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). Sidescan
sonars (100 kHz and 600 kHz) were used to image the bay bottom, and a chirp
sonar (2-12 kHz) was used 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 sidescan
images display strong backscatter in some areas due to coarse sediments in addition
to weak backscatter in areas of fine sediment. The bay bottom was classified using three
levels of sonar backscatter ranging from high to low. Areas of differing sonar
backscatter intensity were sampled with cores and grab-samples. High backscatter
corresponded to coarse shell debris and oyster reefs, medium backscatter corresponded to a sand-silt-shell mixture, and low backscatter corresponded to silty loam. Chirp sonar
records were classified as one of nine different bottom reflection types based on changes
in amplitude and stratigraphy. Parallel, layered sediments are seen filling the bay valley
and resting atop a sharp contact at which the acoustic signal fades out. Along the flanks
of the valley fill the acoustic response revealed an absent or weakly laminated
stratigraphy, 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 included 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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/4896 |
| Date | 25 April 2007 |
| Creators | Maddox, Donald Shea |
| Contributors | Sager, William W. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | 14502256 bytes, electronic, application/pdf, born digital |
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