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Experts say subdivision work damaged Hamilton Pool
Wednesday, May 14, 2008 by Austin Monitor
Outside consultants told Travis County Commissioners Tuesday that erosion and sediment control failures at a subdivision four miles upstream of Hamilton Pool in western
Hamilton Pool and its tributaries – Davis Creek,
Last spring, a combination of failed construction controls on a proposed road and major rain events turned portions of the Hamilton Pool tributaries into a flowing river of mud. A report by Espey Engineering outlines the damage, which resulted, in some places, in silt that was six to eight feet deep in the creek bed.
Espey was brought in to assess the damage. In a discussion with
“What we saw when we did the assessments of the creek bed and the draining tributaries is that the silt source is The Ranches (at Hamilton Pool),” Harkins told the commissioners. “I’m convinced, 100 percent, the silt source is The Ranches. There is a build-up of silt in areas along the creek that feed to Hamilton Pool of several feet. Layers of this silt cake the majority of this creek, and Hamilton Pool has a shelf of about five to six feet of silt along its bottom.”
Commissioners need to act fast because plants are beginning to grow in the displaced sand and silt. Harkins said the county had four options: no action; manual removal of the silt; manual removal of the silt and a wash down; or silt removal from Hamilton Pool, plus the filtration of the water.
The “no action” alternative would be a choice if commissioners wanted natural rain events to move the silt. Numerous rain events would be required to complete the job, said Harkins, who added it’s probably not possible to have a rain event large enough to scour Hamilton Pool itself, which would leave silt in the creek.
Commissioner Sarah Eckhardt asked Harkins what the impact would be if the county chose to move forward with that option.
“I believe that it will be a very long-term effect on Hamilton Pool,” Harkins said. “I believe that the silt shelf will stay there, it will stay cloudy and stay muddy and that during high use, individuals would create a muddy home, from people coming to swim.”
A second option would be the manual removal of the silt, with large amounts of sediment removed from the culvert areas and natural depressions along the creek. That sediment would be easy to recognize – the silt is a different color than the creek bed – and it could be easily removed.
The county also could combine that manual removal with a wash-down of the creek. The manual removal could remove large pockets of sediment and silt. A high-pressure spray could be used to wash down other areas. That spray would displace silt, which would be collected and removed.
If the county chose a full combination of options to restore the creek bed and filtrate the water and use vacuum trucks to remove silt from the bottom of the pool, the price tag could be upwards of $2.3 million. Such an effort would require running the vacuum pumps for a full 45 days to clear the water.
A small portion of the pool is in the Balcones Canyonlands Preserve, so a contractor would not be able to use heavy equipment during part of August, which is considered nesting season for endangered birds. Harkins recommended an initial creek clean-up, then additional filtration after nesting season.
“It may take a little time to amass $2.3 million,” Biscoe said. “So far, heavy praying by the
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