Characterization of sediment movement in tidal creeks adjacent to the gulf intracoastal waterway at Aransas National Wildlife Refuge, Austwell, TX: study of natural factors and effects of barge-induced drawdown currents

摘要

The coastal wetlands at Aransas National Wildlife Refuge near Austwell, Texas,support the last migrating population of whooping cranes during the winter months(October through April). With a population currently at 216 individuals, these are therarest cranes in the world. The wetlands in which they winter are a part of the SanAntonio Bay system, a bay that receives constant fresh water flow from the GuadalupeRiver. Currently there is a plan for using water diverted from the Guadalupe River justbefore it enters San Antonio Bay as a water supply for the greater San Antoniometropolitan area located 200 km to the northwest. The Guadalupe River deliversnutrients and sediment into the estuary along with fresh water. Because of theimportance of sediment within a tidal wetland ecosystem, it is imperative to understandthe sediment budget and underlying forces that drive it if one is to ultimately grasp howthis ecosystem functions. To document natural and anthropogenic factors exertingcontrol over sediment movement in this system, three sites on tidal creeks near theboundary between marsh and bay were chosen. The Gulf Intracoastal Waterwayparallels the marsh edge. Over six, non-consecutive weeks water level and velocity wereautomatically monitored in the tidal creeks. Automated water samplers extracted watersamples that were analyzed for suspended sediment. In addition, bedload traps weredeployed in one creek to monitor sediment movement along the channel bottom. Inflowexceeded outflow during the study. As a result there was a net influx of suspendedsediments into the marsh. Bedload material also moves with current direction, and itappears to move in response to barge induced outflow currents. Barges passing on theGulf Intracoastal Waterway exert influence on water level, flow direction, and velocitywithin tidal creeks. Natural factors such as winds, tides, and freshwater input fromupland runoff or river discharge also impact suspended and bedload sediments.