SOLANO COUNTY, CALIFORNIA, USA AND INCORPORATED AREAS

Transcription

1 VOLUME 1 OF 3 SOLANO COUNTY, CALIFORNIA, USA AND INCORPORATED AREAS COMMUNITY NAME SOLANO COUNTY UNINCORPORATED AREAS COMMUNITY NUMBER CITY OF BENICIA CITY OF DIXON CITY OF FAIRFIELD CITY OF RIO VISTA CITY OF SUISUN CITY OF VACAVILLE CITY OF VALLEJO EFFECTIVE: TBD DRAFT 4/20/2011 FLOOD INSURANCE STUDY NUMBER 06095CV001B Version Number

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3 TABLE OF CONTENTS Volume 1 Page SECTION 1.0 INTRODUCTION The National Flood Insurance Program Purpose of this Flood Insurance Study Report Jurisdictions Included in the Flood Insurance Study Project Considerations for using this Flood Insurance Study Report 7 SECTION 2.0 FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways Base Flood Elevations Non-Encroachment Zones Coastal Flood Hazard Areas Water Elevations and the Effects of Waves Floodplain Boundaries and BFEs for Coastal Areas Coastal High Hazard Areas Limit of Moderate Wave Action 41 SECTION 3.0 INSURANCE APPLICATIONS National Flood Insurance Program Insurance Zones Coastal Barrier Resources System 42 SECTION 4.0 AREA STUDIED Basin Description Principal Flood Problems Non-Levee Flood Protection Measures Levees 53 SECTION 5.0 ENGINEERING METHODS Hydrologic Analyses Hydraulic Analyses Coastal Analyses Total Stillwater Elevations Waves Coastal Erosion Wave Hazard Analyses Alluvial Fan Analyses 84 SECTION 6.0 MAPPING METHODS Vertical and Horizontal Control Base Map Floodplain and Floodway Delineation Coastal Flood Hazard Mapping FIRM Revisions Letters of Map Amendment 110 i

4 6.5.2 Letters of Map Revision Based on Fill Letters of Map Revision Physical Map Revisions Contracted Restudies Community Map History 112 SECTION 7.0 CONTRACTED STUDIES AND COMMUNITY COORDINATION Contracted Studies Community Meetings 118 SECTION 8.0 ADDITIONAL INFORMATION 121 SECTION 9.0 BIBLIOGRAPHY AND REFERENCES 123 Figures Page Figure 1: FIRM Panel Index 10 Figure 2: FIRM Notes to Users 13 Figure 3: Map Legend for FIRM 16 Figure 4: Floodway Schematic 21 Figure 5: Wave Runup Transect Schematic 39 Figure 6: Coastal Transect Schematic 41 Figure 7: Frequency Discharge-Drainage Area Curves 71 Figure 8: 1% Annual Chance Total Stillwater Elevations for Coastal Areas 80 Figure 9: Transect Location Map 83 Tables Page Table 1: Listing of NFIP Jurisdictions 3 Table 2: Flooding Sources Included in this FIS Report 23 Table 3: Flood Zone Designations by Community 42 Table 4: Coastal Barrier Resources System Information 43 Table 5: Basin Characteristics 43 Table 6: Principal Flood Problems 44 Table 7: Historic Flooding Elevations 49 Table 8: Non-Levee Flood Protection Measures 50 Table 9: Levees 55 Table 10: Summary of Discharges 60 Table 11: Summary of Non-Coastal Stillwater Elevations 72 Table 12: Stream Gage Information used to Determine Discharges 73 Table 13: Summary of Hydrologic and Hydraulic Analyses 74 Table 14: Roughness Coefficients 78 Table 15: Summary of Coastal Analyses 78 Table 16: Tide Gage Analysis Specifics 81 Table 17: Coastal Transect Parameters 82 ii

5 Table 18: Summary of Alluvial Fan Analyses 85 Table 19: Results of Alluvial Fan Analyses 85 Table 20: wide Vertical Datum Conversion 86 Table 21: Stream-by-Stream Vertical Datum Conversion 87 Table 22: Base Map Sources 88 Table 23: Summary of Topographic Elevation Data used in Mapping 93 Table 24: Floodway Data 95 Table 25: Flood Hazard and Non-Encroachment Data for Selected Streams 109 Table 26: Summary of Coastal Transect Mapping Considerations 110 Table 27: Incorporated Letters of Map Change 111 Table 28: Community Map History 113 Table 29: Summary of Contracted Studies Included in this FIS Report 114 Table 30: Community Meetings 119 Table 31: Map Repositories 121 Table 32: Additional Information 122 Table 33: Bibliography and References 123 iii

6 Volume 2 Exhibits Flood Profiles Panel Alamo Austin Blue Rock Springs 13 Bucktown Cache Slough Maine Prairie Slough 17 Chabot Clayton 22 Dan Wilson Encinosa 26 Gibson Canyon Gordon Valley Green Valley Horse Industrial 44 Lagoon Drain 45 Laguna Laurel Laurel Diversion Channel 53 Ledgewood Lemon Street Canal Magazine Street Canal Marina Marina Tributary 70 iv

7 Volume 3 Exhibits Flood Profiles Panel McCoy Middle Branch Horse Middle Swale To South Branch Horse 79 Miner Slough 80 North Branch Horse 81 North Fork Rindler Old Alamo Pennsylvania Avenue Pine Tree Pine Tree Split 94 Rindler Sacramento River 99 South Branch Gibson Canyon South Branch Horse 103 South Fork Rindler Steamboat Slough 106 Suisun Sulphur Springs Sulphur Springs Overflow 119 Sutter Slough 120 Tributary to American Canyon 121 Ulatis (Above Cache Slough) Ulatis (Above Leisure Town Road) Union Avenue Union Avenue Diversion Channel Union Watson Hollow 144 Wild Horse 145 Yolo Bypass 146 Sweany Flood Insurance Rate Map (FIRM) Published Separately v

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9 SECTION 1.0 INTRODUCTION FLOOD INSURANCE STUDY REPORT SOLANO COUNTY, CALIFORNIA 1.1 The National Flood Insurance Program The National Flood Insurance Program (NFIP) is a voluntary Federal program that enables property owners in participating communities to purchase insurance protection against losses from flooding. This insurance is designed to provide an insurance alternative to disaster assistance to meet the escalating costs of repairing damage to buildings and their contents caused by floods. For decades, the national response to flood disasters was generally limited to constructing floodcontrol works such as dams, levees, sea-walls, and the like, and providing disaster relief to flood victims. This approach did not reduce losses nor did it discourage unwise development. In some instances, it may have actually encouraged additional development. To compound the problem, the public generally could not buy flood coverage from insurance companies, and building techniques to reduce flood damage were often overlooked. In the face of mounting flood losses and escalating costs of disaster relief to the general taxpayers, the U.S. Congress created the NFIP. The intent was to reduce future flood damage through community floodplain management ordinances, and provide protection for property owners against potential losses through an insurance mechanism that requires a premium to be paid for the protection. The U.S. Congress established the NFIP on August 1, 1968, with the passage of the National Flood Insurance Act of The NFIP was broadened and modified with the passage of the Flood Disaster Protection Act of 1973 and other legislative measures. It was further modified by the National Flood Insurance Reform Act of 1994 and the Flood Insurance Reform Act of The NFIP is administered by the Federal Emergency Management Agency (FEMA), which is a component of the Department of Homeland Security (DHS). Participation in the NFIP is based on an agreement between local communities and the Federal Government. If a community adopts and enforces floodplain management regulations to reduce future flood risks to new construction and substantially improved structures in Special Flood Hazard Areas (SFHAs), the Federal Government will make flood insurance available within the community as a financial protection against flood losses. The community s floodplain management regulations must meet or exceed criteria established in accordance with Title 44 Code of Federal Regulations (CFR) Part 60.3, Criteria for land Management and Use. SFHAs are delineated on the community s Flood Insurance Rate Maps (FIRMs). Under the NFIP, buildings that were built before the flood hazard was identified on the community s FIRMs are generally referred to as Pre-FIRM buildings. When the NFIP was created, the U.S. Congress recognized that insurance for Pre-FIRM buildings would be prohibitively expensive if the premiums were not subsidized by the Federal Government. Congress also recognized that most of these floodprone buildings were built by individuals who did not have sufficient knowledge of the flood hazard to make informed decisions. The NFIP requires that full actuarial rates reflecting the complete flood risk be charged on all buildings constructed or substantially improved on or after the effective date of the initial FIRM for the community or after December 31, 1974, whichever is later. These buildings are generally referred to as Post-FIRM buildings. 1

10 1.2 Purpose of this Flood Insurance Study Report This Flood Insurance Study (FIS) report revises and updates information on the existence and severity of flood hazards for the study area. The studies described in this report developed flood hazard data that will be used to establish actuarial flood insurance rates and to assist communities in efforts to implement sound floodplain management. In some states or communities, floodplain management criteria or regulations may exist that are more restrictive than the minimum Federal requirements. Contact your State NFIP Coordinator to ensure that any higher State standards are included in the community s regulations. 1.3 Jurisdictions Included in the Flood Insurance Study Project This FIS Report covers the entire geographic area of, California. The jurisdictions that are included in this project area, along with the Community Identification Number (CID) for each community and the 8-digit Hydrologic Unit Codes (HUC-8) sub-basins affecting each, are shown in Table 1. The Flood Insurance Rate Map (FIRM) panel numbers that affect each community are listed. If the flood hazard data for the community is not included in this FIS Report, the location of that data is identified. The location of flood hazard data for participating communities in multiple jurisdictions is also indicated in the table. Jurisdictions that have no identified SFHAs as of the effective date of this study are indicated in the table. Changed conditions in these communities (such as urbanization or annexation) or the availability of new scientific or technical data about flood hazards could make it necessary to determine SFHAs in these jurisdictions in the future. 2

11 Table 1: Listing of NFIP Jurisdictions Community CID HUC-8 Sub-Basin(s) Located on FIRM Panel(s) If Not Included, Location of Flood Hazard Data 06095C0025E 06095C0050E 06095C0075E 06095C0100E 06095C0125E 1, Unincorporated Areas , , , C0144E 06095C0150E 06095C0153F 06095C0154F 06095C0158F 06095C0161F 06095C0162F 06095C0163E 06095C0164E 06095C0166F 06095C0167F 06095C0168E 06095C0175F 06095C0200F 06095C0225E 06095C0230E 06095C0234E 06095C0235E 1 N/A 06095C0240E 06095C0241E 06095C0242E 06095C0243E 06095C0244E 06095C0255E C0256E C0257E 06095C0258E C0259E 06095C0261E 06095C0262E 06095C0263E 06095C0264E 06095C0266E 06095C0267E 3

12 Community CID HUC-8 Sub-Basin(s) Located on FIRM Panel(s) If Not Included, Location of Flood Hazard Data 06095C0268E 06095C0269E 06095C0276E 06095C0278E 06095C0279E 06095C0281E 06095C0282E 06095C0283E 00695C0284E 06095C0286E 1, Unincorporated Areas (continued from previous page) C0287E 06095C0288E 06095C0289E 06095C0295E 06095C0320E 06095C0325E 06095C0330E 06095C0335E 06095C0340E 06095C0345E 06095C0355E 06095C0365E 06095C0395E C0415E C0420E 06095C0430E C0431E 06095C0432E 06095C0433E C0434E 06095C 0440E 06095C0445E 06095C0451E 06095C0452E 06095C0453E 06095C0454E 06095C0456E 06095C0457E 06095C0475E 06095C0476E 06095C0500E 4

13 Community, Unincorporated Areas (continued from previous page) CID HUC-8 Sub-Basin(s) Located on FIRM Panel(s) 06095C0525E 06095C0530E 06095C0535E 06095C0537E 06095C0539E 06095C0540E 06095C0541E 06095C0543E C0545E 06095C0555E 06095C0585E C0605E 06095C0610E 06095C0620E C0630E 06095C0633E 06095C0634E 06095C0635E 06095C0640E 06095C0641E 06095C0642E 06095C0653E 06095C0675E 06095C0700E 06095C0705E 06095C0710E 06095C0715E C0720E C0730E If Not Included, Location of Flood Hazard Data City of Benicia C0630E 06095C0633E 06095C0634E 06095C0635E 06095C0641E 06095C0642E 06095C0653E 06095C0675E N/A City of Dixon C0200F N/A 5

14 Community CID HUC-8 Sub-Basin(s) City of Fairfield City of Rio Vista City of Suisun Located on FIRM Panel(s) 06095C0262E 06095C0264E 06095C0266E 06095C0267E 06095C0268E 06095C0269E 06095C0286E C0287E 06095C0288E 06095C0289E 06095C0431E 06095C0432E 06095C0433E C0434E 06095C0445E 06095C0451E 06095C0452E 06095C0453E 06095C0454E 06095C0456E 06095C0530E 06095C0535E 06095C0537E 06095C0539E 06095C0540E 06095C0541E 06095C0543E C0268E 06095C0269E 06095C0288E 06095C0456E 06095C0457E 06095C0476E If Not Included, Location of Flood Hazard Data N/A N/A N/A 6

15 Community CID HUC-8 Sub-Basin(s) Located on FIRM Panel(s) If Not Included, Location of Flood Hazard Data City of Vacaville C0144E 06095C0162F 06095C0163E 06095C0164E 06095C0166F 06095C0167F 06095C0168E 06095C0169E 06095C0257E 06095C0258E C0259E 06095C0267E 06095C0276E 06095C0277E 06095C0278E 06095C0279E 06095C0281E 06095C0282E 06095C0283E 06095C0284E 06095C0286E 1 N/A City of Vallejo C0420E 06095C0440E 06095C0445E 06095C0610E 06095C0630E 06095C0633E 06095C0635E 06095C0640E N/A 1 Panel Not Printed 1.4 Considerations for using this Flood Insurance Study Report The NFIP encourages State and local governments to implement sound floodplain management programs. To assist in this endeavor, each FIS Report provides floodplain data, which may include a combination of the following: 10-, 4-, 2-, 1-, and 0.2-percent annual chance flood elevations (the 1% annual chance flood elevation is also referred to as the Base Flood Elevation (BFE)); delineations of the 1% annual chance and 0.2% annual chance floodplains; and 1% annual chance floodway. This information is presented on the FIRM and/or in many components of the FIS Report, including Flood Profiles, Floodway Data tables, Summary of Non-Coastal Stillwater Elevations tables, and Coastal Transect Parameters tables (not all components may be provided for a specific FIS). 7

16 This section presents important considerations for using the information contained in this FIS Report and the FIRM, including changes in format and content. Figures 1, 2, and 3 present information that applies to using the FIRM with the FIS Report. Part or all of this FIS Report may be revised and republished at any time. In addition, part of this FIS Report may be revised by a Letter of Map Revision (LOMR), which does not involve republication or redistribution of the FIS Report. Refer to Section 6.5 of this FIS Report for information about the process to revise the FIS Report and/or FIRM. It is, therefore, the responsibility of the user to consult with community officials by contacting the community repository to obtain the most current FIS Report components. Communities participating in the NFIP have established repositories of flood hazard data for floodplain management and flood insurance purposes. Community map repository addresses are provided in Table, Map Repositories, within this FIS Report. New FIS Reports are frequently developed for multiple communities, such as entire counties. A countywide FIS Report incorporates previous FIS Reports for individual communities and the unincorporated area of the county (if not jurisdictional) into a single document and supersedes those documents for the purposes of the NFIP. The initial wide FIS Report for became effective on May 4, Refer to Table for information about subsequent revisions to the FIRMs. Selected FIRM panels for the community may contain information (such as floodways and cross sections) that was previously shown separately on the corresponding Flood Boundary and Floodway Map panels. 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) FEMA does not impose floodplain management requirements or special insurance ratings based on Limit of Moderate Wave Action (LiMWA) delineations at this time. The LiMWA represents the approximate landward limit of the 1.5-foot breaking wave. If the LiMWA is shown on the FIRM, it is being provided by FEMA as information only. For communities that do adopt Zone VE building standards in the area defined by the LiMWA, additional Community Rating System (CRS) credits are available. Refer to Section for additional information about the LiMWA. The CRS is a voluntary incentive program that recognizes and encourages community floodplain management activities that exceed the minimum NFIP requirements. Visit the FEMA Web site at or contact your appropriate FEMA Regional Office for more information about this program. Previous FIS Reports and FIRMs may have included levees that were accredited as providing protection from the 1% annual chance flood based on the information available 8

17 and the mapping standards of the NFIP at that time. 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. Since the status of levees is subject to change at any time, the user should contact the appropriate agency for the latest information regarding levees presented in Table 9 of this FIS Report. For levees owned or operated by the U.S. Army Corps of Engineers (USACE), information may be obtained from the USACE national levee database. For all other levees, the user is encouraged to contact the appropriate local community. FEMA has developed a Guide to Flood Maps (FEMA 258) and online tutorials to assist users in accessing the information contained on the FIRM. These include how to read panels and step-by-step instructions to obtain specific information. To obtain this guide and other assistance in using the FIRM, visit the FEMA Web site at 9

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19 0025E 0050E 0075E 0100E Yolo Putah Yolo 80 *0125E Napa 0150E 0175F 9/9/ F 9/9/ E HUC Upper Putah Watershed 0144E 0153F 9/9/ F 9/9/ E 0158F 9/9/9999 Sweany 0161F 0162E 0166F 0167F 9/9/9999 9/9/9999 9/9/9999 9/9/ E 0168E 0169E City of Dixon Railroad 113 HUC Lower Sacramento Watershed City of Dixon **0395E **0585E **0415E Napa 0605E Marin HUC San Pablo Bay Watershed San Pablo Bay 0420E 0610E **0620E Contra Costa 0230E 0240E *0430E 0440E City of Vallejo E *0235E Napa 0630E 0241E 0243E 0431E *0433E E 0242E 0244E City of Fairfield E 0432E 0434E City of Vallejo E 0641E 0642E *0255E 0261E 0263E 0451E 0453E E 0633E 0634E 0653E City of Benicia E 0475E 0454E *0256E *0258E 0266E 0268E Suisun Bay 0257E 0259E City of Fairfield E 0452E 0456E 0278E 0267E *0286E 0269E 0276E 0288E 0457E 0476E 0277E City of Vacaville City of Suisun City E 0287E 0700E 0289E Travis 0281E 0283E AFB 0500E 0295E 12 HUC Suisan Bay Watershed 0282E 0284E SOLANO COUNTY UNINCORPORATED AREAS Contra Costa 0325E City of Vacaville Railroad 0525E 0705E ***0715E 0320E 0710E Sacramento River ***0720E Sacramento 0330E 0340E Cache Slough 0530E City of Rio Vista E 0730E 0537E 0335E 0345E 0535E 0541E 0539E ****0543E 84 Yolo E 0355E 0365E Sacramento 0555E 1 inch = 26,250 feet 1:315,000 Feet 0 7,500 15,000 30,000 45,000 60,000 Map Projection: Universal Transverse Mercator Zone 10 North; North American Datum 1983 COUNTY LOCATOR NATIONAL FLOOD INSURANCE PROGRAM FLOOD INSURANCE RATE MAP INDEX SOLANO COUNTY, CA USA and Incorporated Areas THE INFORMATION DEPICTED ON THIS MAP AND SUPPORTING DOCUMENTATION ARE ALSO AVAILABLE IN DIGITAL FORMAT AT SEE FLOOD INSURANCE STUDY FOR ADDITIONAL INFORMATION * PANEL NOT PRINTED - NO SPECIAL FLOOD HAZARD AREAS ** PANEL NOT PRINTED - AREA ALL WITHIN ZONE A *** PANEL NOT PRINTED - AREA ALL OPEN WATER - ZONE AE (EL 8) **** PANEL NOT PRINTED - AREA ALL OPEN WATER - ZONE AE (EL 9) PANELS PRINTED: 0025, 0050, 0075, 0100, 0144, 0150, 0153, 0154, 0158, 0161, 0162, 0163, 0164, 0166, 0167, 0168, 0169, 0175, 0200, 0225, 0230, 0234, 0240, 0241, 0242, 0243, 0244, 0257, 0259, 0261, 0262, 0263, 0264, 0266, 0267, 0268, 0269, 0276, 0277, 0278, 0279, 0281, 0282, 0283, 0284, 0287, 0288, 0289, 0295, 0320, 0325, 0330, 0335, 0340, 0345, 0355, 0365, 0420, 0431, 0432, 0434, 0440, 0445, 0451, 0452, 0453, 0454, 0456, 0457, 0476, 0500, 0525, 0530, 0535, 0537, 0539, 0540, 0541, 0545, 0555, 0605, 0610, 0630, 0633, 0634, 0635, 0640, 0641, 0642, 0653, 0675, 0700, 0705, 0710, 0730 FEMA MAP NUMBER 06095CIND0B MAP REVISED

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21 Figure 2: FIRM Notes to Users NOTES TO USERS For information and questions about this map, available products associated with this FIRM including historic versions of this FIRM, how to order products, or the National Flood Insurance Program in general, please call the FEMA Map Information exchange at FEMA-MAP ( ) or visit the FEMA Map Service Center website at Available products may include previously issued Letters of Map Change, a Flood Insurance Study Report, and/or digital versions of this map. Many of these products can be ordered or obtained directly from the website. Users may determine the current map date for each FIRM panel by visiting the FEMA Map Service Center website or by calling the FEMA Map Information exchange. Communities annexing land on adjacent FIRM panels must obtain a current copy of the adjacent panel as well as the current FIRM Index. These may be ordered directly from the Map Service Center at the number listed above. For community and countywide map dates, refer to Table 28 in this FIS Report. To determine if flood insurance is available in the community, contact your insurance agent or call the National Flood Insurance Program at PRELIMINARY FIS REPORT: FEMA maintains information about map features, such as street locations and names, in or near designated flood hazard areas. Requests to revise information in or near designated flood hazard areas may be provided to FEMA during the community review period, at the final Consultation Coordination Officer's meeting, or during the statutory 90-day appeal period. Approved requests for changes will be shown on the final printed FIRM. The map is for use in administering the NFIP. It may not identify all areas subject to flooding, particularly from local drainage sources of small size. Consult the community map repository to find updated or additional flood hazard information. BASE FLOOD ELEVATIONS: For more detailed information in areas where Base Flood Elevations (BFEs) and/or floodways have been determined, consult the Flood Profiles and Floodway Data and/or Summary of Stillwater Elevations tables within this FIS Report. Use the flood elevation data within the FIS Report in conjunction with the FIRM for construction and/or floodplain management. Coastal Base Flood Elevations shown on the map apply only landward of 0.0' North American Vertical Datum of 1988 (NAVD 88). Coastal flood elevations are also provided in the Summary of Stillwater Elevations table in the FIS Report for this jurisdiction. Elevations shown in the Summary of Stillwater Elevations table should be used for construction and/or floodplain management purposes when they are higher than the elevations shown on the FIRM. FLOODWAY INFORMATION: Boundaries of the floodways were computed at cross sections and interpolated between cross sections. The floodways were based on hydraulic considerations with regard to requirements of the National Flood Insurance Program. Floodway widths and other pertinent floodway data are provided in the FIS Report for this jurisdiction. 13

22 FLOOD CONTROL STRUCTURE INFORMATION: Certain areas not in Special Flood Hazard Areas may be protected by flood control structures. Refer to Section 4.3 "Non-Levee Flood Protection Measures" of this FIS Report for information on flood control structures for this jurisdiction. PROJECTION INFORMATION: The projection used in the preparation of the map was Universal Transverse Mercator (UTM) Zone 10. The horizontal datum was NAD83, GRS1980 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 the FIRM. ELEVATION DATUM: Flood elevations on the FIRM are referenced to the North American Vertical Datum of These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. For information regarding conversion between the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988, 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) Local vertical monuments may have been used to create the map. To obtain current monument information, please contact the appropriate local community listed in Table 31 of this FIS Report. BASE MAP INFORMATION: Base map information shown on the FIRM was provided by the USDA National Agriculture Imagery Program (NAIP). This information was photogrammetrically compiled at a scale of 1:24,000 from aerial photography dated The following panels used base map information provided by NAIP that was compiled from photography dated 2009: 153, 154, 158, 161, 162, 166, 167, 175, and 200. For information about base maps, refer to Section 6.2 Base Map in this FIS Report. Corporate limits shown on the map are based on the best data available at the time of publication. Because changes due to annexations or de-annexations may have occurred after the map was published, map users should contact appropriate community officials to verify current corporate limit locations. 14

23 NOTES FOR FIRM INDEX REVISIONS TO INDEX: As new studies are performed and FIRM panels are updated within, USA, corresponding revisions to the FIRM Index will be incorporated within the FIS Report to reflect the effective dates of those panels. Please refer to Table 28 of this FIS Report to determine the most recent FIRM revision date for each community. The most recent FIRM panel effective date will correspond to the most recent index date. SPECIAL NOTES FOR SPECIFIC FIRM PANELS This Notes to Users section was created specifically for, USA, effective. Not Applicable for this Update. FLOOD RISK REPORT: A Flood Risk Report (FRR) may be available for many of the flooding sources and communities referenced in this FIS Report. The FRR is provided to increase public awareness of flood risk by helping communities identify the areas within their jurisdictions that have the greatest risks. Although non-regulatory, the information provided within the FRR can assist communities in assessing and evaluating mitigation opportunities to reduce these risks. It can also be used by communities developing or updating flood risk mitigation plans. These plans allow communities to identify and evaluate opportunities to reduce potential loss of life and property. However, the FRR is not intended to be the final authoritative source of all flood risk data for a project area; rather, it should be used with other data sources to paint a comprehensive picture of flood risk. 15

24 Figure 3: Map Legend for FIRM SPECIAL FLOOD HAZARD AREAS: The 1% annual chance flood, also known as the base flood or 100-year flood, has a 1% chance of happening or being exceeded each year. Special Flood Hazard Areas are subject to flooding by the 1% annual chance flood. The Base Flood Elevation is the watersurface elevation of the 1% annual chance flood. The floodway is the channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that the 1% annual chance flood can be carried without substantial increases in flood heights. See note for specific types. If the floodway is too narrow to be shown, a note is shown. Zone A Zone AE Zone AH Special Flood Hazard Areas subject to inundation by the 1% annual chance flood (Zones A, AE, AH, AO, AR, A99, V and VE) The flood insurance rate zone that corresponds to the 1% annual chance floodplains. No base (1% annual chance) flood elevations (BFEs) or depths are shown within this zone. The flood insurance rate zone that corresponds to the 1% annual chance floodplains. Base flood elevations derived from the hydraulic analyses are shown within this zone, either at cross section locations or as static whole-foot elevations that apply throughout the zone. The flood insurance rate zone that corresponds to the areas of 1% annual chance shallow flooding (usually areas of ponding) where average depths are between 1 and 3 feet. Whole-foot BFEs derived from the hydraulic analyses are shown at selected intervals within this zone. Zone AO The flood insurance rate zone that corresponds to the areas of 1% annual chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between 1 and 3 feet. Average whole-foot depths derived from the hydraulic analyses are shown within this zone. Zone AR The flood insurance rate zone that corresponds to areas that were formerly protected from the 1% annual chance flood by a flood control system that was subsequently decertified. Zone AR indicates that the former flood control system is being restored to provide protection from the 1% annual chance or greater flood. Zone A99 The flood insurance rate zone that corresponds to areas of the 1% annual chance floodplain that will be protected by a Federal flood protection system where construction has reached specified statutory milestones. No base flood elevations or flood depths are shown within this zone. Zone V The flood insurance rate zone that corresponds to the 1% annual chance coastal floodplains that have additional hazards associated with storm waves. Base flood elevations are not shown within this zone. Zone VE Zone VE is the flood insurance rate zone that corresponds to the 1% annual chance coastal floodplains that have additional hazards associated with storm waves. Base flood elevations derived from the coastal analyses are shown within this zone as static whole-foot elevations that apply throughout the zone. Regulatory Floodway determined in Zone AE. 16

25 FLOOD INSURANCE IS NOT AVAILABLE FOR STRUCTURES NEWLY BUILT OR SUBSTANTIALLY IMPROVED ON OR AFTER APRIL 8, 1987, IN THE DESIGNATED COLORADO RIVER FLOODWAY OTHER AREAS OF FLOOD HAZARD OTHER AREAS NO SCREEN FLOOD HAZARD AND OTHER BOUNDARY LINES GENERAL STRUCTURES Non-encroachment zone (see Section 2.4 of this FIS Report for more information) The Colorado River Floodway was established by Congress in the Colorado River Floodway Protection Act of 1986, Public Law (100 Statute 1129). The Act imposes certain restrictions within the Floodway. Shaded Zone X: Areas of 0.2% annual chance flood hazards and areas of 1% annual chance flood hazards with average depths of less than 1 foot or with drainage areas less than 1 square mile. Future Conditions 1% Annual Chance Flood Hazard Zone X: The flood insurance rate zone that corresponds to the 1% annual chance floodplains that are determined based on future-conditions hydrology. No base flood elevations or flood depths are shown within this zone. Zone X Protected by Accredited Levee: Areas protected by an accredited levee, dike or other flood control structures. See Notes to Users for important information. Zone D (Areas of Undetermined Flood Hazard): The flood insurance rate zone that corresponds to unstudied areas where flood hazards are undetermined, but possible Unshaded Zone X: Areas determined to be outside the 0.2% annual chance floodplain Flood Zone Boundary (white line) Limit of Study line: Used to indicate the terminus of a 1% annual chance floodplain of a stream or backwater area that presents lower flood risk (e.g. Zone A) Jurisdiction Boundary Limit of Moderate Wave Action (LiMWA): Indicates the inland limit of the area affected by waves greater than 1.5 feet Aqueduct Channel Culvert Storm Sewer Channel, Culvert, Aqueduct, or Storm Sewer 17

26 Dam Jetty Weir Dam, Jetty, Weir Levee, Dike or Floodwall accredited or provisionally accredited to provide protection from the 1% annual chance flood Levee, Dike or Floodwall not accredited to provide protection from the 1% annual chance flood. Bridge Bridge COASTAL BARRIER RESOURCES SYSTEM (CBRS) AND OTHERWISE PROTECTED AREAS (OPA): CBRS areas and OPAs are normally located within or adjacent to Special Flood Hazard Areas. See Notes to Users for important information. CBRS AREA 09/30/2009 Coastal Barrier Resources System Area: Labels are shown to clarify where this area shares a boundary with an incorporated area or overlaps with the floodway. OTHERWISE PROTECTED AREA 09/30/2009 Otherwise Protected Area REFERENCE MARKERS FLOOD PROFILE INFORMATION River mile Markers Lettered Cross Section with 1% annual chance Water Surface Elevation (BFE) Numbered Cross Section with 1% annual chance Water Surface Elevation (BFE) Unlettered/Unnumbered Cross Section with 1% annual chance Water Surface Elevation (BFE) Coastal Transect Profile Baseline: Indicates the modeled flow path of a stream and is shown on FIRM panels for all valid studies with profiles or otherwise established base flood elevation. Coastal Transect Baseline: Used in the coastal flood hazard model to represent the 0.0-foot elevation contour and the starting point for the transect and the measuring point for the coastal mapping. 18

27 Base Flood Elevation Line (shown for flooding sources for which no cross sections or profile are available) ZONE AE (EL 16) ZONE AO (DEPTH 2) ZONE AO (DEPTH 2) (VEL 15 FPS) BASE MAP FEATURES Missouri Static Base Flood Elevation value (shown under zone label) Zone designation with Depth Zone designation with Depth and Velocity River, Stream or Other Hydrographic Feature Interstate Highway U.S. Highway State Highway Highway MAPLE LANE Street, Road, Avenue Name, or Private Drive if shown on Flood Profile RAILROAD Railroad Horizontal Reference Grid Line Horizontal Reference Grid Ticks Secondary Grid Crosshairs Land Grant Name of Land Grant 7 Section Number R. 43 W. T. 22 N. Range, Township Number m E Horizontal Reference Grid Coordinates (UTM) FT Horizontal Reference Grid Coordinates (State Plane) Corner Coordinates (Latitude, Longitude) 19

28 SECTION 2.0 FLOODPLAIN MANAGEMENT APPLICATIONS 2.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1% annual chance (100-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2% annual chance (500-year) flood is employed to indicate additional areas of flood hazard in the community. Each flooding source included in the project scope has been studied and mapped using professional engineering and mapping methodologies that were agreed upon by FEMA and as appropriate to the risk level. Flood risk is evaluated based on factors such as known flood hazards and projected impact on the built environment. Engineering analyses were performed for each studied flooding source to calculate its 1% annual chance flood elevations; elevations corresponding to other floods (e.g. 10-, 4-, 2-, 0.2-percent annual chance, etc.) may have also been computed for certain flooding sources. Engineering models and methods are described in detail in Section 5.0 of this FIS Report. The modeled elevations at cross sections were used to delineate the floodplain boundaries on the FIRM; between cross sections, the boundaries were interpolated using elevation data from various sources. More information on specific mapping methods is provided in Section 6.0 of this FIS Report. Depending on the accuracy of available topographic data (Table 23), study methodologies employed (Section 5.0), and flood risk, certain flooding sources may be mapped to show both the 1% and 0.2% annual chance floodplain boundaries, regulatory water surface elevations (BFEs), and/or a regulatory floodway. Similarly, other flooding sources may be mapped to show only the 1% annual chance floodplain boundary on the FIRM, without published water surface elevations. In cases where the 1% and 0.2% annual chance floodplain boundaries are close together, only the 1% annual chance floodplain boundary is shown on the FIRM. Figure 3, Map Legend for FIRM, describes the flood zones that are used on the FIRMs to account for the varying levels of flood risk that exist along flooding sources within the project area. Table 2 and Table 3 indicate the flood zone designations for each flooding source and each community within, USA, respectively. Table 2Table 2, Flooding Sources Included in this FIS Report, lists each flooding source, including its study limits, affected communities, mapped zone on the FIRM, and the completion date of its engineering analysis from which the flood elevations on the FIRM and in the FIS Report were derived. Descriptions and dates for the latest hydrologic and hydraulic analyses of the flooding sources are shown in Table 13. Floodplain boundaries for these flooding sources are shown on the FIRM (published separately) using the symbology described in Figure 3. On the map, the 1% annual chance floodplain corresponds to the SFHAs. The 0.2% annual chance floodplain shows areas that, although out of the regulatory floodplain, are still subject to flood hazards. 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. The procedures to remove these areas from the SFHA are described in Section 6.5 of this FIS Report. 2.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, 20

29 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 balancing floodplain development against increasing flood hazard. With this approach, the area of the 1% annual chance floodplain on a river is divided into a floodway and a floodway fringe based on hydraulic modeling. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment in order to carry the 1% annual chance flood. The floodway fringe is the area between the floodway and the 1% annual chance floodplain boundaries where encroachment is permitted. The floodway must be wide enough so that the floodway fringe could be completely obstructed without increasing the water-surface elevation of the 1% 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 4. To participate in the NFIP, Federal regulations require communities to limit increases caused by encroachment to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this project are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway projects. Figure 4: Floodway Schematic LINE AB IS THE FLOOD ELEVATION BEFORE ENCROACHMENT. LINE CD IS THE FLOOD ELEVATION AFTER ENCROACHMENT. *SURCHARGE IS NOT TO EXCEED 1.0 FOOT (FEMA REQUIREMENT) OR LESS AMOUNT IF SPECIFIED BY STATE. Floodway widths presented in this FIS Report and on the FIRM were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. For certain stream segments, 21

30 floodways were adjusted so that the amount of floodwaters conveyed on each side of the floodplain would be reduced equally. The results of the floodway computations have been tabulated for selected cross sections and are shown in Table 24, Floodway Data. 22

31 Table 2: Flooding Sources Included in this FIS Report Flooding Source Community Downstream Limit Upstream Limit Airport Drainage Channel, City of Rio Vista Entire drainage channel Entire drainage channel HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis N A 1979 Alamo, City of Vacaville Leisure Town Road Pleasants Valley Drive N AE 1979 Alamo Pleasants Valley Drive 9,000 feet upstream of Pleasants Valley Drive N A 1979 Alamo A-1 Channel Entire Channel Entire Channel N A 1979 American Canyon, City of Fairfield Entire Entire N A 1979 Austin City of Vallejo 80 feet downstream of Stefan Street 1360 feet upstream of Columbus Parkway N AE 1972 Barker Slough Entire slough Entire slough N A 1979 Blue Rock Springs City of Vallejo Confluence with Rindler 640 feet upstream of Admiral Callaghan Lane N AE 1972 Boynton Slough Entire slough Entire slough 3.3 N A 1979 Bucktown, City of Vacaville Confluence with Ulatis 520 feet upstream of Gibson Canyon Road Y AE

32 Flooding Source Community Downstream Limit Upstream Limit Cache Slough Confluence with Sacramento River 13,750 feet upstream of confluence with Hass Slough Calhoun Cut Lindsey Slough 10,000 feet upstream of State Highway 113 HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis N AE N A 1979 Carquinez Strait, City of Benicia Shoreline of City of Benicia Shoreline of City of Benicia N AE, VE 1979 Cat Slough Volanti Slough Cutoff Slough N AE 1979 Chabot, City of Vallejo Confluence with Napa River End of Channel N AE, A as Backwater from Napa River 1972 Chadbourne Slough Wells Slough Frank Horan Slough N AE 1979 Champion Slough Entire slough Entire slough N AE 1979 Clayton Confluence with Ledgewood Clayton Road N AE 1979 Cordelia Slough Green Valley Goodyear Slough N A, AE 1979 Cross Slough Nurse Sough Unnamed Road N A 1979 Cuttoff Slough Montezuma Slough Confluence with unnamed stream N A 1979 Cuttoff Slough Confluence with unnamed stream First Mallard Branch N AE 1979 Dan Wilson, City of Fairfield Confluence with Green Valley Rockville Road Y AE

33 Flooding Source Community Downstream Limit Upstream Limit Dan Wilson Rockville Road 4,300 feet upstream of Suisun Valley Road HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis N A 1979 Denverton Slough Little Honker Bay Denverton N A 1979 Dickson Entire creek Entire creek N A 1979 Dry Arroyo Entire creek Entire creek N A 1979 Duck Slough Entire slough Entire slough N AE 1979 Dutchman Slough Napa River South Slough N A 1979 Encinosa, City of Vacaville Confluence with Alamo Pleasants Valley Road N AE 1979 Encinosa Pleasants Valley Road Pleasants Valley Road N A 1979 First Mallard Branch Suisun slough 3,000 feet upstream of Suisun Slough N AE 1979 First Mallard Branch 3,000 feet upstream of Suisun Slough 8,500 feet upstream of Suisun Slough N A 1979 Frank Horan Slough Entire slough Entire slough N AE 1979 Frost Slough Entire slough Entire slough N AE 1979 Gibson Canyon Confluence with Sweany 2,250 feet downstream of Byrnes Road N A

34 Flooding Source Community Downstream Limit Upstream Limit Gibson Canyon, City of Vacaville 2,250 feet downstream of Byrnes Road Upstream of Browns Valley Road HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis Y A, AE 1979 Goodyear Slough, City of Benicia Entire slough Entire slough N AE 1979 Gordon Valley Confluence with Ledgewood 600 feet upstream of Gordon Valley Road N AE 1979 Green Valley, City of Fairfield 3,700 feet upstream of confluence with Cordelia Slough 500 feet upstream of Green Valley Road Y AE, AO 1979 Hass Slough Confluence with Cache Slough Hass Slough 15,000 feet upstream of confluence with Cache Slough 15,000 feet upstream of confluence with Cache Slough 25,000 feet upstream of confluence with Cache Slough N AE N A 1979 Hastings Cut Lindsey Slough 14,000 feet upstream of confluence with Lindsey Slough N A 1979 Hasting Slough Entire slough Entire slough N A 1979 Hill Slough Entire slough Entire slough N A, AE 1979 Hopkins Ravine Confluence with Unnamed Stream 750 feet upstream of Unnamed Road N A

35 Flooding Source Community Downstream Limit Upstream Limit Horse Confluence with Ulatis 18,125 feet above confluence with Ulatis HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis N A 1979 Horse, City or Vacaville 18,125 feet above confluence with Ulatis 2,100 feet above Allison Parkway Y AE 1979 Howard Slough Entire slough Entire slough N AE 1979 Hunter Cut Montezuma Slough Suisun Slough N AE 1979 Ibis Cut Cordelia Slough Frank Horan Slough N AE 1979 Industrial City of Rio Vista Confluence with Sacramento River 1,860 feet above confluence with Sacramento River Y AE 1979 Industrial City of Rio Vista 1,860 feet above confluence with Sacrament River State Highway N AO 1979 Island Slough Entire slough Entire slough N AE 1979 Lagoon Drain City of Vacaville Confluence with Laguna 2,775 feet upstream of confluence with Laguna Y AE 1979 Laguna, City of Vacaville Confluence with Alamo Pleasants Valley Road Y AE 1979 Lake Chabot City of Vallejo Entire lake Entire lake N AE 1979 Lake Dalwigk City of Vallejo Entire lake Entire lake N AE 1979 Lake Dalwigk Entire lake Entire lake N A 1979 Lake Herman Entire lake Entire lake N A

36 Flooding Source Community Downstream Limit Upstream Limit HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Lake Herman City of Benicia Entire lake Entire lake N A 1986 Date of Analysis Laurel City of Fairfield Confluence with Marina Channel, End of Laurel Diversion Channel State Highway 12, Putah South Canal N AE 1979 Laurel, City of Fairfield Putah South Canal Nelson Road N A 1979 Laurel Diversion Channel, City of Fairfield, City of Suisun Confluence with McCoy Confluence with Laurel Diversion Stub N AE 1979 Ledgewood, City of Fairfield Cordella Road Private Bridge 800 feet upstream of Check Gate N AE, AO 1979 Lemon Street Canal, City of Vallejo Lake Dalwigk 140 feet Upstream of Cabrillo Avenue N AE 1972 Lindsey Slough Cache Slough Hastings Cut N AE 1979 Little Honker Bay Luco Slough Nurse Slough N A 1979 Luco Slough Entire slough Entire slough N A 1979 Magazine Street Canal City of Vallejo Confluence with Lemon Street Canal 800 feet upstream of Fulton Avenue N AE 1972 Maine Prairie Slough Cache Slough 13,700 feet upstream of Cache Slough N AE 1979 Maine Prairie Slough 13,700 feet upstream of Cache Slough 19,000 feet upstream of Cache Slough N A

37 Flooding Source Community Downstream Limit Upstream Limit Mare Island Strait City of Vallejo Mouth of Napa River Mouth of Napa River HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis N AE 1972 Marina City of Rio Vista Confluence with Sacramento river Marina City of Rio Vista 1,250 feet upstream of confluence with Marina Tributary 1,250 feet upstream of confluence with Marina Tributary 2,150 feet upstream of confluence with Marina Tributary Y AE N AO 1979 Marina Tributary City of Rio Vista Confluence with Marina 1,270 feet upstream of confluence with Marina Y AE 1979 Marina Tributary City of Rio Vista 1,270 feet upstream of confluence with Marina 2,700 feet upstream of confluence with Marina N AO 1979 McCoy, City of Fairfield, City of Suisun 600 feet downstream of State Highway 12 Upstream side of Air Base Parkway Y A, AE, AO 1979 McCune Confluence with Sweany 1,900 feet upstream of Interstate N A 1979 Middle Branch Horse City of Vacaville Confluence with Horse feet upstream of Interstate Y AE

38 Flooding Source Community Downstream Limit Upstream Limit Middle Swale to City of Vacaville Confluence with 3,100 feet South Branch South Branch Horse upstream of Horse confluence with South Branch Horse Miner Slough Confluence with Cache Slough Confluence with Sutter Slough HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis Y AE, AO N AE 1979 Montezuma Slough Montezuma Slough Montezuma Slough Sacramento River Nurse Slough N AE 1979 Nurse Slough Cutoff Slough N A 1979 Cutoff Slough Suisun Slough N AE 1979 Mud Slough Entire slough Entire slough N AE 1979 Napa River City of Vallejo Sears Point Road Sears Point Road N AE 1972 Napa River, City of Vallejo Confluence with Dutchman Slough Boundary N A 1979 North Branch Horse City of Vacaville Confluence with Horse Interstate Y AE 1979 North Fork Rindler City of Vallejo Lake Chabot Upstream of Interstate Highway 80 Off Ramp N AE 1972 North Fork Rindler City of Vallejo Upstream of Interstate Highway 80 Off Ramp Exposition Drive N A 1972 Northern Slough Entire slough Entire slough N AE 1979 Noyce Slough Entire slough Entire slough N AE

39 Flooding Source Community Downstream Limit Upstream Limit HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Nurse Slough Montezuma Slough Luco Slough N A 1979 Date of Analysis Old Alamo City of Vacaville Lewis Road End of Channel Y AE 1979 Old Ulatis, City of Vacaville Entire named portion of creek Entire named portion of creek N A 1979 Peltier Slough Unnamed Stream 1,600 feet upstream of Unnamed Stream N AE 1979 Peltier Slough 1,600 feet upstream of Unnamed Stream 5,300 feet upstream of Unnamed Stream A 1979 Peltier Slough 5,300 feet upstream of Unnamed Stream Unnamed Stream AE 1979 Pennsylvania Avenue, City of Fairfield, City of Suisun Confluence with Ledgewood Upstream of On Off Ramp Holiday Lane N AE, AO 1979 Peytonia Slough Suisun Slough Pennsylvania N AE 1979 Pine Tree City of Vacaville Confluence with Horse 900 feet upstream of Browns Valley Parkway Y AE 1979 Pine Tree City of Vacaville 900 feet upstream of Browns Valley Parkway Hilltop Drive N A 1979 Pine Tree Split City of Vacaville Confluence with Pine Tree 1,120 feet upstream of confluence with Pine Tree N AE

40 Flooding Source Community Downstream Limit Upstream Limit HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Pleasants Entire creek Entire creek N A 1979 Date of Analysis Prospect Slough boundary Cache Slough N AE 1979 Pudley Entire creek Entire creek N A 1979 Putah Entire A zone along northern county boundary Entire A zone along northern county boundary N A 1979 Rindler City of Vallejo Lake Chabot 700 feet upstream of Admiral Callaghan Lane N AE 1972 Roaring River Slough Entire slough Entire slough N AE 1972 Sacramento River, City of Rio Vista Confluence with San Joaquin River Boundary N AE, AH 1972 Sacramento Street, City of Vallejo Mouth of City of Vallejo Corporate Boundary N A 1979 San Pablo Bay Entire bay within limits of Solano Entire bay within limits of Solano N A, V, VE 1979 Sheldrake Slough Suisun Slough 6,500 feet upstream of Suisun Slough N A 1979 South Branch Gibson Canyon, City of Vacaville Confluence with Gibson Canyon Browns Valley Road Y AE

41 Flooding Source Community Downstream Limit Upstream Limit South Branch City of Vacaville Confluence with 2,350 feet Horse Horse upstream of Sewer Maintenance Road HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis Y AE, AO 1979 South Fork Putah Boundary Boundary N A 1979 South Fork Rindler City of Vallejo Confluence with Rindler 3,560 feet upstream of Admiral Callaghan Lane N AE 1972 South Slough Napa River Entire slough within Solano N A 1979 Steamboat Slough Confluence with Sacramento River Confluence with Sutter Slough N AE 1979 Suisun Bay Suisun Slough Sulphur Springs N AE, VE 1979 Suisun, City of Fairfield 500 feet downstream of Union Pacific Railroad 8,750 feet upstream of Suisun Valley Road Y AE 1979 Suisun Slough, City of Suisun Suisun Bay First Mallard Branch N AE 1979 Suisun Slough Suisun Slough, City of Suisun, City of Suisun First Mallard Branch Hill Slough N A 1979 Hill Slough Almond Street N AE

42 Flooding Source Community Downstream Limit Upstream Limit Sulphur Springs, City of Benicia 400 feet downstream of Railroad Spur 1,160 feet upstream of Williams and Lane Company HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis Y AE, AO 1979 Sulphur Springs City of Benicia 1,160 feet upstream of Williams and Lane Company Lake Herman N A 1986 Sulphur Springs Lake Herman 6,600 feet upstream of Lake Herman N A 1979 Sulphur Springs Overflow, City of Benicia Confluence with Carquinez Strait Divergence from Sulphur Springs Y AE 1979 Sutter Slough Confluence with Steamboat Slough Sweany Confluence with Ulatis Boundary Confluence with McCune N AE N A 1979 Sweany Confluence with McCune 550 feet upstream of Timm Road N AE 2008 Sweany Overflow to Gibson Canyon Tributary Gibson Canyon Tributary just east of I-505 Sweany west of I N AE, AO 2008 The Big Ditch Entire ditch Entire ditch N A 1979 Tree Slough Montezuma Slough Grizzly Island Road N AE 1979 Tributary to American Canyon City of Vallejo Boundary Upstream of Fairgrounds Drive N AE

43 Flooding Source Community Downstream Limit Upstream Limit Ulatis Confluence with Alamo City of Vacaville Corporate Valley HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis N A 1979 Ulatis (above Leisure Town Road) Ulatis (above Cache Slough), City of Vacaville, City of Vacaville Leisure Town Road Vaca Valley Road Y AE 1979 Vaca Valley Road Bucktown Lane N AE 1979 Ulatis Bucktown Lane 5,500 feet above Pleasants Valley road Y A 1979 Union Avenue City of Fairfield, City of Suisun Confluence with Marina Channel 3,500 feet upstream of Interstate Y AE, AH 1979 Union Avenue Diversion Channel City of Fairfield Confluence with Laurel Putah South Canal N A 1979 Union 300 feet downstream of Hangar Avenue 1,200 feet upstream of Cannon Road N AE 1991 Union, City of Vacaville Union Pacific Railroad Unnamed culvert N A 1979 Volanti Slough Entire slough Entire slough N AE 1979 Watson Hollow City of Rio Vista Unnamed Road Airport Road N A 1979 Watson Hollow City of Rio Vista Airport Road Liberty Island Road N AE

44 Flooding Source Community Downstream Limit Upstream Limit Wells Slough Suisun Slough 6,900 feet upstream of Suisun slough HUC-8 Sub- Basin(s) Length (mi) (streams or coastlines) Area (mi 2 ) (estuaries or ponding) Floodway (Y/N) Zone shown on FIRM Date of Analysis N AE 1979 Wells Slough 6,900 feet upstream of Suisun Slough Wild Horse Confluence with Green Valley Yolo Bypass Confluence with Cache Slough 9,700 feet upstream of Suisun Slough 2,750 feet upstream of confluence with Green Valley Boundary N A Y AE N AE

45 All floodways that were developed for this FIS project are shown on the FIRM using the symbology described in Figure 3. In cases where the floodway and l% annual chance floodplain boundaries are either close together or collinear, only the floodway boundary has been shown on the FIRM. For information about the delineation of floodways on the FIRM, refer to Section Base Flood Elevations The hydraulic characteristics of flooding sources were analyzed to provide estimates of the elevations of floods of the selected recurrence intervals. The Base Flood Elevation (BFE) is the elevation of the 1% annual chance flood. These BFEs are most commonly rounded to the whole foot, as shown on the FIRM, but in certain circumstances or locations they may be rounded to 0.1 foot. Cross section lines shown on the FIRM may also be labeled with the BFE rounded to 0.1 foot. Whole-foot BFEs derived from engineering analyses that apply to coastal areas, areas of ponding, or other static areas with little elevation change may also be shown at selected intervals on the FIRM. Cross sections with BFEs shown on the FIRM correspond to the cross sections shown in the Floodway Data table and Flood Profiles in this FIS Report. BFEs 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. 2.4 Non-Encroachment Zones Some States and communities use non-encroachment zones to manage floodplain development. While not a FEMA designated floodway, the non-encroachment zone represents that area around the stream that should be reserved to convey the 1% annual chance flood event. Nonencroachment determinations are typically shown in Table 25Table, Flood Hazard and Non- Encroachment Data for Selected Streams. This section is not applicable to this FIS project. 2.5 Coastal Flood Hazard Areas For most areas along rivers, streams, and small lakes, BFEs and floodplain boundaries are based on the amount of water expected to enter the area during a 1% annual chance flood and the geometry of the floodplain. Floods in these areas are typically caused by storm events. However, for areas on or near ocean coasts, large rivers, or large bodies of water, BFE and floodplain boundaries may need to be based on additional components, including storm surges and waves. Communities on or near ocean coasts face flood hazards caused by offshore seismic events as well as storm events. Coastal flooding sources that are included in this FIS project are shown in Table Water Elevations and the Effects of Waves Specific terminology is used in coastal analyses to indicate which components have been included in evaluating flood hazards. The stillwater elevation (SWEL or still water level) is the surface of the water resulting from 37

46 astronomical tides, storm surge, and freshwater inputs, but excluding wave setup contribution or the effects of waves. Astronomical tides are periodic rises and falls in large bodies of water caused by the rotation of the earth and by the gravitational forces exerted by the earth, moon and sun. Storm surge is the additional water depth that occurs during large storm events. These events can bring air pressure changes and strong winds that force water up against the shore. Freshwater inputs include rainfall that falls directly on the body of water, runoff from surfaces and overland flow, and inputs from rivers. The 1% annual chance stillwater elevation is the stillwater elevation that has been calculated for a storm surge from a 1% annual chance storm. The 1% annual chance storm surge can be determined from analyses of tidal gage records, statistical study of regional historical storms, or other modeling approaches. Stillwater elevations for storms of other frequencies can be developed using similar approaches. The total stillwater elevation (also referred to as the mean water level) is the stillwater elevation plus wave setup contribution but excluding the effects of waves. Wave setup is the increase in stillwater elevation at the shoreline caused by the reduction of waves in shallow water. It occurs as breaking wave momentum is transferred to the water column. Like the stillwater elevation, the total stillwater elevation is based on a storm of a particular frequency, such as the 1% annual chance storm. Wave setup is typically estimated using standard engineering practices or calculated using models, since tidal gages are often sited in areas sheltered from wave action and do not capture this information. Coastal analyses may examine the effects of overland waves by analyzing storm-induced erosion, overland wave propagation, wave runup, and/or wave overtopping. Storm-induced erosion is the modification of existing topography by erosion caused by a specific storm event, as opposed to general erosion that occurs at a more constant rate. Overland wave propagation describes the combined effects of variation in ground elevation, vegetation, and physical features on wave characteristics as waves move onshore. Wave runup is the uprush of water from wave action on a shore barrier. It is a function of the roughness and geometry of the shoreline at the point where the stillwater elevation intersects the land. Wave overtopping refers to wave runup that occurs when waves pass over the crest of a barrier. 38

47 Figure 5: Wave Runup Transect Schematic Floodplain Boundaries and BFEs for Coastal Areas For coastal communities along the Atlantic and Pacific Oceans, the Gulf of Mexico, the Great Lakes, and the Caribbean Sea, flood hazards must take into account how storm surges, waves, and extreme tides interact with factors such as topography and vegetation. Storm surge and waves must also be considered in assessing flood risk for certain communities on rivers or large inland bodies of water. Beyond areas that are affected by waves and tides, coastal communities can also have riverine floodplains with designated floodways, as described in previous sections. Floodplain Boundaries In many coastal areas, storm surge is the principle component of flooding. The extent of the 1% annual chance floodplain in these areas is derived from the total stillwater elevation (stillwater elevation including storm surge plus wave setup) for the 1% annual chance storm. The methods that were used for calculation of total stillwater elevations for coastal areas are described in Section 5.3 of this FIS Report. Location of total stillwater elevations for coastal areas are shown in Figure 8, 1% Annual Chance Total Stillwater Levels for Coastal Areas. In some areas, the 1% annual chance floodplain is determined based on the limit of wave runup or wave overtopping for the 1% annual chance storm surge. The methods that were used for calculation of wave hazards are described in Section 5.3 of this FIS Report. Table 15 presents the types of coastal analyses that were used in mapping the 1% annual chance floodplain in coastal areas. Coastal BFEs Coastal BFEs are calculated as the total stillwater elevation (stillwater elevation including storm surge plus wave setup) for the 1% annual chance storm plus the additional flood hazard from overland wave effects (storm-induced erosion, overland wave propagation, wave runup and wave overtopping). 39

48 Where they apply, coastal BFEs are calculated along transects extending from offshore to the limit of coastal flooding onshore. Results of these analyses are accurate until local topography, vegetation, or development type and density within the community undergoes major changes. Parameters that were included in calculating coastal BFEs for each transect included in this FIS Report are presented in Table 17, Coastal Transect Parameters. The locations of transects are shown in Figure 9, Transect Location Map. More detailed information about the methods used in coastal analyses and the results of intermediate steps in the coastal analyses are presented in Section 5.3 of this FIS Report. Additional information on specific mapping methods is provided in Section 6.4 of this FIS Report Coastal High Hazard Areas Certain areas along the open coast and other areas may have higher risk of experiencing structural damage caused by wave action and/or high-velocity water during the 1% annual chance flood. These areas will be identified on the FIRM as Coastal High Hazard Areas. Coastal High Hazard Area (CHHA) is a SFHA extending from offshore to the inland limit of the primary frontal dune (PFD) or any other area subject to damages caused by wave action and/or high-velocity water during the 1% annual chance flood. Primary Frontal Dune (PFD) is a continuous or nearly continuous mound or ridge of sand with relatively steep slopes immediately landward and adjacent to the beach. The PFD is subject to erosion and overtopping from high tides and waves during major coastal storms. CHHAs are designated as V zones (for velocity wave zones ) and are subject to more stringent regulatory requirements and a different flood insurance rate structure. The areas of greatest risk are shown as VE on the FIRM. Zone VE is further subdivided into elevation zones and shown with BFEs on the FIRM. The landward limit of the PFD occurs at a point where there is a distinct change from a relatively steep slope to a relatively mild slope; this point represents the landward extension of Zone VE. Areas of lower risk in the CHHA are designated with Zone V on the FIRM. More detailed information about the identification and designation of Zone VE is presented in Section 6.4 of this FIS Report. Areas that are not within the CHHA but are SFHAs may still be impacted by coastal flooding and damaging waves; these areas are shown as A zones on the FIRM. Figure 6, Coastal Transect Schematic, illustrates the relationship between the base flood elevation, the 1% annual chance stillwater elevation, and the ground profile as well as the location of the Zone VE and Zone AE areas in an area without a PFD subject to overland wave propagation. This figure also illustrates energy dissipation and regeneration of a wave as it moves inland. 40

49 Figure 6: Coastal Transect Schematic LiMWA Methods used in coastal analyses in this FIS project are presented in Section 5.3 and mapping methods are provided in Section 6.4 of this FIS Report. Coastal floodplains are shown on the FIRM using the symbology described in Figure 3, Map Legend for FIRM. In many cases, the BFE on the FIRM is higher than the stillwater elevations shown in Table 17 due to the presence of wave effects. The higher elevation should be used for construction and/or floodplain management purposes Limit of Moderate Wave Action Laboratory tests and field investigations have shown that wave heights as little as 1.5 feet can cause damage to and failure of typical Zone AE building construction. Wood-frame, light gage steel, or masonry walls on shallow footings or slabs are subject to damage when exposed to waves less than 3 feet in height. Other flood hazards associated with coastal waves (floating debris, high velocity flow, erosion, and scour) can also damage Zone AE construction. Therefore, a LiMWA boundary may be shown on the FIRM as an informational layer to assist coastal communities in safe rebuilding practices. The LiMWA represents the approximate landward limit of the 1.5-foot breaking wave. The location of the LiMWA relative to Zone VE and Zone AE is shown in Figure 6. The effects of wave hazards in Zone AE between Zone VE (or the shoreline where Zone VE is not identified) and the limit of the LiMWA boundary are similar to, but less severe than, those in Zone VE where 3-foot or greater breaking waves are projected to occur during the 1% annual chance flooding event. Communities are therefore encouraged to adopt and enforce more stringent floodplain management requirements than the minimum NFIP requirements in the LiMWA. The NFIP Community Rating System provides credits for these actions. Where wave runup elevations dominate over wave heights, there is no evidence to date of significant damage to residential structures by runup depths less than 3 feet. Examples of these 41

50 areas include areas with steeply sloped beaches, bluffs, or flood protection structures that lie parallel to the shore. In these areas, the FIRM shows the LiMWA immediately landward of the VE/AE boundary. Similarly, in areas where the zone VE designation is based on the presence of a primary frontal dune or wave overtopping, the LiMWA is delineated immediately landward of the Zone VE/AE boundary. SECTION 3.0 INSURANCE APPLICATIONS 3.1 National Flood Insurance Program Insurance Zones For flood insurance applications, the FIRM designates flood insurance rate zones as described in Figure 3, Map Legend for FIRM. Flood insurance zone designations are assigned to flooding sources based on the results of the hydraulic or coastal analyses. Insurance agents use the zones shown on the FIRM and depths and base flood elevations in this FIS Report in conjunction with information on structures and their contents to assign premium rates for flood insurance policies. The 1% annual chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (e.g. Zones A, AE, V, VE, etc.), and the 0.2% annual chance floodplain boundary corresponds to the boundary of areas of additional flood hazards. Table 3Table 3 lists the flood insurance zones in the unincorporated and incorporated areas of. Table 3: Flood Zone Designations by Community Community, Unincorporated Areas City of Benicia City of Dixon City of Fairfield City of Rio Vista City of Suisun City of Vacaville City of Vallejo Flood Zone(s) A, AE, AO, D, V, VE, X A, AE, X A, X A, AE, AH, AO, D, X A, AE, AO, X A, AE, AO, X A, AE, AO, X A, AE, V, VE, X 3.2 Coastal Barrier Resources System The Coastal Barrier Resources Act (CBRA) of 1982 was established by Congress to create areas along the Atlantic and Gulf coasts and the Great Lakes, where restrictions for Federal financial assistance including flood insurance are prohibited. In 1990, Congress passed the Coastal Barrier Improvement Act (CBIA), which increased the extent of areas established by the CBRA and added Otherwise Protected Areas (OPA) to the system. These areas are collectively referred to as the John. H Chafee Coastal Barrier Resources System (CBRS). The CBRS boundaries are typically listed in This section is not applicable to this FIS project. 42

51 Table 4Table 4, Coastal Barrier Resource System Information. This section is not applicable to this FIS project. Table 4: Coastal Barrier Resources System Information Primary Flooding Source CBRS/OPA Type Date CBRS Area Established FIRM Panel Number(s) N/A N/A N/A N/A SECTION 4.0 AREA STUDIED 4.1 Basin Description Table 5 contains a description of the characteristics of the HUC-8 sub-basins within which each community falls. The table includes the main flooding sources within each basin, a brief description of the basin, and its drainage area. Table 5: Basin Characteristics HUC-8 Sub- Basin Name Lower Sacramento HUC-8 Sub-Basin Number Primary Flooding Source Sacramento River Description of Affected Area Basin encompasses the northeastern half of the county, extends to the Sacramento River on the southern boundary Upper Putah Putah Covers small area of northwest portion of near Cold Canyon Suisun Bay Suisun Bay Basin encompasses the central portion of the county, west of the Lower Sacramento Basin and flows to Suisun Bay. San Pablo Bay San Pablo Bay Covers the western most portion of the county and flows to San Pablo Bay. Drainage Area (square miles) 1, , Principal Flood Problems Table 6 contains a description of the principal flood problems that have been noted for Solano by flooding source. 43

52 Flooding Source Table 6: Principal Flood Problems Description of Flood Problems Alamo The most severe floods in Vacaville occurred in February and March 1940, January 1967, and January In 1940, Alamo and Ulatis s flooded residential properties, requiring evacuation of homes, blocked roads, and disrupted traffic. The 1967 and 1973 floods were of similar magnitude and are considered the largest in recent years. During both flood periods, Alamo overflowed its banks in several locations and flooded streets and lawns, stranded residents, and deposited debris and garbage. Several families were forced to evacuate their homes or apartments when floodwaters covered the lower floors. All sources General rain floods can occur in at any time from October through April. This type of flood results from prolonged heavy rainfall and is characterized by high peak flows of moderate duration and large volume runoff. Flooding is more severe when antecedent rain has resulted in saturated ground conditions and minimal infiltration. Cloudburst storms, sometimes lasting as long as 6 hours, can occur at any time from late spring to early fall, and may occur as an extremely severe sequence within a general rainstorm. Cloudbursts are high-intensity storms that can produce floods characterized by high peak flows, a short duration of flood flows, and a small volume of runoff. In, cloudbursts can produce peak flows substantially greater than those of general rainstorms. In urbanizing areas, flood problems are intensified because new homes and other structures, and new streets, driveways, parking lots, and other paved areas decrease the amount of open land available to absorb rainfall and runoff, thus increasing the volume of water that must be carried away by waterways. has a long history of flooding, but little definitive data are available for specific floods. Stream flow records are essentially nonexistent for the streams under study, and the rural nature of most past flooding precluded detailed news coverage. Information on past floods is based primarily on historical accounts, brief newspaper descriptions, and various published and unpublished reports. The earliest floods mentioned in accounts of the area occurred in 1842, 1861, and As described by General Mariano Vallejo, a government official, flooding in December 1842 was widespread: the whole country was overflowed, as well as all that level part of the country out to the hills at Vacaville. On that day I sailed in a schooner of twenty tons from the present site of Sacramento in a southwesterly direction, passing over what is now elevated farming lands in that section. The Montezuma hills and other highlands were not submerged, but all the other country was. I was able to, and did, sail over with ease where now fine farms are. Several hunters and their horses were drowned, and afterwards found at Benicia when the water subsides. The overflow lasted for several weeks. No crops were then raised, as there were no settlers in the whole region at that date, only a few cattle herders and hunters (Reference 4). 44

53 Flooding Source Cache Slough Dixon Lower sections of Austin, White Slough, and Chabot Lower reaches of Green Valley, Dan Wilson, and Suisun s Description of Flood Problems In general, major floods have inundated highly developed agricultural lands, urban and rural residential properties, and commercial and public facilities. Flooding has damaged orchards, vineyards, pasturelands, and croplands; damaged or destroyed growing crops; deposited debris on agricultural lands; destroyed livestock and poultry; and damaged farm equipment and agricultural improvements such as fences and irrigation systems. Floodwater has entered basements or lower floors of dwellings and commercial structures and deposited debris on lawns and gardens. Numerous streets have been flooded; bridges, roadbeds, and culverts damaged or destroyed; and stream channels and flood control works eroded. Severe flooding occurred in 1861 and Floodwaters swept away the original settlements of Maine Prairie (at the head of navigation on Maine Prairie Slough, now Cache Slough) and Rio Vista. Some Maine Prairie inhabitants rebuilt at the original site, but Rio Vista was relocated approximately 3 miles downstream to its present site. The flood history of Dixon is not well documented, but minor flooding is reported to have occurred in December 1955, April 1958, and January In 1955, floodwater surrounded a few homes in a subdivision is the northwestern sector of town, but no damage was reported. Overflow from Dickson probably contributed to the flooding. In 1958, a cloudburst left water standing in a few streets, but no homes were known to have been flooded. Minor flooding also occurred in 1965, but no damage was reported. Most of Dickson in Dixon flows through underground storm drains. The drains can carry runoff from a storm expected to occur once in approximately 10 years on the long-term average. Less frequent (greater) runoff flows over streets and may pond behind natural or manmade barriers. In the lower sections of the Austin -White Slough-Chabot areas, flooding occurs due to the occasional breaching of the levees. This results in shallow flooding conditions. Runoff from the area east of Interstate Highway 80 is diverted to a storm drain that discharges into Mare Island Strait at the foot of Solano Avenue. However, during periods of high flows, part of the runoff from the Austin area diverted to the Solano Avenue drain overflows into Lake Dalwigk, a holding basin, in the Lemon Street area. Before 1950, flood damage in Fairfield was minor because development in floodplains was limited to areas along Pennsylvania Avenue and Union Avenue s. There was no urban development in the floodplains of Laurel, Ledgewood, and McCoy s at that time. Extensive flooding, particularly along State Highway 12, occurred in December The December 1955 flood, with a recurrence interval of 50 years, produced flows estimated at 2300, 500, and 5000 cubic feet per second (cfs) along the lower reaches of Green Valley, Dan Wilson, and Suisun s, respectively. Green Valley and Dan Wilson s flooded approximately 800 acres of land and caused damage estimated at $40,000. Damage data for other streams in the area are not available. Flood flows in April 1958 peaked at approximately 900, 200, and 2000 cfs along Green Valley, Dan Wilson, and Suisun s, respectively, at U.S. Highway 40 (now Interstate Highway 80). Approximately 1000 acres of farm and orchard land were flooded by these streams. Because of prolonged inundation, field and orchard crops were destroyed or yields were drastically 45

54 Flooding Source Sacramento River Description of Flood Problems reduced. Flood damage totaled approximately $100,000. In 1955, flooding from Suisun, Green Valley, and Dan Wilson s had a recurrence interval of 50 years. The 1958 flood from Suisun and Green Valley s had a recurrence interval of 8 years. The 1958 flood from Dan Wilson had a 10-year recurrence interval. In 1967, flooding from Pennsylvania Avenue and Union Avenue s had a recurrence interval of 15 years. The lower reaches of the Sacramento River are under the influence of tides. Severe flooding along this waterway could result when very high tides and a large volume of stream outflow occur coincidentally, and strong onshore winds generate wave action that would increase the flood hazard above that of the tidal surge alone. The most damaging flood in Rio Vista since 1900 occurred in March A portion of the town was flooded by high flow in the Sacramento River and concurrent high tides. All the buildings in the Waterfront district were flooded, and Front Street was submerged, except at the high southern end. Flooding also occurred in Between 1917 and 1927, the river channel downstream from Rio Vista was enlarged and straightened, so water-surface levels have been greatly reduced since that time. Minor flooding occurred along the waterfront in December 1955, April 1958, and January Several buildings were flooded, but no serious damage was reported. The most recent severe flooding occurred in February 1986, which caused serious damage to the city. The tide gaging station at Rio Vista, with a period of record from 1925 to 1986, had a maximum recorded peak stage of 8.8 feet on February 20, Recurrence intervals were estimated for tide stages along the Sacramento River. The computed frequency for the 1955 flood was 20 years; for the 1958 flood, 10 years; and for the 1973 flood, 25 years. The February 1986 flood was far in excess of the 1-percent annual chance flood frequency. There are no stream flow records for the other streams under study in Rio Vista. The first damaging flood in Rio Vista since the turn of the century occurred in March Flooding also occurred in 1904, 1955, 1958, 1973, and In 1907, a portion of the city was flooded by high flow in Sacramento River and the concurrent high tides. All the buildings in the waterfront district were flooded, and Front Street was submerged except at the high southern end. Between 1917 and 1927, the river channel downstream from Rio Vista was enlarged and straightened, so water-surface levels have been greatly reduced since that time. Minor flooding occurred along the waterfront in December 1955, April 1958, and January Several buildings were flooded, but no serious damage was reported. The highest recorded stages along Sacramento River at Collinsville occurred in January 1909, December 1955, and January In 1955, a combination of high river outflow, high tides, and adverse winds caused a small levee south of Collinsville to fail, and an area was flooded for approximately 5 days. Approximately 1,000 acres of land were flooded along the river upstream from Collinsville. Most of this area is not protected by levees. Information on other 46

55 Flooding Source Description of Flood Problems flood events of record (1950, 1952, and 1958) is not available. On February 20, 1986, flooding along the Sacramento River at Rio Vista reached an elevation of 8.8 feet and was estimated to have exceeded the 1- percent annual chance flood. Southampton Bay Suisun Suisun, Ledgewood, Union Avenue, Pennsylvania Avenue, and Laurel s Suisun Slough There are no streamflow records for the streams under study. Estimated frequencies of major floods along selected streams and computed frequencies of tidal stages along Sacramento River are shown in Table 7, Historic Flood Elevations. In the southwest portion of City of Benicia, Southampton Bay is affected by tidal flooding. Minor damage to suburban residences in the Suisun floodplain occurred, and large amounts of clippings dumped along the stream were picked up by floodwaters and carried downstream to jam on bridges and at other restricted channel sections. The most recent flooding in the Suisun floodplain occurred in Agricultural lands were inundated, and several homes were damaged. Floodwater was reported to be 24 to 30 inches deep at Solano Community College. In January 1967, several streets in Fairfield were flooded, and telephone service was interrupted. A high tide and overflow from Suisun, Ledgewood, Union Avenue, and Laurel s flooded approximately 1100 acres of land in January Many streets were inundated, and a few homes and businesses were evacuated. Damage was estimated at $71,000. Approximately 70 homes in Fairfield were damaged by the January 1973 flood. Water flowed along many streets, and several major roads had to be closed. In January 1967, floodwater covered one-third of the streets of Suisun City and was approximately 2 feet deep in the southern part of town. Flooding from Pennsylvania Avenue and Union Avenue had a recurrence interval of 15 years for the 1967 flood. A high tide and overflow from Laurel and Union Avenue caused flooding in January Suisun Slough is under the influence of tides. The most severe flooding along this waterway would result when very high tides and a large volume of stream outflow occur coincidently. In Fairfield, restrictive outlets into slough areas cause flood flows to pond in low-lying areas, and high tides may delay drainage for several days. Flooding in Fairfield occurred in 1940, 1950, 1955, 1958, 1963, 1966, 1967, 1969, 1970, and Suisun Slough is under the influence of tides. The most severe flooding along this waterway would result when very high tides and a large volume of stream outflow occur coincidently. In Suisun City, restrictive outlets into slough areas cause flood flows to pond in low-lying areas, and high tides may delay drainage for several days. Flooding occurred in Suisun City in 1940, 1950, 1955, 1963, 1966, 1967, 1969, 1970, and Extensive flooding along State Highway 12 occurred in December Floodwater rose to waist depth in a residential development 47

56 Flooding Source when high tides slowed drainage. Description of Flood Problems Suisun Slough, the lower reaches of streams tributary to Suisun Bay, and the lower reaches of Sacramento River are under the influence of tides. The most severe flooding along these waterways would result when very high tides and a large volume of stream outflow occur coincidentally and strong onshore winds generated wave actions. In the Fairfield-Suisun City area, restrictive outlets into slough areas cause flood flows to pond in low-lying areas, and high tides may delay drainage for several days. In urbanized areas, flood problems are intensified because new homes and other structures, new streets, driveways, parking lots, and other paved areas all decrease the amount of open land available to absorb rainfall and runoff, and thus increase the volume of water that must be carried away by waterways. Flood conditions in the Delta are influenced by Pacific Ocean tides, high flood outflow from tributary streams, and strong onshore winds. A single island or a group of islands may flood when the levees protecting them are overtopped or fail as a result of the separate or coincidental occurrence of higher high tides and high stream outflow through the Delta. A fundamental flood problem in the Delta results from the fact that for every square mile of land reclaimed, there is one square mile less of floodplain to contain the volume of the rising tide and outflow from the rivers of the Central valley. The eastern portion of contiguous to the Delta area has a long history of flooding. The major cause of the latest floods was levee instability. The most recent major flood events were those that occurred in 1950, 1955, , 1969, 1972, 1981, 1982, and Sulphur In mid-january 1980, severe rainstorms over central California precipitated high river outflow through the Delta, which, coinciding with gale force winds over the Delta and high tides, resulted in the levee failure and flooding of two tracts (placing approximately 9,600 acres under water). Continued high inflow to the Delta and wind-generated waves increased erosion on all Delta levees, necessitating intensive flood fighting and the temporary curtailment of boat traffic. Then, in late February 1980, three islands at the lower end of the Yolo Bypass and one additional tract were inundated. Recent notable flooding in Benicia occurred in January 1983 and February The 1983 flood caused minor damages to the mobile home park east of H Street. The 1986 flood caused damages to the industrial complex and automobile storage areas along Sulphur Springs (Reference 3). Flooding along Sulphur Springs results from lack of channel capacity and shallow flooding parallel to the channel. The other flooding area is along the waterfront in the southeast part of town by the wastewater treatment plant. At higher tide levels the water will overflow into some of the streets in the Benicia Junior High School area. However, this is expected to be eliminated with the development of a Marina at the foot of East Fifth Street. The rest of the waterline is in the bay itself; a small six to twelve inches of water at infrequent times coming into the lower portions of the town facing on the waterfront. 48

57 Flooding Source Ulatis Description of Flood Problems Thirty-one floods are reported to have occurred in the Ulatis basin, which includes the streams studied, from 1880 through Severe flooding occurred in 1937, 1941, 1943, 1948, 1952, 1955, and Since 1959, flooding has occurred in 1962 (two periods), 1963, 1964 to 1965, 1967, 1969, and In 1958 and 1963, respectively, streams in the basin flooded approximately 20,000 and 26,000 acres of land. During both floods, most of the overflow was below the Union Pacific Railroad and was the result of a commingling of flood flows from several streams. In 1958, the flooded area extended southeast from the railroad for approximately 11 miles, and damage was estimated at $170,000. In 1963, the flooded area extended for approximately 9 miles, and damage was approximately $136,000. Since 1964, flooding has been reduced substantially by a NRCS project comprising channel improvements and levees along selected stream reaches below Interstate Highway 80. It is estimated that the 1967 and 1973 floods have a recurrence interval of 30 years, and the 1969 event has a 7-year recurrence interval. Table 7 contains information about historic flood elevations in the communities within Solano. Table 7: Historic Flooding Elevations Flooding Source Location Historic Peak (Feet NAVD88) Event Date Approximate Recurrence Interval (years) Source of Data Dan Wilson Fairfield-Suisun City Group Data not available Data not available Dan Wilson Fairfield-Suisun City Group Data not available Data not available Green Valley and Suisun s Fairfield-Suisun City Group Data not available Data not available Green Valley and Suisun s Fairfield-Suisun City Group Data not available Data not available Pennsylvania Avenue and Union Avenue s Fairfield-Suisun City Group Data not available Data not available Sacramento River at Collinsville Rio Vista- Collinsville Group Data not available Data not available Sacramento River at Collinsville Rio Vista- Collinsville Group Data not available Data not available 49

58 Flooding Source Sacramento River at Collinsville Location Rio Vista- Collinsville Group Historic Peak (Feet NAVD88) Data not available Event Date Approximate Recurrence Interval (years) Source of Data Data not available Sacramento River at Collinsville Rio Vista- Collinsville Group Data not available Data not available Sacramento River at Collinsville Rio Vista- Collinsville Group Data not available Data not available Sacramento River at Rio Vista Rio Vista- Collinsville Group Data not available Data not available Sacramento River at Rio Vista Rio Vista- Collinsville Group Data not available Data not available Sacramento River at Rio Vista Rio Vista- Collinsville Group Data not available Data not available Ulatis Stream System Vacaville Group Data not available Data not available Ulatis Stream System Vacaville Group Data not available Data not available Ulatis Stream System Vacaville Group Data not available Data not available 4.3 Non-Levee Flood Protection Measures Table 8 contains information about non-levee flood protection measures within such as dams, jetties, and or dikes. Levees are addressed in Section 4.4 of this FIS Report. Flooding Source Table 8: Non-Levee Flood Protection Measures Structure Name Type of Measure Location Description of Measure Alamo New Channel Downstream from Nut Tree Road Alamo Stabilization Structures Along creek Designed to protect against a 10-pecent annual chance flood. Designed to protect against a 10-pecent annual chance flood. 50

59 Flooding Source Dan Wilson s Dixon Structure Name Type of Measure Location Description of Measure Channel improvement Ponding basins Downstream from Rockville Road Just north of Dixon, near the intersection of West H and Almond Streets Dudley Ponding Basin On Dudley south of Interstate 90 Gibson Canyon Green Valley s Horse Horse Channel Improvements Levee and channel improvement Channel Improvements Stabilization Structures From Cache Slough upstream to approximately Interstate Highway 80 Vicinity of Cordelia From Cache Slough upstream to approximately Interstate Highway 80 Along creek Lake Curry Reservoir Suisun, Napa Lakes Madigan, and Frey Lake Madigan and Lake Frey Reservoir Wild Horse, Designed to provide protection against 40-year flood interval. Maintenance of the project is the responsibility of Solano These basins collect runoff from a subdivision area and funnel it through an uncontrolled outlet into an underground conduit carrying Dickson. Small ponding basin Designed to protect against a 10-pecent annual chance flood. Designed to provide protection against 40-year flood interval. Maintenance of the project is the responsibility of Solano Designed to protect against a 10-pecent annual chance flood. Designed to protect against a 10-pecent annual chance flood. This reservoir is operated by the City of Vallejo for domestic water supply and provides incidental flood control by storing runoff that occurs early in the flood season. This reservoir is operated by the City of Vallejo for domestic water supply and provides incidental flood control by storing runoff that occurs early in the flood season. 51

60 Flooding Source Laurel Laurel Ledgewood Ledgewood Marina Channel and other areas near or adjacent to Suisun Slough McCoy McCoy Structure Name Type of Measure Location Description of Measure Improved channel Improved Channel Improved channel Improved channel Sumps Detention basin Improved channel From just north of Air Base Parkway extending upstream approximately 1.1 miles. From the north side of Cement Hill Road upstream for approximately 0.5 mile From the north side of Interstate Highway 80 from approximately 1.1 miles. From Interstate Highway 80 downstream to the Union Pacific Railroad spur. The portion of Suisun City below State Highway 12. Upstream from Air Base Parkway From approximately Prosperity Lane downstream to State Highway 12 These improvements along with channel improvements by private interests, upstream from this reach, will provide 1- percent annual chance flood protection to the land along both banks of the stream beginning approximately 1100 feet upstream from Air Base Parkway and extending upstream for approximately 1.3 miles. The improved channel will provide 1-percent annual chance flood protection to land along the west bank, which has been raised with fill. The east bank has not been raised and is subject to overtopping. Designed to pass a 2- percent annual chance flood flow, including inflow from a proposed diversion from Pennsylvania Avenue. The improved channel protects against the 1-percent annual chance flood. Based on the Fairfield Vicinity Streams flood control project Flood and storm waters collected in the sumps are pumped out into nearby tidal channels. Outflow is regulated by two ungated corrugated metal culverts 6 feet in diameter. The improved channel will contain the 1-percent annual chance flood 52

61 Flooding Source McCoy, Pennsylvania Avenue, Ledgewood, Laurel, and Union Avenue s Old Alamo Pennsylvania Avenue Sacramento River Sulphur Springs Ulatis Ulatis Structure Name Lake Herman Reservoir Type of Measure Location Description of Measure Channel improvements, diversion channels, drop structures, and apparent new bridges and culverts Channel Improvements Detention reservoir Dams and reservoirs Reservoir Channel Improvements Stabilization Structures Along reaches of project flooding sources. From Cache Slough upstream to approximately Interstate Highway 80 Just north of Interstate Highway 80 On main stem and tributary streams from the Sacramento River in Shasta on the north to the American River in Sacramento on the south Sulphur Springs From Cache Slough upstream to approximately Interstate Highway 80 Along creek Known as The Fairfield Vicinity Streams Project. The project was designed to provide protection from the 0.5-percent annual chance flood event. Construction began in 1986 and was completed in Designed to protect against a 10-pecent annual chance flood. Designed to regulate a 15- year flood so that floodwaters can pass under the highway without inundating adjacent areas. Areas adjacent to the river are afforded flood protection directly or indirectly by every flood-control storage project in the Sacramento River Basin Lake Herman Reservoir is located on Sulphur Springs but has no provisions for flood control storage. Designed to protect against a 10-pecent annual chance flood. Designed to protect against a 10-pecent annual chance flood. 4.4 Levees For purposes of the NFIP, FEMA only recognizes levee systems that meet, and continue to meet, minimum design, operation, and maintenance standards that are consistent with comprehensive floodplain management criteria. The Code of Federal Regulations, Title 44, Section (44 CFR 65.10) describes the information needed for FEMA to determine if a levee system provides protection from the 1% annual chance flood. This information must be supplied to FEMA by the community or other party when a flood risk study or restudy is conducted, when FIRMs are revised, or upon FEMA request. FEMA reviews the information for the purpose of establishing 53

62 the appropriate FIRM flood zone. Levee systems that are determined to provide protection from the 1% annual chance flood are accredited by FEMA. FEMA can also grant provisional accreditation to a levee system that was previously accredited on an effective FIRM and for which FEMA is awaiting data and/or documentation to demonstrate compliance with Section These levee systems are referred to as Provisionally Accredited Levees, or PALs. Provisional accreditation provides communities and levee owners with a specified timeframe to obtain the necessary data to confirm the levee s certification status. Accredited levee systems and PALs are shown on the FIRM using the symbology shown in Table 3 and in Table 9. If the required information for a PAL is not submitted within the required timeframe, or if information indicates that a levee system not longer meets Section 65.10, FEMA will de-accredit the levee system and issue an effective FIRM showing the levee-impacted area as a SFHA. FEMA coordinates its programs with USACE, who may inspect, maintain, and repair levee systems. The USACE has authority under Public Law to supplement local efforts to repair flood control projects that are damaged by floods. Like FEMA, the USACE provides a program to allow public sponsors or operators to address levee system maintenance deficiencies. Failure to do so within the required timeframe results in the levee system being placed in an inactive status in the USACE Rehabilitation and Inspection Program. Levee systems in an inactive status are ineligible for rehabilitation assistance under Public Law FEMA coordinated with the USACE, the local communities, and other organizations to compile a list of levees that exist within. Table 9, Levees, lists all levees shown on the FIRM for this FIS Report. The USACE Levee ID provides the identification number of levees that are included in the USACE National Levee Database; the number may not match numbers based on other identification systems that were listed in previous FIS Reports. Levees identified as PALs in the table are labeled on the FIRM to indicate their provisional status. Please note that the information presented in Table 9 is subject to change at any time. For that reason, the latest information regarding any USACE structure presented in the table should be obtained by contacting USACE and accessing the USACE national levee database. For levees owned and/or operated by someone other than the USACE, contact the local community shown in Table. 54

63 Table 9: Levees Community Flooding Source Levee Location Levee Owner Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Ulatis Channel Ulatis By-Pass Ulatis By-Pass Hastings Cut Hastings Cut NP NP NP NP NP Water Agency (SCWA) Reclamation District 2060 Reclamation District 2060 Reclamation District 2060 Reclamation District 2060 USACE Levee ID Covered Under PL84-99 Program? FIRM Panel(s) Levee Status No 06095C0325E ACCREDITED Yes 06095C0320E Yes 06095C0340E PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED No 06095C0320E ACCREDITED No 06095C0340E ACCREDITED Horseshoe Bend NP NP No 06095C0730E ACCREDITED Steamboat Slough Steamboat Slough Steamboat Slough Steamboat Slough Steamboat Slough Hastings Cut Hastings Cut City Of Fairfield Green Valley Left Descending City Of Fairfield Green Valley Left Descending NP NP NP NP NP NP NP Reclamation District 501 Reclamation District 501 Reclamation District 501 Reclamation District 501 Reclamation District 501 Reclamation District 2060 Reclamation District 2060 City of Fairfield Department of Public Works City of Fairfield Department of Public Works Yes 06095C0535E DE-ACCREDITED Yes 06095C0541E DE-ACCREDITED Yes 06095C0365E DE-ACCREDITED Yes 06095C0545E DE-ACCREDITED Yes 06095C0555E DE-ACCREDITED No 06095C0320E ACCREDITED No 06095C0340E ACCREDITED No 06095C0434E ACCREDITED No 06095C0432E ACCREDITED 55

64 Community Flooding Source Levee Location Levee Owner City Of Fairfield Dan Wilson NP Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas City of Fairfield Department of Public Works USACE Levee ID Covered Under PL84-99 Program? FIRM Panel(s) Levee Status No 06095C0451E ACCREDITED Unnamed NP NP No 06095C0715E NP Miner Slough Miner Slough Miner Slough South Fork Putah South Fork Putah City Of Vacaville Alamo Channel NP Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas NP NP NP NP NP Reclamation District 501 Reclamation District 501 Reclamation District 501 CA Department of Water Resources (DWR) CA Department of Water Resources (DWR) Water Agency (SCWA) Yes 06095C0535E DE-ACCREDITED Yes 06095C0365E DE-ACCREDITED Yes 06095C0345E DE-ACCREDITED Yes 06095C0100E Yes 06095C0075E PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED No 06095C0279E ACCREDITED Lindsey Slough NP NP Yes 06095C0530E Lindsey Slough NP NP Yes 06095C0320E Lindsey Slough NP NP Yes 06095C0340E Sacramento River Deep Water Ship Channel South Fork Putah South Fork Putah Cache Slough Cache Slough PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED NP NP No 06095C0730E ACCREDITED NP NP NP NP CA Department of Water Resources (DWR) CA Department of Water Resources (DWR) Reclamation District 2104 Reclamation District Yes 06095C0100E Yes 06095C0075E Yes 06095C0320E Yes 06095C0340E PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED 56

65 Community Flooding Source Levee Location Levee Owner Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Hass Slough Hass Slough Hass Slough Hass Slough Hass Slough Ulatis Channel Ulatis Channel Ulatis Channel Ulatis Channel Wright Cut Wright Cut Sutter Slough Cache Slough Cache Slough NP NP NP NP NP NP NP NP NP NP NP NP NP NP Reclamation District 2104 Reclamation District 2104 Reclamation District 2104 Reclamation District 2104 Reclamation District 2104 Water Agency (SCWA) Water Agency (SCWA) Water Agency (SCWA) Water Agency (SCWA) Reclamation District 2060 Reclamation District 2060 Reclamation District 501 Reclamation District 501 Reclamation District USACE Levee ID Covered Under PL84-99 Program? FIRM Panel(s) Yes 06095C0320E Yes 06095C0325E Yes 06095C0330E Yes 06095C0340E Yes 06095C0325E Levee Status PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED No 06095C0320E ACCREDITED No 06095C0325E ACCREDITED No 06095C0320E ACCREDITED No 06095C0325E ACCREDITED Yes 06095C0530E Yes 06095C0340E PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED Yes 06095C0365E DE-ACCREDITED Yes 06095C0535E DE-ACCREDITED Yes 06095C0541E DE-ACCREDITED Cache Slough NP NP Yes 06095C0340E Unnamed NP NP Yes 06095C0541E PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED Unnamed NP NP No 06095C0541E ACCREDITED

66 Community Flooding Source Levee Location Levee Owner USACE Levee ID Covered Under PL84-99 Program? FIRM Panel(s) Levee Status City Of Rio Vista Unnamed NP NP No 06095C0530E ACCREDITED Unincorporated Areas Unincorporated Areas Unnamed NP NP No 06095C0175F NP Unnamed NP NP No 06095C0175F NP City Of Vacaville Alamo Channel NP City Of Vacaville Alamo Channel NP Unincorporated Areas Unincorporated Areas Unincorporated Areas Unincorporated Areas Alamo Channel Alamo Channel Barker Slough Cache Slough NP NP NP NP Water Agency (SCWA) Water Agency (SCWA) Water Agency (SCWA) Water Agency (SCWA) Reclamation District 2060 Reclamation District No 06095C0279E ACCREDITED No 06095C0283E ACCREDITED No 06095C0283E ACCREDITED No 06095C0284E ACCREDITED Yes 06095C0320E Yes 06095C0340E PROVISIONALLY ACCREDITED PROVISIONALLY ACCREDITED 58

67 SECTION 5.0 ENGINEERING METHODS For the flooding sources 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 at least once on the average during any 10-, 25-, 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-, 25-, 50-, 100-, and 500-year floods, have a 10-, 4-, 2-, 1-, and 0.2% 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 100-year flood (1-percent chance of annual exceedance) during the term of a 30-year mortgage is approximately 26 percent (about 3 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. The engineering analyses described here incorporate the results of previously issued Letters of Map Change (LOMCs) listed in Table 27, Incorporated Letters of Map Change, which include Letters of Map Revision (LOMRs). For more information about LOMRs, refer to Section 6.5, FIRM Revisions. 5.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak elevation-frequency relationships for floods of the selected recurrence intervals for each flooding source studied. Hydrologic analyses are typically performed at the watershed level. Depending on factors such as watershed size and shape, land use and urbanization, and natural or man-made storage, various models or methodologies may be applied. A summary of the hydrologic methods applied to develop the discharges used in the hydraulic analyses for each stream is provided in Table 13. Greater detail (including assumptions, analysis, and results) is available in the archived project documentation. A summary of the discharges is provided in Table 10. Frequency Discharge-Drainage Area Curves used to develop the hydrologic models may also be shown in Figure 7 for selected flooding sources. A summary of stillwater elevations developed for non-coastal flooding sources is provided in Table 11. (Coastal stillwater elevations are discussed in Section 5.3 and shown in Table 17.) Stream gage information is provided in Table

68 Flooding Source Alamo 1 Alamo Alamo Location Pleasant Valley Road Interstate Highway 80 Alamo Drive Drainage Area (Square Miles) Table 10: Summary of Discharges 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance 2,000 * 2,700 2,900 * 3,600 2,700 * 3,200 3,400 * 3,500 4,400 * 5,700 6,200 * 6,700 Alamo Leisure town Road ,500 * 3,700 3,700 * 3,900 Bucktown At Confluence with Ulatis * * 420 Clayton At Clayton Road * * 1,390 Dan Wilson At Cordelia Road * 1,220 1,995 * 3,100 3,4 Dan Wilson 3,4 At Interstate Highway * 1,220 1,995 * 3,100 Dan Wilson At Rockville Road * 795 1,535 * 2,500 3,4 Dickson 5 At Union Pacific Railroad * * 36 Dickson 5 At North 1 st Street * * 689 Dickson 5 Dickson 5 At North Almond Street At Interstate Highway * * * *

69 Flooding Source Encinosa Gibson Canyon Gibson Canyon Gibson Canyon Gibson Canyon Gibson Canyon Gibson Canyon Gibson Canyon Gibson Canyon Gibson Canyon Location At Confluence with Alamo At Browns Valley Road Upstream of Putah South Canal Downstream of Putah South Canal Approximately 675 feet downstream of Eubanks Road At Interstate Highway 505 Upstream of Confluence of South Branch Gibson Canyon At Leisure Town Road At Sewage Treatment Plant Upstream of Interstate Highway 80 Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance * 1,070 1,080 * 1, * * * * 1, * * * * * * * * * * * * ,800 * 2,300 2,400 * 2,900 61

70 Flooding Source Gibson Canyon Gibson Canyon Gordon Valley Location Downstream of Interstate Highway 80 Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance ,800 * 2,300 2,500 * 3,000 At Byrnes Road ,900 * 2,500 2,700 * 3,100 At Gordon Valley Road * 900 1,210 * 2,210 Green Valley At Cordelia Road ,225 * 2,400 2,950 * 4,350 3, 10 Green Valley At 3, 10 Green Valley At 3, 10 Interstate Highway 80 Green Valley Road (Lower Crossing) ,225 * 2,350 3,300 * 5, ,130 * 2,200 3,350 * 6,600 Green Valley At Mason Road * 1,800 2,150 * 2,700 3, 10 Green Valley At Rockville Road * 1,550 1,750 * 2,350 3, 10 Green Valley At 3, 10 Green Valley At 3, 10 Horse Horse Wild Horse Green Valley Road (Upper Crossing) Upstream of Putah South Canal Downstream of Putah South Canal * 1,550 2,500 * 4, * 790 1,260 * 2, * * * *

71 Flooding Source Horse Horse Horse Location Upstream of Confluence with South Branch Horse At Interstate Highway 505 Upstream of Confluence with Pine Tree Horse At Interstate 80 (Westbound) Drainage Area (Square Miles) 10% Annual Chance 63 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance * * * * * * ,200 * 1,300 1,400 * 1,500 Horse At Orange Drive ,600 * 1,900 1,900 * 2,000 9 Horse At Leisure Town Road ,200 * 2,700 2,700 * 3,300 Horse At Willow Avenue ,300 * 2,800 2,900 * 3,500 Industrial Laguna Lagoon Drain Laurel Laurel Laurel At St. Francis Way * * 940 At Confluence with Alamo At Interstate Highway 80 At Union Pacific Railroad At Air Base Parkway At Putah South Canal 8.8 1,700 * 2,400 2,700 * 3, * * 1, * 1,615 2,190 * 4, * 1,400 1,930 * 3, * 1,260 1,740 * 3,130

72 Flooding Source Ledgewood 11 Location At Interstate Highway 80 Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance ,180 * 2,280 4,480 * 12,750 Ledgewood At Abernathy Road ,020 * 2,120 4,100 * 12, Ledgewood 11 At Mankas Corner Road * 770 1,070 * 2,000 Marina At Second Street * * 1,500 Marina Marina Tributary McCoy McCoy Above Marina Tributary * * 1,330 Above Mouth * * 170 At State Highway 12 At Air Base Parkway * 800 1,040 * 1, * * 500 Middle Branch At Mouth * * 570 Horse 12 Middle Branch Horse 12 Middle Branch Horse 12 Middle Swale to South Branch Horse At Interstate Highway 505 At Confluence with Horse At Confluence with South Branch Horse * * * * * *

73 Flooding Source Middle Swale to South Branch Horse Middle Swale to South Branch Horse Location At Putah South Canal At Browns Valley Road Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance * * * * 340 Old Alamo At Lewis Road * * Old Alamo 13 Old Alamo 13 At Union Pacific Railroad At Leisure Town Road * * * * 215 Pennsylvania At Cordelia Road * * 1,000 Avenue 1 Pennsylvania At Crowley Lane * * 1,400 Avenue 1 Pennsylvania Avenue 1 Pine Tree 14 Pine Tree 14 Pine Tree 14 At Interstate Highway 80 Upstream of Union Pacific Railroad Upstream of Putah South Canal Downstream of Putah South Canal * * * * * * * *

74 Flooding Source Pine Tree 14 Pine Tree 14 Location At Interstate Highway 505 At Interstate Highway 80 Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance * * * * 825 Pine Tree At Nut Tree Airport * * 1, Pine Tree 14 Pine Tree 14 South Branch Gibson Canyon South Branch Gibson Canyon South Branch Gibson Canyon South Branch Gibson Canyon South Branch Gibson Canyon At Putah South Canal At Browns Valley Road Upstream of Browns Valley Road Upstream of Putah South Canal Downstream of Putah South Canal At Interstate Highway 505 Upstream of Confluence with Gibson Canyon * * 2, * * * * * * * * * * * *

75 Flooding Source South Branch Horse 15 South Branch Horse 15 South Branch Horse 15 Location Upstream of Confluence of Middle Swale to South Branch Horse Downstream of Putah South Canal Upstream of Confluence with Horse Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance * * * * * * 420 South Branch At Mouth * * 650 Horse 15 South Branch Horse 15 South Branch Horse 15 At Putah South Canal At Browns Valley Road * * * * 680 Suisun 1 At Cordelia Road ,550 * 3,330 3,300 * 3,330 Suisun 1 At Interstate Highway ,550 * 3,610 3,610 * 3,610 Suisun 1 At Rockville Road ,500 * 4,200 4,200 * 4,310 Suisun 1 Suisun 1 Suisun 1 At Suisun Valley Road At Wooden Valley Road At Nap - Limits ,550 * 4,900 5,850 * 6, ,450 * 5,000 6,9000 * 10, ,200 * 4,800 6,500 * 12,400 67

76 Flooding Source Sulphur Springs Sulphur Springs Sulphur Springs Sweany 1 Sweany 1 Sweany 1 Location Downstream of I- 680 Downstream of East 2 nd Street Downstream of Lake Herman Above confluence with McCune At Putah Canal South Bridge At Pleasant Glen Road Drainage Area (Square Miles) 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance ,060 * 3, * 10, ,987 * 3,068 3,900 * 10, ,391 * 2,217 2,856 * 7, * * * 2,660 * * 10.1 * * * 3,780 * * 7.65 * * * 3,830 * * Ulatis 1 Leisure Town Road ,700 * 2,800 2,800 * 2,800 Ulatis 1 Ulatis 1 Ulatis 1 Union Avenue 16 Union Avenue 16 Union Avenue 16 At Interstate Highway 80 Farrell Road Putah South Canal At Marina Boulevard At Union Pacific Railroad At Washington Street ,300 * 4,700 5,200 * 6,100 3,000 * 4,200 4,700 * 5,800 3,300 * 4,400 4,500 * 4, * * * * * * 630 Union Avenue At Travis Boulevard * *

77 Flooding Source Union Avenue 16 Union Avenue 16 Union Avenue 16 Union Avenue 16 Union Avenue 16 Location At Western Pacific Railroad At Air Base Parkway At Putah South Canal At Interstate Highway 80 (Downstream) At Interstate Highway 80 (Upstream) Drainage Area (Square Miles) Union At Airbase Parkway 5.6 Union At Cordero Junction 4.9 Union At Union Pacific Railroad 10% Annual Chance 4% Annual Chance Peak Discharge (cfs) 2% Annual Chance 1% Annual Chance Existing 1% Annual Chance Future 0.2% Annual Chance * * * * * * * * * * 1, * * * * 2,500 * 1,930 * Wild Horse At Mouth * * 1,350 1 Wild Horse 1 At Upstream Limit of Study * 750 1,270 * 2, *Not calculated for this FIS project. 1 Decrease in 2-, 1-, and 0.2-percent annual chance floodflows in downstream direction due to overbank losses 2 Data not available 3 Flows from Green Valley and Dan Wilson s commingle between Interstate Highway 80 and Cordelia Road 4 Flows for 2-, 1, and 0.2 percent annual chance floods include overland flow from Suisun 5 Fluctuation in floodflows due to ponding and (for North First Street and Union Pacific Railroad index stations) overland flow from Dudley 69

78 6 Includes Dudley 7 Increase in area with reduction in discharge due to ponding behind South Putah Canal 8 Increase in area with reduction in discharge due to split flow 9 Value reflects reduction in total discharge due to split flow 10 Fluctuation in 1- and 0.2-percent annual chance floodflows due to tributary inflow and overbank gains and losses 11 Flows for 1- and 0.2-percent annual chance floods at Abernathy Road and Interstate Highway 80 include overland flow from Suisun 12 Decrease in 2- and 1-percent annual chance floodflows in a downstream direction due to overbank losses 13 Fluctuation in 10-percent annual chance floodflow due to tributary inflow above Interstate Highway 80 and to channel routing losses; fluctuation in 2-, 1-, and 0.2-percent annual chance floodflows due to overbank losses and tributary inflow 14 Decrease in 10-percent annual floodflow in a downstream direction due to channel routing losses; fluctuation in 1- and 0.2-percent annual chance floodflows due to overbank losses and large inflow from Ulatis via overflow channelway 15 Fluctuation in 10-percent annual chance floodflow due to channel routing losses and tributary inflow above South Putah Canal; fluctuation in 2-, 1-, and 0.2-percent annual chance floodflows due to overbank losses and tributary inflow 16 Fluctuation in floodflows due to overbank gains and losses 70

79 Figure 7: Frequency Discharge-Drainage Area Curves 71

80 Table 11: Summary of Non-Coastal Stillwater Elevations Elevations (feet NAVD88) Flooding Source Carquinez Strait Location At Interstate Highway 80 Bridge 10% Annual Chance 4% Annual Chance 7.8 * 2% Annual Chance 2 1% Annual Chance 0.2% Annual Chance Lake Chabot Lake Chabot 78.6 * Lake Dalwigk Lake Dalwigk 8.3 * Mare Island Strait At Mouth 8.6 * Napa River At Sears Point Road 7.9 * Sacramento River Sacramento River Sacramento River Sacramento River Sacramento River At Collinsville Tide Gage At Rio Vista Tide Gage At Walnut Grove Tide Gage At Snodgrass Slough Tide Gage At Sacramento (Near I Street Bridge) * * * * * Suisun Slough At Suisun City 8.3 * *Not calculated for this FIS project 1Reflects a static water condition that includes wind set and any other hydrologic action that tends to build up stage levels, but not wave action, which will increase 1-percent annual chance flood stage by 1.5 to 2.0 feet. 2Data not available 72

81 Table 12: Stream Gage Information used to Determine Discharges Flooding Source Gage Identifier Agency that Maintains Gage Site Name Drainage Area (Square Miles) Period of Record From To Not Available Not Available Not Available Not Available Not Available Not Available Not Available 5.2 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. Base flood elevations on the FIRM represent the elevations shown on the Flood Profiles and in the Floodway Data tables in the FIS Report. Rounded whole-foot elevations may be shown on the FIRM in coastal areas, areas of ponding, and other areas with static base flood elevations. These wholefoot elevations may not exactly reflect the elevations derived from the hydraulic analyses. 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. The hydraulic analyses for this FIS 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. For streams for which hydraulic analyses were based on cross sections, locations of selected cross sections are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 6.3), selected cross sections are also listed on Table 24, Floodway Data. A summary of the methods used in hydraulic analyses performed for this project is provided in Table 13. Roughness coefficients are provided in Table 14. Roughness coefficients are values representing the frictional resistance water experiences when passing overland or through a channel. They are used in the calculations to determine water surface elevations. Greater detail (including assumptions, analysis, and results) is available in the archived project documentation. 73

82 Table 13: Summary of Hydrologic and Hydraulic Analyses Flooding Source Alamo Carquinez Strait Clayton Dan Wilson Dan Wilson Dickson Dudley Encinosa Gibson Canyon Gordon Valley Downstream Limit Limits within unincorporated areas of Solano Limits within City of Vallejo Limits within unincorporated areas of Solano Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within Limits within City of Dixon Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Study Limits Upstream Limit Limits within unincorporated areas of Solano Limits within City of Vallejo Limits within unincorporated areas of Solano Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within Limits within City of Dixon Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Hydrologic Model or Method Used Hydraulic Model or Method Used Date Analyses Completed HEC-1 HEC-2 Date not available HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 Approximate Date not methods available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available Flood Zone on FIRM A, AE AE, VE AE AE A A A, AE A A, AE AE Special Considerations 74

83 Flooding Source Green Valley Green Valley Horse Industrial Lagoon Drain Laguna Laurel Ledgewood Ledgewood Mare Island Strait Downstream Limit Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within City of Vallejo Study Limits Upstream Limit Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within City of Vallejo Hydrologic Model or Method Used Hydraulic Model or Method Used Date Analyses Completed HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available Flood Zone on FIRM AE, AO AE, AO A, AE AO AE AE AE AE, AO AE, AO AE Special Considerations 75

84 Flooding Source Marina Marina Tributary McCoy Middle Branch Horse Middle Swale to South Branch Horse Napa River Old Alamo Pennsylvania Avenue Pine Tree Sacramento River Study Limits Hydrologic Model or Method Used Hydraulic Model or Method Used Date Analyses Completed Flood Zone on FIRM AE Downstream Limit Upstream Limit Limits within Limits within HEC-1 HEC-2 Date not unincorporated unincorporated available areas of Solano areas of Solano Limits within Limits within HEC-1 HEC-2 Date not AE unincorporated unincorporated available areas of Solano areas of Solano Limits within Limits within HEC-1 HEC-2 Date not unincorporated unincorporated available areas of Solano areas of Solano Limits within Limits within HEC-1 HEC-2 Date not AE unincorporated unincorporated available areas of Solano areas of Solano Limits within Limits within HEC-1 HEC-2 Date not unincorporated unincorporated available areas of Solano areas of Solano Limits within City Limits within City HEC-1 HEC-2 11/14/1980 AE of Vallejo of Vallejo Limits within Limits within HEC-1 HEC-2 Date not AE unincorporated unincorporated available areas of Solano areas of Solano Limits within Limits within HEC-1 HEC-2 Date not unincorporated unincorporated available areas of Solano areas of Solano Limits within Limits within HEC-1 HEC-2 Date not available Delta Region Delta Region HEC-1 HEC-2 Date not available A, AE, AO AE, AO AE, AO A, AE AE, AH Special Considerations 76

85 Flooding Source Sacramento- San Joaquin Delta Suisun Suisun Slough Sulphur Springs Sweany Ulatis Union Avenue Union Unnamed Tributary to Ulatis Wild Horse Downstream Limit Sacramento-San Joaquin Delta area Limits within Limits within Sulphur Springs within City of Benicia Confluence with McCune Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Study Limits Upstream Limit Sacramento-San Joaquin Delta area Limits within Limits within Sulphur Springs within City of Benicia 930 feet above Timms Road Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Streams within City of Fairfield Limits within unincorporated areas of Solano Limits within unincorporated areas of Solano Hydrologic Model or Method Used Hydraulic Model or Method Used Date Analyses Completed HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 N/A Date not Tidal effects available only, not influenced by stream hydraulics HEC-1 HEC-2 Date not available Flood Zone on FIRM AE, AH AE A AE, AO HEC-1 HEC-RAS AE,AO HEC-1 HEC-2 Date not AE available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available HEC-1 HEC-2 Date not available AE, AH AE AE AE Special Considerations 77

86 Table 14: Roughness Coefficients Flooding Source Channel n Overbank n Gibson Canyon Horse Middle Branch Horse Middle Swale to South Branch Horse North Branch Horse Pine Tree South Branch Gibson Canyon South Branch Horse Sulphur Springs Sweany Coastal Analyses For the areas of that are impacted by coastal flooding processes, coastal flood hazard analyses were performed to provide estimates of coastal BFEs. Coastal BFEs reflect the increase in water levels during a flood event due to extreme tides and storm surge as well as overland wave effects. The following subsections provide summaries of how each coastal process was considered for this FIS Report. Greater detail (including assumptions, analysis, and results) is available in the archived project documentation. Table 15 summarizes the methods and/or models used for the coastal analyses. Refer to Section for descriptions of the terms used in this section. Table 15: Summary of Coastal Analyses Flooding Source From Study Limits To Hazard Evaluated Model or Method Used Date Analysis was Completed Carquinez Strait / Mare Island Strait Entire reach Entire reach Statistical Analyses Not Available Unknown Sutter Slough Entire reach Entire reach Statistical Analyses Delta Tidal Hydrodynamic Computer Model Unknown Steamboat Slough Entire reach Entire reach Statistical Analyses Delta Tidal Hydrodynamic Computer Model Unknown 78

87 Flooding Source From Study Limits To Hazard Evaluated Model or Method Used Date Analysis was Completed Miner Slough Entire reach Entire reach Statistical Analyses Delta Tidal Hydrodynamic Computer Model Unknown Suisun Bay Entire reach Entire reach Statistical Analyses Delta Tidal Hydrodynamic Computer Model Unknown Sacramento River Entire tidal reach Entire tidal reach Statistical Analyses Elevations Interpolated between tidal gaging stations Total Stillwater Elevations The total stillwater elevations (stillwater including storm surge plus wave setup) for the 1% annual chance flood were determined for areas subject to coastal flooding. The models and methods that were used to determine storm surge and wave setup are listed in Table 15. The stillwater elevation that was used for each transect in coastal analyses is shown in Table 17, Coastal Transect Parameters. Figure 8 shows the total stillwater elevations for the 1% annual chance flood that was determined for this coastal analysis. 79

88 Figure 8: 1% Annual Chance Total Stillwater Elevations for Coastal Areas Astronomical Tide Astronomical tidal statistics were generated directly from local tidal constituents by sampling the predicted tide at random times throughout the tidal epoch. Storm Surge Statistics Storm surge is modeled based on characteristics of actual storms responsible for significant coastal flooding. The characteristics of these storms are typically determined by statistical study of the regional historical record of storms or by statistical study of tidal gages. When historic records are used to calculate storm surge, characteristics such as the strength, size, track, etc., of storms are identified by site. Storm data was used in conjunction with numerical hydrodynamic models to determine the corresponding storm surge levels. An extreme value analysis was performed on the storm surge modeling results to determine a stillwater elevation for the 1% annual chance event. Tidal gages can be used instead of historic records of storms when the available tidal gage record for the area represents both the astronomical tide component and the storm surge component. Table 16 provides the gage name, managing agency, gage type, gage identifier, start date, end date, and statistical methodology applied to each gage used to determine the stillwater elevations. For areas between gages, peak stillwater elevations for selected recurrence intervals 80