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Measuring reservoir capacity through sedimentation survey Posted on January 11, 2021



Nathan Leber – Hydrographic Survey Manager, Texas Water Development Board 

In Texas, quite a bit of our water supply is held in reservoirs across the state. Over time, these reservoirs lose capacity due to sedimentation. We need to understand what our current capacity is as well as our future capacity in those reservoirs to be able to properly plan for our future water supply needs.

We offer sedimentation surveys in addition to volumetric surveys. The sedimentation survey is built upon the volumetric survey. The volumetric survey will tell us what the current capacity is while the sedimentation surveys show us what the capacity used to be before the reservoir was impounded or the dam was built.

We can use these to project out a rate of sedimentation over time because we know what it started with and we know what it's at now. Once we have that rate, we can project into the future and determine what the reservoir capacity might be in 10 or 20 years.

Sedimentation is a natural process that occurs in reservoirs across the state. This happens when you have a rain event and anything from a speck of dust to a large boulder is washed into tributaries, and these are ultimately trapped in our reservoir systems.

Josh Duty – Hydrologist, Texas Water Development Board

We will use a vibracore vibrator and a 3-inch core tube attached, and we'll lower that down to the current bottom and, basically, we will vibrate that core tube down into the ground until it stops. Once it stops, we winch it back up and, at that point, we remove it from the vibracore. We measure it to determine how much sediment we collected, and we cap it, and then we bring it back to the office where we'll do post-processing.

Once the core is back at the lab for post-processing, we will cut the core tube in half and we'll analyze the unique sediment layers in that sample.

Nathan Leber – Hydrographic Survey Manager, Texas Water Development Board

Different layers are made up of different types, or textures, or colors of sediment or materials. We look for things like roots or rocks within those layers to determine whether or not that was a pre-impoundment surface that was laid down prior to the reservoir being impounded or if it was laid down after the reservoir was dammed.

Josh Duty – Hydrologist, Texas Water Development Board

Sedimentation is, basically, it's going to be land matter. It could be anything from organic matter such as leaves; it could be large woody debris, different classifications of a rock or gravel, sand, silt, clay, a mixture of anything that you could technically find land based.

Nathan Leber – Hydrographic Survey Manager, Texas Water Development Board

We work with river authorities. We also work with water districts; we work with cities. Any manager or owner of water rights in a reservoir can sponsor a survey with us.

Hydrographic surveys in Texas allow us to monitor the state of our reservoirs. This also helps us plan for the future water needs of our state.

This article is posted in Water Planning / Water Supply / Water Data .