River floods and debris flows can transport car- and even house-sized boulders at high velocity, making them some of the most deadly and costly natural hazards. Despite this hazard, there exist few methods for continuous monitoring or early warning of these destructive processes. One of the main issues in developing monitoring techniques is that floods and debris flows easily destroy any instruments placed in their path. To address this challenge, we have developed a methodology for measuring and quantifying the ground vibrations produced by water and sediment transport, so that we can continuously monitor these hazards from a safe distance. Building on previous modeling efforts, we have identified how random fluctuations in particle speeds within a debris flow result in impacts with the channel bed and thus cause vibrations of the ground. Different segments of the debris flow contribute to the ground motions, and we have found that the predicted signal strength scales with properties of the debris flow in a way that more closely resembles the total damage and erosion caused by a flow than just the volume of flow. This conclusion suggests that the seismic ground motion measurement may be useful for measuring either damage or erosion caused by debris flows.

To test the predictions made by the models we developed, we performed two different sets of laboratory experiments, one using a 15-m long tilting flume to test relatively dilute flows and one using a 100-m long hillside flume to test relatively concentrated debris flows. In the tilting flume experiments, we verified that our previous models for seismic motion accurately predict observations to within a factor of two uncertainty. We also found that particle trajectories are more complex than originally assumed, with multiple impacts often occurring per ejection event. In the hillside flume experiments, we verified that our debris flow ground motion model also accurately predicts observations. Comparisons between predictions and observations were substantially better when data were used to correct for the precise energy decay due to wave propagation from each point of impact to each seismic sensor, and was accomplished by measuring the ground motion from hammer blows along the flume. In the absence of such data, the cruder assumptions that must be made only allow for order-of-magnitude accuracy of the model.

We have also applied our debris flow ground motion model to the natural, catastrophic debris flow event of January 9, 2018 that occurred in Montecito, California. One seismological station of the Southern California Seismic Network was located in Montecito, allowing us to measure the ground motions from the event with a data latency of only a few seconds. Data from the Montecito debris flows confirmed that our model predictions are consistent with field observations, at least to within an order of magnitude, as might be expected given the dearth of accurate wave propagation information. We were able to use the ground motion data in Montecito to estimate the speed of propagation and approximate average size of boulders during the most damaging time. This conclusion suggests that seismic ground motion measurements could potentially be useful for early warning of debris flows, although sensor placement and messaging capabilities would be key to making the warning successful.

Finally, on the educational side, a number of undergraduate and graduate students and postdoctoral scientists were trained, and we have also tested an educational module about the physics of water flows targeting advanced high school students.

Last Modified: 05/30/2019
Modified by: Victor C Tsai