Armed with a two-man pontoon boat, laptop computer, sonar and global positioning system equipment, the Texas Agricultural Experiment Station research assistant is conducting bathymetric surveys of the 27 sediment retention ponds on Fort Hood.
Bathymetry is measuring the water depth relative to some benchmark. In other words, researchers are mapping underwater what would be seen on land in terms of topography, such as the elevation and contours, McAlister said.
"We are surveying the underwater landscape of these ponds without getting our feet wet," he said, "although sometimes that happens anyway."
He and other researchers at the Texas A&M University System's Blackland Research and Extension Center in Temple have been contracted by land managers at Fort Hood – through the Natural Resources Conservation Service – to determine the current holding capacity of the ponds.
The land managers at Fort Hood, or Integrated Training Area Management, want to know if the sediment retention ponds are silted in; if so, to what degree, McAlister said.
The conservation services constructed most of the ponds in the 1990s to capture sediment-laden storm-water runoff that might otherwise wash off the base because of maneuvering of trucks, light armor and tanks.
Fort Hood is on the watershed for Lake Belton, which supplies Fort Hood, Killeen, Temple, Belton and other communities. Comparison of satellite photographs from 1995 until 2000 shows substantial delta formation at the confluence of Fort Hood's Cowhouse Creek and Lake Belton.
Since 2004, McAlister has been doing these surveys by using remote sensing techniques and calculating the decrease in capacity in the ponds.
Remote sensing is "the science of acquiring information without direct physical contact – touching what you can't actually touch or seeing what you can't actually see by some other method," he said.
"It's good data to have," he said. "These are pretty costly structures and they need to be maintained. You need to have an idea of what their current functional health is."
McAlister's work is related to the ancient practice of sounding, or taking measurements of water depth by sinking a piece of lead attached to a line.
He started with a simple fish finder coupled to a global positioning system unit, he said.
"We did it and proved that it could be done very efficiently, so we upgraded our equipment," he said.
McAlister gathers data using sound navigation and ranging, or sonar, to record elevation data, and a global positioning system to record location data.
An electrical signal from the transmitter is converted to a sound wave by a transducer, McAlister said. The sound wave is sent into the water, and when the sound hits the bottom of the pond – or fish or debris in the water – the sound bounces back. The echo hits the transducer which converts it back to an electrical signal. The receiver amplifies the signal and sends it to the laptop computer for display; the echogram is "read" to distinguish what caused the signal to bounce back, he said.
The pond's bottom will send a continuous return signal, and a fish or debris will send back a shallower, sporadic signal, he said.
Thousands of the georeferenced elevation points are collected and paired with coordinates from the global positioning system receiver. From that, a three-dimensional digital terrain model or topographic map is created using geographic information systems, McAlister said.
Alternatives for the land managers at Fort Hood are few and expensive. They could let the ponds fill with silt, rendering them ineffective. They could dredge the ponds, build new ponds, or prevent as much sediment as possible from getting into those that exist.
"Building any more (retention ponds) would impact the maneuver capability" McAlister said. "You really have no choice but to go around them."
The Experiment Station and Texas Water Resources Institute, in cooperation with the conservation service, have been helping with the fourth alternative: preventing more sediment from being washed into the retention ponds and Lake Belton.
The conservation service has installed gully plugs, rock formations across gullies that slow down the flow of sediment after rainfall. Experiment Station researchers and Texas Cooperative Extension faculty have applied excess compost from nearby dairies. That promotes the growth of grasses and prevents soil from washing off the landscape in the first place.
"The bathymetric survey of Fort Hood's sediment retention structures from 2004 has given us a current capacity to compare to what the original planned design was; therefore we can tell to what degree these ponds are silted in," McAlister said. "As well, the current capacity data will be compared to future surveys – like the one currently under way."
The surveys should give land managers an idea of how fast these ponds accumulate sediment.
"This year's set of data will be of particular interest for comparing to future datasets, in that a lot of effort has been spent implementing erosion mitigating practices – gully plugs, contour ripping, and compost/revegetation activities," McAlister said. "The 2006 survey will set the stage for future analysis of the effectiveness of these management practices slowing the sedimentation rates.
"The project benefits not only Fort Hood land managers to maintain quality training lands for the soldiers, but also benefits the surrounding communities in terms of water quality," he said.
Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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