Shoreline Changes on Brazos Island and South Padre Island (Mansfield Channel to Mouth of the Rio Grande), An Analysis of Histor

Abstract
Historical monitoring along Brazos and south Padre Islands 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 1854, 1867, 1879-1880, 1917, 1934) and aerial photographs (taken in 1937, 1960, 1970, 1974) indicates short term changes of accretion and erosion along the beach between the mouth of the Rio Grande 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 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 120-year time period of this study indicates net erosion for south Padre Island; maximum net erosion for this segment was 1,400 feet or approximately 13.1 feet per year. Minimum net erosion, in proximity to the north jetty at Brazos Santiago Pass, was 75 feet or less than 1 foot per year. The shoreline at the southern tip of south Padre Island has undergone accretion since construction of the jetties in 1935. The long-term shoreline trend of Brazos Island has been one of accretion; however, this is attributed to moderate and extreme accretion between 1854 and 1937. After this period, shoreline changes were short-term erosional and accretionary cycles. Because of limitations imposed by the technique used, rates of change are subordinate to t rends 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 change 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 Brazos and south Padre Islands, are relative sea-level rise, compactional subsidence, and a deficit in sediment supply. Changes in position of the vegetation line are primarily related to storms. Studies indicate that changes in shoreline and vegetation line on Brazos and south Padre Islands are largely the result of natural processes, perhaps expedited by man's activities. The apparent effect of Falcon Dam upon the discharge of water and suspended sediment of the Rio Grande is marked, and the entrapment of sediment by the south jetties at Brazos Santiago Pass and Mansfield Channel is obvious. Structures that retard or eliminate sediment transport add to the sediment deficit already present in the littoral drift system. 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.
Authors
Robert A. Morton
Mary Jarrell Pieper
Citation

Morton, R. A., and Pieper, M. J., 1975, Shoreline Changes on Brazos Island and South Padre Island (Mansfield Channel to Mouth of the Rio Grande), An Analysis of Historical Changes of the Texas Gulf Shoreline: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 75-2 39 p. doi.org/10.23867/gc7502D.

ISSN
2475-3637
Number of figures
12
Number of pages
39
Publisher
The University of Texas at Austin, Bureau of Economic Geology
Series
Geological Circular
Year
1975