MAPPING BOTTOM TYPE AND ANTHROPOGENIC IMPACTS ON SEDIMENTS IN GALVESTON BAY, TEXAS

Knowledge of bottom types 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, two 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 9.2 km² was surveyed in two areas, one in Bolivar Roads (6.1 km²) and one near Redfish Bar (3.1 km²). The surveys were done using a side-scan sonar (100 kHz) to image the bay bottom and a 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 show strong contrasts in the amount of sonar return from the bay bottom, highlighting differences in bottom texture and sediment type. In the Bolivar area, the Houston Ship Channel (HSC) and Texas City Ship Channel show strong return, likely as a result of coarse shell debris that coats the channel bottoms. Outside of the channels, sonar return is moderate to low, the former being areas with shell debris and the latter, an area of sand bottom. The HSC has a low to moderate return in the Redfish Bar area, where shell debris is generally absent. Outside of the HSC, return is patchy with sediment ripples and waves being common in some areas and holes being typical of oyster bed areas. Low sonar return areas typically have a muddy bottom. Surprisingly, oyster beds were difficult to image in the Redfish Bar survey area, possibly reflecting the lack of large, densely-populated reefs that would strongly scatter acoustic waves from the sonar. Chirp sonar records generally show parallel, layered sediments filling the bay valley and resting atop the surface formed by erosion prior to filling of the bay. In some places, such as the HSC, some of this sediment pile has been removed. In sandy areas, and much of the Bolivar Roads study site that is mantled with shell debris, the coarse bottom sediments cause acoustic scattering, which masks subsurface layers. Anthropogenic features imaged with the sonar include excavations, such as the ship channels, dredge holes, gouges, and trawl marks, as well as debris, such as probable boats, pipes, and unidentified debris.