Shoreline Movement in the Copano, San Antonio, and Matagorda Bay Systems, Central Texas Coast, 1930s to 2010s

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2016

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Shoreline position and morphology extracted from airborne lidar surveys acquired over the Copano, San Antonio, and Matagorda Bay systems between 2013 and 2015 were used to (1) classify 1,065 km of bay shoreline into 11 common shoreline types, and (2) compare shoreline positions extracted from lidar survey data with previous shoreline positions determined from aerial photographs from the 1930s, 1950s, and 1982 to determine shoreline movement and land-loss rates for a long-term (1930s to 2010s) and more recent (1950s or 1982 to 2010s) period. From higher to lower elevation adjacent to the shoreline, the common shoreline types are high and low Pleistocene clayey sand and sandy clay bluffs, Pleistocene sandy slopes, fan deltas, sandy and shelly beaches and spits, tidal passes, flood-tidal delta marshes and tidal flats, deltaic marshes, and back-barrier and bay-margin marshes and tidal flats. The lower-elevation shoreline types (back-barrier and bay-margin marsh and tidal flats) are the most common shoreline types in the three bay systems, together constituting about 50 percent of the total shoreline length.

Shoreline movement was dominantly erosional over both the long-term and more recent periods, with 80 percent of the nearly 10,000 measurement sites retreating between the 1930s and 2010s and 82 percent retreating during the more recent period. Despite the preponderance of sites undergoing shoreline retreat, the net shoreline movement rate for the long-term period was nearly zero because ubiquitous erosion in the Copano, San Antonio, and Matagorda Bay systems was offset by delta progradation across eastern Matagorda Bay when a river logjam on the Colorado River was removed in 1929. During the more recent period, net shoreline retreat averaged -0.60 m/yr for all bay systems, translating to an average land-loss rate of 63.5 ha/yr. Average shoreline retreat rates were highest in Matagorda Bay at -0.64 m/yr, followed by Copano Bay retreat rates at -0.62 m/yr and San Antonio Bay retreat rates at -0.49 m/yr. Shoreline types experiencing the highest rates of retreat between the 1930s and 2010s were the tidal pass (-0.79 m/yr), sandy and shelly spit (-0.72 m/yr), and high bluff (-0.54 m/yr) shorelines. During the more recent period, shoreline retreat rates increased for all shoreline types. Shorelines retreated most rapidly along tidal passes (-1.67 m/yr), high bluffs (-0.86 m/yr), and spits (-0.84 m/yr) during the most recent period.

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