SIDE-SCAN SONAR IMAGING OF BAY BOTTOM SEDIMENT AND ANTHROPOGENIC IMPACTS IN GALVESTON BAY, TEXAS

Knowledge of surface sediment distribution in Galveston Bay is important because it allows us to better understand how the bay works and how human activities impact the bay and its ecosystems. In this project, six areas of bay bottom were surveyed using acoustic techniques to make maps of bay bottom types and to investigate the types and extent of anthropogenic impacts. A total of 31 km² was surveyed in six areas, one in Bolivar Roads (6.1 km²), one near Redfish Bar (3.1 km²), two in East Bay (12 km²), one southeast of the Clear Lake entrance (5.3 km²), and one in Trinity Bay (4.3 km²). We used side-scan sonars (100 kHz and 600 kHz) to image the bay bottom and chirp sonar (2-12 kHz) to image subsurface sediment layers and bottom topography. In the side-scan records, objects as small as a few meters in extent were visible whereas the chirp sonar records show a vertical resolution of a few tens of centimeters. The side-scan images display strong returns due to coarse sediments and weak returns in areas of fine sediment. The Bolivar Roads survey area displayed strong returns due to coarse shell debris. The remaining survey sites were homogeneous or displayed strong returns from oyster mounds and beds and week returns from bay bottom mud. Chirp sonar records were classified as 9 different bottom reflection types based on different seismic returns and stratigraphy. Parallel, layered sediments are seen filling the bay valley and resting atop the surface formed by erosion prior to filling of the bay. Along the flanks of the valley fill the acoustic response returned an absent or weakly laminated return whereas areas of high oyster productivity produced mounds, strong surface returns, and strong, shallow, subsurface reflectors surrounding current oyster reefs. Anthropogenic features imaged with the sonar include sediment disruptions, such as the ship channels, dredge holes, gouges, and trawl marks, as well as debris, such as submerged boats, pipes, and unidentified objects.