LONG COUNTY, GEORGIA AND INCORPORATED AREAS. Long County. Effective: September 26, 2008 FLOOD INSURANCE STUDY NUMBER 13183CV000A

Transcription

1 LONG COUNTY, GEORGIA AND INCORPORATED AREAS Long County Community Name Community Number LONG COUNTY (UNINCORPORATED AREAS) LUDOWICI, CITY OF Effective: September 26, 2008 FLOOD INSURANCE STUDY NUMBER 13183CV000A

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. Selected Flood Insurance Rate Map panels for this community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map panels (e.g., floodways, cross sections). In addition, former flood hazard zone designations have been changed as follows: Old Zone(s) Al through A30 B C New Zone AE X X Initial Countywide FIS Effective Date: September 26, 2008 i

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...12 ii

4 TABLE OF CONTENTS (Continued) TABLES Table 1 - Streams Studied by Approximate Methods... 2 Table 2 - Vertical Datum Conversion... 7 Table 3 - Community Map History Exhibit 1 - Flood Insurance Rate Map Index Flood Insurance Rate Map EXHIBITS iii

5 FLOOD INSURANCE STUDY LONG 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 Long County, Georgia, including the City of Ludowici and the unincorporated areas of Long County (referred to collectively herein as Long 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 The hydrologic and hydraulic analyses for this countywide FIS were performed by PBS&J, for the Georgia Department of Natural Resources (DNR), under Contract No. EMA-2006-CA-5615 with FEMA. The work was completed in Base map information shown on the Flood Insurance Rate Map (FIRM) was derived from U.S. Geological Survey (USGS) Digital Orthophoto Quadrangles 1

6 produced at a scale of 1:24,000, 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 East (FIPS 1002 Feet), and the horizontal datum used is NAD 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. For this countywide FIS report, the initial meeting was held on December 7, 2005, and was attended by representatives of FEMA, Georgia DNR, PBS&J, Long County, and the City of Ludowici. The results of the study were reviewed at the final meeting held on February 7, 2008, and attended by representatives of Long County, Georgia DNR, FEMA, and PBS&J. All problems raised at that meeting have been addressed. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Long County, Georgia, including the incorporated communities listed in Section 1.1. The flood hazards in Long County were studied by approximate methods. 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 Long County. For this countywide FIS, approximate analyses were used to study those areas having low development potential or minimal flood hazards. The streams studied by approximate methods are presented in Table 1. Table 1 - Streams Studied by Approximate Methods Stream Altamaha River Reach Description From the southern county boundary to the confluence of Beards Creek 2

7 Table 1 - Streams Studied by Approximate Methods (Continued) Stream Doctors Creek Horse Creek Jones Creek Kirkland Creek Kirkland Creek Tributary 3 Little Doctor Creek Little Doctor Creek Tributary 1 Old Millhead Branch Old Millhead Branch Tributary 4 Old Millhead Branch Tributary 5 Payne Creek Reach Description From the confluence with Altamaha River to approximately 5,820 feet upstream of Rye Patch Road From the county boundary to approximately 2,190 feet upstream of Horsecreek Road From the confluence of Still Branch to approximately 2,010 feet upstream of the confluence of Jones Creek Tributary 2.10 From the confluence with Taylors Creek to approximately 9,800 feet upstream of the confluence of Kirkland Creek Tributary 3 From the confluence with Kirkland Creek to approximately 10,380 feet upstream of the confluence with Kirkland Creek From the confluence with Doctors Creek to approximately 3,730 feet upstream of Rye Patch Road From the confluence with Little Doctor Creek to approximately 560 feet upstream of Long County Road From the confluence with Taylors Creek to approximately 1,050 feet upstream of Old Millhead Branch Tributary 5 From the confluence with Old Millhead Branch to approximately 7,200 feet upstream of the confluence with Old Millhead Branch From the confluence with Old Millhead Branch to approximately 4,250 feet upstream of the confluence with Old Millhead Branch From the county boundary to approximately 4,000 feet upstream of the county boundary 3

8 Table 1 - Streams Studied by Approximate Methods (Continued) Stream Taylors Creek Taylors Creek Tributary 2 Taylors Creek Tributary 6 Taylors Creek Tributary 7 Taylors Creek Tributary 12 Taylors Creek Tributary 13 Reach Description From the county boundary to the confluence of Taylors Creek Tributary 13 From the confluence with Taylors Creek to approximately 21,650 feet upstream of the confluence with Taylors Creek From the confluence with Taylors Creek to approximately 12,460 feet upstream of the confluence with Taylors Creek From the confluence with Taylors Creek to approximately 9,580 feet upstream of the confluence with Taylors Creek From the confluence with Taylors Creek to approximately 3,760 feet upstream of the confluence with Taylors Creek From the confluence with Taylors Creek to approximately 2,400 feet upstream of the confluence with Taylors Creek Also for this countywide FIS, all other streams studied by approximate methods were redelineated to better match existing topography. 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 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 The following tabulation presents Letters of Map Correction (LOMCs) incorporated into this countywide study: LOMC Case Number Date Issued Project Identifier Flooding Source LOMR P November 30, 2007 Crawford Development Doctors Creek, Little Doctor Creek, and Doctors Creek Tributary 1 4

9 2.2 Community Description Ludowici, the county seat of Long County, is located approximately 213 miles south of Augusta, Georgia (U.S. Cities Online, 2007). The county is bordered by Wayne County to the southwest, McIntosh County to the southeast, Liberty County to the northeast, and Tattnall County to the northwest. The total area contained within the county is approximately 403 square miles. According to the U.S. Census Bureau in 2000, the population for Long County was 10,304 (U.S. Census Bureau, 2007). Summer weather is characterized by great variability, while stretches of mild weather will alternate with cold spells during the winter months (NOAA, 2007). On average, January is the coldest month with an average low of 39 degrees Fahrenheit ( F) while July is the warmest month with an average high of 94 F. July is also the wettest month with an average of 6.0 inches of rainfall, while the driest month, November, only receives 2.7 inches (The Weather Channel, 2007). Long County is in the Atlantic Coastal Plain, which stretches from Massachusetts to the Florida peninsula and around the Gulf of Mexico. The region is characterized by a flat landscape. The rivers in this region drain into the Atlantic Ocean. As a percentage of total land cover for Long County, evergreen forests, mixed forests, and forested wetlands make up about 72% of the total land. About 13% is clear-cut or sparse lands while less than 10% of the county is urbanized (NARSAL, 2007). Long County is serviced by both state and federal highways. This includes U.S. Highways 25 and 84, and State Highways 57 and 144. The CSX railroad also intersects the county. Drainage in the county typically flows in a south-easterly direction. The county is drained by numerous ditches and rivers, including Jones Creek and Doctors Creek. 2.3 Principal Flood Problems Historically, precipitation which can result in flooding occurs as a result of prolonged, slow-moving low-pressure systems particularly in the cooler months; tropical hurricanes; frontal storm activity, or concentrated thunderstorms, as is particularly the case during the summer months. 2.4 Flood Protection Measures There are no other flood protection measures known to exist in the study area. 5

10 3.0 ENGINEERING METHODS For the flooding sources studied in the county, 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 (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 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. For the approximate analyses listed in Table 1, peak discharges were estimated by published USGS regional regression equations (Stamey and Hess, 1993). Regression equations estimate peak discharges for ungaged streams based on characteristics of nearby gaged streams. The USGS has developed regression equations for both rural and urban areas within Georgia. Based on a review of available aerial photos, all of approximate streams studied are in rural watersheds. Therefore the only physiographic parameter utilized for the regression equations is drainage area (square miles). 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. The 1-percent-annual-chance WSELs for the approximate studies listed in Table 1 were computed using the USACE s HEC-RAS hydraulic model, version (HEC, 2004b). HEC-GeoRAS was used to generate preprocessing data (HEC, 2004a). The hydraulic models were prepared without surveying bathymetric data. 6

11 Channel roughness factors (Mannings n ) used in the hydraulic computations were chosen by engineering judgment and based on field observations of streams and floodplain areas. The hydraulic analyses for this study were based on unobstructed flow. The flood delineations are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 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 models created for this FIS report 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. In this countywide FIS, an average vertical datum conversion of foot was calculated from NGVD to NAVD, using the National Geodetic Survey s (NGS) VERTCON online utility (NGS, 2007). The data points used to determine the conversion are listed in Table 2. Table 2 Vertical Datum Conversion Conversion from Quad Name Corner Longitude Latitude NGVD to NAVD Glissons Millpond SW Willie SW Glennville NE SW Taylors Creek SW Glennville SE SW Walthourville SW Hinesville SW Ludowici SW East of Ludowici SW Riceboro SW Average:

12 For additional information regarding conversion between NGVD and NAVD, visit the National Geodetic Survey website at or contact the National Geodetic Survey at the following address: Vertical Network Branch, N/CG13 National Geodetic Survey, NOAA Silver Spring Metro Center East-West Highway Silver Spring, Maryland (301) 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) , or visit their website at 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-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, although none were mapped for this study. 8

13 For the streams studied by approximate methods, between modeled cross sections, the boundaries were interpolated using topographic maps at a scale of 1:12,000, with a contour interval of 10 feet (USGS, various dates). For the streams studied by approximate methods, only the 1-percent-annualchance floodplain boundary is shown on the FIRM (Exhibit 1). 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. No floodways have been mapped for Long County. 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: 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 X Zone X is the flood insurance risk zone that corresponds to areas outside the 0.2-percentannual-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. 9

14 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. Insurance agents use the zones 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. The countywide FIRM presents flooding information for the entire geographic area of Long 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 3. 10

15 COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDARY MAP REVISION DATE FIRM EFFECTIVE DATE FIRM REVISION DATE Long County (Unincorporated Areas) March 17, 1978 None September 27, 1985 September 26, 2008 Ludowici, City of September 26, 2008 None September 26, 2008 None TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY LONG COUNTY, GA AND INCORPORATED AREAS COMMUNITY MAP HISTORY Table 1 - Community Map History

16 7.0 OTHER STUDIES FIS reports have been prepared for Liberty County, Georgia (Unincorporated Areas) (FEMA, 1983a), McIntosh County, Georgia (Unincorporated Areas) (FEMA, 1983b), and Wayne County, Georgia (Unincorporated Areas) (FEMA, 1988). No previous FIS reports have been prepared for Long County, Georgia or Tattnall County, Georgia. 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 BIBLIOGRAPHY AND REFERENCES Federal Emergency Management Agency, Flood Insurance Study, Liberty County, Georgia (Unincorporated Areas), Flood Insurance Study Report, June 1, 1983a; Flood Insurance Rate Map, December 1, Federal Emergency Management Agency, Flood Insurance Study, Wayne County, Georgia (Unincorporated Areas), September 30, Federal Emergency Management Agency, Flood Insurance Study, McIntosh County, Georgia (Unincorporated Areas), Flood Insurance Study Report, November 15, 1983b; Flood Insurance Rate Map, October 16, Hydrologic Engineering Center, HEC-GeoRAS River Analysis System, Version BETA 6.0, U.S. Army Corps of Engineers, Davis, California, April 2004a. Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.3, U.S. Army Corps of Engineers, Davis, California, April 2004b. NARSAL (Natural Resources Spatial Analysis Laboratory), Georgia Land Use Trends; Long County: 1998 Land Cover. Retrieved January 23, 2007, from 12

17 National Geodetic Survey, VERTCON-North American Vertical Datum Conversion Utility. Retrieved January 23, 2007, from National Oceanic and Atmospheric Administration, NOAA's National Weather Service. Retrieved January 27, 2007, from Stamey, T.C. and C.W. Hess, USGS Water Resources Investigations Report , Techniques for Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia, U.S. Census Bureau, American FactFinder, Long County, Georgia, Retrieved January 19, 2007, from U.S. Cities Online, Key to the City: Ludowici, Long County, Georgia. Retrieved January 23, 2007, from U.S. Geological Survey, 7.5-Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 5 and 10 feet, digitized by Geotechnologies, Inc.: Glissons Millpond, 1993; Glennville, 1993; Glennville NE, 1993; Taylors Creek, 1993; Glennville SW, 1993; Glennville SE, 1993; Walthourville, 1993; Hinesville, 1993; Jesup NW, 1993; Doctortown, 1993; Ludowici, 1993; East of Ludowici, 1993; Riceboro, 1993; Jesup East, 1993; Bug Island, 1993; Townsend, 1993, U.S. Department of the Interior, The Weather Channel, Monthly Averages for Ludowici, GA. Retrieved January 23, 2007, from 13