CHEROKEE COUNTY, GEORGIA

Transcription

1 CHEROKEE COUNTY, GEORGIA AND INCORPORATED AREAS Community Name Community Number *BALL GROUND, CITY OF CANTON, CITY OF CHEROKEE COUNTY (UNINCORPORATED AREAS) HOLLY SPRINGS, CITY OF WALESKA, CITY OF WOODSTOCK, CITY OF Cherokee County *No Flood Hazard Areas Identified Revised: September 29, 2006 FLOOD INSURANCE STUDY NUMBER 13057CV000A

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: July 15, 1988 Revision Dates: September 14, 1990 September 29, 2006

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...37 i

4 TABLE OF CONTENTS (Continued) FIGURES Figure 1 - Floodway Schematic TABLES Table 1 Streams Studied by Detailed Methods... 3 Table 2 Redelineated Streams... 5 Table 3 Streams Studied by Limited Detailed Methods... 6 Table 4 Summary of Discharges Table 5 Summary of Stillwater Elevations Table 6 Vertical Datum Conversion Table 7 Floodway Data Table Table 8 Community Map History EXHIBITS Exhibit 1 - Flood Profiles Avery Creek Canton Creek Clark Creek Etowah River Kellogg Creek Little River Mill Creek Noonday Creek Owl Creek Rocky Creek Rubes Creek Rubes Creek Tributary Smithwick Creek Tate Creek Toonigh Creek Tributary L Tributary M Unnamed Tributary to Smithwick Creek Panels 01P-03P Panel 04P Panels 05P-09P Panels 10P-14P Panels 15P-16P Panels 17P-20P Panels 21P-22P Panels 23P-24P Panels 25P-31P Panels 32P-33P Panels 34P-37P Panels 38P-39P Panel 40P Panel 41P Panels 42P-44P Panels 45P-47P Panels 48P-49P Panel 50P Exhibit 2 - Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY CHEROKEE 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 Cherokee County, including the Cities of Ball Ground, Canton, Holly Springs, Waleska, Woodstock; and the unincorporated areas of Cherokee County (referred to collectively herein as Cherokee 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, Please note that the majority of the Cities of Mountain Park and Nelson are geographically located in Fulton and Pickens Counties, respectively. Therefore they were not included in this FIS report. See the separately printed FIS report and Flood Insurance Rate Maps (FIRMs) for flood hazard information. Please note that the City of Ball Ground does not have any flood hazard areas identified. 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. 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 initial countywide study (Reference 1) were performed by the U.S. Army Corps of Engineers (USACE), Mobile District, for the Federal Emergency Management Agency (FEMA), under Interagency Agreement No. EMW-E-1153, Project Order No. 1. The work was completed in July

6 The hydraulic analyses for the September 14, 1990, revision (Reference 2) were performed by Browning Engineering Associates, Inc., by the USACE, Charleston District, and by Braswell Engineering, Inc. For this revision, the work done for the redelineated streams and the streams studied by limited detailed methods were performed by PBS&J, for the Georgia Department of Natural Resources (DNR), under Contract No. EMA-2003-GR with FEMA. The work was completed in April The hydrologic and hydraulic analyses for Tate Creek was performed by PBS&J, for FEMA, under Contract No. EMW-88-C The study was completed in May 1989 (Reference 3). 1.3 Coordination The initial coordination meeting for the initial countywide FIS report was held on August 9, 1983, and attended by representatives of FEMA, the USACE, Mobile District, and Cherokee County. The purpose of the initial meeting was to discuss the scope of the FIS. The results of the study were reviewed at the final meeting held on December 11, 1986, and attended by representatives of FEMA, the USACE, Mobile District, and Cherokee County. All problems raised at that meeting were addressed. For this countywide FIS report, a scoping meeting was held on September 22, 2004, and attended by representatives of Cherokee County, the City of Canton, the Georgia DNR, FEMA, Black and Veatch, and PBS&J. The purpose of this meeting was to discuss the scope of the FIS. A final coordination meeting was held on December 8, Attending the meeting were representatives of Cherokee County, the Georgia DNR, FEMA, and PBS&J. All problems raised at that meeting have been addressed. 2.0 AREA STUDIED 2.1 Scope of Study For this revision the FIS covers the geographic area of Cherokee County, Georgia, including the incorporated communities listed in Section 1.1. The areas studied by detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through July Table 1 lists the streams studied by detailed methods in this FIS report. 2

7 Table 1 Streams Studied by Detailed Methods Avery Creek Canton Creek Clark Creek Etowah River Kellogg Creek Little River Mill Creek Stream Limits of Detailed Study From the confluence with Mill Creek to approximately 4,000 feet upstream of Hickory Road From the confluence with Etowah River to approximately 4,650 feet upstream of Georgia Northeastern Railroad From approximately 1,450 downstream of State Highway 92/Cherokee Road to approximately 2,250 feet upstream of Dunnwood Drive From approximately 4,300 feet downstream of State Highway 20/Knox Bridge Highway to approximately 40,200 feet upstream of Georgia Northeastern Railroad From the confluence with Allatoona Lake to approximately 2,320 feet upstream of Woodstock Road From Interstate Highway 575/State Highway 5 to approximately 6,150 feet upstream of the confluence of Rocky Creek From the confluence with Little River to approximately 1,200 feet upstream of Hickory Flat Highway/State Highway 140 Noonday Creek Owl Creek From the confluence with Little River to the county boundary From the confluence with Allatoona Lake to approximately 150 feet upstream of Dogwood Lane Rocky Creek From the confluence with Little River to approximately 3,200 feet upstream of Cox Road Rubes Creek From the confluence with Little River to approximately 5,150 feet upstream of Alabama Road/State Highway 92 Rubes Creek Tributary Smithwick Creek From the confluence with Rubes Creek to approximately 550 feet upstream of Justin Drive From approximately 5,000 feet upstream of Old Darnell Road to approximately 9,100 feet upstream of Old Darnell Road 3

8 Table 1 Streams Studied by Detailed Methods Stream Limits of Detailed Study Tate Creek From approximately 250 feet upstream of Interstate Highway 575/State Highway 5 to approximately 5,100 feet upstream of Flooding Retarding Structure No. 17 Toonigh Creek From the confluence with Little River to approximately 1,650 feet upstream of Hickory Road Tributary L From the confluence with Little River to approximately 1,350 feet upstream of Cherecobb Drive Tributary M From the confluence with Mill Creek to approximately 450 feet upstream of South Holly Spings Road Unnamed Tributary to Smithwick Creek From the confluence with Smithwick Creek to approximately 710 feet upstream of the confluence with Smithwick Creek Initial Countywide FIS Report (July 15, 1988) In the initial countywide FIS report, flooding caused by the overflow of Avery Creek, Canton Creek, Clark Creek, the Etowah River, Kellogg Creek, the Little River, Mill Creek, Mill Creek Tributary, Noonday Creek, Owl Creek, Rocky Creek, Rubes Creek, Tate Creek, Toonigh Creek, Tributary L, and Tributary M were studied by detailed methods. First Countywide FIS Revision (September 14, 1990) For the September 14, 1990, FIS report, portions of the following flooding sources were restudied: Little River, Rocky Creek, Etowah River, and Noonday Creek. The Little River was restudied from the confluence of Rocky Creek to a point located about 1.3 miles upstream. Rocky Creek was restudied from its mouth to a point located about 1.3 miles upstream. The restudy was performed in order to delineate new 1-percent and 0.2-percent-annual-chance flood boundaries, as well as a floodway boundary, in areas of the Little River and Rocky Creek where the Settindown Creek Golf Course has been constructed. These areas had been previously studied by detailed methods. The Etowah River was restudied from about 0.85 mile downstream of Interstate Highway 575 to about 8.4 miles upstream of Interstate Highway 575, to account 4

9 for the effects of fill placed in the overbank areas downstream of Interstate Highway 575 for the Cherokee County Industrial Park. Revised 1-percent-annualchance flood and floodway boundaries were developed for this reach, which was previously studied by detailed methods. Noonday Creek was restudied from 0.5 mile downstream of Alabama Road to the upstream county boundary to account for the effects of fill placed in the overbank areas just downstream and upstream of Alabama Road. Revised 1-percent-annualchance flood and floodway boundaries were developed for this reach, which was previously studied by detailed methods. This Countywide Revision Tate Creek is in both Cherokee and Cobb Counties. For this countywide revision, flooding information for Tate Creek was updated with the more recent Tate Creek analysis from the Cobb County FIS report (Reference 3). For this countywide revision, reaches of streams that have been studied by detailed methods were selected for redelineation based on more recent topography. The topographic mapping was provided by the City of Canton and was produced in 2004 (Reference 4). The topographic data is comprised of 55 square miles mapped at a 2 foot contour interval (in parts of the city), and another 56 square miles mapped at a 5 foot contour interval (outside of the city). This data has a vertical accuracy within 1 foot (for the 2 foot contour mapped area) and 2.5 feet (for the 5 foot contour mapped area). The City of Canton also provided orthophotos produced at 3 inch-pixel for the 2 foot mapping areas and 0.8 inch-pixel for the 5 foot mapping area. The reaches that were redelineated in this revision are shown in Table 2, Redelineated Streams. Canton Creek Stream Table 2- Redelineated Streams Reach Description From the confluence with the Etowah River to approximately 4,500 feet upstream of Georgia Northeastern Railroad Etowah River From the confluence with Allatoona Lake to approximately 2.5 miles upstream of miles upstream of Georgia Northeastern Railroad 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 February The streams studied by limited detailed methods are listed in Table 3, Streams Studied by Limited Detailed Methods. 5

10 Table 3- Streams Studied by Limited Detailed Methods Stream Reach Description Etowah River From approximately 19,080 feet downstream of East Cherokee Drive to approximately 8,170 feet upstream of East Cherokee Drive Hickory Creek From the confluence with Etowah River to approximately 2,000 feet upstream of Interstate Highway 575/State Highway 5 Rubes Creek Tributary was studied by detailed methods as part of Letter of Map Revision (LOMR) Case No P, dated May 9, LOMR Case No P, dated August 3, 1999, revised a portion of Noonday Creek. Smithwick Creek and an Unnamed Tributary to Smithwick Creek were studied by detailed methods as part of LOMR Case No P, dated October 20, The Hollis Q. Lathem Reservoir was studied by approximate methods as part of LOMR Case NO P, dated November 1, Blankets Creek was studied by approximate methods as part of LOMR Case No P, dated October 29, All of these LOMRs were incorporated into this revision. Please note that as a result of this countywide revision, the paneling scheme has changed. Therefore, the flood hazards revised in the September 14, 1990, revision now fall on different panels, as shown below: Previous FIRM Number 13057C0230C 13057C0308C 13057C0310C 13057C0335C Current FIRM Number 13057C0251D 13057C0252D 13057C0253D 13057C0254D 13057C0333D 13057C0331D 13057C0332D 13057C0334D 13057C0356D 13057C0358D 13057C0360D 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 Cherokee County. 6

11 2.2 Community Description Cherokee County lies in the northwest part of Georgia, on the northern fringe of the Atlanta metropolitan area. It is bordered by Pickens County to the north, Dawson and Forsyth Counties to the east, Fulton and Cobb Counties to the south, and Bartow County to the west. The county is bisected by the Etowah River, which runs southwesterly through the county and eventually empties into Allatoona Lake, southwest of Canton. Railroad service to the county is provided by the Georgia Northeastern Railroad. The main highways servicing Cherokee County are State Highways 205, 92, 5, 20, 108, 140 and 372 and Interstate Highways 575 and 75. According to the 2000 Census, the population of Cherokee County was 141,903 (Reference 5). Cherokee County offers a unique balance between the three main branches of economic activity: agriculture, industry and commerce. The county seat, Canton, has been a focal point for agriculture and industry. Canton was incorporated under the name of Etowah in 1833, and its present name was approved by the legislature the following year. The leading industry since 1899 has been Canton Textile Mills. Some of the minerals produced in the county are granite, marble, copper, iron, asbestos, graphite, talc, mica and soapstone. The terrain may be characterized as typical foothill country. It consists mainly of well-defined floodplains with relatively steep sides rising to the surrounding hills. The climate of Cherokee County consists of long, warm summers, with the highest mean monthly temperature of 89 degrees Fahrenheit ( F), occurring in July. While summer temperatures are frequently quite warm, prolonged periods of hot weather are rare. The winters are short and mild, with the lowest mean monthly temperature of 30 F, occurring in January (Reference 6). The average annual rainfall in Cherokee County is 48 inches. The wettest month is March with an average of 6.87 inches while October is the driest with 3.23 inches. The average annual snowfall is about 1.5 inches. However, a snow of 4 inches or more occurs about once every five years (Reference 7). 2.3 Principal Flood Problems Portions of Cherokee County are subject to periodic flooding from storms causing disruptions to vehicular traffic and inconveniences to residents. 7

12 2.4 Flood Protection Measures The Natural Resource Conservation Service (NRCS), formerly the Soil Conservation Service, completed watershed work-plans for streams in Cherokee County, ultimately constructing flood retarding reservoirs in both the Avery and Tate Creek watersheds. These projects are designed to provide significant flood peak attenuation. 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. Initial Countywide FIS Report (July 15, 1988) The analyses were accomplished by using the following three methods: (1) a regional discharge-frequency study published by the U.S. Geological Survey (USGS) for the State of Georgia (Reference 8); (2) discharge frequency analyses using USGS river gaging records; and (3) discharges from FIS reports for adjoining counties that had been computed for streams that flow into Cherokee County. Stream gaging records were available for the Etowah River and the Little River and provided the basis for discharge frequency analyses performed to determine discharges for those streams. 8

13 The regional study, Floods in Georgia, Magnitude and Frequency (Reference 8), was used to determine discharges for Avery Creek, Canton Creek, Clark Creek, Etowah River, Kellogg Creek, Little River, Mill Creek, Owl Creek, Smithwick Creek, Toonigh Creek, Tributary L, and Tributary M. Since the boundary line between Region 1 and Region 2 of the regional study follows the Etowah River, bisecting Cherokee County, frequency discharges for the two regions were averaged to arrive at discharges applicable to the county s streams. Discharges for Noonday Creek at the county line were taken from the FIS report for Cobb County (Reference 9). Discharges for the upstream portion of Rubes Creek covered in this study were taken from the FIS report for Cobb County. Discharges for the downstream portion were obtained from the regional discharge frequency study. The Avery Creek basin contains three flood retarding reservoirs: Bradshaw Lake, Soil Conservation Service (SCS) Dam No. 2, located on Avery Creek at the upper limit of the study area; SCS Dam No. 6, located on Avery Creek just downstream of Bradshaw Lake; and SCS Dam No. 12, located on a tributary that enters Avery Creek between SCS Dam No. 6 and the mouth. A HEC-1 runoff model (Reference 10) of the basin was developed to determine the storage effects of the reservoirs. SCS unit hydrographs and SCS infiltration rate curve numbers were used in the model. Frequency rainfall was taken from the National Weather Service Technical Papers 40 (Reference 11) and 49 (Reference 12) and applied to the runoff model. The resulting discharges were considerably higher than those computed using the regional equations. To obtain discharges more consistent with the rest of the study, the HEC-1 runoff model was used to model the storage routing through the reservoirs. The outflows from the routing were then added to the discharges computed with the regional equation for the uncontrolled area below Dams No. 6 and 12. Discharges for Rocky Creek at Lake Cherful Dam were taken from the FIS report for Fulton County (Reference 13) and the FIS report for the City of Mountain Park (Reference 14). Discharges at the mouth were obtained by taking the difference in regional frequency discharges (discharge at the mouth less the discharge at the dam), and combining those with the corresponding discharges at the dam. First Countywide FIS Revision (September 14, 1990) The hydrologic analyses for the September 14, 1990, FIS revision used the same flow rates as those used in the original study. 9

14 This Countywide Revision Discharge-frequency relationships for Tate Creek were determined using the HEC-1 computer program. Runoff hydrographs entered into HEC-1 were developed according to methods outlined by USGS (References 15 and 16). Hickory Creek and a portion of the Etowah River were the 2 streams studied by limited detailed methods. For Hickory Creek, regression equations were used to compute the peak discharges for the 1-percent-annual chance flood (Reference 17). The watershed area for Hickory Creek was calculated using USGS 30 meter resolution Digital Elevation Models (DEMs) gathered from the National Elevation Dataset (NED). The limited detailed study for the Etowah River used effective discharges from the existing FIS. The LOMRs listed in Section 2.1 should be consulted to identify the hydrologic methods used for those streams. 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 community are presented in Table 4. 10

15 Table 4 - Summary of Discharges Flooding Source and Location AVERY CREEK At the confluence with Mill Creek CANTON CREEK Approximately 1,300 feet upstream of Marietta Road/State Highway 20 CLARK CREEK At the confluence with Allatoona Lake ETOWAH RIVER Just downstream of State Highway 20/Knox Bridge Highway Just downstream of Marietta Highway/State Highway 140 KELLOGG CREEK At the confluence with Allatoona Lake LITTLE RIVER Just downstream of Rope Mill Road Approximately 16,465 feet upstream of the confluence of Rocky Creek MILL CREEK At the confluence with Little River NOONDAY CREEK At the confluence with Little River OWL CREEK At the confluence with Allatoona Lake ROCKY CREEK At the confluence with Little River RUBES CREEK At the confluence with Little River Drainage Area (square miles) Peak Discharges (cubic feet per second) 10-Percent- Annual-Chance 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance 9.0 1,120 2,140 2,720 5,250 2,550 4,100 4,800 6, ,425 2,250 2,600 3, ,320 39,115 45,960 63, ,300 36,000 42,300 58, ,000 1,175 1, ,310 8,200 9,570 13, ,480 5,370 6,270 8, ,900 6,050 7,000 9, ,454 8,588 9,372 10, ,050 1,225 1, ,280 2, ,190 3,400 3,900 5,400 Data not available 11

16 Table 4 - Summary of Discharges (Cont d) Flooding Source and Location RUBES CREEK TRIBUTARY At the confluence with Rubes Creek Approximately 500 feet upstream of Independence Way Drainage Area (square miles) Peak Discharges (cubic feet per second) 10-Percent- Annual-Chance 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance ,030 1,200 1, ,280 SMITHWICK CREEK Approximately 5,015 feet upstream of Old Darnell Road TATE CREEK At confluence with Noonday Creek TOONIGH CREEK At the confluence with Little River TRIBUTARY L At the confluence with Little River TRIBUTARY M At the confluence with Mill Creek UNNAMED TRIBUTARY TO SMITHWICK CREEK At the confluence with Smithwick Creek 8.1 3, ,246 1,471 2, ,325 2,080 2,425 3, ,450 1,700 2, ,210 1,430 1, * * 225 * Data not available The stillwater elevation for the 1-percent-annual-chance flood event for Allatoona Lake is shown in Table 5. Table 5 - Summary of Stillwater Elevations Water Surface Elevations (Feet NAVD88 1 ) 10-Percent- Annual-Chance 2-Percent- Annual-Chance 1-Percent- Annual-Chance 0.2-Percent- Annual-Chance Flooding Source ALLATOONA LAKE At Allatoona Lake Dam North American Vertical Datum of 1988 Data not available 12

17 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. Initial Countywide FIS Report (July 15, 1988) Cross sections were compiled by photogrammetric methods (Reference 18), and were located at close intervals above and below bridges in order to compute the backwater effects of these structures. Bridge opening geometry was established by field measurements. Water surface elevations (WSELs) of floods of the selected recurrence intervals were computed through the use of the USACE s HEC-2 step-backwater computer program (Reference 19). Starting WSELs were determined using the slope/area method. The elevation was then compared with the Allatoona Lake normal pool level, and the higher of the two was adopted as starting WSEL. First Countywide FIS Revision (September 14, 1990) The hydraulic analyses for the Little River and Rocky Creek were revised to reflect changes in topography due to the construction of a golf course. The hydraulic analyses for the Etowah River were revised to reflect fill in the overbanks for the Cherokee County Industrial Park. The hydraulic analyses for Noonday Creek were revised to reflect fill in the overbanks just upstream and downstream of Alabama Road. Cross sections for Little River and Rocky Creek in the area from the original FIS were resurveyed, and additional new cross sections were surveyed. New cross sections were also added for two golf cart bridges on the Little River. For Etowah River, additional cross sections were inserted into the analysis based on mass grading plans for the Industrial Park, which were prepared at a scale of 1:1,200 with a contour interval of 2 feet (Reference 20). For Noonday Creek, additional cross sections were used in conjunction with revised bridge and valley sections to refine the floodplain. The revised hydraulic analyses for the 10-, 2-, 1-, and 0.2-percent chance floods for the Little River and Rocky Creek were performed using the HEC-2 stepbackwater computer program (Reference 21). The revised hydraulic analyses for 13

18 Noonday Creek is contained in a report prepared by Braswell Engineering, Inc. (Reference 22). The starting WSELs for Little River and Rocky Creek were not revised from the original hydraulic analyses used for the initial countywide FIS (July 15, 1988). This Revision For this revision, cross sections for the flooding sources studied by limited detailed methods were obtained using digital topography and field measurements. The 1-percent-annual-chance WSELs were computed using the USACE s HEC-RAS hydraulic model, version (Reference 23), and HEC- GeoRAS was used to delineate the 1-percent-annual-chance floodplain (Reference 24). The hydraulic model was prepared using digital elevation data (Reference 4), without surveying bathymetric data. Where bridge or culvert data were readily available, these data were reflected in the hydraulic model. Where these structural data were not readily available, field measurements of these structures were made to approximate their geometry in the hydraulic models. This model does not include field surveys that determine specifics on channel and floodplain characteristics. A limited detailed study is a buildable product that can be upgraded to a full detailed study at a later date by verifying stream channel characteristics, bridge and culvert opening 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. For Tate Creek, the water surface elevations of floods of the selected recurrence intervals were computed using HEC-2 step back-water computer program (Reference 25). The LOMRs listed in Section 2.1 should be consulted to identify the hydraulic analyses that were used. The channel and overbank n values for the streams studied by detailed methods are shown in the following tabulation, Summary of Roughness Coefficients. 14

19 Summary of Roughness Coefficients Stream Channel n Overbank n Avery Creek Canton Creek Clark Creek Etowah River Kellogg Creek Little River Mill Creek Noonday Creek Owl Creek Rocky Creek Rubes Creek Rubes Creek Tributary Smithwick Creek Tate Creek Toonigh Creek Tributary L Tributary M Unnamed Tributary to Smithwick Creek 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). Flood profiles were drawn showing the computed WSELs for floods of the selected recurrence intervals. In cases where the 2-percent and 1-percent annual chance flood elevations were close together, only the 1-percent annual chance flood profile has been shown due to limitations of the profile scale. 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 the National Geodetic Vertical Datum of 1929 (NGVD29). With the finalization of the North American Vertical Datum of 1988 (NAVD88), many FIS reports and FIRMs are being prepared using NAVD88 as the referenced vertical datum. 15

20 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 that was used to convert the data in this FIS report to NAVD88 was calculated using the National Geodetic Survey s VERTCON online utility (Reference 26). The data points used to determine the conversion factor are listed in Table 6 Vertical Datum Conversion. Table 6 Vertical Datum Conversion Conversion from Quad Name Corner Longitude Latitude NGVD29 to NAVD88 Fairmount SE feet Ludville SE feet Jasper SE feet Nelson SE feet Ball Ground East SE feet Ball Ground East SW feet Waleska SE feet Waleska SW feet South Canton SE feet South Canton SW feet Canton 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. 16

21 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1-percent-annual-chance (100-year) flood elevations and delineations of the 1- and 0.2-percent-annual-chance (500-year) floodplain boundaries and 1-percent-annual-chance floodway to assist communities in developing floodplain management measures. This information is presented on the FIRM and in many components of the FIS report, including Flood Profiles, Floodway Data Table, and Summary of Stillwater Elevations Table. Users should reference the data presented in the FIS report as well as additional information that may be available at the local map repository before making flood elevation and/or floodplain boundary determinations. 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1-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. Initial Countywide FIS Report (July 15, 1988) For each stream studied by detailed methods, the 1- and 0.2-percent-annualchance floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:4,800, with a contour interval of 4 feet (Reference 27). First Countywide FIS Revision (September 14, 1990) The 1- and 0.2-percent-annual-chance floodplain boundaries for Little River and Rocky Creek were revised using topographic mapping at a scale of 1:1,200 photoreduced to 1:2,400, with a contour interval of 2 feet (Reference 28). This Revision For this revision, the City of Canton submitted 2 foot and 5 foot contour interval topographic data (Reference 4). The Etowah River and Canton Creek were redelineated using the 2 foot contour interval topographic data. For the streams studied by limited detailed methods, Hickory Creek used the 2 foot contour interval data, and the limited detailed portion of the Etowah River used 5 foot contour interval data. 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 17

22 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 and limited detailed methods, only the 1-percent-annual-chance floodplain boundary is shown on the FIRM (Exhibit 2). 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 1-percent-annual-chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1-percent-annual-chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1.0 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 and are shown in Table 7, Floodway Data. 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. Portions of the floodways for the Little River, Rock Creek, Rubes Creek, Tate Creek, and Clark Creek lie outside the county boundary. 18

23 FLOODING SOURCE CROSS SECTION DISTANCE WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) AVERY CREEK A , B 1, , C 1, D 3, , E 5, , F 7, , G 9, H 11, I 11, J 13, , K 14, , CANTON CREEK A 3, , B 3, , C 6, D 7, E 8, Feet above confluence with Mill Creek 2 Elevation computed without consideration of backwater effects from Mill Creek 3 Feet above confluence with Etowah River 4 Elevation computed without consideration of backwater effects from Etowah River TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA AVERY CREEK - CANTON CREEK

24 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) CLARK CREEK A 9, , B 10, C 13, , D 14, E 17, F 17, G 19, , H 21, I 21, J 22, K 26, L 27, M 28, N 28, Feet above confluence with Allatoona Lake 2 Elevation computed without consideration of backwater effects from Allatoona Lake TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA CLARK CREEK

25 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) ETOWAH RIVER A 8, , B 15, , C 19, , D 23, , E 27, , F 31, , G 35, , H 37, , I 38, , J 42, , K 44, , L 49, , M 53, , N 56, , O 58,390 1,326 20, P 65, , Q 65, , R 70,350 1,036 16, S 73, , T 74, , Feet above confluence of Downing Creek 2 Elevation computed without consideration of backwater effects from Allatoona Lake 3 Actual floodway width differs from modeled floodway width due to redelineation TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA ETOWAH RIVER

26 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) ETOWAH RIVER (CONT D) U 75, , V 77, , W 78, , X 79, , Y 86, , Z 94,095 1,009 14, AA 103, , AB 108, , AC 113, , AD 116, , AE 119, , Feet above confluence of Downing Creek TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA ETOWAH RIVER

27 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) KELLOGG CREEK A 1, B 2, C 2, D 3, E 4, F 5, G 6, H 6, I 7, , J 8, K 8, L 9, M 10, N 10, O 11, Feet above confluence with Allatoona Lake 2 Elevation computed without consideration of backwater effects from Allatoona Lake TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA KELLOGG CREEK

28 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) LITTLE RIVER A , B , C 1, , D 1, , E 1, , F 2, , G 3, , H 4, , I 5, , J 5, , K 6, , L 7, , M 8, , N 8, , O 9, , P 10, , Q 13, , R 16, , S 17, , T 17, , U 18, , Feet above Interstate Highway Elevation computed without consideration of backwater effects from Allatoona Lake TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA LITTLE RIVER

29 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) LITTLE RIVER (CONT D) V 19, , W 21, , X 23,620 1,537 8, Y 25, , Z 26, , AA 26, , AB 26, , AC 27, , AD 29, , AE 30, , AF 32, , AG 33, , AH 35, , AI 36, , AJ 38,000 1,212 8, AK 39, , AL 40, , AM 40,900 1,135 4, AN 43, , AO 44,300 1,606/ , AP 45, / , Feet above Interstate Highway Total width/ width within county TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA LITTLE RIVER

30 FLOODING SOURCE CROSS SECTION DISTANCE 1 WIDTH (FEET) FLOODWAY SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) 1-PERCENT-ANNUAL-CHANCE-FLOOD WATER SURFACE ELEVATION WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) MILL CREEK A 3, , B 3, , C 4, , D 6, , E 11, F 14, , G 17, , H 20, , I 22, , J 23, , K 26, , L 28, , M 29, N 30, , O 31, , P 32, , Q 32, , R 33, , S 37, , T 39, Feet above confluence with Little River 2 Elevation computed without consideration of backwater effects from Little River TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY CHEROKEE COUNTY, GA AND INCORPORATED AREAS FLOODWAY DATA MILL CREEK