Shoreline changes in the vicinity of the Brazos River Delta ...

Bureau Publication GC7504 - to purchase a print copy please go to: https://store.beg.utexas.edu/geologic-circulars/414-gc7504.htmlHistorical monitoring in the vicinity of the Brazos River delta (San Luis Pass to Brown Cedar Cut) 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 1852-56) and aerial photographs (taken in 1930-37, 1956-57, 1965, and 1974) indicates short-term changes of accretion and erosion along the Gulf shoreline between San Luis Pass and Brown Cedar Cut. 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 122-year time period of this study indicates a decrease in net erosion from 1,325 feet at San Luis Pass to 250 feet at a point approximately 3.5 miles east of the Freeport jetties. Net accretion recorded for the shoreline segment from 3.5 miles east of the jetties to just east of the San Bernard River varied from 325 feet on the old Brazos delta to 6,000 feet on the new Brazos delta. Net accretion was recorded in the vicinity of the old Brazos delta because the shoreline accreted after construction of the jetties in 1896. Subsequent shoreline erosion has not reached the pre-jetty (1852-53) shoreline position. A diversion channel constructed in 1929 rerouting the Brazos River 6.5 miles west of the Freeport jetties led to the destruction of the old Brazos delta and construction of the new Brazos delta. After 1929, the old delta entered into an erosional phase, and the new delta began prograding at the mouth of the diversion channel. Erosion predominated along then remaining segment of shoreline from the San Bernard River to Brown Cedar Cut. Maximum net erosion for this segment was 1,850 feet; minimum net erosion was 500 feet. Both erosional and accretionary rates of shoreline change have been high for this segment of the Texas Coast. Average net erosion east of the old Brazos delta was 769 feet or 6.3 feet per year, whereas average net accretion along the deltaic shoreline was 2,317 feet or 19 feet per year. Average net erosion westward from the new Brazos delta to Brown Cedar Cut was 1,541 or 12.7 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 the shoreline between San Luis Pass and Brown Cedar Cut, are a deficit in sediment supply, relative sea-level rise, and compactional subsidence. Changes in position of the vegetation line are primarily related to storms. Studies indicate that changes in shoreline and vegetation line between San Luis Pass and Brown Cedar Cut are largely the result of natural processes. The only exceptions are accretion and erosion associated with the jetties at the old Brazos River and construction and destruction of the old and new Brazos deltas subsequent to diversion of the Brazos River in 1929. 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.Bureau of Economic GeologyUT Librarie