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GC7702. Shoreline Changes on Central Padre Island (Yarborough Pass to Mansfield Channel): An Analysis of Historical Changes of the Texas Gulf Shoreline, by R. A. Morton and M. J. Pieper. 35 p., 9 figs., 1 table, 3 appendices, 1977. ISSN: 0082-3309. Print.
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Historical monitoring along central Padre Island records the nature 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 accomplished 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 1879-81) and aerial photographs (taken in 1937, 1960, 1969, and 1975) indicates short-term changes of accretion and erosion along central Padre Island between Yarborough Pass and Mansfield Channel. 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 minor short-term cycles of retreat related to storms (primarily hurricanes) and recovery during intervening years of low storm incidence. Major changes in vegetation along this particular coastal segment result from formation and migration of active dunes and blowouts which are largely controlled by climatic fluctuations.
Long-term trend or direction of shoreline changes averaged over the 96-year time period of this study indicates that net accretion ranging from 25 to 400 feet and averaging 210 feet was predominant from Yarborough Pass to a point 25 miles north of Mansfield Channel. Net accretion along this segment was influenced by substantial accretion between 1879-81 and 1937. Both erosion and accretion occurred from 1937 to 1960, but after 1960, shoreline changes have been erosional. Net shoreline changes along the southern half of central Padre Island (from Mansfield Channel to a point 19 miles north of the channel) were erosional with net erosion ranging from 25 to 1,150 feet. Maximum net erosion occurred north (downdrift) of Mansfield Channel for 3.5 miles. Average net erosion for this shoreline segment was approximately 795 feet, whereas average net erosion for the remaining shoreline not directly affected by the jetties was about 205 feet.
The shoreline segments experiencing net accretion and net erosion were separated by a transition zone extending for approximately 6 miles. Maximum net shoreline changes within the transition zone were 100 feet, but most net changes were less than 50 feet. Net rates of change along central Padre Island were low except immediately downdrift from Mansfield Channel where net erosion ranged from 4.9 to 12.0 feet per year. Excluding points adjacent to the jetties, net erosion varied from less than 1 foot per year to 4.2 feet per year and averaged 2.0 feet per year. Net rates of accretion also ranged from less than 1 foot per year to 4.2 feet per year and averaged 2.0 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 central Padre Island, are relative sea-level rise, compactional subsidence, and changes in sediment supply.
Studies indicate that changes in shoreline and vegetation line on central Padre Island are largely the result of natural processes, perhaps expedited by man's activities. 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 coast.
Keywords: erosion, Mansfield Channel, Padre Island, shoreline change, shorelines, Texas, Texas Gulf Coast
Morton, R. A., and Pieper, M. J., 1977, Shoreline Changes on Central Padre Island (Yarborough Pass to Mansfield Channel): An Analysis of Historical Changes of the Texas Gulf Shoreline: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 77-2, 35 p.