CURRY COUNTY, OREGON AND INCORPORATED AREAS

Transcription

1 CURRY COUNTY, OREGON AND INCORPORATED AREAS COMMUNITY NAME COMMUNITY NUMBER BROOKINGS, CITY OF CURRY COUNTY UNINCORPORATED AREAS GOLD BEACH, CITY OF PORT ORFORD, CITY OF Effective September 25, 2009 Federal Emergency Management Agency Flood Insurance Study Number 41015CV000A

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. Selected Flood Insurance Rate Map panels for the 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 A1 through A30 V1 through V30 B C New Zone AE VE X (shaded) X (unshaded) Part or all of this may be revised and republished at any time. In addition, part of this FIS may be revised by a Letter of Map Revision process, which does not involve republication or redistribution of the FIS. It is, therefore, the responsibility of the user to consult with community officials and to check the community repository to obtain the most current FIS report components. This FIS report was revised on September 25, User should refer to Section 10.0, Revision Descriptions, for further information. Section 10.0 is intended to present the most up-to-date information for specific portions of this FIS report. Therefore, users of this FIS report should be aware that the information presented in Section 10.0 supersedes information in Sections 1.0 through 9.0 of this FIS report

3 TABLE OF CONTENTS Page 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgements 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 APPLICATION FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES REVISION DESCRIPTION First Revision (Effective March 1998) Countywide Study (Effective September 25, 2009) 45 FIGURES Figure 1 Floodway Schematic 20 i

4 TABLES Table 1 Initial, Intermediate, and Final CCO Meetings 2 Table 2 Flooding Sources Studied by Detailed Methods 3 Table 3 Flooding Sources Studied by Approximate Methods 4 Table 4 Summary of Discharges 11 Table 5 Range of Manning s Roughness Values 13 Table 6 Summary of Elevations 16 Table 7 Floodway Data 21 Table 8 Flood Insurance Zones Within Each Community 33 Table 9 Community Map History 35 Table 10 Revised Study Descriptions 45 EXHIBITS Exhibit 1 Flood Profiles Chetco River Panels 01P-05P Elk River Panels 06P-19P Hunter Creek Panels 20P-25P Pistol River Panels 26P-30P Rogue River Panels 31P-39P Rogue River at Agness Panel 40P Sixes River Panels 41P-42P Winchuck River Panels 43P-45P Flood Insurance Rate Map Index Flood Insurance Rate Map PUBLISHED SEPARATELY ii

5 1.0 INTRODUCTION 1.1 Purpose of Study FLOOD INSURANCE STUDY CURRY COUNTY AND INCORPORATED AREAS This Flood Insurance Study revises and updates information on the existence and severity of flood hazards in the geographic area of Curry County, including the Cities of Brookings, Gold Beach, and Port Orford; and the unincorporated areas of Curry County (referred to collectively herein as Curry 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. 1.2 Authority and Acknowledgments The sources of authority for this Flood Insurance Study are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of The hydrologic and hydraulic analyses for the Elk River, Pistol River, Chetco River, Winchuck River, and Hunter Creek were performed by the U.S. Soil Conservation Service (SCS) for Curry County. These studies were completed at various times from 1979 to The hydrologic and hydraulic analyses for the Rogue River, Sixes River, Pacific Ocean, and the conversion of the SCS analyses for this study were performed by CH2M HILL NORTHWEST, INC. This study, completed in October 1981, was performed for the Federal Emergency Management Agency (FEMA) under Contract No. EMW-C Approximate flood boundaries for the unincorporated areas of Curry County were determined in 1977 by SCS, the Curry County Planning Department, the Oregon Department of Geology and Mineral Industries, Southern Oregon College, and private consultants, under contract to the Federal Insurance Administration. Coastal flood information was revised in February and March of 1998 for portions of the City of Gold Beach and unincorporated Curry County just north of Gold Beach. The portion of the analysis within the City of Gold Beach extended from Cunniff Creek at the south end of the Curry County Fairgrounds to the southern city limit of the City of Gold Beach. The portion of the analysis within unincorporated Curry County is a 4,300-foot reach along Rogue Shores. These analyses were performed by CH2M Hill, Inc. under FEMA Contract No. EMW-94-C-4526 and were completed in October and December 1995, respectively. The countywide update was performed by WEST Consultants, Inc. for FEMA under Contract No. EMS CO The update was completed in August

6 1.3 Coordination The dates of the initial, intermediate, and final CCO meetings held for the previous FIS reports for Curry County and the incorporated communities within its boundaries are shown in Table 1, Initial, Intermediate, and Final CCO Meetings. They were attended by representatives of FEMA, the communities, and the study contractor. Table 1. Initial, Intermediate, and Final CCO Meetings Community Initial CCO Date Interim CCO Date Final CCO Date Brookings, City of May 1979 October 27, 1982 July 19, 1983 Curry County, unincorporated areas May 1979 October 27, 1982 July 20, 1983 Gold Beach, City of May 1979 October 27, 1982 July 20, 1983 Port Orford, City of N/A N/A N/A Streams and coastal areas requiring detailed study were identified at a meeting attended by representatives of the study contractor, FEMA, and representatives of Curry County in May Results of the hydrologic analyses were coordinated with SCS, the U.S. Geological Survey (USGS), and the U.S. Army Corps of Engineers (USACE). An intermediate coordination meeting was held on October 27, 1982, and was attended by representatives of FEMA, the study contractor, and Curry County. No problems or questions were raised at that time. On July 19 and 20, 1983, the results of the study were reviewed at the final meeting attended by representatives of the study contractor, FEMA, and community officials. The study was acceptable to the community with one exception. The City of Brookings requested the elimination of the approximate flooding on Ransom Creek. This request was denied by FEMA. The results of the 1998 coastal revision performed by CH2M Hill, Inc. were reviewed at a final Consultation Coordination Office meeting held on February 12, 1997 which was attended by representatives of FEMA, the City of Gold Beach, Curry County, and the study contractor. All problems raised at the meeting were addressed in the restudy. Countywide Update An initial community coordination meeting for Curry County was held on March 15, This meeting was attended by representatives of the City of Port Orford, Curry County, State of Oregon, FEMA and WEST Consultants. The results of the study were reviewed at the final Consultation Coordination Officer [CCO] meeting held on November 6, 2008, and attended by representatives of FEMA, the Oregon Department of Land Conservation and Development, Curry County, the City of Brookings and the City of Port Orford. All problems raised at that meeting have been addressed in this study. 2.0 AREA STUDIED 2.1 Scope of Study This Flood Insurance Study covers the geographic area of Curry County, Oregon, including the incorporated communities listed in Section

7 The flooding sources studied by detailed methods in Curry County were selected with priority given to all known flood hazards and areas of projected development or proposed construction through October The limits of detailed studies in Curry County were determined by FEMA with community and study contractor consultation at the meeting in May Table 2 lists the flooding sources studied in detail and the included segments. Table 2. Flooding Sources Studied by Detailed Methods Flooding Source Chetco River Elk River Hunter Creek Pacific Ocean Pistol River Rogue River Sixes River Winchuck River Limits of Detailed Study From 100 feet downstream of the U.S. Highway 101 Bridge to Elk Creek (River Mile 0.8 to 10.7) From its mouth at the Pacific Ocean to Anvil Creek (River Mile 0.0 to 13.5) From its mouth at the Pacific Ocean to Conn Creek (River Mile 0.0 to 7.0) From Otter Point State Park to the mouth of the Rogue River (2.5 miles); from the southern corporate limit of Gold Beach to the mouth of Hunter Creek (0.5 mile); and from the mouth of the Winchuck River to the Oregon-California Border (0.5 mile) From the U.S. Highway 101 Bridge to a point 1.5 miles downstream from Glade Creek (River Mile 0.5 to 4.5) From its mouth at the Pacific Ocean to Libby Creek (River Mile 0.0 to 10.0) and 1 mile at Agness (River Mile 27.5 to 28.5) From Summers Creek to a point 0.75 mile upstream of U.S. Highway 101 (River Mile 4.6 to 6.1) From its mouth at the Pacific Ocean to Salmon Creek (River Mile 0.0 to 9.5) Approximate analyses were used to study flooding sources in areas having a low development potential or minimal flood hazards. These analyses were adopted from previously effective flood hazard boundary maps (Reference 1). Table 3 lists the flooding sources, grouped by watershed, which were studied by approximate methods. 3

8 Table 3. Flooding Sources Studied by Approximate Methods 1. Chetco River, North Fork Chetco River, Jack Creek, Dry Creek, Big Emily Creek 2. Elk River 3. Euchre Creek, Cedar Creek, Miller Creek, Boulder Creek 4. Hubbard Creek, North Fork Hubbard Creek 5. New River, New Lake, Floras Creek, Willow Creek, Floras Lake 6. Pacific Ocean 7. Pistol River, Deep Creek 8. Ransom Creek, Riley Creek 9. Rogue River, Lobster Creek, Illinois River, Lawson Creek, Shasta Costa Creek, Lone Tree Creek, Scott Creek, Foster Creek, 10. Sixes River, Crystal Creek 2.2 Community Description Curry County is located in the southwest corner of the State of Oregon. It is bounded on the west by the Pacific Ocean, on the south by Del Norte County, California, on the east by Josephine County, and on the north by Coos and Douglas Counties. Varying in width from 14 to 34 miles and with an approximately 67-mile length, the county has an area on 1,629 square miles. The eastern two-thirds of the county lie in the Siskiyou National Forest. Curry County s population was estimated to be 17,100 in 1980, of which 10,920 people lived in the unincorporated areas and the remaining 6,180 lived in the incorporated cities of Port Orford, Gold Beach, and Brookings (Reference 2). The estimated population in 2000 was 21,137 (Reference 3). Curry County s economy is based on the wood products industry, agriculture, commercial fishing, and tourism. The main streams draining the area in the northern portion of the county are Elk River, Sixes River, and Floras Creek. Pistol River, Hunter Creek, Rogue River, and Euchre Creek drain the central area and the Chetco and Winchuck Rivers drain in the southern portion. The streams flow in a westerly direction to the Pacific Ocean. Development within the flood plains studied is mostly agricultural, scattered residential, and limited industrial and commercial. Curry County has a temperate marine climate and relatively wet winters and dry summers. The average winter temperature is 48 F and the average summer temperature is 59 F (Reference 4). The average annual precipitation varies from 80 inches along the coast to 120 inches in the mountains of eastern Curry County (Reference 5). Most of the precipitation occurs during the months of November through March. Summer winds are steady with moderate breezes from the north and northwest. Winter winds are normally steady breezes from the south and southeast, with occasional strong onshore gales from the southwest. Marine sedimentary rocks, predominant throughout most of the county, consist of alluvium, siltstone, mudstone, sandstone, and shale. Igneous and metamorphic rocks found in the county include basalt, diorite, peridotite, schist, and serpentine (Reference 6). The topography of Curry County is mostly rolling foothills and mountains peaking near 5,000 4

9 feet. There are some level areas along the coast and in some of the river valleys. City of Brookings The City of Brookings was incorporated in 1951 and is located in the southern portion of Curry County about 3 miles north of the Oregon-California border. The city is surrounded by unincorporated areas of Curry County to the north and east, and by the Pacific Ocean to the west and south. Most of the city is located on a bluff 80 feet above the ocean rising to 300 feet in the northeast corner. According to population estimates, the July 1980 population was 3,420 (Reference 2) and 5,447 as of the 2000 national census (Reference 3). The economy depends primarily on the wood products industry, fishing, and tourism. The Chetco River, which forms the eastern corporate limits, is one of the principal rivers in southern Oregon. Much of the 352 square mile drainage area of Chetco River is located within the Siskiyou National Forest. The approximate length of the Chetco River is 66 miles (Reference 7). Ransom Creek, which flows through the northern part of the city, has a drainage area of 0.74 square mile and a length of approximately 2 miles. The total land area within the corporate limits of Brookings is approximately 3.6 square miles. Development in the flood plain is predominantly commercial and industrial. Harbor facilities and the airport are operated by the Port of Brookings. U.S. Highway 101 is the only major highway serving the Brookings area. The City of Brookings has a temperate marine climate, with temperatures ranging from an average of 48 F in winter and 59 F in summer. Rainfall averages 75 inches per year with more than 10 inches per month in the period of November through February (Reference 8). The soils in the study area are Orford silty clay loam, Gardiner fine sandy loam, and Nehalem silty loam. Natural forest vegetation includes willow, red alder, Oregon myrtle wood, maple, spruce, Douglas fir, and various shrubs and grasses (Reference 7). City of Gold Beach The City of Gold Beach is located on the southern coast of Oregon, on the southern bank of the Rogue River at its mouth on the Pacific Ocean. The city was incorporated in 1951 and lies approximately 43 miles north of the Oregon-California border and 78 miles south of Coos Bay in the center of Curry County. It is surrounded by unincorporated areas of Curry County to the north, east, and south, and by the Pacific Ocean to the west. The elevation varies from less than 10 feet along the coast to more than 300 feet in the hills on the eastern corporate limits. The population of Gold Beach was estimated to be 1,690 in 1980 (Reference 2) and 1,897 as of the 2000 national census (Reference 3). The economy depends primarily on tourism, the wood products industry, and commercial fishing The Rogue River, which forms the northern corporate limit of Gold Beach, flows westerly to the Pacific Ocean. The Rogue River is one of the principle rivers in southern Oregon and has a drainage area of 5,170 square miles at its mouth on the Pacific Ocean. The total land area within the corporate limits of Gold Beach is 2.5 square miles, 31 percent 5

10 of which is dune land along the Pacific Ocean (Reference 9). Development in the flood plain consists of a number of residential, commercial, and industrial structures. Harbor facilities and the airport are operated by the Port of Gold Beach. U.S. Highway 101 is the only major highway serving the Gold Beach area. The City of Gold Beach has a temperate marine climate. The average summer temperature is 58 F, and the average winter temperature is 48 F. Average annual precipitation is 74 inches, with an average of over 10 inches per month in the period of November through March (Reference 8). Most of the city is located on a marine terrace consisting of indistinctly bedded sand, silt, and clay. Along the beach, there are areas of both stable and active dunes, which are mainly fine sand (Reference 2). Vegetation of the area includes Sitka spruce, shore pine, manzanita, scotchbroom, evergreen huckleberry, dune grass, and forbs. City of Port Orford The City of Port Orford is located approximately 14 miles south of the Curry-Coos County border. It was incorporated in 1935 and had an estimated population of 1,153 as of the 2000 census (Reference 3). The majority of Port Orford sits atop a bluff overlooking the Pacific Ocean. U.S. Highway 101 is the only major highway serving the city. The only sources of flooding for the city are the Pacific Ocean and Garrison Lake which forms a natural coastal lagoon. These were studied by approximate methods and are designated as Zone A. 2.3 Principal Flood Problems Riverine flooding in Curry County usually occurs during the months of November through February. Minor flooding occurs annually in some areas, but produces very little damage. The major damaging floods are the result of winter storms producing a sustained intense rainfall which, when combine with higher than normal tides, increase the damaging effects in the lower reaches of the rivers. In the Rogue River Basin, this intense rainfall is sometimes augmented by snowmelt in the Cascade Mountains. Severe flooding occurs at about 10-year intervals, usually resulting from several days of sustained heavy rainfall and is sometimes augmented by snowmelt in the Cascade Mountain Range. Flooding lasts for 2 to 3 days with peak stages lasting for only a few hours (Reference 10). Ocean flooding is the result of higher than normal sea levels resulting from storms or a seismic disturbance on the ocean floor. Storms during the months of November through February produce the storm surge and wind generated waves which combine with the astronomical tide to cause the most frequent and serious flooding. Seismic sea waves or tsunamis, which can occur at any time during the year, are the most destructive type of ocean flooding. Physical characteristics of the continental shelf and the shoreline affect all types of waves, focusing wave energy at some locations and dissipating energy in other areas. The flood of December 1964 on the Rogue River equaled or exceeded the historic floods of record of the 1800 s. The USACE estimated the peak flow of the Rogue River at Gold Beach during this flood to be 500,000 cubic feet per second (cfs) (Reference 11). The estimated peak flow was revised to 550,000 cfs, with a return period of approximately 130 years. In the vicinity of Gold Beach, 24 commercial enterprises were heavily damaged, some with complete loss of structures; high velocities at the mouth scoured the river bottom to -40 feet 6

11 NGVD causing over $500,000 damage to the North Jetty (Reference 11). January 1974 flooding caused minor damage to the Port of Gold Beach facilities. No estimate was made of the peak flow in 1974, but based on data in the upstream reaches the flood had a recurrence interval of approximately 15 years (Reference 12). Flooding in January 1997 was comparable in magnitude to the flooding of January The largest flood of recent record on the Chetco River occurred in December The peak discharge was 85,400 cfs with an estimated return interval of 150 years (Reference 7). This discharge was estimated by the SCS for the USGS gaging site located approximately 0.6 miles upstream of Elk Creek. Damages resulting from this flood were approximately $1.5 million. The storm of 1964 was a long duration, and the Chetco River remained high for several days. The combination of high tides and the intense storm produced the record flood in the Brookings area. Most of the city is located on a high bluff and areas within the floodplain were undeveloped, so only minor damage was caused by this flood. The U.S. Highway 101 Bridge was declared unsafe because of log debris. After 1964, the bridge was replaced at a higher elevation. Due to the steep terrain in the Chetco River watershed, the water can rise rapidly. The flood duration is relatively short, usually less than two days. Occasionally, during an intense, long duration rainfall, the flood duration will be longer. The highest discharge floods are usually associated with frozen ground, an accumulation of snow at the higher elevations, and a warming trend with rainstorms. Other notable floods on the Chetco River occurred in December 1861, February 1890, December 1955, December 1965, January 1971, and January The return interval for the 1955 and 1971 floods is approximately 30 years. No estimate was made of the peak flow for the January 1974 flood, but based on data in the upstream reaches the flood had a recurrence interval of approximately 15 years (Reference 12). The most recent significant flooding event occurred in November On Hunter Creek, the flood of record was the December 1964 flood. Several areas experienced extremely deep flooding, but very little damage was reported (Reference 13). The December 1964 flood was the largest of recent record on the Winchuck River, but there were no damage reports (Reference 14). The largest flood of recent record on Elk River and Pistol River occurred in December Extensive flooding resulted from several days of heavy rainfall, but there were no reports of residences being flooded (References 15 and 16). Flooding from the Pacific Ocean also occurred in A tsunami brought 9-foot waves into the mouth of the Rogue River, causing $40,000 worth of damage to the businesses along the banks (Reference 11). No significant damage resulted from the tsunami within the City of Gold Beach. 2.4 Flood Protection Measures The Rogue River has three flood control reservoirs located in the upper reaches of the watershed. Lost Creek and Applegate Reservoirs were constructed by USACE. Available storage for flood control in Lost Creek Reservoir is 315,000 acre-feet and in Applegate Reservoir is 75,000 acre-feet (Reference 17). Emigrant Reservoir, constructed by the U.S. 7

12 Bureau of Reclamation, is located in the upper reaches of the Applegate River approximately 110 miles upstream of Agness and has 20,000 acre-feet of usable storage (Reference 18). Both Emigrant Reservoir and Lost Creek Reservoir are located approximately 120 miles upstream of Agness in the Cascade Mountain Range. The Rogue River flood flows in Curry County are not significantly affected by the reservoirs because of the small percentage of the total drainage basin controlled and the overall rainfall patterns in the Rogue River Basin. Various streams have levees and rock riprap protection, but these measures do not affect the 100-year flood elevations. There is a breakwater on the south side of the Rogue River which forms a barrier between the main river flow and the Port of Gold Beach harbor area. This breakwater does not provide flood protection for the northern portion of Gold Beach. Curry County is currently in the emergency flood insurance program as administered by FEMA. Through the use of flood plain ordinances, development is regulated in identified flood hazard areas. The National Weather Service (NWS) in Medford, Oregon, issues flood warnings when forecasts indicate near bank-full stages for several rivers in southwestern Oregon. The flood warnings are issued at regular intervals until the streams recede below bank full. NWS maintains close cooperation with State, county, and city authorities, Federal agencies, civil defense officials, private and public utilities, and the news media. The Portland Weather Forecast Office issues storm tide warnings indicating expected tidal stages above mean lower low water or departure from normal high tide, degree of flooding, possible wave or surf battering, and significant beach erosion. The National Weather Service prepares warnings and advisories of tsunamis. The local officials have the responsibility for advising the local population. 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 100-year flood (1-percent chance of annual exceedence) 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. 8

13 For the Chetco River, Elk River, Pistol River, Winchuck River, and Hunter Creek, two sets of the various recurrence interval flows were considered. One set of flows was determined by a regional analysis of gaged stream basins similar to the study basins. The analyses were conducted by the standard Log-Pearson Type III Method as outlined by the Water Resources Council (Reference 19). The second set of flows was obtained using the SCS TR-20 Flood Routing Computer Program (Reference 20). The two sets of flows were then compared. The flows calculated by TR-20 were determined to be more representative of actual conditions and were, therefore, used in the studies of the Chetco River, Hunter Creek, Winchuck River, Elk River, and Pistol River (References 7, 13, 14, 15, and 16). The flows for the Sixes River were determined using the method for ungaged streams as outlined by the USGS Open-File Report (Reference 21), because the period of discharge records at the time of the analysis covered only 3 years. The method is based on regression analysis relating drainage area, area of lakes and ponds, forest cover, and precipitation intensity to peak discharges by empirical equations. For the detailed study of the Rogue River, the discharges were based on statistical analysis of flow records covering an 18-year period at the Agness gaging station (No ) operated by USGS (References 22, 23, and 24). This analysis followed the standard Log- Pearson Type III Method as outlined by the Water Resources Council (Reference 19). The discharges from the gage analysis were transferred downstream to the study areas on the Rogue River using a ratio of drainage areas: Q u = Q g (A u /A g ) n Where Q g and A g are the discharge and drainage area at the gage, Q u and A u are the discharge and drainage area at the ungaged location, and the exponent n is determined using a log/log plot of the runoff per square mile of drainage basin for all gaged streams in the south coast region of Oregon. The value determined for the exponent was Drainage areas at various points in the detailed study areas were measured using USGS topographic maps (Reference 9). Based on this analysis, the December 1964 flood had a 10-year reoccurrence interval at the Agness gage. Regionally, this flood equaled or exceeded a 100-year event, so additional analysis was performed to determine the reasonableness of the estimated peak discharge. The additional analysis included comparison of high water marks and estimated flow of the 1964 flood to the predicted gage flows transferred downstream. The recorded flows were also reanalyzed with various modifications because the gage is affected by backwater from the Illinois River at high flow. The methodology developed by CH2M HILL for the study of Pacific Northwest storms was used to study the coastal flooding in Curry County. This method involves statistical analysis of the various components of ocean flooding caused by storms and a combined probability analysis to determine the effect of each component of flood levels. Predicted astronomical tides were calculated on an hourly basis for the study areas based on the National Oceanic and Atmospheric Administration Tide Tables (Reference 25). The hourly predicted tides were used to compute the astronomical tide height histogram 9

14 (Reference 26). Significant storm surge-producing events were selected from 3-hour surface weather maps for the period 1942 to 1980 (Reference 27). The storm surge heights were computed for these events and grouped into three wind direction classes. Storm-surge frequency distributions were computed from a population of the highest storm surges for each class. A wave forecasting computer program was used to compute wind-generated wave height (Reference 28). The program uses wind speed, direction, and fetch data from the surface weather maps to compute significant wave height and period at 6-hour intervals. Frequency curves were plotted for the three wind direction classes of both sea wave and swell wave height values. Hydrology was updated for 1998 revisions of coastal flooding near Gold Beach. The stillwater level (SWL) was determined using 51 years of annual maximum-observed tidal data from a coastal tide station at Crescent City, Oregon, with a comparison of coinciding tides at Charleston, Oregon, from 1971 to The storm-tide elevations at both stations are very similar once a tide-correction factor of 1.10 is applied. The revised area is located between the two stations and experiences essentially the same storm tides as those measured at Crescent City. The USACE Automated Coastal Engineering System program Extremal Significant Wave Height Analysis (Reference 29) was used to determine the return-period stillwater values from the storm-tide elevations. Peak discharge-drainage area relationships for flooding sources studied in detail in Curry County are shown in Table 4. 10

15 Table 4. Summary of Discharges Flooding Source and Location Drainage Area (Square Miles) 10-percentannual-chance Peak Discharges (cfs) 2-percentannual-chance 1-percentannual chance 0.2-percentannual chance Chetco River Mouth on Pacific Ocean ,400 84,100 94, ,100 Upstream of Jack Creek ,600 81,600 91, ,600 Upstream of North Fork Chetco River ,200 74,200 83, ,300 Upstream of Big Emily Creek ,700 70,800 79,600 99,500 Elk River Mouth on Pacific Ocean ,800 22,100 25,100 31,900 Upstream of River Mile ,400 21,500 24,400 31,000 Upstream of River Mile ,900 20,900 23,700 30,100 Hunter Creek Mouth on Pacific Ocean ,200 18,200 20,200 24,900 Upstream of Swinging Bridge Road ,300 17,100 19,000 23,400 Pistol River Mouth on Pacific Ocean ,800 35,300 39,700 47,600 Upstream of Crook Creek ,300 34,700 39,000 46,800 Upstream of River Mile ,000 34,300 38,500 46,200 Upstream of River Mile ,600 33,800 38,000 45,600 Rogue River Mouth of Pacific Ocean 5, , , , ,000 Downstream of Lobster Creek 5, , , , ,000 Rogue River at Agness Downstream of Illinois River 4, , , , ,100 Rogue River near Agness (Gage No ) 3, , , , ,000 11

16 Flooding Source and Location Table 4. Summary of Discharges (continued) Drainage Area (Square Miles) 10-percentannual-chance Peak Discharges (cfs) 2-percentannual-chance 1-percentannual chance 0.2-percentannual chance Sixes River Downstream of Crystal Creek ,600 26,700 29,100 35,200 Downstream of Highway ,300 23,800 26,200 31,400 Winchuck River Mouth on Pacific Ocean ,700 28,000 31,800 41,700 Upstream of South Fork Winchuck River ,600 25,000 28,400 37,200 Upstream of Moser Creek ,800 23,800 27,100 35,500 Upstream of Deer Creek ,200 23,100 26,200 34,300 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 Flood Insurance Rate Map (FIRM) represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data tables 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 sections for the backwater analyses were obtained by field measurement. Cross sections used in the hydraulic analysis of the Rogue River were surveyed by CH2M Hill Northwest, Inc., in July 1978, for a USACE sedimentation study. Hydraulic structures were field checked to obtain elevation data and 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. Channel roughness factors (Manning s n ) used in the hydraulic computations were chosen by engineering judgment based on field observations of the streams and flood plain areas. The range of roughness values used for all floods is shown in Table 5. The acceptability of all assumed data was checked by computations that duplicated historic floodwater profiles. Table 5. Range of Manning s Roughness Values Flood Source Main Channel Flood Plain Chetco River Elk River Hunter Creek Pistol River Rogue River Sixes River Winchuck River Water-surface flood elevations of the selected recurrence intervals for the Rogue River and Sixes River were computed using of the USACE HEC-2 step-backwater computer program (Reference 30). Water-surface elevations for the Chetco River, Hunter Creek, Winchuck River, Elk River, and Pistol River were computed for the flood hazard studies in Curry County conducted by the SCS (References 7, 13, 14, 15, and 16). The elevations in the SCS studies were determined using the WSP-2 step-backwater computer program (Reference 31). Starting water surface elevations for the Sixes River and Rogue River were calculated using the slope-area method. The slope was determined from high-water marks set by the historic flood of December For the Winchuck River, Hunter Creek, Pistol River, and Chetco River, starting water-surface elevations were at critical depth for all floods. The starting water surface elevations for the Elk River were one or two feet above critical depth based on high water marks in the lower portion of the watershed. In areas of both riverine and ocean flooding, the higher elevation was plotted on the flood profiles. Flood profiles were drawn showing computed water-surface elevations for floods of the selected recurrence intervals. 13

18 The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. Stillwater levels along the coastline of Curry County were computed by combining the astronomical tide heights and storm surge heights through use of the computer program COAST (Reference 32). This program is a modification of the NWS Program SPLASH to accommodate Pacific Northwest storm types (References 33, 34, and 35). The COAST program includes offshore water depths, atmospheric pressure, pressure gradient fields, and other parameters obtained from Jelesnianski (Reference 36). The parameters were adjusted to calibrate the program to observed winds and high-water marks. Pressure fields from representative surge-producing storms were input to the calibrated model to calculate storm surge heights in the study area. Height-frequency relationships were calculated from the storm surge heights for three wind direction classes. The astronomical tide and storm surges were combined by a procedure similar to that employed to combine tsunamis and astronomical tides (References 37 and 38). A cumulative histogram was computed for four surge heights, and the probability of occurrence was multiplied by the surge probability. An enveloping curve was drawn to produce the stillwater probability curve for each of the three wind direction classes. The curves for the three wind direction classes were combined to obtain the stillwater levels for the study areas. Waves of various recurrence intervals were tracked from deep-water to shore using the wave refraction and shoaling computer program WAVES 2, which is a modified version of the program WAVES (Reference 39). Data required for this program, which were obtained from USACE, are ocean-bottom topography, wave height, period, direction, and starting location (Reference 40). Wave setup and wave runup were calculated for sea waves as specified in the USACE Shore Protection Manual (Reference 41). The effective beach slope values used in the runup computations were obtained by matching surge and wave projections to open coast highwater marks. The values of wave runup for certain recurrence intervals were added to the stillwater levels to obtain the combined sea-surge/tide flood elevation/frequency curves. The swell/tide curves were statistically combined with the sea/surge/tide curves to obtain the open coast flood elevation-frequency curves for the study areas. Tsunami and storm flood events were considered to be independent events because tsunami waves can occur at any time during the year and storm waves are seasonal. Initial calculations to statistically combine these two events show an adjustment of less than 0.5 feet to the flood elevation. Because of the uncertainties involved in combining these events, this adjustment was not made. Instead, the open coast flood levels were compared to the tsunami flood levels determined by USACE (Reference 42) and the higher of the two elevations were used for this study. The results of the coastal analyses are summarized in Table 6. Coastal flood information was revised in February and March of 1998 for portions of the City of Gold Beach and unincorporated Curry County just north of Gold Beach. The portion of the analysis within the City of Gold Beach extended from Cunniff Creek at the south end of the Curry County Fairgrounds to the southern city limit of the City of Gold Beach. The portion of the analysis within unincorporated Curry County was a 4,300-foot reach along Rogue Shores. 14

19 Wave data were taken from information published by the USACE, Coastal Engineering Center, in the report entitled Wave Information Study 17-Pacific Coast Hindcast Phase III North Wave Information (Reference 43). These data were used to determine the significant wave heights, which were then related to deepwater unrefracted wave heights using USACE procedures. Average wave heights and periods were determined from the deepwater unrefracted wave heights. Using these data and the FEMA Runup Program, runup heights were developed. The 100-year runup elevation was determined to occur from either a combination of the mean runup height from a 2-year wave and the 50-year SWL or the mean runup height from a 10- year wave and the 10-year SWL (depending on the transect location). Based on observations of drift-log and debris deposition behind the dunes, it was determined that the 100-year mean runup elevations (which were all well below the dune crest or foreshore crest) did not represent the actual flood elevations. Therefore, the mean runup was related to the maximum runup through a series of regression equations. The maximum runup was used for mapping purposes. The effects of tsunamis were also analyzed and were determined to be less severe than the maximum wave-runup elevation due to a coastal storm without the tsunami effects. A field survey was performed between June and July 1994 to obtain elevation data for the seven beach transects used in this study. Transects were established to be representative of the topographic conditions along a segment. Mapping of the flood zones was done according to the procedures discussed in FEMA s Guidelines and Specifications for Wave Elevation Determination and V Zone Mapping (Reference 44). A Letter of Map Revision issued on December 4, 1995, for an area along the east bank of the Rogue River from just downstream of Saunders Creek to an unnamed tributary of the Rogue River was incorporated as part of this revision. That revision was based on surveyed information and was used to correct a mapping error. 15

20 Flooding Source and Location Table 6. Summary of Elevations Elevation (Feet, NAVD 88) 2-percentannual-chance 10-percentannual-chance 1-percentannual-chance 0.2-percentannual-chance Pacific Ocean Rogue Shores 9.4 / / / / Hunter Creek 12.5 / / / / 23.8 Winchuck River 12.8 / / / / Stillwater elevation / elevations including maximum wave runup Countywide vertical conversion from NGVD29 to NAVD88 is ft. 16

21 Approximate elevations for flooding from the Pacific Ocean were estimated based on the detailed studies for unincorporated areas of Curry County north and south of the corporate limits of Gold Beach (Reference 45). 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 used for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD 29). With the completion of the North American Vertical Datum of 1988 (NAVD 88), many FIS reports and FIRMs are now prepared using NAVD 88 as the referenced vertical datum. Flood elevations shown in this FIS report and on the FIRMs are referenced to NAVD 88. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. For information regarding conversion between the NGVD and the NAVD, visit the National Geodetic Survey website at or contact the National Geodetic Survey at the following address: NGS Information Services NOAA, N/NGS12 National Geodetic Survey SSMC-3, # East-West Highway Silver Spring, Maryland (301) (301) (fax) The conversion factor from NGVD to NAVD for all flooding sources in this report is feet. 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 the FIRMs 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 the FIRMs, please contact 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. To assist in this endeavor, each FIS report provides 1-percent annual-chance floodplain data, which may include a combination of the following: 10-, 2-,1-, and 0.2-percent-annual-chance flood elevations; delineations of the 1-percent-annual-chance and 0.2-percent-annual-chance floodplains; and 1-percent-annual-chance floodway. This information is presented on the FIRM and in many 17

22 components of the FIS report, including Flood Profiles, Floodway Data tables and Summary of Stillwater Elevation tables. Users should reference the data presented in the FIS report as well as additional information that may be available at the local community 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 (100-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent-annual-chance (500-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, the 1- and 0.2-percent-annual-chance floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at scales of 1: 4,800 and 1:12,000 (References 7, 13, 14, 15, 16, and 46). For streams studied by approximate methods, the boundary of the 1-percent-annual-chance flood was taken from a USGS Flood-Prone Area Map (Reference 47), Special Flood Hazard Area Maps (Reference 48), Geologic Hazard Maps (Reference 5), field investigations, and engineering judgment. Approximate delineations made by the Curry County Planning Department in conjunction with the U.S. Soil Conservation Service, the Oregon Department of Geology and Mineral Industries, Southern Oregon State College, and private consultants were incorporated into the approximate 1-percent-annual-chance flood boundaries. These approximate flood boundaries were adopted from a previously effective Flood Insurance Study (FIS) for Curry County (Reference 1). For Ransom Creek, studied by approximate methods, the boundary of the 1-percent-annualchance flood was developed from normal depth calculations, field inspection, and a topographic map at a scale of 1:62,500 with a contour interval of 80 feet (Reference 9). Some flood hazard areas on the Flood Hazard Boundary Map (Reference 49) are not shown correctly and therefore were either revised or determined to be areas of minimal flooding (unshaded Zone X) for this study. In the coastal study areas, the boundaries of the 1- and 0.2-percent-annual-chance flood elevations were determined using topographic maps at a scale of 1:4,800 with a contour interval of 5 feet (Reference 46). Flood boundaries for the Pacific Ocean in areas studied by approximate methods were determined using stereo pairs of aerial photographs (Reference 50). The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the Flood Insurance Rate Map (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, AE, V, and VE), 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-annualchance floodplain boundaries are close together, only the 1-percent-annual-chance floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. For the streams studied by approximate methods, only the 1-percent-annual-chance floodplain boundary is shown on the Flood Insurance Rate Map. The approximate 1-percent- 18

23 annual-chance tidal boundaries for the Pacific Ocean were determined using information from the detailed coastal study of Curry County (Reference 45) and the topographic map (Reference 9). Countywide Update As part of the countywide update, floodplain boundaries were digitized from the effective FIRM and Floodway panels. USGS topographic maps (Reference 51) and aerial photography (Reference 52) were used to adjust floodplain and floodway boundaries where appropriate. In accordance with FEMA Procedure Memorandum 36 (Reference 53), profile baselines have been included in all areas of detailed study. Profile baselines are shown in the location of the original stream centerline or original profile baseline without regard to the adjusted floodplain position on the new base map. This was done to maintain the relationship of distances between cross sections along the profile baseline between the hydraulic models, flood profiles and floodway data tables. 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 100-year flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1 foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this study 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 are tabulated for selected cross sections (see Table 7). In cases where the floodway and 1-percent-annual-chance floodplain boundaries are either close together or collinear, only the floodway boundary is shown. The floodway was not computed downstream of cross section F on the Elk River or downstream of U.S. Highway 101 for all other flooding sources because of uncertainties in flow distribution due to tidal influence. 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 water-surface elevation 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. 19