COLUMBIA COUNTY, GEORGIA

Transcription

1 COLUMBIA COUNTY, GEORGIA AND INCORPORATED AREAS Community Name Community Number COLUMBIA COUNTY (UNINCORPORATED AREAS) GROVETOWN, CITY OF HARLEM, CITY OF COLUMBIA COUNTY Effective: September 19, 2007 FLOOD INSURANCE STUDY NUMBER 13073CV000A

2 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 Countywide FIS Effective Date: September 19, 2007

3 TABLE OF CONTENTS 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgments Coordination AREA STUDIED Scope of Study Community Description Principal Flood Problems Flood Protection Measures ENGINEERING METHODS Hydrologic Analyses Hydraulic Analyses Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES...48 i

4 TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Floodway Schematic...17 TABLES Table 1 - Streams Studied by Detailed Methods... 3 Table 2 - Streams Studied by Limited Detailed Methods... 4 Table 3 - Incorporated Letters of Map Correction... 4 Table 4 - Summary of Discharges... 7 Table 5 - Vertical Datum Conversion Table 6 - Floodway Data Table 7 - Community Map History Exhibit 1 - Flood Profiles EXHIBITS Bettys Branch Panels 01P-03P Bettys Branch Tributary Panel 04P Bonaire Heights Tributary Panel 05P Bowen Pond Tributary Panels 06P-07P Columbia Road Tributary East Panel 08P Columbia Road Tributary West Panel 09P Crawford Creek Panels 10P-12P El Cordero Estates Tributary Panel 13P Furys Ferry Road Tributary East Panel 14P Furys Ferry Road Tributary North Panel 15P Furys Ferry Road Tributary South Panels 16P-17P Furys Ferry Road Tributary West Panel 18P Gibbs Road Tributary Panel 19P Holiday Park Tributary Panels 20P-21P Jones Creek Panels 22P-25P Jones Creek Tributary No. 1 Panel 26P Jones Creek Tributary No. 2 Panel 27P Jones Creek Tributary No. 3 Panel 28P Marshall Pond Tributary Panel 29P Mt. Enna Branch Panels 30P-31P Oak Lake Tributary East Panel 32P Oak Lake Tributary West Panel 33P Oakley Pirkle Road Tributary Panel 34P Old Belair Road Tributary East Panels 35P-36P Old Belair Road Tributary West Panel 37P ii

5 Exhibit 1 - Flood Profiles (Continued) TABLE OF CONTENTS (Continued) Old Evans Road Tributary Panel 38P Owens Road Tributary Panels 39P-40P Polar Bridge Creek Panel 41P Reed Creek Panels 42P-45P Savannah River Panels 46P-48P Seaboard Railroad Tributary Panels 49P-50P Stevens Creek Road Tributary Panel 51P Tudor Branch Panels 52P-54P Uchee Creek (Lower Reach) Panels 55P-56P Uchee Creek (Middle Reach) Panel 57P Uchee Creek (Upper Reach) Panel 58P Uchee Creek Tributary No. 1 Panel 59P Uchee Creek Tributary No. 2 Panel 60P Uchee Creek Tributary No. 3 Panel 61P Upper Reed Creek Tributary Panel 62P Walton Branch Panels 63P-66P Walton Branch Tributary Panel 67P Washington Road Tributary Panels 68P-69P Watery Branch Panels 70P-72P Watery Branch Tributary Panel 73P Westhampton Tributary No. 1 Panel 74P Westhampton Tributary No. 2 Panel 75P Westhampton Tributary No. 3 Panel 76P West Lake Tributary Panel 77P Wymberly Tributary Panel 78P Wynngate Tributary Panels 79P-80P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map iii

6 FLOOD INSURANCE STUDY COLUMBIA 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 Columbia County, including the Cities of Grovetown and Harlem; and the unincorporated areas of Columbia County (referred to collectively herein as Columbia County), and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 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, 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 For the initial May 1, 1980, FIS report for the unincorporated areas of Columbia County (Federal Insurance Administration, 1980), the hydrologic and hydraulic analyses were performed by Parson Brinckerhoff Quade and Douglas, Inc., for the Federal Insurance Administration (FIA), under Contract No. H The work was completed in October

7 For the October 16, 1984, revision for the unincorporated areas of Columbia County (FEMA, 1984), the hydraulic analysis for the Savannah River was performed by the FEMA, using data provided by Hydro Studies, Inc. For the September 20, 1995, revision for the unincorporated areas of Columbia County (FEMA, 1995), the hydrologic analysis for the entire Savannah River was performed by the U.S. Army Corps of Engineers (USACE), Savannah District, for FEMA. The hydraulic analysis for the entire Savannah River was prepared by FEMA. Technical data was submitted by the City of North Augusta in support of an appeal to the hydrologic and hydraulic analyses, which FEMA reviewed and accepted. For this countywide study, the work done for the streams studied by limited detailed methods was performed by PBS&J, for the Georgia Department of Natural Resources (DNR), under Contract No. EMA-2003-GR-5369 with FEMA. The work was completed in January Coordination For the May 1, 1980, FIS for Columbia County, an initial meeting was held on June 17, 1976, and attended by representatives of the FIA and the County. The purpose of an initial meeting is to discuss the scope of the FIS. Regional hydrologic analyses were coordinated by Parsons Brinckerhoff with those being developed concurrently by the U.S. Geological Survey (USGS) for other communities in northern Georgia. The results of the study were reviewed at the final meeting held on Februrary 1, 1979, and attended by representatives of the USACE, the County, and the FIA. For the September 20, 1995, FIS revision, the county was notified by letter on August 8, 1994, that a revision to the FIS and Flood Insurance Rate Map (FIRM) would be performed. For this countywide FIS, a scoping meeting was held on October 27, 2004, and attended by representatives of Columbia County, the Georgia DNR, FEMA, and PBS&J. The purpose of the meeting was to discuss the scope of the FIS. A final meeting was held on August 14, Attending the meeting were representatives of Columbia County, the Georgia DNR, FEMA, and PBS&J. All issues raised at the meeting have been addressed. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Columbia County, Georgia, including the incorporated communities listed in Section 1.1. The areas studied by detailed 2

8 methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction at the time of the study. Streams studied by detailed methods in this FIS report are indicated in Table 1. The limits of detailed study are shown on the Flood Profiles (Exhibit 1), and on the FIRM (Exhibit 2). Table 1 - Streams Studied by Detailed Methods Bettys Branch Owens Road Tributary Bettys Branch Tributary Reed Creek Bonaire Heights Tributary Polar Bridge Creek Bowen Pond Tributary Savannah River Columbia Road Tributary East Seaboard Railroad Tributary Columbia Road Tributary West Stevens Creek Road Tributary Crawford Creek Tudor Branch El Cordero Estates Tributary Uchee Creek (Lower Reach) Furys Ferry Road Tributary East Uchee Creek (Middle Reach) Furys Ferry Road Tributary North Uchee Creek (Upper Reach) Furys Ferry Road Tributary South Uchee Creek Tributary No. 1 Furys Ferry Road Tributary West Uchee Creek Tributary No. 2 Gibbs Road Tributary Uchee Creek Tributary No. 3 Holiday Park Tributary Upper Reed Creek Tributary Jones Creek Walton Branch Jones Creek Tributary No. 1 Walton Branch Tributary Jones Creek Tributary No. 2 Washington Road Tributary Jones Creek Tributary No. 3 Watery Branch Marshall Pond Tributary Watery Branch Tributary Mt. Enna Branch Westhampton Tributary No. 1 Oak Lake Tributary East Westhampton Tributary No. 2 Oak Lake Tributary West Westhampton Tributary No. 3 Oakley Pirkle Road Tributary West Lake Tributary Old Belair Road Tributary East Wymberly Tributary Old Belair Road Tributary West Wynngate Tributary Old Evans Road Tributary For this revision, the vertical datum was converted from the National Geodetic Vertical Datum of 1929 (NGVD29) to the North American Vertical Datum of 1988 (NAVD88). In addition, the Universal Transverse Mercator coordinates, previously referenced to the North American Datum of 1927, are now referenced to the North American Datum of The areas studied by limited detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through January The streams studied by limited detailed methods are listed in Table 2. 3

9 Table 2 Streams Studied by Limited Detail Methods Broad Branch Broad Branch Tributary No. 3 Broad Branch Tributary No. 1 Long Branch Broad Branch Tributary No. 1A Long Branch Tributary No. 1 Broad Branch Tributary No. 1A2 Long Branch Tributary No. 2 Broad Branch Tributary No. 2 Long Branch Tributary No. 2A Broad Branch Tributary No. 2A Long Branch Tributary No. 2B Broad Branch Tributary No. 2B Long Branch Tributary No. 3 Broad Branch Tributary No. 2C Uchee Creek 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 Columbia County. Letters of Map Revision were incorporated as appropriate, and are listed in Table 3. Table 3 - Incorporated Letters of Map Correction Case No. Date Project Identifier P 10/18/2001 Uchee Creek, Uchee Creek Tributary No. 1 and No. 2 in the vicinity of the Ivy Falls subdivision P 11/1/2001 Uchee Creek from the confluence of Broad Branch to Harlem-Grovetown Road and and Uchee Creek Tributary No. 3 from the confluence with Uchee Creek to a point 4,200 feet upstream P 7/10/2003 Uchee Creek Tributary No. 2 from the confluence with Uchee Creek to a point 1,250 feet upstream P 11/29/2006 Harris Ridge Developement 2.2 Community Description Columbia County is located in the Central Savannah River area of Georgia, approximately 135 miles east of the City of Atlanta, and five miles northwest of the City of Augusta. The county has a total land area of 290 square miles. It is bordered by Lincoln County on the north, the City of Augusta and Richmond County on the south, Edgefield and McCormick Counties, South Carolina, on the east, and McDuffie County, on the west. Columbia County is part of the Standard Metropolitan Statistical Area, which includes Richmond and Aiken Counties, 4

10 South Carolina. According to the U.S. Census Bureau, in 2004 the population for Columbia County and its incorporated areas was 100,589 (U.S. Census Bureau, 2004). The climate of the area is generally warm, with humid summers and mild, short winters. The average annual temperature is approximately 62 degrees Fahrenheit. The mean annual precipitation is approximately 48 inches (The Weather Channel, 2006). The county lies in the Piedmont and Atlantic Coastal Plain physiographic provinces, which are separated by the Fall Line. The Fall line is an irregular line along which granites, gneisses, and schists of the Piedmont are overlapped by the sands, clays, and limestones of the Atlantic Coastal Plain. The differences in the composition, structure, and time exposure of the Piedmont and Coastal Plain provinces have yielded somewhat different topographic features, but in passing from one to the other along the Fall Line the contrast is subdued. 2.3 Principal Flood Problems There is no history of flood damage in Columbia County. Flooding along principal streams and tributaries in Columbia County has occurred as a consequence of heavy rains usually resulting from localized thunderstorms. 2.4 Flood Protection Measures Columbia County has recognized the need for proper floodplain management and has adopted ordinances restricting the construction of residential structures within the floodplain. The county does not allow new residences to be located within the 1-percent-annual-chance floodplain. All new developments must submit an engineer s floodplain study showing the 1-percent-annual-chance flood elevation for the area and no construction is permitted below that elevation. The county has also complied with provisions of the NFIP with the adoption of a soil and erosion control ordinance, which requires any operation greater than one acre or 500 cubic yards of excavation or fill to submit a soil erosion and sediment control plan. This ordinance is intended to reduce silt and debris accumulation in the natural streams of the area. Columbia County is protected by Thurmond Dam, the Stevens Creek Dam and the Augusta City Lock and Dam along the Savannah River. The county is afforded partial flood protection from the Savannah River by several flood control reservoirs. In December 1951, the J. Strom Thurmond (formerly Clarks Hill) Reservoir began impounding water. Since that time, it has controlled discharges into the Savannah River. Additional control is afforded by the Hartwell and Richard B. Russell Reservoirs, which began operating in 1961 and 1984, 5

11 respectively. Numerous private ponds along many of the streams studied herein serve as retention areas for excessive runoff. 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 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 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. Flood flow frequencies for the Savannah River were calculated by the USACE, Savannah District, using procedures described in the USGS report of the Savannah River flood frequencies (U.S. Geological Survey, 1990). Savannah River discharge values for areas upstream of Stevens Creek were reduced based on a HEC-5 simulation of reservoir operations provided by the USACE. For Uchee Creek (Upper Reach), Uchee Creek Tributary No. 1, Uchee Creek Tributary No. 2, and Uchee Creek Tributary No. 3, peak discharges for the selected flood intervals were calculated using regression equations published in Techniques for Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia (U.S. Geological Survey, 1993). The hydrologic analysis for all other studied streams in detail were based upon utilization of the current USGS regional regression equations relating discharge to drainage area for natural streams in various physiographic provinces in the State of Georgia. These equations were determined by synthesizing 75 years of flood record from short-term and long-term streamflow and rainfall data, applying the 6

12 log-pearson Type III distribution with regional skew coefficients as recommended by the Water Resources Council, and regionalizing by multiple regression techniques (U.S. Geological Survey, 1976 and Water Resources Council, 1976). For this countywide study, the following regression equation was used to compute the peak discharges for the selected flood intervals for Broad Branch, Broad Branch Tributary No. 1, Broad Branch Tributary No. 1A, Broad Branch Tributary No. 1A2, Broad Branch Tributary No. 2, Broad Branch Tributary No. 2A, Broad Branch Tributary No. 2B, Broad Branch Tributary No. 2C, and the portion of Uchee Creek studied by limited detailed methods: Q 100 = 794A 0.605, where Q 100 = 1-percent-annual-chance peak discharge (cubic feet per second), and A = cumulative drainage area (square miles) (U.S. Geological Survey, 1993). For Long Branch, Long Branch Tributary No. 1, Long Branch Tributary No. 2, Long Branch Tributary No. 2A, Long Branch Tributary No. 2B, and Long Branch Tributary No. 3, the following regression equation was used to compute the peak discharges for the selected flood intervals: Q 100 = 644A 0.70 TIA 0.17, where Q 100 = 1- percent-annual-chance peak discharge (cubic feet per second), A = cumulative drainage area, and TIA = total impervious area (percentage) (U.S. Geological Survey, 1993). Columbia County provided 2 and 4 foot digital topographic data for the portions of the watersheds within the county boundary (Columbia County, 2005). ArcGIS generated watershed and sub-basin areas using elevation data from the National Elevation Dataset (NED) (ESRI, 2005 and U.S. Geological Survey, 1999). The county s digital topographic data was then merged with the NED data to produce the working Digital Elevation Model that was used in the hydrologic analysis. Peak discharge-drainage area relationships for the 10-, 2-, 1-, and 0.2-percent annual-chance floods of each flooding source studied in detail in the county 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 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance BETTYS BRANCH At confluence with Savannah River Approximately 1,530 feet upstream of Hardy McManus Road Approximately 400 feet downstream of the confluence of Gibbs Road Tributary BETTYS BRANCH TRIBUTARY At confluence with Washington Road Tributary ,480 2,365 2,750 3, ,265 2,025 2,355 3, ,010 1,

13 Table 4 Summary of Discharges (Continued) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance Peak Discharges (cubic feet per second) 2-Percent- 1-Percent- Annual-Chance Annual-Chance 0.2-Percent- Annual-Chance BONAIRE HEIGHTS TRIBUTARY At confluence with Wynngate Tributary BOWEN POND TRIBUTARY At confluence with Reed Creek ,120 1,560 Just downstream of the confluence of Westhampton Tributary No ,015 COLUMBIA ROAD TRIBUTARY EAST At confluence with Crawford Creek COLUMBIA ROAD TRIBUTARY WEST At confluence with Crawford Creek CRAWFORD CREEK At confluence with Tudor Branch Approximately 375 feet downstream of the confluence of Oakley Pirkle Road Tributary Just upstream of the confluence of Oak Lake Tributary East EL CORDERO ESTATES TRIBUTARY At confluence with Wynngate Tributary ,385 2,210 2,575 3, ,375 1,610 2, ,140 1, FURYS FERRY ROAD TRIBUTARY EAST At confluence with Reed Creek Approximately 1,080 feet upstream of the confluence with Reed Creek FURYS FERRY ROAD TRIBUTARY NORTH At confluence with Jones Creek FURYS FERRY ROAD TRIBUTARY SOUTH At confluence with Jones Creek FURYS FERRY ROAD TRIBUTARY WEST At confluence with Reed Creek Approximately 250 feet upstream of Inverness Way GIBBS ROAD TRIBUTARY At confluence with Bettys Branch HOLIDAY PARK TRIBUTARY At confluence with Reed Creek ,140 1,590 8

14 Table 4 Summary of Discharges (Continued) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance Peak Discharges (cubic feet per second) 2-Percent- 1-Percent- Annual-Chance Annual-Chance 0.2-Percent- Annual-Chance HOLIDAY PARK TRIBUTARY (CONTINUED) Approximately 300 feet downstream of Columbia Road/ State Highway 232 JONES CREEK At confluence with Savannah River Just upstream of confluence of Jones Creek Tributary No. 1 Just upstream of confluence of Marshall Pond Tributary ,075 1,730 2,020 2, ,075 1,265 1, JONES CREEK TRIBUTARY NO. 1 At confluence with Jones Creek Approximately 3,400 feet upstream of the confluence with Jones Creek JONES CREEK TRIBUTARY NO. 2 At confluence with Jones Creek JONES CREEK TRIBUTARY NO. 3 At confluence with Jones Creek MARSHALL POND TRIBUTARY At confluence with Jones Creek MT. ENNA BRANCH At confluence with Bettys Branch Approximately 40 feet downstream of Silver Lake Drive Approximately 2,840 feet upstream of Washington Road/ State Highway 104 OAK LAKE TRIBUTARY EAST At confluence with Crawford Creek OAK LAKE TRIBUTARY WEST At confluence with Crawford Creek OAKLEY PIRKLE ROAD TRIBUTARY At confluence with Crawford Creek OLD BELAIR ROAD TRIBUTARY EAST At confluence with Crawford Creek OLD BELAIR ROAD TRIBUTARY WEST At confluence with Crawford Creek ,145 1,350 1, ,055 1,

15 Table 4 Summary of Discharges (Continued) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance Peak Discharges (cubic feet per second) 2-Percent- 1-Percent- Annual-Chance Annual-Chance 0.2-Percent- Annual-Chance OLD EVANS ROAD TRIBUTARY At confluence with Reed Creek OWENS ROAD TRIBUTARY At confluence with Holiday Park Tributary POLAR BRIDGE CREEK 0.42 * * 310 * Upstream of confluence of Sadie Morris Creek REED CREEK At confluence with Augusta Canal Just upstream of the confluence of Furys Ferry Road Tributary East Approximately 1,060 feet downstream of Old Evans Road Approximately 2,430 feet upstream of Columbia Road State Highway 232 SAVANNAH RIVER At confluence of Stevens Creek Dam ,200 3,500 4,040 5, ,800 2,870 3,325 4, ,340 2,150 2,500 3, ,040 1,225 1,710 7,173 44,000 75, , ,000 SEABOARD RAILROAD TRIBUTARY At confluence with Jones Creek ,295 STEVENS CREEK ROAD TRIBUTARY At confluence with Reed Creek TUDOR BRANCH At confluence with Uchee Creek Just upstream of confluence of Crawford Creek Approximately 50 feet downstream of Columbia Road State Highway 232 UCHEE CREEK (LOWER REACH) At confluence with Savannah River Just upstream of confluence of Long Branch UCHEE CREEK (MIDDLE REACH) Just upstream of State Highway 232/ Columbia Road UCHEE CREEK (UPPER REACH) Just upstream of confluence of Broad Branch ,190 3,475 4,010 5, ,400 2,240 2,610 3, ,130 1, ,060 4,740 5,455 7, ,750 4,255 4,910 6, * * 6,870 * 19.2 * * 4,745 * * No Data 10

16 Table 4 Summary of Discharges (Continued) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance Peak Discharges (cubic feet per second) 2-Percent- 1-Percent- Annual-Chance Annual-Chance 0.2-Percent- Annual-Chance UCHEE CREEK TRIBUTARY NO. 1 At Dam 0.26 * * 360 * UCHEE CREEK TRIBUTARY NO. 2 At Dam 0.83 * * 710 * UCHEE CREEK TRIBUTARY NO. 3 At Dam 0.50 * * 522 * UPPER REED CREEK TRIBUTARY At confluence with Reed Creek WALTON BRANCH At confluence with Tudor Branch Just upstream of confluence of Walton Branch Tributary Approximately 930 feet downstream of Interstate Highway 20/ State Highway 402 WALTON BRANCH TRIBUTARY At confluence with Walton Branch WASHINGTON ROAD TRIBUTARY At confluence with Bettys Branch WATERY BRANCH At confluence with Savannah River Approximately 4,020 feet upstream of Point Comfort Road WATERY BRANCH TRIBUTARY At confluence with Watery Branch WESTHAMPTON TRIBUTARY NO. 1 At confluence with Bowen Pond Tributary WESTHAMPTON TRIBUTARY NO. 2 At confluence with Bowen Pond Tributary WESTHAMPTON TRIBUTARY NO. 3 At confluence with Bowen Pond Tributary ,015 1,630 1,905 2, ,300 1,530 2, , , * No Data 11

17 Table 4 Summary of Discharges (Continued) Flooding Source and Location Drainage Area (square miles) 10-Percent- Annual-Chance Peak Discharges (cubic feet per second) 2-Percent- 1-Percent- Annual-Chance Annual-Chance 0.2-Percent- Annual-Chance WEST LAKE TRIBUTARY At confluence with Reed Creek Approximately 1,010 feet upstream of West Lake Drive WYMBERLY TRIBUTARY At confluence with Crawford Creek WYNNGATE TRIBUTARY At confluence with Reed Creek ,015 1,425 Just upstream of confluence of Bonaire Heights Tributary 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. For the Savannah River, cross-sections were obtained by field survey. For all other streams, cross-sections for the flooding sources studied by detailed methods were obtained by combining photogrammetrically prepared overbank floodplains with field surveyed channels. All bridges and culverts were field surveyed to obtain elevation data and structural geometry. Water surface elevations (WSELs) of the floods of the selected recurrence intervals were computed using the USACE HEC-2 step-backwater computer program (Hydraulic Engineering Center, 1973, 1974, 1991 and U.S. Army Corps of Engineers, 1959). The HEC-2 model for the Savannah River was prepared using the 1988 HEC-2 model, which was prepared by the USACE for the Savannah River channel capacity study by combining a number of previously unrelated HEC-2 models for reaches of the Savannah River (USACE, 1988). The 1988 model contained 10 new surveyed cross-sections between the Augusta City Dam and the New Savannah Bluff Lock and Dam, which is located in Richmond County. Additional changes were made to the HEC-2 model to incorporate more detailed modeling of the Sandbar Ferry Road and Seaboard Coastline Railroad bridges, located in Richmond County (Georgia Department of Transportation, 1988 and Seaboard Coast Line Railroad Company, 1976). Starting WSELs for the Savannah River were determined by the slope-area 12

18 method. Starting WSELs for Uchee Creek (Upper Reach), Uchee Creek Tributary No. 1, and Uchee Creek Tributary No. 3 were determined using the slope-area method. A known WSEL was used for the starting WSEL for Uchee Creek Tributary No. 2. Starting WSELs for all other streams were obtained from elevations at the confluence with the main stream. Roughness coefficients (Manning s n ) for the Savannah River were based on the calibration to published flood data from the 5 th Street Bridge gage and the Butler Creek gage (No ) at the City of Augusta, Georgia. The channel n values ranged from to 0.033, and the overbank n value was (Walden, Ashworth, & Associates, 1994). For all other streams, roughness coefficients for the computations were assigned on the basis of field inspections and engineering judgment. The channel n values ranged from to 0.040, and the overbank n values ranged from to Flood profiles were drawn showing the computed WSELs for floods of the selected recurrence intervals. In cases where the 2- and 1-percent-annual-chance flood elevations are close together, due to limitations of the profile scale, only the 1-percent-annual-chance profile has been shown. The 1-percent-annual-chance flood elevations for the flooding sources that were studied by approximate methods were determined by using the flood depthfrequency drainage area relations determined by the USGS (U.S. Geological Survey, 1977). 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). Detailed studied streams that were not restudied as part of this map update may include a "profile baseline" on the maps. This "profile base line" provides a link to the flood profiles included in the FIS report. The detailed studied stream centerline may have been digitized or redelineated as part of this revision. The "profile baselines" for these streams were based on the best available data at the time of their study and are depicted as they were on the previous FIRMs. In some cases where improved topographic data was used to redelineate floodplain boundaries, the "profile baseline" may deviate significantly from the channel centerline or may be 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. 13

19 For this countywide study, cross sections for streams listed in Table 2, Streams Studied by Limited Detailed Methods, were obtained using digital topography and field surveys. For Broad Branch, Broad Branch Tributary No. 1, Broad Branch Tributary No. 1A, Broad Branch Tributary No. 1A2, Broad Branch Tributary No. 2, Broad Branch Tributary No. 2A, Broad Branch Tributary No. 2B, and Broad Branch Tributary No. 2C, the 1-percent-annual-chance WSELs were computed using the USACE s HEC-RAS hydraulic model, version (Hydrologic Engineering Center, 2004a). For Long Branch, Long Branch Tributary No. 1, Long Branch Tributary No. 2, Long Branch Tributary No. 2A, Long Branch Tributary No. 2B, Long Branch Tributary No. 3, and Uchee Creek, the 1-percent-annual-chance WSELs were computed using the USACE s HEC- RAS hydraulic model, version (Hydrologic Engineering Center, 2004b). HEC-GeoRAS was used to delineate the 1-percent-annual-chance floodplain (Hydrologic Engineering Center, 2000). The hydraulic model was prepared using digital elevation data (U.S. Geological Survey, 1999), without surveying bathymetric data. Where bridge or culvert data were readily available, these data were reflected in the hydraulic model. Where structure data were not readily available, field measurements were made to approximate the geometry in the hydraulic models. Models do not include field surveys that determine the specifics of channel and floodplain geometry. A limited detailed study can be upgraded to a full detailed study at a later date by verifying stream channel and overbank geometry, bridge and culvert geometry, and by analyzing multiple recurrence intervals. Flood profiles have been developed for streams studied by limited detailed methods to be used for floodplain management and flood insurance rating purposes. The flood profiles for the streams studied by limited detailed methods were published separately from this FIS report. Please contact your local floodplain administrator for more information. 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 NGVD29. With the finalization of the NAVD88, many FIS reports and FIRMs are being prepared using NAVD88 as the referenced vertical datum. All flood elevations shown in this FIS report and on the FIRM are referenced to NAVD88. Structure and ground elevations in the community must, therefore, be referenced to NAVD88. It is important to note that adjacent communities may be referenced to NGVD29. This may result in differences in Base Flood Elevations (BFEs) across the corporate limits between the communities. The average conversion factor of feet that was used to convert the data in this FIS report to NAVD88 was calculated using the National Geodetic Survey s 14

20 VERTCON online utility (National Geodetic Survey, 2005). The data points used to determine the conversion are listed in Table 5. Table 5 - Vertical Datum Conversion Conversion from Quad Name Corner Latitude Longitude NGVD29 to NAVD88 Woodlawn SE feet Leah SE feet Clarks Hill SE feet Winfield SE feet Appling SE feet Evans SE feet Harlem SE feet AVERAGE = feet For more information on NAVD88, see the FEMA publication entitled Converting the National Flood Insurance Program to the North American Vertical Datum of 1988 (FEMA, June 1992), or contact the Vertical Network Branch, National Geodetic Survey, Coast and Geodetic Survey, National Oceanic and Atmospheric Administration, Silver Spring, Maryland (Internet address 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. 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. 15

21 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1-percentannual-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. Between cross sections for the Savannah River, the boundaries were interpolated between cross sections using topographic maps at a scale of 1:12,000, with a contour interval of 5 feet (Columbia County Engineering Services Department, 1994). For all other detailed studied streams, between cross-sections, the boundaries were interpolated using topographic maps at a scale of 1:2,400, with a contour interval of 5 feet (Federal Emergency Management Administration, 1995). For this countywide study, Columbia County submitted 2 foot and 5 foot contour interval topographic data, which was used to delineate the floodplains for the streams studied by limited detailed methods (Columbia County, 2005). 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 and limited detail 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 16

22 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. 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 17

23 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) BETTYS BRANCH A 4, , * * * B 4, , * * * C 5, , * * * D 6, , * * * E 7, , * * * F 7, , * * * G 8, , * * * H 10, , I 12, J 14, K 15, L 19, M 20, N 21, O 22, , P 22, Q 23, R 24, S 25, T 25, U 26, Feet above confluence with Savannah River *Data not available TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA BETTYS BRANCH

24 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) BETTYS BRANCH TRIBUTARY A B C D 1, E 1, Feet above confluence with Washington Road Tributary TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA BETTYS BRANCH TRIBUTARY

25 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) BONAIRE HEIGHTS TRIBUTARY A B C Feet above confluence with Wynngate Tributary 2 Elevation computed without consideration of backwater effects from Wynngate Tributary TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA BONAIRE HEIGHTS TRIBUTARY

26 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) BOWEN POND TRIBUTARY A 1, , B 1, , C 1, , D 2, E 3, F 3, G 3, H 4, I 5, J 5, K 5, L 6, M 7, N 7, O 7, P 8, Feet above confluence with Reed Creek TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA BOWEN POND TRIBUTARY

27 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) COLUMBIA ROAD TRIBUTARY EAST A B C 1, D 1, COLUMBIA ROAD TRIBUTARY WEST A B C 1, Feet above confluence with Crawford Creek TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA COLUMBIA ROAD TRIBUTARY EAST- COLUMBIA ROAD TRIBUTARY WEST

28 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) CRAWFORD CREEK A , B 2, C 3, D 5, E 6, F 8, G 9, H 9, I 11, J 13, K 13, L 13, M 14, N 15, O 15, P 16, Q 16, R 16, S 17, , T 18, , U 18, Feet above confluence with Tudor Branch TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA CRAWFORD CREEK

29 FLOODING SOURCE CROSS SECTION DISTANCE WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) CRAWFORD CREEK (CONTINUED) V 19, W 19, X 20, Y 20, Z 21, AA 21, , AB 21, AC 21, , AD 22, AE 22, AF 23, AG 23, , AH 24, EL CORDERO ESTATES TRIBUTARY A B C 1, Feet above confluence with Tudor Branch 3 Elevation computed without consideration of backwater effects from Wynngate Tributary 2 Feet above confluence with Wynngate Tributary TABLE 6 DATA COLUMBIA COUNTY, GA AND INCORPORATED AREAS CRAWFORD CREEK EL CORDERO ESTATES TRIBUTARY FEDERAL EMERGENCY MANAGEMENT AGENCY

30 FLOODING SOURCE CROSS SECTION DISTANCE WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) FURYS FERRY ROAD TRIBUTARY EAST A B 1, C 1, FURYS FERRY ROAD TRIBUTARY NORTH A B C D 1, E 1, F 1, G 1, Feet above confluence with Reed Creek 2 Feet above confluence with Jones Creek TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA FURYS FERRY ROAD TRIBUTARY EAST- FURYS FERRY ROAD TRIBUTARY NORTH

31 FLOODING SOURCE CROSS SECTION DISTANCE WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT WITH INCREASE (FEET) FURYS FERRY ROAD TRIBUTARY SOUTH A B C 1, D 1, E 1, FURYS FERRY ROAD TRIBUTARY WEST A B C 1, D 1, Feet above confluence with Jones Creek 2 Feet above confluence with Reed Creek TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY COLUMBIA COUNTY, GA AND INCORPORATED AREAS DATA FURYS FERRY ROAD TRIBUTARY SOUTH- FURYS FERRY ROAD TRIBUTARY WEST