Shoreline Changes on Galveston Island (Bolivar Roads to San Luis Pass)--An Analysis of Historical Changes

GC7402. Shoreline Changes on Galveston Island (Bolivar Roads to San Luis Pass)--An Analysis of Historical Changes of the Texas Gulf Shoreline, by R. A. Morton. 34 p., 12 figs., 3 appendices, 1974. ISSN 00823309: Print.

To purchase this publication as a PDF download, please order GC7402D.



ABSTRACT
Historical monitoring along Galveston Island records the type and magnitude of changes in position of the shoreline and vegetation line and provides insight into the factors affecting those changes.

Documentation of changes is aided by the compilation of shoreline and vegetation line position from topographic maps, aerial photographs, and coastal charts of various vintages. Comparison of shoreline position based on topographic charts (dated 1851-52) and aerial photographs (taken in 1930, 1956, 1965, and 1973) indicates short-term changes of accretion and erosion along Galveston Island between the south jetty at Bolivar Roads and San Luis Pass. Erosion produces a net loss in land, whereas accretion produces a net gain in land. Comparison of the vegetation line based on the aforementioned aerial photographs indicates definite short-term cycles of erosion related to storms (primarily hurricanes) and recovery during intervening years of low storm incidence.

Long-term trend or direction of shoreline changes averaged over the 135-year time period of this study indicates net accretion along East Beach from the south jetty to the seawall; maximum net accretion on East Beach was greater than 6,000 feet. The beach in front of the seawall experienced net erosion ranging .from 30 to 875 feet. Most of this net erosion is attributed to reorientation of the shoreline between 1850 and 1930. Net erosion was also recorded from the end of the seawall west approximately 3.75 miles; maximum net erosion was 1,260 feet or approximately 10.3 feet per year. Minimum net erosion for this segment was 140 feet or about 1.2 feet per year. Minor net accretion was recorded along the next 4.75 miles of beach; maximum net accretion was 100 feet. Net accretion at the other points was 90 feet or less and averaged about 75 feet. Thus, rates of change along the 4.75 miles of beach are less than 1 foot per year. Net erosion, which dominated the remaining 10 miles of beach westward to San Luis Pass, ranged from 20 to 210 feet and averaged about 150 feet. Rates of change for this western segment over the 120-year time interval ranged from less than 1 foot per year to 1.7 feet per year and averaged 1.3 feet per year.

Because of limitations imposed by the technique used, rates of change are subordinate to trends or direction of change. Furthermore, values determined for long-term net changes should be used in context. The values for rates of net change are adequate for describing long-term trends; however, rates of short-term changes may be of greater magnitude than rates of long-term changes, particularly in areas where both accretion and erosion have occurred.

Major and minor factors affecting shoreline changes include: (1) climate, (2) storm frequency and intensity, (3) local and eustatic sea-level conditions, (4) sediment budget, and (5) human activities. The major factors affecting shoreline changes along the Texas Coast, including Galveston Island, are a deficit in sediment supply and relative sea-level rise or compactional subsidence. Changes in the vegetation line are primarily related to storms.

Studies indicate that shoreline and vegetation line changes on Galveston Island are largely the result of natural processes. A basic comprehension of these physical processes and their effects is requisite to avoid or minimize physical and economic losses associated with development and use of the beach.

Keywords: Bolivar Roads, San Luis Pass, shorelines, vegetation, Galveston, erosion, sea level, storms, beaches


Citation
Morton, R. A., 1974, Shoreline Changes on Galveston Island (Bolivar Roads to San Luis Pass)--An Analysis of Historical Changes of the Texas Gulf Shoreline: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 74-2, 34 p.