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Walton County

WALTON COUNTY, GEORGIA AND INCORPORATED AREAS Community Community Name Number Between, Town of 130410 Good Hope, City of 130411

*Jersey, Town of 130412 Loganville, City of 130326 Monroe, City of 130227 Social Circle, City of 130505 Walnut Grove, Town of 130413 Walton County 130185

(Unincorporated Areas) *No Flood Hazards Areas Identified

FLOOD INSURANCE STUDY NUMBER 13297CV000A

Revised: May 18, 2009

NOTICE TO FLOOD INSURANCE STUDY USERS

Communities participating in the National Flood Insurance Program have established repositories of flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study (FIS) report may not contain all data available within the Community Map Repository. Please contact the Community Map Repository for any additional data. The Federal Emergency Management Agency (FEMA) may revise and republish part or all of this FIS report at any time. In addition, FEMA may revise part of this FIS report by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS report. Therefore, users should consult with community officials and check the Community Map Repository to obtain the most current FIS report components. Initial Effective Date: February 16, 1990 Revised Date: February 16, 1995 May 18, 2009

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TABLE OF CONTENTS

1.0 INTRODUCTION ................................................................................................................ 1

1.1 Purpose of Study............................................................................................................. 1

1.2 Authority and Acknowledgments ................................................................................... 1

1.3 Coordination ................................................................................................................... 2

2.0 AREA STUDIED .................................................................................................................. 3

2.1 Scope of Study ................................................................................................................ 3

2.2 Community Description................................................................................................ 13

2.3 Principal Flood Problems.............................................................................................. 13

2.4 Flood Protection Measures ........................................................................................... 13

3.0 ENGINEERING METHODS ............................................................................................ 13

3.1 Hydrologic Analyses..................................................................................................... 14

3.2 Hydraulic Analyses....................................................................................................... 20

3.3 Vertical Datum.............................................................................................................. 24

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS ..................................................... 26

4.1 Floodplain Boundaries .................................................................................................. 26

4.2 Floodways..................................................................................................................... 27

5.0 INSURANCE APPLICATIONS ....................................................................................... 33

6.0 FLOOD INSURANCE RATE MAP ................................................................................. 34

7.0 OTHER STUDIES .............................................................................................................. 36

8.0 LOCATION OF DATA...................................................................................................... 36

9.0 BIBLIOGRAPHY AND REFERENCES ......................................................................... 36

TABLE OF CONTENTS (Continued)

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FIGURES

Figure 1 - Floodway Schematic ........................................................................................................ 33

TABLES

Table 1 – Detailed Streams Incorporated into this Revision .............................................................. 4 Table 2 – Streams Studied by Approximate Methods ........................................................................ 4 Table 3 – Incorporated Letters of Map Change ................................................................................ 12 Table 4 – Summary of Discharges.................................................................................................... 16 Table 5a – Vertical Datum Conversion ............................................................................................ 24 Table 5b – Vertical Datum Conversion ............................................................................................ 25 Table 6 – Floodway Data.................................................................................................................. 28 Table 7 – Community Map History .................................................................................................. 35

EXHIBITS

Exhibit 1 - Flood Profiles

Alcovy River Panels 01P-03P Apalachee River Panels 04P-08P Bay Creek Panel 09P Big Flat Creek Panels 10P-11P Big Haynes Creek Panel 12P Brushy Fork Creek Panels 13P-17P Cedar Creek Southeast Panel 18P Grubby Creek Panel 19P Jacks Creek Tributary No. 1 Panel 20P Jacks Creek Tributary No. 2 Panel 21P Jacks Creek Tributary No. 3 Panel 22P Little Haynes Creek Panels 23P-24P Mountain Creek (Lower Reach) Panel 25P Mountain Creek (Upper Reach) Panels 26P-31P Mountain Creek Tributary No. 1 Panels 32P-34P Mountain Creek Tributary No. 2 Panels 35P-36P Shoal Creek Panels 37P-41P Tributary No. 1 to Beaverdam Creek Tributary No. 3 Panel 42P Tributary No. 2 to Tributary No. 1 to Beaverdam Creek Panel 43P

Tributary No. 3 Unnamed Tributary to Mountain Creek Panel 44P

Exhibit 2 - Flood Insurance Rate Map Index

Flood Insurance Rate Map

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FLOOD INSURANCE STUDY WALTON COUNTY, GEORGIA AND INCORPORATED AREAS

1.0 INTRODUCTION

1.1 Purpose of Study

This Flood Insurance Study (FIS) revises and updates information on the existence and severity of flood hazards in the geographic area of Walton County, including the Cities of Good Hope, Loganville, Monroe, and Social Circle; the Towns of Between, Jersey, and Walnut Grove; and the unincorporated areas of Walton County (referred to collectively herein as Walton County), and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973. This study has developed flood-risk data for various areas of the community that will be used to establish actuarial flood insurance rates and to assist the community in its efforts to promote sound floodplain management. Minimum floodplain management requirements for participation in the National Flood Insurance Program (NFIP) are set forth in the Code of Federal Regulations at 44 CFR, 60.3. Please note that the City of Loganville is also geographically located in Gwinnett County and the City of Social Circle is also geographically located in Newton County. The Cities of Loganville and Social Circle are included in their entirety in this FIS report. Please note that the Town of Jersey has no mapped flood hazard areas. In some states or communities, floodplain management criteria or regulations may exist that are more restrictive or comprehensive than the minimum Federal requirements. In such cases, the more restrictive criteria take precedence and the State (or other jurisdictional agency) will be able to explain them. The Digital Flood Insurance Rate Map (DFIRM) and FIS report for this countywide study have been produced in digital format. Flood hazard information was converted to meet the Federal Emergency Management Agency (FEMA) DFIRM database specifications and Geographic Information System (GIS) format requirements. The flood hazard information was created and is provided in a digital format so that it can be incorporated into a local GIS and be accessed more easily by the community.

1.2 Authority and Acknowledgments

The sources of authority for this FIS are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973.

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Previous Countywide Analyses The hydrologic and hydraulic analysis for the original February 16, 1990, countywide FIS report (FEMA, 1990) were performed by W.L. Jorden & Company, Inc., for FEMA, under Contract No. EMW-86-C-0015. The work was completed in June 1987. Additional hydrologic and hydraulic analyses were taken from a U.S. Army Corps of Engineers (USACE) floodplain information report (USACE, 1975). The hydrologic and hydraulic analysis for the Apalachee River for the February 16, 1995, countywide revision (FEMA, 1995) was performed by Neel-Schaffer, Inc., for FEMA, under Contract No. EMW-90-C-3129. The work was completed in May 1993. This Countywide FIS Revision For this countywide FIS revision, hydrologic and hydraulic analyses for streams restudied by approximate methods were performed by PBS&J, for the Georgia Department of Natural Resources (DNR), under Contract No. EMA-2006-CA-5615, with FEMA. All other streams previously studied by approximate methods were redelineated to better match existing topography. The work was completed in June 2007. Base map information shown on the Flood Insurance Rate Map (FIRM) was derived from U.S. Geological Survey (USGS) Digital Orthophoto Quadrangles produced at a scale of 1:12,000, from National Aerial Photography Program black and white photography dated 1999 or later. The projection used in the preparation of this map is State Plane Georgia West, and the horizontal datum used is North American Datum of 1983.

1.3 Coordination An initial meeting is held with representatives from FEMA, the community, and the study contractor to explain the nature and purpose of a FIS, and to identify the streams to be studied or restudied. A final meeting is held with representatives from FEMA, the community, and the study contractor to review the results of the study. The initial meeting for the February 16, 1990, FIS report was held on January 21, 1986, and attended by representatives of FEMA, Northeast Georgia Area Planning and Development Commission, W.L. Jorden & Company, Inc., Walton County, and the City of Monroe. The results of the study were reviewed at the final meeting held on April 11, 1989, and attended by representatives of FEMA, W.L. Jorden & Company, Inc., and Walton County.

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For the February 16, 1995, revision, the county was notified by letter on April 15, 1993, that a revision to the FIS and FIRM would be performed using the analyses prepared by Neel-Schaffer, Inc. For this countywide revision, a scoping meeting was held on January 18, 2006, in the City of Winder, Barrow County, Georgia. The meeting was attended by representatives of FEMA, the Georgia DNR, PBS&J and the communities. The results of the study were reviewed at the final meeting held on January 29, 2008, and attended by representatives of Walton County, the Town of Between, the Cities of Good Hope, Loganville, and Monroe, Georgia DNR, and PBS&J. All problems raised at the meeting have been addressed.

2.0 AREA STUDIED

2.1 Scope of Study

This FIS covers the geographic area of Walton County, Georgia, including the incorporated communities listed in Section 1.1. The areas studied by detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through the time of this study. The following streams are studied by detailed methods in this FIS report: Alcovy River Little Haynes Creek Apalachee River Mountain Creek (Lower Reach) Bay Creek Mountain Creek (Upper Reach) Big Flat Creek Mountain Creek Tributary No. 1 Big Haynes Creek Mountain Creek Tributary No. 2 Brushy Fork Creek Shoal Creek Cedar Creek Southeast Tributary No. 1 to Beaverdam Creek Grubby Creek Tributary No. 3 Jacks Creek Tributary No. 1 Tributary No. 2 to Tributary No. 1 to Jacks Creek Tributary No. 2 Beaverdam Creek Tributary No. 3 Jacks Creek Tributary No. 3 Unnamed Tributary to Mountain Creek

For this countywide revision, detailed studies were incorporated from the FIS for Gwinnett County, Georgia and Incorporated Areas (FEMA, 2006) and are listed in Table 1.

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Table 1 – Detailed Studies Incorporated into this Revision Stream Reach Description Alcovy River From approximately 525 feet downstream

of confluence of Cedar Creek Southeast to approximately 485 feet upstream of county boundary

Apalachee River From approximately 2,600 feet downstream of upstream county boundary to the upstream county boundary

Bay Creek From approximately 345 feet downstream of county boundary to approximately 670 feet upstream of county boundary

Big Haynes Creek From downstream county boundary to the upstream county boundary

Brushy Fork Creek From approximately 1,740 feet downstream of Centerville Rosebud Road to approximately 2,550 feet upstream of Old Loganville Road

Cedar Creek Southeast From confluence with Alcovy River to approximately 50 feet upstream of county boundary

The limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). The areas restudied by approximate methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through June 2007. The streams studied by approximate methods are listed in Table 2.

Table 2 – Streams Studied by Approximate Methods Stream

Reach Description

Alcovy River Tributary 20.21 From confluence with Mountain Creek to approximately 3,750 feet upstream of Bold Springs Road

Alcovy River Tributary 20.3 From the confluence with Mountain Creek Tributary No. 2 to approximately 2,300 feet upstream of Adamson Road

Table 2 – Streams Studied by Approximate Methods (Continued)

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Stream

Reach Description

Alcovy River Tributary 48.4 From confluence with Bay Creek to approximately 6,570 feet upstream of Grady Smith Road

Alcovy River Tributary 48.8 From confluence with Bay Creek to approximately 11,370 feet upstream of confluence with Bay Creek

Apalachee River Tributary 16.6.1 From confluence with Little Shoal Creek to approximately 2,270 feet upstream of confluence with Little Shoal Creek











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Stream

Reach Description








Apalachee River Tributary 16.16 From confluence with Shoal Creek to approximately 520 feet upstream of Macedonia Church Road

Apalachee River Tributary 16.31 From confluence with Shoal Creek to approximately 4,300 feet upstream of confluence with Shoal Creek



Apalachee River Tributary 16.33.1 From confluence with Shoal Creek Tributary 16.33 to approximately 2,070 feet upstream of confluence with Shoal Creek Tributary 16.33


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Stream

Reach Description

Apalachee River Tributary 16.33.2 From confluence with Apalachee River Tributary 16.33 to approximately 100 feet upstream of Summerset Place




Bay Creek From confluence with Alcovy River to approximately 2,010 feet upstream of State Highway 81

Big Flat Creek From Old Zion Cemetery Road to approximately 1,480 feet upstream of State Highway 81

Big Flat Creek Tributary 18.12 From confluence with Little Flat Creek to approximately 970 feet upstream of Youth Monroe Road

Cornish Creek From county boundary to approximately 2,470 feet upstream of Robertson Road

Cornish Creek Tributary 16 From confluence with Cornish Creek to approximately 6,050 feet upstream of Paul Smith Road

Cornish Creek Tributary 19 From confluence with Cornish Creek to approximately 3,820 feet upstream of State Highway 138

Grubby Creek From confluence with Jacks Creek to Veralyn Drive; From approximately 700 feet upstream of Poplar Street to approximately 390 feet upstream of Pine Park Street

Hard Labor Creek From county boundary to approximately 4,010 feet upstream of Old Monroe Madison Road


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Stream

Reach Description

Hard Labor Creek Tributary 3.7 From confluence with Means Creek to approximately 4,360 feet upstream of confluence of Means Creek

Hard Labor Creek Tributary 3.8 From confluence with Means Creek to approximately 4,980 feet upstream of Fairplay Drive

Hard Labor Creek Tributary 3.9 From confluence with Means Creek to approximately 640 feet upstream of Wood Street

Hard Labor Creek Tributary 3.10 From confluence with Means Creek to approximately 2,730 feet upstream of Fairplay Drive

Hard Labor Creek Tributary 16 From confluence with Hard Labor Creek to approximately 3,630 feet upstream of Dial Road

Indian Creek From confluence with Jacks Creek to approximately 1,760 feet upstream of State Highway 83

Jacks Creek From county boundary to approximately 4,010 feet upstream of Gratis Road

Jacks Creek Tributary No. 1 From East Marble Street to approximately 1,590 feet upstream of John Deere Road

Jacks Creek Tributary 2.3 From confluence with Indian Creek to approximately 7,780 feet upstream of Lamar Poss Road

Jacks Creek Tributary 2.7 From confluence with Indian Creek to approximately 5,890 feet upstream of confluence with Indian Creek

Jacks Creek Tributary 19 From confluence with Jacks Creek to approximately 1,210 feet upstream of Old Good Hope Road

Jacks Creek Tributary 20 From confluence with Jacks Creek to approximately 3,630 feet upstream of confluence with Jacks Creek


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Stream

Reach Description

Jacks Creek Tributary 21 From confluence with Jacks Creek to approximately 320 feet upstream of Stillbrook Road

Jacks Creek Tributary 21.1 From confluence with Jacks Creek Tributary 21 to approximately 2,000 feet upstream of Pleasant Valley Road

Jacks Creek Tributary 21.1.1 From confluence with Jacks Creek Tributary 21 to approximately 2,300 feet upstream of confluence with Jacks Creek Tributary 21

Jacks Creek Tributary 21.1.2 From confluence with Jacks Creek Tributary 21 to approximately 1,000 feet upstream of Pleasant Valley Road

Jacks Creek Tributary 24 From confluence with Jacks Creek to approximately 1,860 feet upstream of Mount Vernon Road

Jacks Creek Tributary 29 From confluence with Jacks Creek to approximately 4,960 feet upstream of Old Athens Highway

Jacks Creek Tributary 29.3 From confluence with Jacks Creek Tributary 29 to approximately 450 feet upstream of Old Athens Highway

Jacks Creek Tributary 34 From confluence with Jacks Creek to approximately 2,730 feet upstream of confluence with Jacks Creek

Jacks Creek Tributary 37.10 From confluence with Jacks Creek Tributary No. 1 to approximately 3,500 feet upstream of confluence with Jacks Creek Tributary No. 1

Jacks Creek Tributary 37.11 From confluence with Jacks Creek Tributary No. 1 to Phillips Drive

Jacks Creek Tributary 39 From confluence with Jacks Creek to approximately 1,230 feet upstream of John Deere Road

Little Flat Creek From confluence with Big Flat Creek to approximately 1,400 feet upstream of Partridge Lane


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Stream

Reach Description

Little Haynes Creek From approximately 240 feet upstream of Green Drive to Overlook Drive

Little Haynes Creek Tributary 16 From approximately 260 feet upstream of Claude Brewer Road to approximately 5,570 feet upstream of Claude Brewer Road

Little Haynes Creek Tributary 16.1 From confluence with Haynes Creek Tributary 16 to approximately 720 feet upstream of Baker Carter Drive

Little Haynes Creek Tributary 17 From confluence with Little Haynes Creek to approximately 280 feet upstream of Donald Drive

Little Haynes Creek Tributary 17.4 From confluence with Little Haynes Creek Tributary 17 to approximately 820 feet upstream of State Highway 20

Little Haynes Creek Tributary 17.4.1 From confluence with Little Haynes Creek Tributary 17.4 to approximately 1,930 feet upstream of confluence with Little Haynes Creek Tributary 17.4

Little Haynes Creek Tributary 17.6 From confluence with Little Haynes Creek Tributary 17 to the county boundary

Little Haynes Creek Tributary 17.6.1 From confluence with Little Haynes Creek Tributary 17.6 to approximately 2,460 feet upstream of confluence with Little Haynes Creek Tributary 17.6

Little Haynes Creek Tributary 18 From confluence with Little Haynes Creek to approximately 2,290 feet upstream of confluence with Little Haynes Creek

Little River Tributary 3 From confluence with Little River to approximately 1,620 feet upstream of Vine Circle

Little Shoal Creek From approximately 1,320 feet downstream of Michael Road to approximately 3,400 feet upstream of confluence of Apalachee River Tributary 16.6.17


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Stream

Reach Description

Maple Creek From confluence with Alcovy River to approximately 3,340 feet upstream of Azalea Drive

Means Creek From confluence with Hard Labor Creek to approximately 9,500 feet upstream of Hawkins Academy Road

Mountain Creek From confluence with Alcovy River to approximately 4,600 feet upstream of Ammons Bridge Road; From approximately 4,340 feet downstream of Double Springs Church Road to approximately 3,100 feet upstream of Meadow Wood Lane

Mountain Creek Tributary No. 1 From approximately 730 feet downstream of Plaza Drive to approximately 700 feet upstream of Mayfield Drive

Mountain Creek Tributary No. 2 From confluence with Mountain Creek to Criswell Road

Reedy Creek From confluence with Hard Labor Creek to approximately 2,430 feet upstream of Grady Lemonds Road

Richland Creek From confluence with Alcovy River to approximately 2,960 feet upstream of Tipp Drive

Sandy Creek From county boundary to approximately 3,720 feet upstream of Walker Bennett Road

Sandy Creek Tributary 5 From confluence with Sandy Creek to approximately 1,230 feet upstream of Pointer Road

Shoal Creek From approximately 4,480 feet downstream of State Highway 11 to approximately 5,160 feet upstream of State Highway 11

Also, all other existing areas studied by approximate methods were redelineated as part of this countywide revision. The 20 foot contour interval topographic maps were obtained from the USGS (USGS, various dates).

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Approximate analyses were used to study those areas having low development potential or minimal flood hazards. The scope and methods of study were proposed to and agreed upon by FEMA and the communities. Table 3 presents Letters of Map Change (LOMCs) incorporated into this countywide study.

Table 3 – Incorporated Letters of Map Change

LOMC Case Number Date Issued Project Identifier

LOMR 96-04-303P October 21, 1996 Barrington Manor Subdivision; Approximately

0.5 mile southwest of the intersection of Johnson Road

and Johnson Spur

LOMR 97-04-129P August 29, 1997 From East Tributary to Sandy Creek from the upstream side

of Johnson Road to approximately 2,000 feet

upstream of Johnson Road; From West Tributary to Sandy Creek from the upstream side

of Johnson Road to approximately 1,550 feet

upstream of Johnson Road

LOMR 98-04-289P December 5, 1998 Area along unnamed tributary of Sandy Creek extends from

approximately 1,500 feet north 1,600 feet south of

Johnson Road

LOMR 05-04-A009P September 25, 2006 Double Springs Church Road at Friendship Road

LOMR 06-04-B578P May 31, 2006 Highway 78 and Unisia Drive

LOMR 07-04-0192P May 31, 2007 Crossroads at Monroe

LOMR 07-04-1176P June 29, 2007 Ansley Offices

LOMR 08-04-1976P August 15, 2008 Grand Haven

For this countywide FIS, the FIS report and FIRM were converted to countywide format, and the flooding information for the entire county, including both incorporated and unincorporated areas, is shown. Also, the vertical datum was

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converted from the National Geodetic Vertical Datum of 1929 (NGVD) to the North American Vertical Datum of 1988 (NAVD). In addition, the Transverse Mercator, State Plane coordinates, previously referenced to the North American Datum of 1927, are now referenced to the North American Datum of 1983.

2.2 Community Description

Walton County is in north-central Georgia on the Piedmont Plateau. It is approximately 41 miles east of Atlanta and approximately 25 miles southwest of Athens. The county is bordered by Barrow and Oconee Counties on the north, Morgan County on the southeast, Rockdale and Newton Counties on the southwest, and Gwinnett County on the northwest. According to the U.S. Census Bureau, the 2006 population in Walton County was 79,388 (U.S. Census, 2007). The climate of Walton County is generally mild, with an average high of 87 degrees Fahrenheit (°F) during the summer months and an average low of 32°F during the winter months (The Weather Channel, 2007). The average monthly precipitation in the county is approximately 4.13 inches with highest amounts occurring in the winter and early spring months (The Weather Channel, 2007). Walton County is primarily agricultural, with cotton, corn, and grain being the principle crops. Dairy cattle, beef cattle, and poultry are also important to the local economy.

2.3 Principal Flood Problems

Damaging floods in the region often occur during the winter and spring, when flooding is most likely. The most severe flooding in the area occurred in January 1946, but large floods have occurred in November 1948 and March 1971. Major flooding in the area is caused primarily by runoff from rain and thunderstorms, but occasionally large floods are caused by hurricanes and tropical storms.

2.4 Flood Protection Measures There are several detention ponds and two dams, the Monroe Dam and the Deer Acres Lake Dam, located on Jacks Creek Tributary No. 1. Neither dam has significant flood storage volume to provide flood peak attenuation in a significant hydro-meteorological event including the 1-percent-annual-chance flood. No other flood protection measures are known to exist within the study area.

3.0 ENGINEERING METHODS

For the flooding sources studied by detailed methods in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude that are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence

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interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term, average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood that equals or exceeds the 1-percent-annual-chance (100-year) flood in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community. Previous Countywide Analyses

The discharge-frequency relationships for Alcovy River from approximately 7,995 feet downstream of New Hope Church Road to approximately 4,460 feet upstream of State Highway 81, Big Flat Creek, Grubby Creek, Jacks Creek Tributary Nos. 1 through 3, Little Haynes Creek, Mountain Creek (Upper Reach), Mountain Creek Tributary Nos. 1 and 2, and Shoal Creek were developed using regional frequency equations and methodologies developed for non-urbanized basins (USGS, 1979). Peak flows for urbanized areas were developed using the modified Sauer equations (USGS, 1979). Only those areas with existing development reflect urbanized flows. Future development possibilities were not taken into consideration when computing stream flows. The hydrologic analysis for the Apalachee River from the downstream county boundary/State Highway 186 to approximately 100 feet upstream of U.S. Highway 78 Westbound was performed by estimating the magnitude and frequency of floods with 10-, 2-, 1-, and 0.2-percent-annual-chance recurrence intervals using the regional regression equations developed by the USGS (USGS, 1979). The USGS method of analysis estimates the peak discharges developed for rural streams and adjustments are made with gage data. The equations for rural streams applicable to Walton County, Georgia are (USGS, 1979):

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RQ10 = 446A0.59 RQ50 = 727A0.58

RQ100 = 862A0.57

Where: RQT = peak discharge, in cubic feet per second (cfs), for a flood frequency of

magnitude “T” years A = drainage area, in square miles Adjustments in peak discharges estimated using the regional regression equations were performed by applying the following data from USGS gage No. 02217200 on the Middle Oconee River near Jefferson, Georgia:

Q10 = 8,370 cfs Q50 = 12,400 cfs Q100 = 14,100 cfs.

Gage data was not available for a suitable site on the Apalachee River from the downstream county boundary/State Highway 186 to approximately 100 feet upstream of U.S. Highway 78 Westbound; therefore, USGS gage No. 02217200 was chosen because of the hydrological similarity of the Middle Oconee and Apalachee River basins.

This Countywide Revision For the Apalachee River, from approximately 2,600 feet downstream of the upstream county boundary to the upstream county boundary, Big Haynes Creek, and Brushy Fork Creek, the USACE’s Hydrologic Engineering Center (HEC) HEC-1 computer program Version 4.1 (HEC, 1998) was used to calculate flood discharges. For the Alcovy River, from approximately 525 feet downstream of confluence of Cedar Creek Southeast to approximately 485 feet upstream of the county boundary, Bay Creek, and Cedar Creek Southeast, the Natural Resources Conservation Service (NRCS), formerly the U.S. Soil Conservation Service (SCS), unit hydrograph method was used in conjunction with the NRCS TR-55 methodology (SCS, 1986) in the USACE’s HEC-HMS program Version 2.2.1 (HEC, 2002) to calculate flood discharges. NRCS curve numbers were assigned for each land use category in each soil type according to Tables 2-5(a) and 2-7 in the Gwinnett County Storm Water Design Manual (SWDM) (Gwinnett County, 2004), which are taken from TR-55 (SCS, 1986). Composite curve numbers for each sub-basin were calculated based on the area computed for each land use in each soil type. Time of concentration values were computed based on the NRCS TR-55 methodology, as well as localized guidance in the Gwinnett County SWDM.

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For Mountain Creek (Lower Reach) and Unnamed Tributary to Mountain Creek, peak flows were obtained using USGS regression equations for rural streams (USGS, 1993). For Tributary No. 1 to Beaverdam Creek Tributary No. 3 and Tributary No. 2 to Tributary No. 1 to Beaverdam Creek Tributary No. 3, the USACE HEC-HMS Version 3.0.0 (HEC, 2005b) was used to calculate the discharges. The NRCS Curve Number method was used to determine the runoff generated within each basin (SCS, 1972). For the approximate study streams, peak flows were determined using the rural regression equations for Georgia (USGS, 1993). Peak discharge-drainage area relationships for the detailed studied streams in Walton County, Georgia, are shown in Table 4.

Table 4 - Summary of Discharges

Peak Discharges (cubic feet per second)

Flooding Source and Location Drainage Area (square miles)

10-Percent-Annual-Chance



0.2-Percent-Annual-Chance

ALCOVY RIVER

Approximately 1.5 miles downstream of New Hope Church Road

83.72 6,079 9,479 10,753 14,440

Approximately 1.5 miles downstream Bullock Bridge Road

81.14 5,967 9,309 10,563 14,189

Approximately 1.74 miles downstream of State Highway 81

60.61 5,024 7,860 8,945 12,051

Approximately 0.64 mile upstream of State Highway 81

57.18 4,854 7,599 8,652 11,664

Approximately 1,640 feet upstream of confluence of Cedar Creek Southeast

50.99 9,228 15,271 17,096 24,336

APALACHEE RIVER

At State Highway 186 156.95 9,150 13,800 15,600 18,700 Approximately 440 feet

upstream of State Highway 186

155.21 9,110 13,700 15,500 18,200

Approximately 6,170 feet upstream of State Highway 186

150.95 9,000 13,500 15,300 17,200

At Snows Mill Road 146.32 8,880 13,300 15,100 16,600 Approximately 4,000 feet

upstream of Mount Carmel Church Road

144.36 8,830 13,200 15,000 16,400

Table 4 – Summary of Discharges (Continued)

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APALACHEE RIVER (continued)

Approximately 3,500 feet upstream of Mount Carmel Church Road

139.82 8,710 13,000 14,700 16,300

Just downstream of U.S. Highway 78 / State Highway 10

133.34 8,530 12,700 14,400 16,200

Just upstream of U.S. Highway 78 / State Highway 10

131.33 8,470 12,600 14,300 16,100

Approximately 1,800 feet upstream of county boundary

28.94 4,936 7,636 8,398 9,136

BAY CREEK


6.92 3,353 5,255 5,823 7,855

BIG FLAT CREEK

Approximately 0.83 mile downstream of Hodges Road

6.95 1,658 2,550 2,936 3,950

Just downstream of U.S. Highway 78 / State Highway 10

5.18 1,429 2,192 2,526 3,396

Just downstream of Old Zion Cemetery Road

3.70 1,200 1,836 2,119 2,848

BIG HAYNES CREEK

Approximately 2,800 feet upstream of upstream county boundary

18.07 3,963 6,531 7,325 10,191

BRUSHY FORK CREEK

Approximately 0.4 mile upstream of Rosebud Road (Gwinnett County)

11.08 2,096 3,213 3,534 4,676

Approximately 0.5 mile upstream of Beaver Road (Gwinnett County)

8.13 1,851 2,706 2,941 3,742

Just upstream of Lake Carlton South (Gwinnett County)

5.65 1,046 1,645 1,768 2,249

Approximately 0.4 mile upstream of State Highway 78 / State Highway 10 / Athens Highway (Gwinnett County)

1.42 911 1,353 1,489 1,970

CEDAR CREEK SOUTHEAST


3.29 1,868 3,133 3,442 4,525


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GRUBBY CREEK

At Poplar Street 1.25 * * 840 2,010 JACKS CREEK TRIBUTARY NO. 1

At confluence with Jacks Creek

3.56 942 1,518 1,778 2,464

Approximately 1,000 feet downstream of East Marble Street

3.14 876 1,412 1,655 2,296

Approximately 55 feet upstream of East Marble Street

2.67 796 1,285 1,509 2,097

At Deer Acres Lake 2.19 * * 1,210 2,890 JACKS CREEK TRIBUTARY NO. 2

At confluence with Jacks Creek

1.45 * * 920 2,200

At South Hammond Drive 0.58 * * 520 1,250 At State Highway 10

Business 0.29 * * 350 830

JACKS CREEK TRIBUTARY NO. 3

At Baker Street 0.37 * * 400 950 At State Highway 10

Business 0.26 * * 320 770

LITTLE HAYNES CREEK

Approximately 1.1 miles downstream of Center Hill Church Road

15.50 2,247 3,564 4,111 5,615

Approximately 1.2 miles upstream of Center Hill Church Road

9.93 1,728 2,753 3,190 4,376

Approximately 3.0 miles upstream of Center Hill Church Road

5.18 1,177 1,887 2,201 3,040

MOUNTAIN CREEK (LOWER REACH)

Approximately 1,000 feet upstream of the confluence of Unnamed Tributary to Mountain Creek

6.59 1,206 1,982 2,288 3,227

MOUNTAIN CREEK (UPPER REACH)

Approximately 0.887 mile upstream of Ammons Bridge Road

5.64 1,526 2,331 2,682 3,597

Just downstream of McDaniel Street

5.44 1,508 2,300 2,645 3,544

*Data not available


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MOUNTAIN CREEK (UPPER REACH) (continued)

At confluence of Mountain Creek Tributary No. 1

4.52 1,352 2,065 2,379 3,192

Approximately 1,000 feet upstream of State Highway 10 Business

3.16 1,039 1,609 1,865 2,528

Approximately 700 feet upstream of U.S. Highway 78 / State Highway 10

2.57 921 1,429 1,659 2,253

MOUNTAIN CREEK TRIBUTARY NO. 1

At confluence with Mountain Creek (Upper)

1.20 644 985 1,144 1,543

Approximately 300 feet downstream of State Highway 10 Business

1.05 625 948 1,098 1,474


At Calvary Church Road 1.76 * * 1,040 2,490 At Crisswell Road 1.52 * * 950 3,370 At Walker Drive 0.69 * * 580 1,380

SHOAL CREEK

Approximately 0.84 mile downstream of Bradley Gin Road

18.44 2,490 3,942 4,539 6,189

Approximately 1.12 miles upstream of Bradley Gin Road

12.34 1,964 3,122 3,610 4,942

Approximately 1,680 feet upstream of Carl Moon Road

10.00 1,735 2,764 3,203 4,393

Approximately 0.68 mile upstream of Mount Vernon Road

7.02 1,408 2,251 2,618 3,604

Just upstream of Dry Pond Road

4.53 1,088 1,746 2,039 2,820

Approximately 2,175 feet upstream of John Stowe Road

2.29 727 1,176 1,382 1,924

TRIBUTARY NO. 1 TO BEAVERDAM CREEK TRIBUTARY NO. 3

At confluence with Beaverdam Creek Tributary No. 3

0.18 * * 159 *

*Data not available


20







TRIBUTARY NO. 2 TO TRIBUTARY NO. 1 TO BEAVERDAM CREEK TRIBUTARY NO. 3

At confluence with Tributary No. 1 to Beaverdam Creek Tributary No. 3

0.07 * * 71 *

UNNAMED TRIBUTARY TO MOUNTAIN CREEK

Approximately 800 feet upstream of confluence with Mountain Creek (Lower Reach)

0.12 141 184 223 321

*Data not available

3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data Table in the FIS report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this FIS report in conjunction with the data shown on the FIRM. Previous Countywide Analyses Cross sections for the Alcovy River, from approximately 7,995 feet downstream of New Hope Church Road to approximately 4,460 feet upstream of State Highway 81, Big Flat Creek, Grubby Creek, Jacks Creek Tributary Nos. 1 through 3, Little Haynes Creek, Mountain Creek (Upper Reach), Mountain Creek Tributary Nos. 1 and 2, Shoal Creek, Tributary No. 1 to Beaverdam Creek Tributary No. 3, and Tributary No. 2 to Tributary No. 1 to Beaverdam Creek Tributary No. 3 were obtained by field survey. Additional cross sections were interpolated using field-surveyed cross sections and topographic maps. Structural geometry and elevations for bridges and road sections were obtained from field surveys. Cross sections for the Apalachee River from the downstream county boundary/State Highway 186 to approximately 100 feet upstream of U.S. Highway 78 Westbound were obtained from field surveys. All bridges, dams,

21

and culverts were field surveyed to obtain elevation data and structural geometry. Water surface elevations (WSELs) of floods of the selected recurrence intervals for the Alcovy River, from approximately 7,995 feet downstream of New Hope Church Road to approximately 4,460 feet upstream of State Highway 81, the Apalachee River, from the downstream county boundary/State Highway 186 to approximately 100 feet upstream of U.S. Highway 78 Westbound, Big Flat Creek, Jacks Creek Tributary No. 1, Little Haynes Creek, Mountain Creek (Upper Reach), Mountain Creek Tributary No. 1, and Shoal Creek were computed using the HEC-2 step-backwater computer program (HEC, 1991). The upstream WSELs for Grubby Creek, Jacks Creek Tributary Nos. 1, 2, and 3, Mountain Creek Tributary No. 1 (upstream of East Spring Street), and Mountain Creek Tributary No. 2 were taken from a floodplain information report (USACE, 1975). Starting WSELs for the Alcovy River, from approximately 7,995 feet downstream of New Hope Church Road to approximately 4,460 feet upstream of State Highway 81, Apalachee River, from the downstream county boundary/State Highway 186 to approximately 100 feet upstream of U.S. Highway 78 Westbound, Big Flat Creek, Little Haynes Creek, Mountain Creek (Upper Reach), Mountain Creek Tributary No. 1 (upstream of East Spring Street), and Shoal Creek were calculated using the slope-area method. This Countywide Revision For the Alcovy River, from approximately 525 feet downstream of the confluence of Cedar Creek Southeast to approximately 485 feet upstream of the county boundary, the Apalachee River, from approximately 2,600 feet downstream of the upstream county boundary to the upstream county boundary, Bay Creek, Big Haynes Creek, Brushy Fork Creek, and Cedar Creek Southeast, new cross sections were either field surveyed in full across the entire floodplains, or surveyed 50 feet outside of each channel bank, with overbank geometry added from 2 foot contour interval topographic maps (Gwinnett County, 2000). For Tributary No. 1 to Beaverdam Creek Tributary No.3 and Tributary No. 2 to Tributary No. 1 to Beaverdam Creek Tributary No. 3, cross sections were surveyed using aerial photography (Jack Berry & Associates, 2006) along with USGS quadrangle maps for Between and Monroe, Georgia. For Mountain Creek (Lower Reach) and Unnamed Tributary to Mountain Creek, cross sections were selected using topographic maps with a 1 foot contour interval (Professional Engineering Consultants, 2007).

22

For the portion of Big Flat Creek Tributary 18.12 from approximately 700 feet downstream of Youth Monroe Road to the upstream limit, cross section data was obtained using topographic maps (True Vine Engineering, Inc., 2007). For the Alcovy River, from approximately 525 feet downstream of confluence of Cedar Creek Southeast to 485 feet upstream of the county boundary, Apalachee River, from approximately 2,600 feet downstream of the upstream county boundary to the upstream county boundary, Bay Creek, Big Haynes Creek, Brushy Fork Creek, and Cedar Creek Southeast, HEC-RAS Version 3.1.1(HEC, 2003) was used to compute the WSELs. HEC-RAS Version 3.1.2 (HEC, 2004) was used to compute the WSELs for Tributary No. 1 to Beaverdam Creek Tributary No.3 and Tributary No. 2 to Tributary No. 1 to Beaverdam Creek Tributary No. 3. For Mountain Creek (Lower Reach) and Unnamed Tributary to Mountain Creek, HEC-RAS Version 3.1.3 was used to compute WSELs (HEC, 2005a). For the portion of Big Flat Creek Tributary 18.12 from approximately 700 feet downstream of Youth Monroe Road to the upstream limit, HEC-RAS Version 3.1.3 was used to compute WSELs (HEC, 2005a). For Alcovy River, from approximately 525 feet downstream of confluence of Cedar Creek Southeast to 485 feet upstream of the county boundary, Bay Creek, Big Haynes Creek, Brushy Fork Creek, and Cedar Creek Southeast, starting WSELs were determined by the slope-area method or by known starting WSELs were coincident peaks were observed in the HEC-HMS model. Starting WSELs for Apalachee River, from approximately 2,600 feet downstream of the upstream county boundary to the upstream county boundary, were calculated using the slope-area method in HEC-RAS. Starting WSELs for Tributary No. 1 to Beaverdam Creek Tributary No. 3, Tributary No. 2 to Beaverdam Creek Tributary No. 3, Mountain Creek (Lower Reach), and Unnamed Tributary to Mountain Creek were based on normal depth. For the portion of Big Flat Creek Tributary 18.12 from approximately 700 feet downstream of Youth Monroe Road to the upstream limit, starting WSELs were based on critical depth. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the FIRM (Exhibit 2).

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For the approximate study streams, cross section data was obtained from the topography. Low flow channels were added to the cross section data, based on the estimated depth of the 50-percent-annual-chance flow. Roads appearing on the topographic maps were modeled as weirs; top of road elevations were estimated from the topography. The studied streams were modeled using HEC-RAS version 3.1.3 (HEC, 2005a). Channel roughness factors (Mannings “n”) used in the hydraulic computations were chosen by field observation and by Cowan’s Method (Cowan, 1956). The Manning’s “n” values for all detailed studied streams are listed in the following table:

Manning's "n" Values

Stream Channel “n” Overbank “n” Alcovy River 0.030-0.060 0.055-0.170 Apalachee River 0.030-0.065 0.035-0.130 Bay Creek 0.030-0.055 0.060-0.110 Big Flat Creek 0.035-0.050 0.055-0.170 Big Haynes Creek 0.030-0.055 0.060-0.110 Brushy Fork Creek 0.020-0.050 0.050-0.150 Cedar Creek Southeast 0.050 0.100 Grubby Creek * * Jacks Creek Tributary Nos. 1, 2, and 3 0.040-0.070 0.055-0.170 Little Haynes Creek 0.035-0.050 0.055-0.170 Mountain Creek (Lower Reach) 0.035 0.080 Mountain Creek (Upper Reach) 0.040-0.070 0.055-0.170 Mountain Creek Tributary Nos. 1 and 2 0.040-0.070 0.055-0.170 Shoal Creek 0.035-0.050 0.055-0.170 Tributary No. 1 to Beaverdam Creek Tributary No. 3 0.040 0.100 Tributary No. 2 to Tributary No. 1 to Beaverdam Creek

Tributary No. 3 0.040 0.100

Unnamed Tributary to Mountain Creek 0.035 0.080 *Data not available

The profile baselines depicted on the FIRM represent the hydraulic modeling baselines that match the flood profiles on this FIS report. As a result of improved topographic data, the profile baseline, in some cases, may deviate significantly from the channel centerline or appear outside the Special Flood Hazard Area. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail.

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3.3 Vertical Datum

All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum in use for newly created or revised FIS reports and FIRMs was NGVD. With the finalization of NAVD, many FIS reports and FIRMs are being prepared using NAVD as the referenced vertical datum. All flood elevations shown in this FIS report and on the FIRM are referenced to NAVD. Structure and ground elevations in the community must, therefore, be referenced to NAVD. It is important to note that adjacent communities may be referenced to NGVD. This may result in differences in Base Flood Elevations (BFEs) across the corporate limits between the communities. The average conversion factor that was used to convert the data in this FIS report to NAVD was calculated using the National Geodetic Survey’s VERTCON online utility (National Geodetic Survey, 2007). The data points used to determine the conversion are listed in Table 5. Gwinnett County’s vertical datum conversion was used for the Alcovy River, from approximately 525 feet downstream of confluence of Cedar Creek Southeast to 485 feet upstream of the county boundary, Apalachee River, from approximately 2,600 feet downstream of the upstream county boundary to the upstream county boundary, Bay Creek, Big Haynes Creek, Brushy Fork Creek, and Cedar Creek Southeast and is shown in Table 5a.

Table 5a –Vertical Datum Conversion

Quad Name Corner Latitude Longitude

Conversion from NGVD29 to

NAVD88

Cumming SE 34.13 -84.12 0.138 feet Buford Dam SE 34.13 -84.00 0.128 feet

Flowery Branch SE 34.13 -83.87 0.043 feet Roswell SE 34.00 -84.25 0.190 feet Duluth SE 34.00 -84.13 0.180 feet

Suwanee SE 34.00 -84.00 0.105 feet Hog Mountain SE 34.00 -83.87 0.039 feet Lawrenceville SE 33.88 -83.87 0.016 feet

Luxomni SE 33.88 -84.00 0.095 feet Norcross SE 33.88 -84.13 0.157 feet Chamblee SE 33.88 -84.25 0.246 feet Snellville SE 33.75 -84.00 0.082 feet

Average: 0.118 feet

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The vertical datum conversion calculated for Walton County, Georgia, was used for all other streams, and is shown in Table 5b.

Table 5b – Vertical Datum Conversion

Quad Name

Corner

Longitude

Latitude

Conversion from NGVD29 to

NAVD88

Lawrenceville SE 33.875 -83.875 0.020 feet Loganville SE 33.750 -83.875 0.003 feet

Bold Springs SE 33.875 -83.750 -0.036 feet Between SE 33.750 -83.750 -0.059 feet Jersey SE 33.625 -83.750 -0.066 feet

Winder South SE 33.875 -83.625 -0.095 feet Monroe SE 33.750 -83.625 -0.095 feet

Social Circle SE 33.625 -83.625 -0.131 feet Average: -0.057 feet

For additional information regarding conversion between NGVD and NAVD, visit the National Geodetic Survey website at www.ngs.noaa.gov, or contact the National Geodetic Survey at the following address:

Vertical Network Branch, N/CG13 National Geodetic Survey, NOAA Silver Spring Metro Center 3 1315 East-West Highway Silver Spring, Maryland 20910 (301) 713-3191

Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with the FIS report and FIRM for this community. Interested individuals may contact FEMA to access these data. To obtain current elevation, description, and/or location information for benchmarks shown on this map, please contact the Information Services Branch of the NGS at (301) 713-3242, or visit their website at www.ngs.noaa.gov.

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4.0 FLOODPLAIN MANAGEMENT APPLICATIONS

The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1-percent-annual-chance (100-year) flood elevations and delineations of the 1- and 0.2-percent-annual-chance (500-year) floodplain boundaries and 1-percent-annual-chance floodway to assist communities in developing floodplain management measures. This information is presented on the FIRM and in many components of the FIS report, including Flood Profiles, Floodway Data Table, and Summary of Stillwater Elevations Table. Users should reference the data presented in the FIS report as well as additional information that may be available at the local map repository before making flood elevation and/or floodplain boundary determinations.

4.1 Floodplain Boundaries

To provide a national standard without regional discrimination, the 1-percent-annual-chance flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent-annual-chance flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, the 1- and 0.2-percent-annual-chance floodplain boundaries have been delineated using the flood elevations determined at each cross section. For Alcovy River, from approximately 525 feet downstream of confluence of Cedar Creek Southeast to approximately 485 feet upstream of the county boundary, Apalachee River, from approximately 2,600 feet downstream of the upstream county boundary to the upstream county boundary, Bay Creek, Big Haynes Creek, Brushy Fork Creek, and Cedar Creek Southeast, the boundaries were interpolated using topographic maps at a scale of 1:100, with a contour interval of 2 feet (Gwinnett County, 2006). For Tributary No. 1 to Beaverdam Creek Tributary No. 3 and Tributary No. 2 to Tributary No. 1 to Beaverdam Creek Tributary No. 3, the boundaries were interpolated using topographic maps at a scale of 1:120 and 1:200, with a contour of 2 feet (Jack Berry & Associates, 2006). For Mountain Creek (Lower Reach) and Unnamed Tributary to Mountain Creek, floodplain boundaries were delineated using topographic mapping at a scale of 1:4,800, with a contour interval of 2 feet (Professional Engineering Consultants, 2007). For the portion of Big Flat Creek Tributary 18.12 from approximately 700 feet downstream of Youth Monroe Road to the upstream limit, floodplain boundaries were interpolated using topographic maps at a scale of 1:720, with a contour interval of 2 feet.

27

For all other streams studied by detailed and approximate methods, the boundaries were interpolated using topographic maps at a scale of 1:24,000, with a contour interval of 20 feet (USGS, various dates). The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 1-percent-annual-chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, and AE), and the 0.2-percent-annual-chance floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 1- and 0.2-percent-annual-chance floodplain boundaries are close together, only the 1-percent-annual-chance floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. For the streams studied by approximate methods, only the 1-percent-annual-chance floodplain boundary is shown on the FIRM (Exhibit 2).

4.2 Floodways

Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 1-percent-annual-chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1-percent-annual-chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1 foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this FIS report and on the FIRM were computed for certain stream segments on the basis of equal-conveyance reduction from each side of the floodplain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. The results of the floodway computations have been tabulated for selected cross sections (Table 6). In cases where the floodway and 1-percent-annual-chance floodplain boundaries are either close together or collinear, only the floodway boundary has been shown. Floodways were not computed for the Alcovy River, from approximately 7,995 feet downstream of New Hope Church Road to approximately 4,460 feet

FLOODING SOURCE FLOODWAY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION

CROSS SECTION DISTANCE WIDTH (FEET)

SECTION AREA

(SQUARE FEET)

MEAN VELOCITY (FEET PER SECOND)

REGULATORY (FEET NAVD)

WITHOUT FLOODWAY (FEET NAVD)

WITH FLOODWAY (FEET NAVD)

INCREASE (FEET)

ALCOVY RIVER A-N1 O 47,8002 359 5,817 3.1 783.1 783.1 783.6 0.5 APPALACHEE RIVER A 203 244/1184 2,065 7.6 623.4 623.4 624.3 0.9 B 1,0813 245/1274 1,750 8.9 628.4 628.4 628.4 0.0 C 2,6403 181/1184 2,179 7.2 642.3 642.3 643.3 1.0 D 5,5553 207/1014 3,003 5.2 648.9 648.9 649.8 0.9 E 8,5353 235/984 3,955 3.9 652.5 652.5 653.4 0.9 F 11,0903 412/604 5,775 2.6 654.4 654.4 655.4 1.0 G 13,4603 348/2034 5,406 2.8 655.9 655.9 656.9 1.0 H 16,0153 203/1064 3,307 4.6 657.7 657.7 658.7 1.0 I 18,7253 224/504 3,493 4.4 660.4 660.4 661.4 1.0 J 20,9753 195/854 3,074 5.0 663.0 663.0 664.0 1.0 K 23,3903 223/1314 3,436 4.4 666.0 666.0 666.8 0.8 L 24,0603 172/584 2,456 6.1 667.5 667.5 668.4 0.9 M 26,3503 197/904 3,428 4.4 670.9 670.9 671.7 0.8 N 28,0403 109/504 1,849 8.1 672.1 672.1 673.0 0.9 O 30,4753 298/1514 3,672 4.1 676.1 676.1 677.1 1.0 P 32,1453 227/724 3,678 4.1 679.1 679.1 680.0 0.9 Q 34,0903 200/1234 3,415 4.4 680.8 680.8 681.8 1.0

1Floodway not computed 4Total width/Width within county 2Feet above confluence of Beaverdam Creek 3Feet above county boundary/State Highway 186

FLOODWAY DATA

TAB

LE 6

FEDERAL EMERGENCY MANAGEMENT AGENCY

WALTON COUNTY, GA AND INCORPORATED AREAS

ALCOVY RIVER – APALACHEE RIVER



SECTION AREA

(SQUARE FEET)





INCREASE (FEET)

APALACHEE RIVER

(CONTINUED)

R 36,5201 227/1102 3,699 4.1 683.2 683.2 684.2 1.0 S 39,9801 883/1472 7,503 2.0 685.7 685.7 686.7 1.0 T 41,6951 395/2112 6,207 2.4 686.8 686.8 687.8 1.0 U 44,4951 1,306/7452 18,456 0.8 687.5 687.5 688.5 1.0 V 47,1501 471/2412 6,181 2.4 688.2 688.2 689.2 1.0 W 51,9151 206/1242 2,714 5.3 691.4 691.4 692.4 1.0 X 98,0081 862 5,586 1.5 822.1 822.1 822.7 0.6 BAY CREEK A 24,4003 360 2,297 2.7 792.4 792.4 793.2 0.8

1Feet above county boundary/State Highway 186 2Total width/Width within county 3Feet above confluence with Alcovy River

FLOODWAY DATA

TAB

LE 6



APALACHEE RIVER – BAY CREEK



SECTION AREA

(SQUARE FEET)





INCREASE (FEET)

BIG FLAT CREEK A 2,6001 533 3,248 0.9 860.5 860.5 861.5 1.0 B 4,0801 510 3,412 0.9 861.2 861.2 862.1 0.9 C 6,2421 475 2,316 1.3 862.5 862.5 863.5 1.0 D 7,0361 377 1,343 2.2 867.0 867.0 867.4 0.4 E 8,1501 303 1,547 1.9 869.5 869.5 870.0 0.5 F 8,9681 240 2,492 1.0 876.1 876.1 876.7 0.6 G 10,7701 350 2,701 0.9 876.1 876.1 876.8 0.7 BIG HAYNES CREEK A 7952 190 2,357 4.1 847.7 847.7 848.2 0.5 BRUSHY FORK CREEK A 14,3303 310 1,225 2.8 887.7 887.7 888.3 0.6

CEDAR CREEK SOUTHEAST

A 1,1914 336 1,621 2.1 782.4 776.65 777.6 1.0

1Feet above Marce Camp Road 4Feet above confluence with Alcovy River 2Feet above county boundary 5Elevation computed without consideration of backwater effects from Alcovy RIver 3Feet above confluence with Big Haynes Creek

FLOODWAY DATA TAB

LE 6


WALTON COUNTY, GA

AND INCORPORATED AREAS

BIG FLAT CREEK – BIG HAYNES CREEK – BRUSHY FORK CREEK – CEDAR CREEK SOUTHEAST


CROSS SECTION DISTANCE1 WIDTH (FEET)

SECTION AREA

(SQUARE FEET)





INCREASE (FEET)

LITTLE HAYNES CREEK

A 5,425 200 1,681 2.4 712.0 712.0 713.0 1.0 B 8,192 420 2,297 1.8 719.0 719.0 719.6 0.6 C 9,503 350 1,793 2.3 722.6 722.6 723.4 0.8 D 10,484 225 697 5.9 727.0 727.0 728.0 1.0 E 11,444 105 1,039 4.0 743.2 743.2 743.9 0.7 F 14,681 310 1,600 2.6 747.9 747.9 748.9 1.0 G 17,510 250 1,346 2.4 754.1 754.1 754.7 0.6 H 20,706 340 1,339 2.4 760.5 760.5 761.4 0.9 I 21,699 340 1,360 2.3 763.0 763.0 763.6 0.6 J 24,440 55 424 7.5 770.8 770.8 771.8 1.0 K 25,635 83 484 6.6 777.2 777.2 777.6 0.4 L 27,386 340 1,209 1.8 782.5 782.5 783.4 0.9 M 29,630 48 294 7.5 789.5 789.5 790.3 0.8

1Feet above county boundary

FLOODWAY DATA

TAB

LE 6



LITTLE HAYNES CREEK



SECTION AREA

(SQUARE FEET)





INCREASE (FEET)

MOUNTAIN CREEK (UPPER)

A 4,6001 191 1,303 2.1 743.9 743.9 744.9 1.0 B 6,3001 252 1,496 1.8 746.9 746.9 747.9 1.0 C 6,8351 242 1,320 2.0 747.5 747.5 748.5 1.0 D 7,5901 36 404 6.5 753.1 753.1 753.8 0.7 E 8,8361 193 1,096 2.2 755.3 755.3 755.9 0.6 F 10,3391 200 889 2.7 761.7 761.7 761.7 0.0 G 10,9411 303 1,159 1.6 762.6 762.6 763.0 0.4 H 11,9171 287 893 2.1 766.6 766.6 766.6 0.0 I 12,7941 51 273 6.8 771.5 771.5 771.9 0.4 J 13,2061 30 205 9.1 778.5 778.5 778.5 0.0 K 13,3261 33 211 8.8 779.9 779.9 780.0 0.1 L 13,8781 33 236 7.9 782.8 782.8 783.7 0.9 M 16,5391 27 166 10.0 795.8 795.8 796.2 0.4


A 8432 22 83 13.8 757.0 757.0 757.2 0.2 B 9592 22 249 4.6 764.1 764.1 764.1 0.0 C 3,3052 74 175 6.3 775.4 775.4 775.4 0.0 D 3,6112 8 56 10.3 778.2 778.2 778.5 0.3 E 3,6682 64 317 1.8 781.5 781.5 782.3 0.8

1Feet above Ammons Bridge Road 2Feet above confluence with Mountain Creek (Upper)

FLOODWAY DATA

TAB

LE 6



MOUNTAIN CREEK (UPPER) –


33

upstream of State Highway 81, Grubby Creek, Jacks Creek Tributary Nos. 1, 2, and 3, Mountain Creek (Lower Reach), Mountain Creek Tributary No. 2, Shoal Creek, Tributary No. 1 to Beaverdam Creek Tributary No. 3, Tributary No. 2 to Tributary No. 1 to Beaverdam Creek Tributary No. 3, and Unnamed Tributary to Mountain Creek. Along streams where floodways have not been computed, the county must ensure that the cumulative effect of the development in the floodplain will not cause more than 1.0-foot increase in the base (1-percent-annual-chance) flood elevations at any point within the county. The area between the floodway and 1-percent-annual-chance floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increasing the WSEL of the 1-percent-annual-chance flood more than 1 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 1.

Figure 1 - Floodway Schematic

5.0 INSURANCE APPLICATIONS

For flood insurance rating purposes, flood insurance zone designations are assigned to a community based on the results of the engineering analyses. These zones are as follows:

34

Zone A Zone A is the flood insurance risk zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS by approximate methods. Because detailed hydraulic analyses are not performed for such areas, no BFEs or base flood depths are shown within this zone. Zone AE Zone AE is the flood insurance risk zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS by detailed methods. In most instances, whole-foot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone. Zone X Zone X is the flood insurance risk zone that corresponds to areas outside the 0.2-percent-annual-chance floodplain, areas within the 0.2-percent-annual-chance floodplain, areas of 1-percent-annual-chance flooding where average depths are less than 1 foot, areas of 1-percent-annual-chance flooding where the contributing drainage area is less than 1 square mile, and areas protected from the 1-percent-annual-chance flood by levees. No BFEs or base flood depths are shown within this zone.

6.0 FLOOD INSURANCE RATE MAP

The FIRM is designed for flood insurance and floodplain management applications. For flood insurance applications, the map designates flood insurance risk zones as described in Section 5.0 and, in the 1-percent-annual-chance floodplains that were studied by detailed methods, shows selected whole-foot BFEs or average depths. Insurance agents use the zones and BFEs in conjunction with information on structures and their contents to assign premium rates for flood insurance policies. For floodplain management applications, the map shows by tints, screens, and symbols, the 1- and 0.2-percent-annual-chance floodplains, floodways, and the locations of selected cross sections used in the hydraulic analyses and floodway computations. The countywide FIRM presents flooding information for the entire geographic area of Walton County. Previously, FIRMs were prepared for each incorporated community and the unincorporated areas of the County identified as flood-prone. Historical data relating to the maps prepared for each community are presented in Table 7.

COMMUNITY NAME

INITIAL IDENTIFICATION

FLOOD HAZARD BOUNDARY MAP REVISION DATE

FIRM EFFECTIVE DATE

FIRM REVISION DATE

Between, Town of February 16, 1990 None February 16, 1990 None

Good Hope, City of April 4, 1975 None June 17, 1986 February 16, 1990

Jersey, Town of* N/A None N/A None

Loganville, City of April 4, 1975 None July 16, 1982 None

Monroe, City of June 28, 1974 February 6, 1976 February 16, 1990 None

Social Circle, City of February 16, 1990 None February 16, 1990 None

Walnut Grove, Town of February 16, 1990 None February 16, 1990 None

Walton County

(Unincorporated Areas) February 20, 1976 July 30, 1976 February 16, 1990 February 16, 1995

*No flood hazard areas identified

TAB

LE 7



COMMUNITY MAP HISTORY

Table 2 - Community Map History

36

7.0 OTHER STUDIES

This report either supersedes or is compatible with all previous studies on streams studied in this report and should be considered authoritative for purposes of the NFIP.

8.0 LOCATION OF DATA

Information concerning the pertinent data used in the preparation of this study can be obtained by contacting FEMA, Federal Insurance and Mitigation Division, Koger Center – Rutgers Building, 3003 Chamblee Tucker Road, Atlanta, Georgia 30341.

9.0 BIBLIOGRAPHY AND REFERENCES Cowan, W. L., Estimating Hydraulic Roughness Coefficients, Agricultural

Engineering, Volume 37, No. 7, pp. 473-475, July 1956. Federal Emergency Management Agency, Flood Insurance Study, Walton County,

Georgia and Incorporated Areas, February 16, 1990. Federal Emergency Management Agency, Flood Insurance Study, Walton County,

Georgia and Incorporated Areas, February 16, 1995. Federal Emergency Agency, Flood Insurance Study, Gwinnett County, Georgia and

Incorporated Areas, September 29, 2006. Gwinnett County, Georgia, Planimetric and Topographic Maps, Scale 1:100, Contour

Interval 2 feet, Spring 2000. Gwinnett County, Georgia, Gwinnett County Stormwater Design Manual, Revised

January 27, 2004. Available at http://www.co.gwinnett.ga.us. Hydrologic Engineering Center, HEC-2 Water-Surface Profiles, U.S. Corps of

Engineers, Davis, California, May 1991. Hydrologic Engineering Center, HEC-1 Flood Hydrograph Package, Version 4.1, U.S.

Army Corps of Engineers, Davis, California, 1998. Hydrologic Engineering Center, Hydrologic Modeling Systems, HEC-HMS, Version

2.2.2, U.S. Army Corps of Engineers, Davis, California, October 2002. Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.1, U.S.

Army Corps of Engineers, Davis, California, May 2003.

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Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.2, U.S. Army Corps of Engineers, Davis, California, April 2004.

Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.3, U.S. Army Corps of Engineers, Davis, California, May 2005a. Hydrologic Engineering Center, Hydrologic Modeling Systems, HEC-HMS, Version

3.0.0, U.S. Army Corps of Engineers, Davis, California, December 2005b. Jack Berry & Associates, Aerial Topography, Scale 1:120; 1:200, Contour Interval 2

feet, Peachtree City, Georgia, August 2006. National Geodetic Survey, VERTCON-North American Vertical Datum Conversion

Utility. Retrieved January 18, 2007, from http://www.ngs.noaa.gov/. Professional Engineering Consultants, Flood Section Map, Scale 1:4,800, Contour

Interval 2 feet, Loganville, Georgia, December 3, 2007. Soil Conservation Service, National Engineering Handbook, Section 4, Hydrology,

U.S. Department of Agriculture, 1972. Soil Conservation Service, TR-55, Urban Hydrology for Small Watersheds, U.S.

Department of Agriculture, June 1986. The Weather Channel, Monthly Averages for the City of Monroe, Georgia. Retrieved

January 30, 2007, from http://www.weather.com. True Vine Engineering, Inc., Floodplain Map, Scale 1:720, Contour Interval 2 feet,

Loganville, Georgia, April 11, 2007. U.S. Army Corps of Engineers, Savannah District, Floodplain Information Report,

Jacks Creek, Grubby Creek, Mountain Creek and Tributaries, City of Monroe, Georgia, June 1975.

U.S. Census Bureau, American Fact Finder, Walton County, Georgia, 2006. Retrieved

January 30, 2007, from http://factfinder.census.gov. U.S. Geological Survey, 7.5 Minute Series Topographic Maps, Scale 1:24,000,

Contour Interval 20 feet; Monroe, Georgia, 1985; Statham, Georgia, 1964, photorevised 1985; Between, Georgia, 1964; Bold Springs, Georgia, 1964; Winder South, Georgia, 1964; Loganville, Georgia, 1964; High Shoal, Georgia, 1964, photorevised 1985, U.S. Department of the Interior, various dates.

U.S. Geological Survey, Floods in Georgia, Magnitude and Frequency, Water-

Resource Investigation Report No. 78-137, U.S. Department of the Interior, 1979.

38

U.S. Geological Survey, Techniques For Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia, Water-Resources Investigations Report 93-4016, U.S. Department of the Interior, 1993.

U.S. Geological Survey, Flood-Frequency Relations for Urban Streams in Georgia-

1994 Update, Water-Resources Investigations Report 95-4017, U.S. Department of the Interior, 1995.

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