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Rediscovering the South Llano River through innovative data acquisition Posted on March 08, 2022


Dr. Mark Wentzel - Hydrologist, Texas Water Development Board

It's a historic river. It’s a big part of the history of Texas. Our rivers are vital to us as a water resource. They're vital to us for recreation, for contact with the environment. The South Llano is just a great place to go to either the state park or along the entire length of the river for recreation and just connection with nature. Anyone who’s out there falls in love with it.

On the South Llano, previously, we've had cross-section data that might have been collected at specific locations, but not a full, comprehensive survey of the entire length of the river and what the entire shape of the bed banks and bottom of the river would look like.

Joey Thomas - Strategic Mapping Project Manager, Texas Natural Resources Information System

We’re doing a riverine bathymetric study where we collect Lidar data of the river bottom surface. It allows us to get a full picture of not only what the bottom of the river looks like but the banks and the surrounding area as well. 

So, Lidar is collected by a sensor on the bottom of a plane, which pulses a laser. And as that laser reflects off of the ground surfaces, we get a timing for that. And that timing is used to calculate the elevation of the reflected surface. For Bathymetric Lidar collections, it's actually a different type of sensor than the topographical Lidar.

Dr. Mark Wentzel - Hydrologist, Texas Water Development Board

The South Llano River is an excellent fit for this type of project because it is generally a clear water stream, and that'll allow the bathymetric Lidar to accurately penetrate the column and get a good measurement.

Previous traditional Lidar would just give us a flat elevation across the surface of the river. To know anything about the channel, we have to go out in the field to survey that data. It would take us a month of data collection to get detail in maybe a mile or two miles of river. With this bathymetric Lidar collection, they're able in one day to collect 30 miles of river data. The efficiency is just fantastic in comparison to what we would do, collecting it by hand.

Joey Thomas - Strategic Mapping Project Manager, Texas Natural Resources Information System

So, in addition to acquiring the Lidar data, we also hire a third party. The folks on the ground are actually using survey-grade data collection.  So, they are going and they’re collecting survey-grade GPS data points for the bottom surfaces of the river to validate the elevation values that we get from the Lidar.

Dr. Mark Wentzel - Hydrologist, Texas Water Development Board

Really, this is foundational-type data. So, it’s similar for a study on land, the Lidar data would be useful for land zoning, for any kind of development work that you do, rainfall, runoff modeling, any of that type of work. So, by adding that aspect of having the bathymetric or the bottom-of-the-channel data, we can now use that data also for flood studies; very useful information for doing a hydraulic study to know different flow rates; what would be the water surface elevation associated with those flood levels. If we were able to get a complete bathymetric dataset of rivers in Texas, that would be very helpful for flood planners.

One of the ways, as a river scientist, that I'm excited about is just looking at the shape of the river and, then maybe, coming back in 10 or 20 years and re-flying the bathymetric Lidar, collecting that data again and having that comparison with over one or two decades and see how the river is changing—what impacts human developments may or may not be having on the river. And that type of analysis is only possible with a great data set like this.

This article is posted in Flood / Technology / Water Data .