PUTNAM COUNTY, GEORGIA

Transcription

1 PUTNAM COUNTY, GEORGIA AND INCORPORATED AREAS Community Name Community Number Eatonton, City of Putnam County (Unincorporated Areas) Putnam County Effective: September 26, 2008 FLOOD INSURANCE STUDY NUMBER 13237CV000A

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 26, 2008

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 Principal Flood Problems Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATIONS FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES...11 i

4 TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Floodway Schematic... 9 TABLES Table 1 - Summary of Discharges... 5 Table 2 - Vertical Datum Conversion... 7 Table 3 - Community Map History EXHIBITS Exhibit 1 - Flood Profiles Rooty Creek Panel 01P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY PUTNAM 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 Putnam County, including the City of Eatonton and the unincorporated areas of Putnam County (referred to collectively herein as Putnam 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 the June 19, 1989, study for the City of Eatonton were performed by the U.S. Geological Survey (USGS), Water Resource Division, for FEMA, under Interagency Agreement No. EMW-85-E-1823, Project Order No. 16, Amendment No. 1. The work was completed in February 1987 (FEMA, 1989). 1

6 For this countywide FIS, streams restudied by approximate methods and redelineation of floodplain boundaries for streams studied by approximate methods was 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 May Base map information shown on the Flood Insurance Rate Map (FIRM) was provided for Putnam County by Pond and Company dated February 2001 and captured at a resolution of 1-foot within Putnam County and 0.5-foot within the City of Eatonton. The projection used in the preparation of this map is Georgia State Plane West Zone (FIPS zone 1002), and the horizontal datum used is NAD 83, GRS 80 spheroid. 1.3 Coordination An initial meeting was 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 was held with representatives from FEMA, the community, and the study contractor to review the results of the study. For the June 19, 1989, study, the initial meeting was held on March 12, 1985, and attended by representatives of FEMA and the City of Eatonton. The results of the study were reviewed at the final meeting held on July 13, 1988, and attended by representatives of the Study Contractor, FEMA, and the community. All problems raised at that meeting have been addressed. For this countywide study, the initial meeting was held on January 19, 2006, and attended by representatives from the Georgia DNR, PBS&J, and Putnam County. The results of the study were reviewed at the final meeting held on November 2, 2007, and attended by representatives of Putnam County, Georgia DNR, 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 Putnam County, Georgia, including the incorporated communities listed in Section 1.1. The areas studied by 2

7 detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction. Rooty Creek is studied by detailed methods in this FIS report. The limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Exhibit 2). 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). Approximately 88 stream miles were newly studied by approximate analyses. The streams studied include Glady Creek, Little River, and Rooty Creek as well as numerous tributaries feeding into these three parent streams. Existing approximate study was redelineated. Lake Sinclair and Lake Oconee were also newly studied by approximate methods. 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 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, PBS&J and Putnam County. 2.2 Community Description Putnam County is located in the central portion of Georgia. Putnam County has a population of 18,812 according the 2000 census (U.S. Census Bureau, 2000). The City of Eatonton is the county seat for Putnam County. The City of Eatonton is located in northwestern Putnam County, in north central Georgia. The city is 36 miles north of Macon, Georgia, and about 12 miles east of Monticello, Georgia. Eatonton is served by State Highway 16, State Highway 44, U.S. Highways 129 and 441, and the Norfolk Southern Railway. According to the 2000 census the population of Eatonton is 6,764 (U.S. Census Bureau, 2000). 2.3 Principal Flood Problems Flooding problems in Eatonton are due primarily to the overflow of Rooty Creek, which flows south along the eastern city limits. Major floods in Eatonton are the result of intense rainfall associated with frontal passages. There are no known gage records for streams in Eatonton. The largest 3

8 known floods occurred in 1902, 1908, and These floods produced discharges that had a 2-percent-annual-chance recurrence. There are no records of elevations or discharges for larger floods in the study area (FEMA, 1989). 2.4 Flood Protection Measures Flood protections measures are not 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 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. The discharge-frequency relationships for the 1-percent-annual-chance flood on Rooty Creek were developed using USGS techniques (USGS, 1978). No adjustments were made for urbanization because the discharge adjustments were less than 10 percent. Land use for the watersheds was determined from topographic maps (USGS, 1972), recent city maps, and field inspections. The percentage of impervious area (used where greater than 10 percent), including areas served by storm sewers, was determined from land use in the watersheds according to USGS methods (USGS, 1977). 4

9 Peak discharge-drainage area relationships for Rooty Creek are shown in Table 1. Table 1 - 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 Rooty Creek At Martin Luther King Jr * * 3,060 * Drive At Oconee Springs Road 7.37 * * 2,700 * Approximately 1,600 feet 6.25 * * 2,460 * downstream of Sparta Highway / State Highway 16 / State Highway 44 *Data not available For the approximate study streams, peak flows were determined using the rural regression equations for Georgia (USGS, 1993). 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. Cross-section data for Rooty Creek were obtained by field surveys. Additional sections were estimated from adjacent surveyed sections and topographic maps (USGS, 1972). All bridges and culverts were field surveyed to obtain elevations and structural geometry. 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). Water surface elevations (WSELs) for the 1-percent-annual-chance flood on Rooty Creek were computed using WSPRO, a U.S. Department of Transportation (USDOT) step-backwater computer program (USDOT, 1986). Water surface profile elevations through culverts were computed by 5

10 USGS computer program A526 (USGS, 1968, 1976). The starting elevation for the 1-percent-annual-chance flood profile was determined by stepbackwater methods. Channel roughness factors (Manning s n ) used in the hydraulic computations for Rooty Creek were chosen based on field observations of channel and floodplain areas. Roughness values for the main channel ranged from to depending on width, depth of flow, curves and vegetation. Roughness values for the floodplain ranges from for the grassy areas to for the densely wooded areas. The profile baseline for Rooty Creek depicted on the FIRM represents the hydraulic modeling baselines that matches the flood profile in 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. 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 (HEC, 2004). 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. 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 6

11 National Geodetic Survey s 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 Latitude Longitude NGVD to NAVD Rock Eagle Lake NE Rock Eagle Lake SW Rock Eagle Lake SE Harmony NE Harmony SE Eatonton SW Eatonton SE Liberty SE Meda SE Average: 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 7

12 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-annualchance (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 or approximate methods, the 1- and 0.2-percent-annual-chance floodplain boundaries have been delineated using the flood elevations determined at each cross section where available. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:24,000, with contour intervals of 10 and 20 feet (USGS, 1972). 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-percentannual-chance floodplain boundary is shown on the FIRM (Exhibit 2). 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces floodcarrying 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 8

13 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 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.Fl Figure 1 - Floodway Schematic 9

14 No floodways were computed for Putnam 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 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, wholefoot 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-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. 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. 10

15 The countywide FIRM presents flooding information for the entire geographic area of Putnam 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 OTHER STUDIES A FIS has been published for the City of Eatonton (FEMA, 1989). The Putnam County study incorporates the information for Rooty Creek. Neighboring county FIS reports have been prepared for Baldwin County, Georgia (Unincorporated Areas) (FEMA, 1990), and Morgan County, Georgia and Incorporated Areas (FEMA, 2002). 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, City of Eatonton, Putnam County, Georgia, June 19, Federal Emergency Management Agency, Flood Insurance Study, Baldwin County, Georgia (Unincorporated Areas), January 3, Federal Emergency Management Agency, Flood Insurance Study, Morgan County, Georgia and Incorporated Areas, February 15, Hydrologic Engineering Center, HEC-GeoRAS, Version 3.0, U.S. Army Corps of Engineers, April

16 COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDARY MAP REVISION DATE FIRM EFFECTIVE DATE FIRM REVISION DATE Eatonton, City of February 21, 1975 December 12, 1975 June 19, 1989 September 26, 2008 Putnam County (Unincorporated Areas) September 26, 2008 None September 26, 2008 None TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY PUTNAM COUNTY, GA AND INCORPORATED AREAS COMMUNITY MAP HISTORY Table 2 - Community Map History

17 Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.3, U.S. Army Corps of Engineers, Davis, California, April National Geodetic Survey, VERTCON-North American Vertical Datum Conversion Utility. Retrieved January 10, 2007, from U.S. Census Bureau, American Fact Finder, Putnam County, Georgia, Retrieved May 9, 2007, from U.S. Department of Transportation, Federal Highway Administration, Report No. FHWA/RD-86/108, Bridge Waterways Analysis Model: Research Report, J.O. Shearman, W.H. Kirby, V.R. Snyder, and H.N. Flippo, July U.S. Geological Survey, Preparation and Input Data for Automatic Computation of State-Discharge Relations at Culverts, Water Resources Investigations, U.S. Department of the Interior, H.F. Matthai, H.E. Stull, and Jacob Davidian, Book 7, Chapter 3, U.S. Geological Survey, 7.5 Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 20 feet: Eatonton, Georgia; Medea, Georgia, U.S. Department of the Interior, U.S. Geological Survey, Measurement of Peak Discharge at Culverts by Indirect Methods, Water Resources Investigations, U.S. Department of the Interior, G.L. Bodhaine, Book 3, Chapter A3, U.S. Geological Survey, Preliminary Flood-Frequency Relations for Urban Streams in Metropolitan Atlanta, Georgia, Water Resources Investigations Report 77-57, U.S. Department of the Interior, H.G. Golden, U.S. Geological Survey, Flood in Georgia, Magnitude and Frequency, Water Resources Investigations Report , U.S. Department of the Interior, McGlone Price, U.S. Geological Survey, Techniques for Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia, Water Resources Investigations Report , U.S. Department of the Interior, Stamey, T.C. and G.W. Hess,

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