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Measuring lake capacity through volumetric survey Posted on July 06, 2020



Josh Duty – Hydrologist, Texas Water Development Board:

I'm in a program that maps reservoirs for the state to help water-controlling authorities or water resource managers figure out how much water they have for future generations with increasing population. Hydrographic survey is basically a two-part survey where you have the volumetric survey as one part, and then you have the sedimentation survey as a second part. And depending on the needs of the client, they can decide if they just want us to go in and do a volumetric survey or we can do both.

Volumetric survey is basically an inexpensive, accurate way for water resource managers or controlling authorities to determine current capacity of the reservoirs by looking for current surfaces. The current surface is the current bottom surface of the reservoir. Once you find the current surface, part of data analysis is we go back in and calculate water storage capacity as part of the report. The data we collect often times can be used for state and regional water plans.

The first thing that we do on a volumetric survey is we have to generate a lake boundary. And the way we do that is we take aerial photographs and digitize what we would consider a current lake boundary. Once we have our boundary, then we create our preplanned lines or transect files that are perpendicular to the historical river channel.

When we go out and survey, we basically go out, we set up our equipment, which is a multi-frequency single beam sonar, and we calibrate that equipment. We calculate speed of sound using a velocity profiler. And then, once we've calibrated, we calibrate the transducer for depth. So, we're making sure that if we're in 15 feet of water, our transducer is reading 15 feet of water. Once that's calibrated, then we're ready to survey.

Once we begin surveying, we basically survey from bank to bank across the reservoir. And then, once that transect's finished, we go to the next one until we've mapped all the transects throughout the reservoir. From the boat, we're collecting depth data. That data will be tied to a specific GPS coordinate as we move across the reservoir. So, each depth has a GPS coordinate tied to it. And with one boat on the water we can typically survey a thousand surface acres a day. We finished Lake Travis in 21 to 25 days and Lake Buchanan in about the same timeframe, and those reservoirs average around 20,000 surface acres.

Once that data is collected, we bring it back to the office and that's where our data analysts start processing the data. So, the data analysts actually go back in and run models on that data to where they build a model that shows the depth contours of the lake.

Reservoirs lose capacity over time due to increased sediment levels due to natural runoff and high-flow events. We recommend surveying reservoirs every 10 years or after a major high-flow event. Completing hydrographic surveys for reservoir owners as well as water resource managers helps these entities figure out water storage capabilities for increased population growth.

This article is posted in Water Planning / Water Supply .