Haiyan Yang Department of Geography CE394K/GIS in Water Resources     Beachfront Development at Jamaica Beach, Galveston Bay   Contents Introduction Objective Study Area Data Source Methods and Processing Georeference in ArcMap Findings and Conclusion Reference

Introduction Texas, especially the Gulf of Mexico area, is prone to flood hazards. A hurricane or tropical storm will strike Texas every 2.6 years, and a severe hurricane will hit about every ten years (Ellis, 1986). For most communities in Texas, floods are by far the most common and serious natural hazards the citizens must face. Floods have taken hundreds of lives over the years and caused huge economic loss as well. The 1900 Galveston hurricane killed over 6000 people, making it the greatest disaster in the history of the United States. During the 1915 hurricane 275 lives were lost even though the Weather Bureau had issued warnings 24 hours in advance. The $2 billion in damages made '1983 Hurricane Alicia' the costliest hurricane in Texas history (Ellis, 1986). Coastal flood caused countless loss of people's lives and property. However, people enjoy living in these hazard areas, where they have easy access to rivers, lakes, and the ocean. Typically, these people move back to these waterfront and beachfront areas even after a severe flood regardless of future flood risks. However, do people really know the influence of hurricane disasters? How often does that happen? What was the development pattern during the last couple of decades? Shall people pay enough attention to the coastal disasters?   Objective Using ArcGIS as a tool, this research is to examine how exposure to flood hazards had affected the beachfront area at Jamaica beach, Galveston bay, what was the pattern of beachfront development in Texas during last 40 years and give some suggestion on what people should do in the future.

Study Area Galveston county is on the Upper Coast of Gulf of Mexico. Jamaica beach in on the middle part of the Galveston Island.

Data Source Bureau of Economic Geology(BEG) Raster Data: There are 1:24,000 historical aerial photographs of Jamaica Beach available at the Bureau of Economic Geology, the University of Texas at Austin. They are adequate photographs from 1950s to 2000s. Vector Data: Shapefiles for shorelines, highways, counties are also available at BEG. Texas General Land Office(GLO) Raster Data: Registered digital Orthophoto of Jamaica beach in January, 2002 is available at Texas General Land Office. It is georeferenced into UTM zone 15.

Methods and Processing To be able to show the influence of coastal hazards to beachfront area, two standards are taken: shoreline changed over a period of almost 40 years (1965-2002) and washover away from the shoreline during Hurricane Carla and Hurricane Alicia. Raster Data Processing: First, the historical aerial photographs are dug out from the Bureau of Economic Geology. Four of them are selected to be scanned at 800 dpi. These photographs were taken on 05/05/1958, 10/21/65, 08/22/1983, 4 days after Hurricane Alicia, and 09/14/1961, 4 days after Hurricane Carla. Upper left: Original photograph after scanning. Upper right and Below: Since the file size is too big at 800 dpi, image needs to be cut to get the necessary area. This will increase the processing speed. After scanning them, they will be registered and georeferenced into UTM zone 15 projection. The GLO January 2002 photo will be used as the base image. Vector Data Creating: The next thing is to digitize the specific shorelines of 1965 and 2002. The shoreline change will be displayed from the difference. The past coastal development will also be clearly demonstrated. The washover caused by the 1961 Hurricane Carla and the 1983 Hurricane Alicia will also be digitized to illustrate the danger of living under specific hurricane.

Georeference in ArcMap This is the key part of this project. Below is to demonstrate how to georeference aerial photographs in ArcMap. Scanned maps don't usually contain information as to where the area represented on the map fits on the surface of the earth; the locational information delivered with aerial photos is often inadequate to perform analysis or display in proper alignment with other data. Thus, in order to use this types of raster data in conjunction with other spatial data, users often need to align—or georeference—it to a map coordinate system. Georeferencing raster data is to define how the data is situated in map coordinates. This process includes assigning a coordinate system that associates the data with a specific location on the earth. Georeferencing raster data allows it to be viewed, queried, and analyzed with other geographic data. In this case, all photographs will be georeferenced into UTM zone 15. Steps: 1. Add the layers residing in map coordinates (target data, in this case, it is the 2002 January photo) and the raster to be georeferenced. 2. From the Georeferencing toolbar, click the Layer dropdown arrow and click the raster layer to be georeferenced. 3. Click Georeferencing and click Fit to Display. This will display the raster in the same area as the target layers. The Shift and Rotate tools can be used to move the raster as needed.   4. Click the Control Points button to add control points. 5. To add a link, click the mouse pointer over a known location on the raster, then over a known location on the target data.   6. Add enough links for the transformation order. According to ArcMap help, a minimum of three links for a first-order transformation, six links for a second order, and 10 links for a third order are needed.   7. Click View Link Table to evaluate the transformation. The residual error for each link and the RMS error can be examined. From my own experience, if the RMS error is less than 15, the effect would be satisfatory. 8. Click Georeferencing and click Update Georeferencing to save the transformation information with the raster. This creates a new file with the same name as the raster but with an .aux file extension. It can be converted into .tiff file or whatever the user wants it to be.

Findings and Conclusion 1958, only two houses were on the beachfront area. 1961, more than 25 buildings were built. Washover after Hurricane Carla is clearly demonstrated.   8/22/1983, 4 days after Hurricane Alicia. At this time, more and more buildings were constructed. The damage to the constructions are clearly seen.   After all photographs are georeferenced, the shores of 1965 and 2002 are digitized. Washover after Hurricane Carla and Hurricane Alicia are also digitized. Shoreline erosion is almost greater than 80 meters over the fourty years. This is a very shocking number. Definitely people should consider this if they want to build another house on the beachfront area now. Conclusion: For those who plans to buy land and build house on the beachfront area in Gulf of Mexico, they definitely need to think about the hurricane disaster and shoreline erosion first.

Reference Ellis, Michael J., 1986, The Hurricane Almanac, Texas Edition. Printing: Caribbean Blue, Inc.

 

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