MODOC COUNTY, CALIFORNIA

Transcription

1 MODOC COUNTY, CALIFORNIA AND INCORPORATED AREAS COMMUNITY NAME COMMUNITY NUMBER ALTURAS, CITY OF MODOC COUNTY (UNINCORPORATED AREAS) REVISED: PRELIMINARY: FEBRUARY 14, 2013 FLOOD INSURANCE STUDY NUMBER 06049CV000B 06021CV001A

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). Former flood hazard zone designations have been changed as follows: Old Zone A1 through A30 B C New Zone AE X X 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. This FIS report was revised on. Users should refer to Section 10.0, Revisions Description, 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 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. Initial Countywide FIS Effective Date: June 4, 2010 Revised Dates:

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 REVISIONS DESCRIPTION First Revision a. HydrologicAnalysis...30 b. Hydraulic Analysis...31 c. Bibliography and References i

4 TABLE OF CONTENTS (cont.) FIGURES Figure 1 - Floodway Schematic TABLES Table 1 Initial and Final CCO Meetings... 3 Table 2 Detailed Studied Streams... 3 Table 3 Summary of Discharges... 8 Table 4 Manning s n Value Table Table 5 List of Levees Requiring Flood Hazard Revision Table 6 Vertical Datum Conversion Table 7 Floodway Data Table 8 Community Map History Table 9 Revised Detailed Flooding Sources Table 10 Summary of Revised Discharges Exhibit 1 Flood Profiles EXHIBITS Ash Creek Bidwell Creek Butte Creek Dry Creek Pit River North Fork Pit River South Fork Pit River Panel 01P Panels 02P 03P Panel 04P Panel 05P Panel 06P Panels 06P 08P Panels 09P 10P Exhibit 2 Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY MODOC COUNTY, CALIFORNIA AND INCORPORATED AREAS 1.0 INTRODUCTION 1.1 Purpose of Study This Flood Insurance Study (FIS) revises and supersedes the FIS reports and/or Flood Insurance Rate Maps (FIRMs) in the geographic area of Modoc County, California, including the City of Alturas, and the unincorporated areas of Modoc County (hereinafter referred to collectively as Modoc 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. This information will also be used by Modoc County to update existing floodplain regulations as part of the Regular Phase of the National Flood Insurance Program (NFIP), and by local and regional planners to further promote sound land use and floodplain development. Minimum floodplain management requirements for participation in the 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 FIS are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This FIS was prepared to include all jurisdictions within Modoc County in a countywide FIS. The authority and acknowledgments prior to this countywide FIS were compiled from the previously identified FIS reports for floodprone jurisdictions within Modoc County and are shown below.

6 Modoc County (Unincorporated Areas) The hydrologic and hydraulic analyses for the original study were performed by the U.S. Army Corps of Engineers (USACE), Sacramento District, for FEMA, under Interagency Agreement No. EMW- 89-E-2994, Project Order No. 8. This study was completed in August For the first time countywide FIS, MAP-IX Mainland compiled existing data to convert the previous Modoc County FIS and FIRM into digital format. MAP-IX completed this work in May 22, 2009 under Contract No. EMF-2003-CO In addition, approximate mapping of without levee flooding was also conducted for some areas to indicate the extent of the without levee floodplains per the Code of Federal Regulations, Title 44, Section (44 CFR 65.10), titled Mapping of Areas Protected by Levee Systems. Additional guidance is provided by FEMA s Procedure Memorandum No. 34 Interim Guidance for Studies Including Levees, Procedure Memorandum No. 43 Guidelines for Identifying Provisionally Accredited Levees, and Procedure Memorandum No. 52 Guidance for Mapping Process Associated with Levee Systems. Mapping conducted within the City of Alturas was conducted by the FEMA Service Group (FSG), while other behind levee flooding was performed by MAP-IX Mainland. Base map information shown on the FIRMs was derived from multiple sources. This information was compiled from the California Spatial Information Library, 1997; U.S. Geological Survey (USGS), 1989, 1999, and 2006; National Geodetic Survey, 2002; National Atlas, 2003 and 2006; U.S. Census Bureau, 2006; FEMA, 1996 and 1999; and CADWR, 2002 and Additional Information was photogrammetrically compiled at a scale of 1:12,000 from U.S. Department of Agriculture aerial photography dated The projection used in the preparation of this map was Universal Transverse Mercator (UTM) Zone 10N. The horizontal datum was North American Datum of 1983 (NAD83), GRS80 spheroid. Differences in datum, spheroid, projection or State Plane zones used in the production of FIRMs for adjacent jurisdictions may result in slight positional differences in map features across jurisdiction boundaries. These differences do not affect the accuracy of this FIRM. 1.3 Coordination An initial Consultation Coordination Officer's (CCO) meeting was held with representatives from FEMA, the community, and the study contractor to explain the nature and purpose of a FIS, and to identify the streams to be studied by detailed methods. A final CCO meeting was held with representatives from FEMA, the community, and the study contractor to review the results of the study. The dates of the pre-countywide initial and final CCO meetings held for Modoc County and the incorporated communities within its boundaries are shown in Table 1, Initial and Final CCO Meetings. 2

7 Table 1 Initial and Final CCO Meetings COMMUNITY NAME INITIAL MEETING FINAL MEETING Alturas, City of * November 19, 1998 Modoc County (Unincorporated Areas) May 9, 1989 July 30, 1992 (1 st Revision) * (2 nd Revision) March 3, 1992 * (1 st Revision) November 19, 1998 (2 nd Revision) * Data Not Available For this countywide FIS, an initial CCO meeting was held on December 21, 2006, and was attended by representatives of the community, the study contractor, and FEMA. The final CCO meeting was held on July 13, 2009, and was attended by representatives of the community, the study contractor and FEMA. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Modoc County, California. All or portions of the flooding sources listed in Table 2, Detailed Studied Streams, were studied by detailed methods. Limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRM (Published Separately). Table 2 Detailed Studied Streams Ash Creek Bidwell Creek Butte Creek Dry Creek North Fork Pit River Approximate analyses were used to study those areas having a low development potential or minimal flooding hazards. The scope and methods of study were proposed to, and agreed upon, by FEMA and the communities. 3

8 2.2 Community Description Modoc County is located in the northeastern corner of northern California and is contiguous to the States of Oregon and Nevada. The County is bordered by Klamath and Lake Counties to the north; Washoe County to the east; and Lassen, Shasta, and Siskiyou Counties to the south, southwest, and west, respectively. The City of Alturas serves as the County seat. Reno is located approximately 180 miles to the south-southeast. Logging and mining were early endeavors in the area, and the lumber industry remains the most important present-day focus of the local economy. Ranching activities have spread out in the Big Valley region adding to the economic base, and recent impetus has been given to rejuvenating the long-dormant gold-mining industry. In 2006, the total estimated population of the County was 9,597, of which 6,802 lived in the unincorporated areas and 2,776 lived in the City of Alturas (U.S. Census Bureau, 2011). The climate in the area is characterized as high desert, with cold, wet winters and warm, dry summers. The average high and low temperatures for January are 40 F and 16 F, respectively, while for July they are 88 F and 45 F, respectively. Regional precipitation varies from approximately 8 inches in the low valley areas to approximately 35 inches in the higher mountain areas. City of Alturas The City of Alturas has been the County seat of Modoc County since its incorporation in The City is located near the northeast corner of California, approximately 39 miles south of the Oregon border, 43 miles west of the Nevada border, and 305 miles northeast of Sacramento, California. The City of Alturas is serviced by U.S. Highway 395 from the north and south and State Highway 299 from the west and east. The SPRR runs north-south and west through the City. The City has a municipal airport serving general aviation traffic. The City covers an area of approximately 24 square miles, and is located on the Modoc Plateau at an elevation of approximately 4,300 feet. The City of Alturas serves as the government and commercial center for the surrounding area. The primary economic activities in the area are agriculture, including livestock, hay and grain, and vegetable crops; outdoor recreation such as hunting and fishing; lumbering; and some mining of gravel and other minerals. Modoc County (Unincorporated Areas) The area is served by State Highways 139 and 299, several County roads, and a network of Town streets. In addition, bus service, provided by Greyhound Bus Lines, is available at the City of Alturas; Amtrak connections are available in Reno, Nevada; and the City of Alturas has a municipal airport. 4

9 The community of Adin is located in the southwestern quarter of Modoc County. Just south of the community, State Highways 139 and 299 join from the south and west, respectively, with State Highway 299 continuing in a northerly direction. The community of Adin has a public airport. The nearest communities to Adin are Indian Springs, approximately 7 miles to the north; Lookout and Bieber, approximately 10 miles to the west; Hayden Hill, approximately 15 miles to the south; and Madeline, approximately 28 miles to the east. The City of Alturas, the only incorporated community in Modoc County, is approximately 40 miles to the northeast. Adin lies at an elevation of approximately 4,200 feet at the northeastern edge of an area known as Big Valley. Just to the north and east of the community are steep, sloped hills separated by a narrow valley that traverses in a northeasterly direction, through which State Highway 299 travels. From the base of the adjacent hills, the terrain drains to the west through Big Valley carrying the combined flows of Ash, Dry, and Butte Creeks. Ash, Butte, and Dry Creeks originate in the high mountain elevations east and southeast of Adin. Dry Creek flows into Ash Creek in the northern part of Adin and Butte Creek joins Ash Creek west of the community. Ash Creek continues westerly from Adin for approximately 10 miles, flowing into the Pit River. The community of Fort Bidwell is in the northeastern corner of Modoc County, approximately 42 miles northeast of the City of Alturas. Fort Bidwell is reached from the City of Alturas by taking State Highway 299 to Cedarville and traveling 20 miles on Surprise Valley Road. The nearest communities are Lake City, approximately 15 miles south on Surprise Valley Road, and Willow Creek, approximately 20 miles west. There is a small airfield located north of Fort Bidwell. Fort Bidwell, with a population estimated to be less than 100, is the site of a 19thcentury U.S. Cavalry fort (Hoover, Rensch, Abeloe, and Kyle, 1990). The Fort Bidwell Reservation is located west of the community and is occupied by approximately 175 Northern Paiute Indians (Engle, 1992). Fort Bidwell lies at an elevation of approximately 4,500 feet. The surrounding area is agricultural, primarily irrigated pasture and hay and grain lands. There are some mining, hunting, fishing, and other outdoor recreation activities in the Warner Mountains to the north and west of Fort Bidwell. 2.3 Principal Flood Problems Flooding in the area can occur any time from fall to spring as a result of the occurrence of general rainstorms. General rain floods result from prolonged, heavy rainfall over tributary areas and are characterized by high peak flows and moderate duration and a large volume of runoff. Flooding is more severe when antecedent rainfall has resulted in saturated ground conditions, when the ground is 5

10 frozen and infiltration is minimal, or when rain on snow in the higher elevations adds snowmelt to rain flood runoff. No significant flood problems have been experienced in the City of Alturas since the completion of the USACE channel modification and levee improvement project in Flood Protection Measures City of Alturas Flood protection measures for the area have been constructed by the USACE. The last major project, which was completed in 1972, included the following (USACE, April 1972): Channel excavation from approximately 4,400 feet downstream of the east-west SPRR spur track to approximately 500 feet upstream of the SPRR north-south main track. The total project was approximately 1.9 stream miles. New levee constructed and levee improvements at several locations throughout the project area, including the construction of a wing levee upstream of the SPRR north-south main track. The total project was approximately 1.9 stream miles. Channel erosion prevention facilities included riprap and sacked concrete slope protection at numerous locations throughout the project. This flood control project is maintained by Modoc County under an agreement with the USACE. The maintenance is performed in accordance with an Operation and Maintenance Manual prepared by the USACE (USACE, February 1973). These activities include channel debris and vegetation removal and maintenance and repair of slope protection and levees. The USACE periodically inspects the project facilities and provides the County with direction regarding maintenance and repair. These inspections are to ensure that the project continues to comply with the USACE standard. During January 1995, a significant storm event was experienced in the area. No flooding occurred; however, there was some channel erosion, sediment deposition, and slope protection damage in the project area. As a result, the USACE performed a rehabilitation project under Public Law 84-99, which included sediment removal and the repair of eroded areas and slope protection. This work was completed in 1996 (USACE, August 1995 and USACE, July 1996). 6

11 The levees associated with this flood control project are no longer accredited by FEMA as providing protection against the 1-percent-annual-chance flood. Modoc County (Unincorporated Areas) Tule Lake affords some degree of manmade storage near the Siskiyou County line. Effects on the 1- and 0.2-percent-annual-chance flood events from this storage lake cannot be determined though it does reduce discharge volumes downstream. There are numerous levee systems in Modoc County; however none of them are accredited by FEMA as providing protection against the 1-percent-annual-chance flood. 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-annual-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 the peak discharge frequency relationships for floods of the selected recurrence intervals for each flooding source studied in detail affecting the county. Peak discharge-drainage area relationships for the 10-, 2-, 1-, and 0.2-percentannual-chance floods for each stream studied by detailed methods are presented in Table 3, Summary of Discharges. For each community within Modoc County that had a previously printed FIS report, the hydrologic analyses described in those reports have been compiled and are summarized below. 7

12 FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) Table 3 Summary of Discharges PEAK DISCHARGES (cfs) 10%- 2%- 1%- ANNUAL- ANNUAL- ANNUAL- CHANCE CHANCE CHANCE 0.2%- ANNUAL- CHANCE ASH CREEK At State Highway 299 Bridge * * 5300 * BIDWELL CREEK At Main street (Surprise Valley Road) 29.2 * * 715 * BUTTE CREEK At State Highway 299 Bridge 40.3 * * 1700 * DRY CREEK At State Highway 299 Bridge 8.3 * * 580 * * Data not available Modoc County (Unincorporated Areas) Procedures used during these analyses included the unit-hydrograph method and available streamflow-data analyses and the HEC-1 computer program (USACE, September 1981). Flood hydrographs and peak flows for the 1-percent-annyal-chance floods on Ash, Butte, and Dry Creeks were computed from a 24-hour general rainstorm. The general rainflood peak flow for Ash Creek was based on an analysis of available streamflow records and a hydrologic analysis of the drainage basin using the HEC-1 computer program. There are no streamflow records for Dry and Butte Creeks; therefore, the peak flows for those two streams were derived from hydrologic analyses of those basins using the HEC-1 computer program. The HEC-1 computer program utilizes rainfall, loss rates, unit hydrographs, base flow, and routing criteria as input. Rainfall computations were based on precipitation-frequency data and areal reduction factors derived from the National Oceanic and Atmospheric Administration (NOAA) Atlas 2 for California (NOAA, 1973). Rainfall for the 1-8

13 percent-annyal-chance, 24-hour general rainstorm over the three drainage basins is: Dry Creek at State Highway 299 culvert inches, Butte Creek at State Highway 299 bridge-3.35 inches, and Ash Creek at State Highway 299 bridge inches. Two 24-hour storm distributions were used for this study-one for Dry and Butte Creeks (approximately 8.3 and 40.3 square miles, respectively) and one for Ash Creek (258 square miles). The distributions were derived from the 96- hour standard-project storm distributions for areas between 1 and 100 square miles and 101 and 500 square miles. Loss rates were based on the initial and constant infiltration loss concept and analyses of soil cover and land use. The drainage basins of all three streams were considered to have saturated soils at the beginning of the storm. The initial loss rate used was 0.00 inch and the constant loss rate used was 0.15 inch per hour. These loss rates are comparable to loss rates used in adjacent watersheds. Unit hydrographs were synthetically derived and computed based on the "Modified Los Angeles S-Graph Procedure" described in a November 1989 hydrology study for the community of Adin (USACE, November 1989). The S- graph (Spanish Springs Valley) and basin characteristics used for this study were transposed from an area with similar physical characteristics. A 1-hour time interval was used in the computations. The Muskingum method was used to route 1-percent-annyal-chance flood hydrographs through valley areas. The Muskingum routing data were estimated based on the assumption that the average flood-wave velocity was 6 feet per second and the flood hydrographs had maximum attenuation in valley areas. An estimated 1-percent-annyal-chance peak flow of 5,290 cfs was based on a statistical analysis of the recorded peak flows on Ash Creek at the community of Adin. The peak flow compares favorably with the peak flow of 5,300 cfs developed using the HEC-1 computer program. For the hydrologic analysis for the first revision, the USACE HEC-1 computer program (USACE, September 1990(a)) computed the synthetic rainfall-runoff volumes and unit and flood hydrographs for Bidwell Creek. HEC-1 then routed the flood hydrographs using the modified-puls technique to tabulate the frequency peak discharges. The Natural Resources Conservation Service (NRCS) (formerly the Soil Conservation Service) unit-hydrograph option in the HEC-1 computer program was used. The storage-discharge relationship was determined using multiple-discharge hydraulic computations. Total precipitation was determined from NOAA isohyetal maps (NOAA, 1972). Rainfall distribution was determined based on statistical data developed by the CADWR (CADWR, 1976). The distribution was based on the nearest rainfall gage (Alturas Ranger Station) for which statistics have been developed. A 9

14 balanced 24-hour nested storm pattern was used with a time increment of 5 minutes through 24 hours. Infiltration losses were determined based on watershed soil types and NRCS guidelines (U.S. Department of Agriculture, October 1981). The soils in the watershed area consist of hydrologic soil groups B and C. The resulting curvenumber values range from 58 to 67. Times of concentration (Tcs) and basin lag times were determined using NRCS component methodology (U.S. Department of Agriculture, October 1981). This method consists of estimating travel times for the sheet, shallow-concentrated, and channel-flow segments of the runoff. In accordance with NRCS criteria, lag time was set to equal 0.6 x Tc. Watershed areas were determined using a watershed boundary map that was developed from USGS quadrangle mapping (USGS, n.d. (a)). The watershed was divided into nine subbasins. Soil type and permeability rates were determined from NRCS soil-survey maps (U.S. Department of Agriculture, April 1974). 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 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 in conjunction with the data shown on the FIRM. For each community within Modoc County that had a previously printed FIS report, the hydraulic analyses described in those reports have been compiled and are summarized below. 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). 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. Channel and overbank roughness factors (Manning's "n") used in the hydraulic computations were established by USACE and USGS guidelines (Chow, 1959 and USGS, 1987), determined by engineering judgment, and based on field 10

15 observations of the streams and floodplain areas. Table 4, Manning s n Values, shows the channel and overbank n values for the streams studied by detailed methods. Table 4 Manning s n Value Table STREAM CHANNEL OVERBANK Ash Creek Bidwell Creek Butte Creek Dry Creek Modoc County (Unincorporated Areas) For the original study and the first revision, analyses of the hydraulic characteristics of flooding from Ash, Bidwell, Butte, and Dry Creeks were carried out to provide estimates of the elevations of the base (1-percent-annual-chance) flood. Water-surface elevations were computed through the use of the USACE HEC-2 computer program (USACE, September 1982 and USACE, September 1990(b)). Starting water-surface elevations for Ash, Bidwell, and Butte Creeks were developed by the slope-area method. For Dry Creek, the 1-percent-annualchance stage in Ash Creek was used. Cross sections for backwater analyses were located at close intervals upstream and downstream from bridges, culverts, and other hydraulically significant features in order to establish the backwater effect of such structures in areas presently urbanized or subject to development. Additional cross sections were located at other representative locations in the study area. Cross-section data were derived from field surveys. HEC-2 normal and special bridge routines were used to model existing road crossings. Structural data for bridges and culverts were obtained during a field reconnaissance of the study area. Supercritical flow conditions can occur in some channel reaches. In accordance with FEMA guidelines (FEMA January 1995), subcritical analyses were conducted to determine base flood elevations (BFEs) for all stream reaches. For the first revision, split-flow routines were used to determine discharges for overbank flow paths that are hydraulically separated from the main channel. The split was based on the weir option with a coefficient of 2.6. Separate HEC-2 analyses were conducted for two overbank areas along Bidwell Creek where flow splits off of the main channel and floods additional areas during the 1-percentannual-chance storm event. In areas subject to flooding from split flow, the average depths were determined to be less than 1 foot. HEC-2 special culvert routines were used to model the existing road crossings, with a weir coefficient of 2.6 used to determine road overtopping. 11

16 For the first revision, contraction and expansion coefficients of 0.1 to 0.3 were used for open- channel sections in accordance with USACE guidelines. Contraction coefficients at culverts ranged from 0.3 to 0.5, depending on configuration. Channel and overbank cross sections were determined from surveyed channel cross sections and topographic mapping obtained for this study (Geonex, Inc., June 1993 (a and b)). For the first revision, floodway limits between Cross Sections A through F on Bidwell Creek could not be defined. For the October 20, 1999 study of the North Fork Pit River, Water-surface elevations were computed through the use of the USACE HEC-2 computer program (USACE, September 1990(b)). Channel and overbank cross sections were determined from the USACE as-built cross section for the channel reach where sediment was removed due to the 1995 storm event (Snow, 1986). Overbank sections were digitized or interpolated from topographic mapping at a scale of 1:2,400, with a contour interval of 2 feet (Geonex, Inc., January 1996). Digitized channel cross sections were supplied and verified by field surveys. HEC-2 split flow analyses were performed using either the normal depth or rating curve options. The grade line slopes used in the normal depth option were estimated based on the topographic maps. The rating curve used for the split flow that will pass through the Parker Creek Road (County Road 56) culvert, south of the stream upstream of the north-south SPRR tracks, was determined based on manual culvert hydraulic calculations. This Countywide Study In 2008, the CADWR completed Awareness Floodplain Mapping studies for all California counties. The intent of the project was to identify flood hazard areas for all regions for mapped under FEMA s NFIP. The awareness maps identify the 1- percent-annual-chance flood hazard areas using approximate assessment procedures. These floodplains are shown as floodprone areas without specific depths and other flood hazard data (CADWR, 2009). Some flood hazard information presented in prior FIRMs and in prior FIS reports for Modoc County and its incorporated communities was based on flood protection provided by levees. Based on the information available and the mapping standards of the NFIP at the time that the prior FISs and FIRMs were prepared, FEMA accredited the levees as providing protection from the flood that has a 1-percent chance of being equaled or exceeded in any given year. For FEMA to continue to accredit the identified levees with providing protection from the base flood, the levees must meet the criteria of the Code of Federal Regulations, Title 44, Section (44 CFR 65.10), titled Mapping of Areas Protected by Levee Systems. On August 22, 2005, FEMA issued Procedure Memorandum No Interim Guidance for Studies Including Levees. The purpose of the memorandum was to help clarify the responsibility of community officials or other parties seeking recognition of a levee by providing information identified during a study/mapping project. Often, documentation regarding levee design, accreditation, and the 12

17 impacts on flood hazard mapping is outdated or missing altogether. To remedy this, Procedure Memorandum No. 34 provides interim guidance on procedures to minimize delays in near-term studies/mapping projects, to help our mapping partners properly assess how to handle levee mapping issues. While 44 CFR Section documentation is being compiled, the release of more up-to-date FIRM panels for other parts of a community or county may be delayed. To minimize the impact of the levee recognition and certification process, FEMA issued Procedure Memorandum No Guidelines for Identifying Provisionally Accredited Levees on March 16, These guidelines will allow issuance of preliminary and effective versions of FIRMs while the levee owners or communities are compiling the full documentation required to show compliance with 44 CFR Section The guidelines also explain that preliminary FIRMs can be issued while providing the communities and levee owners with a specified timeframe to correct any maintenance deficiencies associated with a levee and to show compliance with 44 CFR Section FEMA contacted the communities within Modoc County to obtain data required under 44 CFR to continue to show the levees as providing protection from the flood that has a 1-percent chance of being equaled or exceeded in any given year. FEMA understood that it may take time to acquire and/or assemble the documentation necessary to fully comply with 44 CFR Therefore, FEMA put forth a process to provide the communities with additional time to submit all the necessary documentation. For a community to avail itself of the additional time, it had to sign an agreement with FEMA. Levees for which such agreements were signed are shown on the final effective FIRM as providing protection from the flood that has a 1-percent chance of being equaled or exceeded in any given year and labeled as a Provisionally Accredited Levee (PAL). Communities have two years from the date of FEMA s initial coordination to submit to FEMA final accreditation data for all PALs. Following receipt of final accreditation data, FEMA will revise the FIS and FIRM as warranted. FEMA coordinated with the USACE, the local communities, and other organizations to compile a list of levees that exist within Modoc County. Table 5 lists all levees previously shown on an effective FIRM, to include PALs, for which corresponding flood hazard revisions were made. On April 24, 2009 FEMA issued Procedure Memorandum No. 52 Guidance for Mapping Processes Associated with Levee Systems. This memorandum provides guidelines for mapping landward of levee systems. Approximate analyses of without levee flooding were conducted for all the levees in Table 5 to indicate the extent of the without levee floodplains in accordance with Procedure Memorandum No 52. The methodology used in these analyses is discussed below. 13

18 Table 5 List of Levees Requiring Flood Hazard Revision Community Flood Source Levee Inventory Identification Number USACE Levee City of Alturas North Fork Pit River 311 Yes City of Alturas North Fork Pit River 312 Yes City of Alturas North Fork Pit River 313E Yes City of Alturas North Fork Pit River 313W Yes Modoc County (Unincorporated Areas) Modoc County (Unincorporated Areas) Modoc County (Unincorporated Areas) Modoc County (Unincorporated Areas) Modoc County (Unincorporated Areas) Modoc County (Unincorporated Areas) Modoc County (Unincorporated Areas) Tule Lake 1911 No Tule Lake 2141 No Tule Lake 2142 No South Fork Pit River 2264 No Fairchild Swamp 826 No Fairchild Swamp 1532 No Fairchild Swamp 2518 No The approximate levee analysis for Modoc County (Unincorporated Areas) was conducted by MAP-IX using information from existing flood maps and USGS topographic data. The extent of the 1-percent-annual-chance flood for the without levee condition was determined using this data. The 1-percent-annualchance floodplain was traced along the contour lines representing each base flood elevation as determined by portions of the floodplain not constrained by levees. The approximate floodplain boundary limits were refined using orthophotography and topographic features such as high ground. In the area around Tule Lake, levee 2142 on the north side of the lake was treated as a dam rather than a levee. No behind levee analysis was conducted. This is consistent with how this levee is treated in Siskiyou County (FEMA, Unpublished (c)). 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 14

19 Geodetic Vertical Datum of 1929 (NGVD29). With the completion of the North American Vertical Datum of 1988 (NAVD88), many FIS reports and FIRMs are now prepared using NAVD88 as the referenced vertical datum. Prior versions of the FIS report and FIRM were referenced to NGVD29. When datum conversion is effected for an FIS report and FIRM, the flood profiles, and BFEs, reflect the new datum values. To compare structure and ground elevations to 1-percent-annual-chance (100-year) flood elevations shown in the FIS and on the FIRM, the subject structure and ground elevations must be referenced to the new datum values. The conversion from NGVD29 to NAVD88 ranged between and for Modoc County. Accordingly, due to the statistically significant range in conversion factors, an average conversion factor could not be established for the entire county. The elevations shown in the FIS report and on the FIRM were, therefore, converted to NAVD88 using a stream-by-stream approach. In this method, an average conversion was established for each flooding source and applied accordingly. The conversion factor for each flooding source in the county may be found in Table 6, Vertical Datum Conversions. Table 6 Vertical Datum Conversion Stream Name Conversion Factor Ash Creek Bidwell Creek Butte Creek Dry Creek North Fork Pit River The BFEs shown on the FIRM represent whole-foot rounded values. For example, a BFE of will appear as 102 on the FIRM and will appear as 103. Therefore, users that wish to convert the elevations in this FIS to NGVD29 should apply the stated conversion factor(s) to elevations shown on the Flood Profiles and supporting data tables in the FIS report, which are shown at a minimum to the nearest 0.1 foot. For additional information regarding conversion between the NGVD29 and NAVD88, visit the National Geodetic Survey website at or contact the National Geodetic Survey at the following address: 15

20 NGS Information Services NOAA, N/NGS12 National Geodetic Survey, SSMC-3, # East-West Highway Silver Spring, Maryland (301) Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with the FIS report and FIRM for this community. Interested individuals may contact FEMA to access these data. 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 and Floodway Data 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. 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. The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 1-percent-annual-chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, and AE), and the 0.2-percent-annual-chance floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 1- and 0.2- percent-annual-chance floodplain boundaries are close together, only the 1-percent-annual-chance floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. 16

21 For streams studied by approximate methods, only the 1-percent annual-chance floodplain boundary is shown on the FIRM. Modoc County (Unincorporated Areas) Between cross sections, the boundaries were interpolated from the study contractor's field observations and topographic workmaps. The approximate 1-percent annual-chance floodplain boundaries in some portions of the study area were taken directly from the FIRM for Modoc County (FEMA, October 1999 (a-b)). These areas were adjusted as necessary to match the background aerial photography. 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 1-percent-annual-chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1-percent-annual-chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1 foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this FIS report and on the FIRM were computed for certain stream segments on the basis of equal-conveyance reduction from each side of the floodplain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. The results of the floodway computations have been tabulated for selected cross sections (Table 7, Floodway Data ). The computed floodways are shown on the FIRM. In cases where the floodway and 1-percent-annual-chance floodplain boundaries are either close together or collinear, only the floodway boundary has been shown. The area between the floodway and 1-percent-annual-chance floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increasing the 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. Encroachment into areas subject to inundation by floodwaters having hazardous velocities aggravates the risk of flood damage and heightens potential flood hazards by further increasing velocities. A listing of stream velocities at selected 17

22 cross sections is provided in Table 7. To reduce the risk of property damage in areas where the stream velocities are high, the community may wish to restrict development in areas outside the floodway. Figure 1 - Floodway Schematic 18

23 FLOODING SOURCE FLOODWAY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) CROSS SECTION DISTANCE WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY WITHOUT FLOODWAY WITH FLOODWAY INCREASE ASH CREEK A , , , B , , , C , , , , D , , , E , , , F , , , G , , , H , , , BIDWELL CREEK A-F 2 G , , , H , , , I , , , J , , , K , , , L , , , M , , , N , , , O , , , P , , , Q , , , Feet Above Confluence of Butte Creek 2 Floodway Not Determined 3 Feet Above Confluence With Upper Lake TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY MODOC COUNTY, CA AND INCORPORATED AREAS FLOODWAY DATA ASH CREEK / BIDWELL CREEK

24 FLOODING SOURCE FLOODWAY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) CROSS SECTION DISTANCE 1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY WITHOUT FLOODWAY WITH FLOODWAY INCREASE BUTTE CREEK A 1, , , , B 1, , , , C 1, , , , D 2, , , , E 2, , , , F 2, , , , G 3, , , , DRY CREEK A , , , B , , , C , , , D , , , E , , , F , , , G , , , H 1, , , , I 1, , , , Feet Above Confluence With Ash Creek 2 Elevation computed without consideration of backwater effects from Ash Creek TABLE 7 FEDERAL EMERGENCY MANAGEMENT AGENCY MODOC COUNTY, CA AND INCORPORATED AREAS FLOODWAY DATA BUTTE CREEK / DRY CREEK

25 Along streams where floodways have not been computed, the community must ensure that the cumulative effect of development in the floodplains will not cause more than a 1.0-foot increase in the BFEs at any point within the county. No floodways were computed for the North Fork Pit River. 5.0 INSURANCE APPLICATIONS For flood insurance rating purposes, flood insurance zone designations are assigned to a community based on the results of the engineering analyses. The zones are as follows: Zone A Zone A is the flood insurance rate zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS report by approximate methods. Because detailed hydraulic analyses are not performed for such areas, no BFEs or depths are shown within this zone. Zone AE Zone AE is the flood insurance rate zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS report by detailed methods. Whole-foot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone. Zone AH Zone AH is the flood insurance risk zone that corresponds to the areas of 1-percentannual-chance shallow flooding (usually areas of ponding) where average depths are between 1 and 3 feet. Whole-foot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone. Zone AO Zone AO is the flood insurance risk zone that corresponds to the areas of 1-percentannual-chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between 1 and 3 feet. Average whole-foot base flood depths derived from the detailed hydraulic analyses are shown within this zone. Zone X Zone X is the flood insurance rate zone that corresponds to areas outside the 0.2-percent-annual-chance floodplain, areas within the 0.2-percent-annual-chance floodplain, areas of 1-percent-annual-chance flooding where average depths are less than 1 foot, areas of 1-percent-annual-chance flooding where the contributing drainage area is less than 1 square mile (sq. mi.), and areas protected from the base flood by levees. No BFEs or depths are shown within this zone. Zone D Zone D is the flood insurance rate zone that corresponds to unstudied areas where flood hazards are undetermined, but possible. 6.0 FLOOD INSURANCE RATE MAP The FIRM is designed for flood insurance and floodplain management applications. 23

26 For flood insurance applications, the map designates flood insurance risk zones as described in Section 5.0 and, in the 1-percent-annual-chance floodplains that were studied by detailed methods, shows selected whole-foot BFEs or average depths. Insurance agents use the zones and BFEs in conjunction with information on structures and their contents to assign premium rates for flood insurance policies. For floodplain management applications, the map shows by tints, screens, and symbols, the 1- and 0.2-percent-annual-chance floodplains, floodways, and the locations of selected cross sections used in the hydraulic analyses and floodway computations. The countywide FIRM presents flooding information for the geographic area of Modoc County. Previously, FIRMs were prepared for each incorporated community of the County identified as flood-prone. This countywide FIRM also includes flood hazard information that was presented separately on Flood Boundary and Floodway Maps (FBFMs), where applicable. Historical data relating to the maps prepared for each community are presented in Table 8, Community Map History. 7.0 OTHER STUDIES This FIS report either supersedes or is compatible with all previous studies published on streams studied in this report and should be considered authoritative for the purposes of the NFIP. Hydrologic studies of the North Fork Pit River were performed by the USACE in 1967 (USACE, July 1967) for the planning and design of the Alturas Flood Control Project. In 1989, a hydrologic flood insurance study was performed by the USACE that was not adopted by FEMA. Hydrologic and hydraulic analyses for the October 20, 1999 study of the City of Alturas were performed by Ensign & Buckley, for the Federal Emergency Management Agency (FEMA), under Contract No. EMW-90-C This study was completed in April (FEMA, October 1999(a)). This study was revised to incorporate detailed flood-hazard information along the North Fork Pit River from the east-west Southern Pacific Railroad (SPRR) to approximately 1.3 stream miles upstream of the north-south SPRR tracks. The hydrologic and hydraulic analyses for the North Fork Pit River were performed by Ensign & Buckley, for FEMA, under Contract No. EMW-90-C This work was completed in April (FEMA, October 1999(b)). As mentioned previously some data from CADWR Awareness Floodplain Mapping Studies was incorporated into this study. The Awareness Floodplain Maps are not FEMA regulatory floodplain maps; however, at the request of the community FEMA will include the data on the county s maps (CADWR, 2009). Flood data presented are in agreement with the FIS currently in production for Lassen, Shasta, and Siskiyou, California (FEMA, Unpublished (a-c)). Flood data is also compatible with Washoe, Nevada (FEMA, March 2009) Klamath, Oregon (FEMA, June 1984). 24