“These are expensive projects, but there’s not too much of a scientific understanding of what really is going on,” Liu said. “How does the water move around the in-stream structures? How does it carry solute and sediment?”
In contrast to restoration projects that use concrete and steel, water and sediment are able to flow through the pores in the nature-mimicking structure, creating unique turbulent flow patterns.
“The porosity creates more complexity and richness in the flow features,” Liu said. “Water can go through them and around them. This complex flow field is important for the functionality and longevity of the structure.”
As these flow patterns develop, sediment is transported and sometimes filtered by the wood and debris. This sediment movement around the log jam can also result in scour holes that can become habitats and shelters for fish, a desirable characteristic of nature-based solutions. However, Liu noted, these holes develop differently than they would around traditional impervious structures, such as bridge piers, and can have an effect on the longevity of the structure.
For engineers looking to install an engineered log jam, the lack of fundamental understanding of these complex flow patterns means relying on educated guess. Formulas that currently exist to predict scour hole size and depth do not account for porosity.
“It’s a lot of trial and error at this stage right now,” Liu said.
Based on preliminary results, Liu developed his own formula using porosity as a parameter to help predict scour hole size for nature-based solutions. He will test this by developing a high-fidelity 3D model to simulate the flow and sediment dynamics in a river containing an engineered log jam.
Mathematically resolving all of the geometric details found in an engineered log jam requires a lot of computing power, so Liu will rely on the ICS-ACI, Penn State’s high-performance research cloud, to run the simulations.
“The mathematical equations in the model are just the descriptions of the physical processes in this problem,” Liu said. “Flow carries the sediment and creates a hole. When holes are enlarged, water has more space to go. Our model describes this co-evolution with the presence of a complex restoration structure.”
Liu will also run physical experiments in a 15-meter flume, an artificial water channel in the Penn State Hydraulics Laboratory, using scaled-down engineered log jam models. After each experiment, he will drain the flume and use a laser to scan the bed. The results of the flume experiments will then be cross-referenced with the computational model and with field measurements to validate results.
If Liu succeeds and is able to establish a fundamental understanding of the physical processes occurring around these structures, future research will then be able to link that to ecological processes, which should give scientists a better idea of how well these solutions are achieving their sustainability goals.
“That’s the final goal,” Liu said. “Hopefully, with the introduction of nature-based solutions, nature can start to re-establish itself. When nature is working, you don’t need too much human intervention.”