Basin A1 – Offsite is a 15.80-acre basin composed of an offsite area that drains towards the site. The basin is a meadowland in good condition. In this hydrology analysis, the basin was modeled as running to the stormwater management facility, and the pond & outlet control structure were analyzed & designed to handle the flows from this basin.

Please refer to the post-developed map, calculated post-developed times of concentrations, and composite CN calculations table included in Appendix B of this report.

Hydrograph Post-Development Total to Pond A was produced by combining the hydrographs of Post- Development Basin A and Post-Development Basin A1 - Offsite.

Hydrograph Pond A - Routed was created by routing the hydrograph Post-Development Total to Pond A through the modified stormwater management facility, Pond A.

The post-development improvements for this site will increase on-site impervious coverage, therefore measures will be taken to meet and/or exceed the standards set forth in Section 1.2 of the Georgia Stormwater Management Manual. To accomplish this, the existing stormwater management facility will be analyzed and modified to improve stormwater run-off quality, provide stream channel protection, overbank flood protection, extreme flood protection, and erosion and sedimentation control.

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 13

WATER QUALITY

In accordance with the Georgia Stormwater Management Manual the proposed development is required to provide water quality treatment for the average 85th percentile of annual rainfall events. For the state of Georgia, this is achieved by retaining the first 1.2 inches of rainfall. All required water quality calculations are based on the procedures established in section 2.1.7.1 of the Georgia Stormwater Management Manual and all stormwater BMPs meet the specifications set forth in Chapter 3 of that document.

WQV = P * (

𝟏

𝟏𝟐 ) * (0.5+0.009 * I) * A

Where:

WQV = Water quality volume in cubic-feet

P = Target Precipitation (1.2 inches) I = Percent impervious cover A = Site area in square feet

Water quality treatment for Post-Development Basin A is obtained through the wet extended detention pond.

See Appendix C for water quality calculations.

Required WQv = 48,711 cf Proposed WQv = 62,704 cf

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 14

CHANNEL PROTECTION

Extended Detention of the Channel Protection volume is provided for Basin A. All required channel protection calculations are based on the procedures established in section 2.2.5.2 of the Georgia Stormwater Management Manual and all stormwater BMPs meet the specifications set forth in Chapter 3 of the Manual.

The existing wet extended detention (ED) pond has been modified to provide 100% of the required channel protection volume (CPv) for Post-Development Basin A. Calculations are provided below.

Required CPv: 87,123 cf Proposed CPv: 90,126 cf

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 15

STORMWATER MANAGEMENT

Overbank Flood Protection requires that the existing 25 year, 24-hour runoff peak flow rates be detained to or below exiting rates. Detention of the 2, 5, & 10 year, 24-hour peak flow rate to existing rates is required in basins where extended detention of the Channel Protection Volume is not provided. Extreme flood protection requires that peak flow rates for the 50- & 100-year storm event be safely conveyed through project & downstream conveyances. Flows in Basin A and A1 – Offsite are detained to the existing rate through the 100-

yr. storm through the use of an outlet control structure with a series of orifices and weirs. The pond & outfall pipe have been analyzed and designed to safely discharge the proposed 100 yr. peak flow rate.

The volumes and outlet control structures for each pond have been designed using a series of orifices and weirs to meet the requirements of the Unified Stormwater Sizing Criteria for the design of stormwater management facilities. The basic parameters of each facility are as outlined below:

Stormwater Pond A: Wet Extended Detention Pond

Outlet Control Device: An Outlet Control Structure with a series of orifices and weirs.

25-Year Storage Capacity: 214,990 ft3 at elevation 767.70 ft.

100-Year Storage Capacity: 284,814 ft3 at elevation 768.85 ft.

After detaining the storm water runoff, the pond will discharge the runoff through the existing headwall and rip-rap apron and then will discharge at the northeastern boundary of the. See Appendix D for pond modifications, outlet control structure details, and a pond report generated utilizing Hydraflow Hydrographs.

Oconee Veterans Park Tennis Courts Date: April 16, 2025 Page 16

EXECUTIVE SUMMARY

The proposed site improvements known as Oconee Veterans Park Tennis Court Expansion will not negatively impact downstream properties based on the analysis provided above. Table 1 below summarizes the results of the hydraulic and hydrologic analysis for the proposed development. Detailed calculations, hydrograph summaries, and drainage basin maps can be found in the Appendix of this report.

Table 1. Summary of Site Flows

Summary of Site Flows (cfs) Storm Frequency 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr

Point of Analysis “A” Study Point A Original Pre-Developed* - 4.11 9.17 14.65 31.11 51.86 61.99

Calculated Pre-Developed 3.071 4.572 8.071 15.92 37.80 58.90 81.46 Post-Developed 1.552 2.737 6.222 12.72 26.71 40.39 55.23

*”Original Pre-Developed” flows were calculated based on the 2005 report provided bv W.K. Dickson & Co. Rainfall data for Athens was used in the Original study, therefore the “Calculated Pre-Developed” flows from W&A’s Hydraflow model using Watkinsville rainfall data slightly differ due to the use of data from Appendix A.

The proposed stormwater management plan satisfies the performance criteria set forth by Oconee County. All non-local permits will be obtained prior to beginning construction.

DOWNSTREAM ANALYSIS

As shown above in Table 1, the proposed site improvements will not negatively impact downstream properties.

The post-developed flows for the 1, 2, 5, 10, 25, 50 and 100-year are below those of the pre-developed condition.

Since the proposed flows are not increasing, no downstream analysis is required.

CONCLUSION

The development of this site will not affect the downstream properties because the release rates and velocities have been reduced in the post-developed condition as compared to the pre-developed condition. The installation and use of the proposed stormwater management BMPs will meet or exceed the requirements set forth in the Georgia Stormwater Management Manual (GSMM), Volume I and II.

A pp en d ix

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VOL 2

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The following discussion will give some guidance for adjusting curve numbers for different types of impervious areas.

Connected Impervious Areas

The CNs provided in Table 3.1.5-1 for various land cover types were developed for typical land use relationships based on specific assumed percentages of impervious area. These CN values were developed on the assumptions that:

(a) Pervious urban areas are equivalent to pasture in good hydrologic condition, and

(b) Impervious areas have a CN of 98 and are directly connected to the drainage system.

If all of the impervious area is directly connected to the drainage system, but the impervious area percentages or the pervious land use assumptions in Ta-ble 3.1.5-1 are not applicable, use Figure 3.1.5-3 to compute a composite CN.

For example, Table 3.1.5-1 gives a CN of 70 for a 1/2-acre lot in hydrologic soil group B, with an assumed impervious area of 25%. However, if the lot has 20% impervious area and a pervious area CN of 61, the composite CN obtained from Figure 3.1.5-3 is 68. The CN difference between 70 and 68 reflects the difference in percent impervious area.

Unconnected Impervious Areas

Runoff from these areas is spread over a pervious area as sheet flow. To deter-mine CN when all or part of the impervious area is not directly connected to the drainage system, (1) use Figure 3.1.5-4 if total impervious area is less than

30% or (2) use Figure 3.1.5-3 if the total impervious area is equal to or greater than 30%, because the absorptive capacity of the remaining pervious areas will not significantly affect runoff.

Table 3.1.5-1 Runoff Curve Numbers1

Cover description Curve numbers for hydrological soil groups

Cover type and hydrologic condition Average percent impervious area2

A B C D

Cultivated land: without conservation treatment with conservation treatment

Pasture or range land: poor condition good condition

Meadow: good 30 58 71 78

Wood or forest land: thin stand, poor cover good cover

Open space (laws, parks, golf courses, cemeteries, etc)3:

Poor condition (grass cover

<50%)

Fair condition (grass cover 50% to 75%)

Good condition (grass cover

>75%)

Impervious areas: Paved parking lots, roofs, driveways, etc (excluding right-of-way)

98 98 98 98

Streets and roads: Paved; curbs and storm drains

(excluding right-of-way)

Paved; open ditches (including right-of-way)

Gravel (including right-of-way)

Dirt (including right-of-way)

Urban Districts: Commercial and business

Industrial

85%

72%

Residential districts by average lot size:

1/8 acre or less (townhouses)

1/4 acre

1/3 acre

1/2 acre

1 acre

2 acres

65%

38%

30%

25%

20%

12%

Developing urban ar-eas and newly graded areas (pervious areas only, no vegetation)

77 86 91 94

1Average runoff condition, and I a = 0.2S

2The average percent impervious area shown was used to develop the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not connected, the NRCS TR-55 method has an adjustment to reduce the effect.

3CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type.

dramirez Rectangle dramirez Highlight Open space (laws, parks, golf courses, cemeteries, etc) 3:

dramirez Highlight Fair condition (grass cover 50% to 75%) dramirez Highlight dramirez Highlight Impervious areas: Paved parking lots, roofs, driveways, etc (excluding right-of-way) dramirez Highlight dramirez Highlight Streets and roads:

dramirez Highlight Dirt (including right-of-way) dramirez Highlight dramirez Highlight dramirez Highlight good cover dramirez Highlight Wood or forest land:

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355 Oneta Street, Suite D100 Athens, GA 30601 P: (706) 310-0400 waengineering.com

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ALL DRAWINGS SHALL REMAIN THE PROPERTY OF W&A ENGINEERING.

THESE INSTRUMENTS OF SERVICE ARE TO BE USED SOLELY FOR THIS

SPECIFIC PROJECT. W&A ENGINEERING SHALL RETAIN ALL LEGAL

RIGHTS TO THE USE OF THE INSTRUMENTS OF SERVICE AND SHALL

RETAIN FULL PROTECTION UNDER UNITED STATES COPYRIGHT LAW.

W&A PROJECT #: 240044

INITIAL SUB. DATE:

ISSUANCE DATE:

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FOR REVIEW

4/1 6/2

SCALE 1" = 0 50 100' 100 200

AutoCAD SHX Text

PRE DEVELOPMENT BASIN A

AutoCAD SHX Text

COVER DESCRIPTION

AutoCAD SHX Text

COVER AREA (SF)

AutoCAD SHX Text

COVER AREA (ACRES)

AutoCAD SHX Text

CN*

AutoCAD SHX Text

OPEN SPACE, GOOD COVER

AutoCAD SHX Text 710,028.00

AutoCAD SHX Text 16.30

AutoCAD SHX Text

AutoCAD SHX Text

WOOD OR FOREST LAND: GOOD COVER

AutoCAD SHX Text 0.00

AutoCAD SHX Text 0.00

AutoCAD SHX Text

AutoCAD SHX Text

IMPERVIOUS AREAS

AutoCAD SHX Text 222,156.00

AutoCAD SHX Text 5.10

AutoCAD SHX Text

AutoCAD SHX Text

COMPOSITE:

AutoCAD SHX Text 932,184.00

AutoCAD SHX Text 21.40

AutoCAD SHX Text

AutoCAD SHX Text

PRE DEVELOPMENT BASIN A1 - OFFSITE

AutoCAD SHX Text

COVER DESCRIPTION

AutoCAD SHX Text

COVER AREA (SF)

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COVER AREA (ACRES)

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CN

AutoCAD SHX Text

MEADOW, GOOD COVER

AutoCAD SHX Text 688,248.00

AutoCAD SHX Text 15.80

AutoCAD SHX Text

AutoCAD SHX Text

WOOD OR FOREST LAND: GOOD COVER

AutoCAD SHX Text 0.00

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BASIN A1 - OFFSITE*

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AutoCAD SHX Text FOR THE PURPOSES OF TH IS PROJECT AND STUDY, THE IS PROJECT AND STUDY, THE PRE-DEVELOPMENT CONDITIONS ON THE SITE ARE ASSUMED TO BE THE POST-DEVELOPMENT CONDITIONS FROM THE ORIGINAL HYDROLOGY STUDY PROVIDED BY W.K. DIXON & CO. INC. DATED JANUARY 2005. THE BASIN AREAS, CURVE NUMBERS AND TIMES OF CONCENTRATION WERE OBTAINED FROM THE ORIGINAL STUDY. THESE CONDITIONS WERE USED TO ENSURE THE PEAK FLOWS RELEASED FROM THE POND WILL MEET THE INTENT OF THE ORIGINAL DESIGN.

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SCALE 1" = 0 50 100' 100 200

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POST-DEVELOPMENT

BASIN MAP

C352

355 Oneta Street, Suite D100 Athens, GA 30601 P: (706) 310-0400 waengineering.com

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COMMENTDATE

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ALL DRAWINGS SHALL REMAIN THE PROPERTY OF W&A ENGINEERING.

THESE INSTRUMENTS OF SERVICE ARE TO BE USED SOLELY FOR THIS

SPECIFIC PROJECT. W&A ENGINEERING SHALL RETAIN ALL LEGAL

RIGHTS TO THE USE OF THE INSTRUMENTS OF SERVICE AND SHALL

RETAIN FULL PROTECTION UNDER UNITED STATES COPYRIGHT LAW.

W&A PROJECT #: 240044

INITIAL SUB. DATE:

ISSUANCE DATE:

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01/09/2025

FOR REVIEW

3/1 9/2

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AutoCAD SHX Text N/F THE MISSY PETERS REVOCABLE LIVING TRUST PARKER CREEK RD PARCEL B 05 043 DB 1774 PG 465 PB 2019 PG 13

AutoCAD SHX Text N/F JAMES ANDRA & WANDA DICKENS 3710 HOG MOUNTAIN RD PARCEL B 05 003F DB 630 PG 260 PB 24 PG 140

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N/F SANDRA D. & LARRY ALLEN 3610 HOG MOUNTAIN RD PARCEL B 05 003G

AutoCAD SHX Text N/F OCONEE COUNTY, GEORGIA 197.43 ACRES PARCEL B 05 004 DB 638 PG 214 PB 34 PG 151

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OPEN SPACES (LAWNS, PARKS, ETC): FAIR CONDITION GRASS COVER 50% TO 75%

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POST DEVELOPMENT BASIN TRAVEL TIME FLOW PATH ANALYSIS

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38.7 en d

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Project:

Project No. 240044 Date: 3/18/2025 Engineer:

39.87 ac

10.21 ac

1.2 in 50 %

I 25.6 %

RV 0.28

WQv-ed 1.2*Rv*A/12 48711 ft3

WQv-pp Permanent Pool Volume 24355 ft3

24 hr

38369 ft3

81622 ft3

765.00 ft

764.0 ft

0.6

H 0.50 ft Qwq Target Water Quality Discharge Rate 0.444 cfs Ao 0.130 ft2

D 4.75 in

Qp 0.42 cfs (Rounded down to 1/4")

Water Quality Peak Discharge CAo(2gH)^0.5

WQv-ed /t / 3600

Water Quality Orifice Area Qp / (C x (2gH)^0.5)

Water Quality Orifice Diameter 24 x (pi / Ao)^0.5

Water Quality Orifice Elevation (Eo) Water Quality Orifice Coef. (C)

Compute Water Quality Orifice Diameter

Average Extended Detention Head (Ev-Eo)/2

Extended Detention Period (t)

Design ED Water Quality Volume

Water Quality Elevation (Ev)

Compute the Target Water Quality Volume & Component Volumes (Vfb, Vpp, Ved)

Water Quality Volume

Water Quality Pond Parameters

Design Permanent Pool Volume

Target Water Quality Rainfall Depth (P)

Compute Runoff Coefficient

Percent Impervious Area Impervious Area/Drainage Area Runoff Coefficient 0.05+0.009(I)

Target Permanent Pool

WET DETENTION POND A

Oconee Veterans Park Tennis Park Expansion

Water Quality Volume Inputs

Drainage Area (A) Impervious Area

Project:

Project No. 240044 Date: 3/18/2025 Engineer:

Compute Stream Channel Protection (CPV)

39.87 ac 71.3

3.26 in

14.8 min 4.03 0.81 0.25 675 ft3/mi2/in (See Figure 3.1.5-6)

0.93 in

0.025 (See Figure 3.3.5-1)

VS/VR 0.65

87123 ft3

24 hr 90,126 765.20 764.00

0.6 0.444

H 0.60 ft Qcp Target Channel Protection Discharge 1.008 cfs

Qcp-wq Remaining Allowable Qcp 0.564 Ao 0.151 ft2

D 5.25 in

Channel Protection Orifice Coef. (C)

(Ev-Eo)/2

Qp / (C x (2gH)^0.5)

Compute Channel Protection Orifice Diameter

Design Channel Protection

Channel Protection Orifice Diameter 24 x (pi / Ao)^0.5

Average Extended Detention Head

CPv /t / 3600

Channel Protection Orifice Area

Target Water Quality Discharge (Qwq)

Qcp-Qwq

Channel Protection Volume Elevation (Ev) Channel Protection Orifice Elevation (Eo)

POND A CPv OVER WQed Oconee Veterans Park Tennis Park Expansion

Drainage Area (A) Curve Number (CN)

Extended Detention Period (t)

1-Year Precipitation Time of Concentration (TC) Soil Retention Initial Abstraction (IA)

IA/P

Unit Peak Discharge (QU)

Channel Protection Pond Parameters

Runoff (QD)

QO/QI

Name of Developer: Date Submitted:

Development Name: Permit Number:

Site Location / Address: Developer Contact:

Phone Number:

Name of Engineer(s):

Development Type: Maintenance Responsibility:

24.07 24.07 24.07 0.00

I (ac) P (ac) CA (ac) Basin A DB 1 10.21 13.86 0.00

Drainage Basin 2 DB 2 0.00 0.00 0.00

Drainage Basin 3 DB 3 0.00 0.00 0.00

Drainage Basin 4 DB 4 0.00 0.00 0.00

Drainage Basin 5 DB 5 0.00 0.00 0.00

Drainage Basin 6 DB 6 0.00 0.00 0.00

Drainage Basin 7 DB 7 0.00 0.00 0.00

Drainage Basin 8 DB 8 0.00 0.00 0.00

Drainage Basin 9 DB 9 0.00 0.00 0.00

Drainage Basin 10 DB 10 0.00 0.00 0.00

10.21 13.86 0.00

No Yes

37,725

45,270

80%

Tracking #: Conditions of Approval:

Reviewed By:

Date Approved:

Oconee Veterans Park Tennis Courts 3500 Hog Mountain Road

Institutional, Public & Semi Public

Oconee County

Watkinsville, GA

Total Pre-Development Area (ac):

I = Impervious Area, P = Pervious Area, CA = Conservation Area

Target Runoff Reduction Volume Achieved?

Target TSS Removal Achieved?

Total Target Water Quality Volume (cf)

% TSS Removal Achieved

Total Target Runoff Reduction Volume (cf) Runoff Reduction Volume Achieved (cf)

Total Treated Area (ac):

Total Untreated Area (ac):

TOTAL

Georgia Stormwater Management Manual Stormwater Quality Site Development Review Tool

Version 2.2

General Information 3/19/2025

W&A Engineering

Site Summary

Official Use Only

Total Post-Development Area (ac):

0% 0% 0% 0% 0% 0% 0% 0% 0%

0%

20%

40%

60%

80%

100%

DB 1 DB 2 DB 3 DB 4 DB 5 DB 6 DB 7 DB 8 DB 9 DB 10

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S

Re du ct io n

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0% 0% 0% 0% 0% 0% 0% 0% 0% 0%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90%

100%

DB 1 DB 2 DB 3 DB 4 DB 5 DB 6 DB 7 DB 8 DB 9 DB 10

R

R Ta rg et

M et

Runoff Reduction (RR)

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

30 11.95 55 70 77 11.95 50% 30 12.12 58 71 78 12.12 50% 0 0 0 0 0.00 0% 0 0 0 0 0.00 0% 0 0 0 0 0.00 0%

0.00 0% 57 0.00 0%

0.00 24.07 0.00 0.00 24.07 100%

0.00

7.70

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

98 10.21 98 98 98 10.21 42% 39 61 74 80 0.00 0% 49 11.58 69 79 84 11.58 48% 39 61 74 80 0.00 0% 30 2.28 55 70 77 2.28 9%

0.00 0%

0.00 0%

0.00 24.07 0.00 0.00 24.07 100%

10.21 0.43

2.50

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

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.

Oconee Veterans Park Tennis Courts

Basin A

Impervious (ac) Weighted CN

Impervious (ac) Rv

Weighted CN

Select a land cover type… Local Jurisdiction Input

Total

Conservation Area Credits

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

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

Cover Type

Woods - Good Condition Meadow - continuous grass, protected from grazing and generally mowed for hay

Select a land cover type… Select a land cover type…

Potential Max Soil Retention, Spre (in)

Potential Max Soil Retention, Spost (in)

Open space - Good condition (grass cover > 75%) Open space - Fair condition (grass cover 50% to 75%)

Other Total

*HSG = hydrologic soil group

Cover Type

Impervious

Woods - Good Condition Local Jurisdiction Input

Other constant values

Site Data

Georgia Stormwater Management Manual

Stormwater Quality Site Development Review Tool, v2.2 Oconee Veterans Park Tennis Courts

Basin A

Target Runoff Reduction Storm (in) 1.00 Total Site Area for Water Quality Volume (acres) 24.07

Target Runoff Reduction Volume (cf) 37,725

Target Water Quality Volume (cf) 45,270

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 13.86 10.21 15.80 361,574 37,725 0 37,725 0% 0 37,725 45,270 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 13.86 10.21 15.80 37,725 0 45,270

UNTREATED AREA (acres) 0.00 0.00

Target Runoff Reduction Volume (cf) 37,725 Target Achieved? No

Remaining Runoff Reduction Volume (cf) 37,725

Target Water Quality Volume (cf) 45,270 % TSS Removal Achieved 80%

Target Achieved? Yes!

Remaining TSS Removal % 0%

1-yr, 24-hr storm

2-yr, 24-hr storm

25-yr, 24-hr storm

100-yr, 24-hr storm

Target Rainfall Event (in) 3.26 3.72 6.31 8.08

1-yr, 24-hr storm

2-yr, 24-hr storm

25-yr, 24-hr storm

100-yr, 24-hr storm

Storage Volume Provided by

BMP

(cf)

Area Draining to Each BMP RR Conveyance

Volume Provided by

BMP

(cf)

WQ CalculationsRunoff Reduction Calculations

Channel and Flood Protection Calculations

Down-stream

BMP

Select BMPs for Runoff Reduction and Water Quality

Water Quality Goals constant values

Site Data

Georgia Stormwater Management Manual

Stormwater Quality Site Development Review Tool, v2.2 Oconee Veterans Park Tennis Courts

Basin A

0.31 0.48 1.83 3.01

1.45 1.81 4.06 5.70

1.45 1.81 4.06 5.70

Adjusted CN 80 80 80 80

*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) en d

D

P d

L ay ou t

O

C S

D et ai ls

, P on d R ep or t

G

AS

G

AS

GA

S

GAS

GAS

GA

S

GA

S

P

P P

P

A0 B1

B0

STORMWATER

MANAGEMENT PLAN

C321

N

S

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SCALE 1" =

8020 40

40'

FOR SURVEYING & STAKING

INFORMATION:

CONTACT: JOHN BREWER, RLS

Ph: (706) 310-0400 / survey@waengineering.com

355 Oneta Street, Suite D100 Athens, GA 30601 P: (706) 310-0400 waengineering.com

X :\P ro je ct s\

\T o be re

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REVISIONS

COMMENTDATE

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B

C

D

E

F

G

H

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ALL DRAWINGS SHALL REMAIN THE PROPERTY OF W&A ENGINEERING.

THESE INSTRUMENTS OF SERVICE ARE TO BE USED SOLELY FOR THIS

SPECIFIC PROJECT. W&A ENGINEERING SHALL RETAIN ALL LEGAL

RIGHTS TO THE USE OF THE INSTRUMENTS OF SERVICE AND SHALL

RETAIN FULL PROTECTION UNDER UNITED STATES COPYRIGHT LAW.

W&A PROJECT #: 240044

INITIAL SUB. DATE:

ISSUANCE DATE:

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O N

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V

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A N

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PA

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N

N

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C O

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7.

A C

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01/09/2025

01/09/2025

FOR REVIEW

3/1 9/2

AutoCAD SHX Text P

AutoCAD SHX Text P

AutoCAD SHX Text

EX. CB TOP:780.01 IE3"PVC IN:777.87 IE48"CMP IN:770.57 IE48"CMP OUT:770.51

AutoCAD SHX Text

EXISTING ASPHALT PATH

AutoCAD SHX Text

EXISTING GRAVEL DRIVE

AutoCAD SHX Text

EX. YI TOP:778.86 IE20"HDPE:772.12

AutoCAD SHX Text 10"

AutoCAD SHX Text 1"

AutoCAD SHX Text 1"

AutoCAD SHX Text 1"

AutoCAD SHX Text 1"

AutoCAD SHX Text 1"

AutoCAD SHX Text

EXISTING CROSSWALK SIGN

AutoCAD SHX Text

EXISTING STOP SIGN

AutoCAD SHX Text

EXISTING CROSSWALK SIGN

AutoCAD SHX Text

EXSTING PARK MAP SIGN

AutoCAD SHX Text

EX. IE30"RCP:757.67

AutoCAD SHX Text

EX. OCS TOP: 770.84 (SEE INSET)

AutoCAD SHX Text

25' STATE WATERS BUFFER

AutoCAD SHX Text

25' STATE WATERS BUFFER

AutoCAD SHX Text

EXISTING CREEK

AutoCAD SHX Text CONTRACTOR TO INSPECT EXISTING OUTFALL CONDITIONS AND MODIFY AS NECESSARY TO MEET SPECIFICATIONS SHOWN ON THIS SHEET. CONTRACTOR TO NOTIFY ENGINEER OF FINDINGS BEFORE MODIFICATIONS ARE MADE.

AutoCAD SHX Text 15.0'

AutoCAD SHX Text

MODIFIED EXISTING STORMWATER MANAGEMENT POND

AutoCAD SHX Text

EXISTING OUTFALL PIPE TO REMAIN

AutoCAD SHX Text EXISTING OUTLET CONTROL STRUCTURE TO REMAIN & BE MODIFIED. SEE DETAILS ON THIS SHEET FOR MORE INFORMATION

AutoCAD SHX Text

SPLILLWAY

AutoCAD SHX Text 15.0'

AutoCAD SHX Text

POND ACCESS PATH

AutoCAD SHX Text

NTS

AutoCAD SHX Text MODIFIED EXISTING "WEST" POND OUTLET CONTROL STRUCTURE AND SPILLWAY SECTION DETAIL

AutoCAD SHX Text

NTS

AutoCAD SHX Text

EXISTING "WEST" POND OUTLET CONTROL STRUCTURE ISOMETRIC VIEW

AutoCAD SHX Text A

AutoCAD SHX Text C321

AutoCAD SHX Text WQv AND CPv ORIFICE CAP DETAIL

AutoCAD SHX Text H

AutoCAD SHX Text C321

AutoCAD SHX Text E

AutoCAD SHX Text…