FINAL REPORT Coastal Hazards Atlas of Texas: A Tool for Hurricane Preparedness and Coastal Management – Volume 1 The Southeast Coast James C. Gibeaut, William A. White, and Thomas A. Tremblay Development and compilation of Geographic Information System (GIS) database by Sarah B. Dale, Gregory J. Jeffers, John R. Andrews Under the supervision of Thomas A. Tremblay, Senior GIS Analyst A Report of the Texas Coastal Coordination Council pursuant to National Oceanic and Atmospheric Administration Award No. NA770Z0202 Bureau of Economic Geology Scott W. Tinker, Director The University of Texas at Austin Austin, Texas 78713-8924 June 2000 Contents Introduction......................................................................................................................... 1 Relative Sea-Level Rise...................................................................................................... 3 The Moving Gulf of Mexico Shoreline .............................................................................. 5 Long-Term Change.......................................................................................................... 6 Short-Term Change ......................................................................................................... 7 Episodic Shoreline Retreat .............................................................................................. 8 The Pattern of Shoreline Change Today and the Effects of Human-Made Structures.... 9 The Moving Bay Shoreline............................................................................................... 12 Tropical Storms and Hurricanes ....................................................................................... 13 Storm-Surge Penetration................................................................................................ 16 Storm Washover Features.............................................................................................. 16 Subsidence ........................................................................................................................ 17 Problems ........................................................................................................................ 17 Location......................................................................................................................... 17 Causes............................................................................................................................ 20 Measuring Subsidence................................................................................................... 21 Faulting ............................................................................................................................. 24 Problems ........................................................................................................................ 24 Locations and Characteristics........................................................................................ 24 Causes............................................................................................................................ 27 Locating and Mapping Active Faults ............................................................................ 27 Summary of Subsidence and Faulting .............................................................................. 29 References......................................................................................................................... 29 Appendix: Geographic Information System Documentation Files................................... 32 I. Bay Erosion............................................................................................................... 33 II. Cities ........................................................................................................................ 35 III. County Boundaries .................................................................................................. 35 IV. Digital Orthophoto Mosaics (doqq) ........................................................................ 35 V. Environmental Sensitivity Index (ESI) Shoreline .................................................... 36 VI. Faulting.................................................................................................................... 37 VII. Gulf of Mexico Shoreline Erosion (gomerosion) .................................................. 52 VIII. Hurricane Surge and Flooding.............................................................................. 55 IX. National Wetland Inventory (nwi) .......................................................................... 63 X. Shorelines ................................................................................................................. 63 XI. Subsidence.............................................................................................................. 65 XII. Washover Features................................................................................................. 68 ......................................................................................................................................68 Introduction This report accompanies the CD-ROM of the Texas Coastal Hazards Atlas – Volume 1, 2000. The atlas is being developed in response to the need for technical information by coastal planners and to increase public awareness of coastal processes. The area covered in volume 1 (fig. 1) is the southeast coast from the Brazos River on the west to Sabine Lake on the east. The atlas consists of Geographic Information System files in ArcView format. The maps may be viewed and customized on a personal computer using ArcView or ArcExplorer software. Following is the database structure: I. Bay Erosion II. Cities III. County Boundaries A. County Boundaries IV. Digital Orthophoto Mosaics (doqq) A. North 1. Bolivar 2. Caplen 3. Flake 4. High Island 5. Mud Lake 6. Oyster Bayou 7. Robinson Lake 8. Star Lake 9. Clam Lake 10. Sabine Pass 11. North Galveston 12. South Galveston 13. Hoskins Mound 14. Jamaica Beach 15. Jones Creek 16. San Luis Pass 17. Surfside 18. Virginia Point 19. West Bay 20. Christmas Bay 21. North Christmas Bay 22. Christmas Point V. Environmental Sensitivity Index (ESI) Shoreline 1 VI. Faulting A. BEG Faults B. Environmental Geologic Atlas of Texas Faults C. USGS Faults VII. Gulf of Mexico Shoreline Erosion (gomerosion) VIII. Hurricane Surge and Flooding A. Computer Model Surge Data 1. Maximum Surge Lines a. Level 1-5 2. Net Inundation Area a. Level 2-5 B. Hurricanes Beulah and Carla Flood Areas IX. National Wetland Inventory (nwi) A. Brazoria County 1. NWI data for 7.5' quads B. Chambers County 1. NWI data for 7.5' quads C. Galveston County 1. NWI data for 7.5' quads D. Harris County 1. NWI data for 7.5' quads E. Jefferson County 1. NWI data for 7.5’ quads X. Shorelines A. Historic Shorelines B. 2006 Shoreline C. 2026 Shoreline D. 1996 Shoreline E. 2056 Shoreline F. Net Projected Shoreline Erosion and Accretion 1996-2056 XI. Subsidence A. Approximate Land-Surface Subsidence 1906-1987 B. Approximate Land-Surface Subsidence 1983-1987 C. Extensometer Measurement Sites D. TIF Image Files of Extensometer Graphs XIII. Washover Features A. Washover Channels and Interdune Drainages (Polygons) B. Washover Areas (Arcs) Note: All data are in UTM projection Zone 15, NAD83 Subsequent volumes of the atlas will extend coverage to the entire Texas coast. The following sections describe the hazards mapped along the southeast Texas coast. The text is intended for the general public, but more technical references are provided for those interested. The appendix contains copies of documentation files found on the CD- 2 ROM. These files provide technical descriptions of the data and how the data were processed in the Geographic Information System. Figure 1. The upper Texas coast. Relative Sea-Level Rise A rise in the surface of the ocean, a lowering of the land surface or both may cause relative sea-level rise. By looking at sediments once deposited along ancient coasts and now buried beneath more recent sediments or submerged on the continental shelf, 3 geologists know that 18,000 years ago, when the last ice age ended, sea level was about 400 ft lower than it is today.(1) Since that time and on a scale of several thousands of years, the addition of water to the oceans by melting continental ice sheets has caused sea level to rise and the shoreline to move landward. During the 20th century along the upper Texas coast, the rate of relative sea-level rise has been 0.022 ft/yr (about 1 foot in 46 years) as measured by the Pier 21 tide gauge on Galveston Island (fig. 2). This rise is caused by compaction of sediments causing a lowering of the land surface (see the subsidence section) and by a raising in the global ocean surface caused by melting glaciers and thermal expansion of seawater. The upper Texas coast has a relatively high rate of relative sea-level rise compared to other Gulf of Mexico locations because of the high rate of land subsidence. Global warming scenarios predict an increase in the rate of global sea-level rise, but even if that does not happen and relative sea-level rise continues at its present rate there is reason for concern and special planning. 7 Tropical Storm Frances, Sept. 1998 6 Hurricane Carla, Sept. 1961 m 5 2 ft bove Station Datu A 4 Feet 3 2 1900 1920 1940 1960 1980 2000 Years Figure 2. Monthly average sea level since 1909 as measured by the Pier 21 tide gauge in Galveston