Future Scholar profile / Paul riding new wave of hydraulics research

Linet Paul came to South Dakota State University with a bachelor’s degree in civil engineering, five years of experience as a hydraulic engineer in her native Kenya and a desire to grow.

She was looking for a U.S. graduate program where she could earn her master’s degree and gain valuable lab experience that she could turn into know-how for the construction of water engineering systems in her home country. 

Paul earned her master’s degree in December 2024 and continued her academic track as a doctoral student under Professor Francis Ting in the Department of Civil and Environmental Engineering with a focus on fluid mechanics and hydraulic engineering. Ting, a 31-year veteran at SDSU, is recognized as an expert in breaking waves, bridge hydraulics and scour. 

Paul’s doctoral experience includes participating in the college’s Future Scholars of America program, which is designed to create graduate students who can discover, teach and engage with equal intention.

Ting said, “The Future Scholars of America program is  one of the best investment and most rewarding efforts in graduate education and research infrastructure building in the College of Engineering that I have seen in my whole career at SDSU.” 

Her research project involves conducting  meticulous velocity measurements of breaking waves using the particle image velocimetry technique.  Utilizing a 25-meter-long wave flume in the department’s hydraulics laboratory, Paul aims to understand the structure of turbulent motion in the thin region adjacent to the bottom of a simulated surf zone to improve the methods used for calculating bed friction.

With that knowledge, engineers can more accurately determine the amount of support needed for bridges and other coastal structures at the shore’s edge.

One experiment can last 12 hours

In addition to research, she serves as a graduate teaching assistant, helping undergraduates set up and conduct basic laboratory experiments in fluid mechanics and hydraulics. For example, they learn how to measure flow discharge using a sharp-crested weir.

The experience also provides a good opportunity for the doctoral student to improve her teaching and communication skills, she said.

While Paul has gotten involved in the American Society of Civil Engineers, her big focus has been her wave experiments. One experiment can take 12 hours to run. Last spring, Paul and Ting conducted a series of flume runs over successive weeks. That produced enough data to keep them busy all summer doing data analysis.

“It’s a lot of data crunching,” Paul said with a smile.

Paul was familiar with some of the concepts behind measuring bed shear stress, but she had never done this type of experiment before coming to SDSU. 

The work would be easy if one could simply point an instrument at a spot adjacent to the flume bottom, where the flow velocity decreases rapidly due to the friction force that creates erosion. That’s not possible because that boundary layer is extremely thin, typically less than a couple of millimeters. 

Measuring water speed: a complicated process

Therefore, seeding particles (think fine powder) are added to the flume as fluid tracers. A camera capturing images from multiple angles can then create a 3D image, Ting said. That’s just the start of the process. Software then converts the images to a vector map to show water direction and particle speed. 

Those data points on the vector map are then analyzed using computer scripts written in MATLAB (a computing platform that is used for engineering and scientific applications) to calculate bed shear stress, the fluid force acting on the bottom, Paul said.

Ting said, “The precision required for these measurements pushes experimental techniques to the limits.” 

The flume has been part of the hydraulics lab since 2012. Previous research had measured fluid velocity in the interior of the flow, where the velocity changes relatively slowly. “It’s very challenging to measure velocity at the bottom of a breaking wave,” where flow velocity changes rapidly with distance and time as turbulent eddies generated by breaking waves zoom by and complicates data analysis and interpretation.

Calculations for rough is tougher

Earlier research examined bed shear stress when the sea floor is a smooth surface, and Ting wrote a paper about it. Paul helped with data collection while she was a master’s degree student. Calculating bed shear stress on a rough surface is more complicated. That’s where gravels come into play. 

Paul covered the flume bed with fine gravels — from .8 millimeters to 3.5 mm — to mimic coastlines protected by ripraps, a coarse sand or a pebble beach. The rough bed extends two meters in length. Paul started collecting data last year and now is analyzing it. That process is about 40% complete.

“I will see later if I need to collect more data,” she said.

Paul said her research hopes to understand how the roughness of the sea floor increases the bed shear stress under breaking waves, which hasn’t been measured before. Among the variables in her research are changing the depth of the water and the wave height and period. 

Ting said being able to calculate the distribution of instantaneous bed shear stress along the bottom of a smooth or rough surface in a free surface flow will also be useful for understanding the movement of air or water over immersed bodies. “So, our research findings would be useful not only in river or coastal hydraulics but also in the automotive and aeronautics industry,” he said.

As for Paul’s future, she expects to complete her doctorate in December 2027 and then return to Kenya to practice hydraulic engineering.