Josh Duty (left) of the Texas Water Development Board conducts a hydrosurvey with a Lower Colorado River Authority staff member. The board’s hydrosurvey program helps the state determine how much sediment has accumulated in reservoirs over the years and how much water storage could be available in the future. The TWDB just completed its mapping of Lake Buchanan with results expected in May 2020. Photo courtesy of Texas Water Development Board
STAFF WRITER JARED FIELDS
Nathan Leber is all about finding the dirt on Lake Buchanan and other Texas reservoirs, or, more precisely, the dirt at the bottom.
Leber, manager of the TexMesoNet and Hydrosurvey programs for the Texas Water Development Board, is in charge of a crew that’s currently surveying sedimentation in Lake Buchanan.
Sedimentation surveys in Texas reservoirs help the state understand how water resources will look 50-100 years into the future.
“We provide the data; (state leaders) make the decisions,” Leber said.
It also helps water management organizations such as the Lower Colorado River Authority identify potential issues with sedimentation.
The survey crew uses a multi-frequency depth sounder to collect sedimentation thickness and location data from the lake. The data allows researchers to compare how much sediment has settled on the bottom of the lake since the reservoir’s impoundment.
In Lake Buchanan, that thickness can vary from nearly 2 meters in the northern part to tens of centimeters in the southern part.
The Texas Water Development Board uses this information to determine a reservoir’s storage volume and sedimentation level over time.
Lake Buchanan’s last hydrographic survey was published in 2006. The board has completed 182 hydrographic surveys on 113 unique reservoirs since the Texas Legislature authorized the surveys in 1991. When possible, the board aims to perform a hydrographic survey on each lake every 10 years or so. Once the crew completes the Lake Buchanan survey, it will move to Lake Travis.
After the Lake Travis survey, the board will survey lakes Marble Falls and LBJ in 2020 and Lake Austin and Inks Lake in 2021. The LCRA is sponsoring the Highland Lakes surveys.
According to the 2006 Lake Buchanan survey measurements, the reservoir had 34,275 acre-feet of sediment. The reservoir’s total water storage capacity when full is 875,588 acre-feet. As of July 19, Lake Buchanan is at 95 percent of its capacity, or 831,021 acre-feet. An acre-foot of water is the volume of water that would cover one acre to the depth of one foot.
The amount of sediment in Lake Buchanan isn’t an issue in terms of storage, Leber said.
What the water development board and the LCRA will determine with this survey is where the sediment is located and if the increase is outside the expected sedimentation level.
The long-term trend for Lake Buchanan, Leber said, is about 420 acre-feet of additional sedimentation per year. If a major discrepancy is noticed, Leber said the board would help LCRA determine a possible course of action.
Dredging, typically, is not such a course.
“In most circumstances, dredging is economically infeasible; it costs too much money for what you get for just storage capacity,” Leber said.
However, if sediment is building up by a dam and could cut off intake, or cut off flow from one region of a lake to another, then perhaps dredging makes sense to alleviate those issues.
With a close-to-full lake at the moment, the amount of sediment in Lake Buchanan might not be an issue. If, like the recent drought showed, the lake level was historically low, then the LCRA would need to know how much water was remaining and if they had enough clearance from the top of the lake level to the bottom of the lake to continue getting clean water out of the reservoir without pulling out sediment.
If high sedimentation rates are discovered, Leber said one way to mitigate future sedimentation would be to work with entities like LCRA to communicate with landowners upstream and implement best practices on their properties.
The hydrographic survey is performed by driving across the reservoir in a line about every 500 feet to see an underwater cross-section. The data from the multi-frequency depth soundings are correlated with sediment core samples also taken from different parts of the lake bottom.
The survey team typically can cover about 1,000 surface-acres per day. Lake Buchanan, when full, covers over 22,000 surface-acres. The report from this year’s survey, Leber said, could be published around May 2020.
More information on the hydrographic survey program can be found here, and the 2006 Volumetric and Sedimentation Survey of Lake Buchanan is available here.