During the past couple of months, I have been lucky enough to get to go in to lab and start setting up flume experiments on permafrost riverbank thaw. Kim Litwin Miller, the Caltech Earth Surface Dynamics lab manager, has helped me to set up and run a couple of test experiments to study thaw-limited versus entrainment-limited bank erosion. Our lab is a large warehouse with fans and a highbay door going directly outside, so it is an excellent place to work at a social distance. The photos below show our setup for test experiments we ran over the last couple of weeks.
Looking downstream in the flume to the end tank – for scale, the flume is 75cm wide. The end tank is filled with water and has a pool cover on top and insulation on the sides to try and cool down a large volume of water to run through the experiment. The white tubes lining the flume test section circulate glycol chilled to 5F by the same type of chiller used to make ice rinks. We form the bank by shoveling in saturated sand and then freezing it in layers. The metal rails are tracks for a large instrument cart to move across the flume and take measurements.
Running a test, with water flowing left to right out of the headbox, past immobile riprap and into the experiment test section. The cart sitting on top of the flume has a laser and sonar that we use to take scans of the bank before and after the experiment, and measure the water surface slope at intervals throughout the experiment. Overhead cameras are used to track bank erosion via automated image processing, so we dye the water blue to enhance the contrast between the channel and the eroding bank made of frozen sand and ice. The sides of the flume are insulated to keep the flowing water and icy bank cold.
We are testing a heat transfer model by freezing an array of temperature sensors into the bank as we build it, which you can see as the red wires going into the frosty bank. Similar to our observations of permafrost rivers in the field, thermal erosion of the banks only extends to the water surface and the river gradually undercuts its bank, forming a “thermo-erosional niche.” Comparing our lab results to a numerical model will allow us to scale up our findings to real rivers in the Arctic, as well as investigate how river bank erosion may respond to a range of climate change scenarios.