Kimberly Hill Studies Particles That Can Bring Down Mountains

Debris flows – rapid, gravity-driven events involving a mixture of boulders, gravel, sand, and mud – can cause tremendous loss of life and property. In January 2023, an article published by Oregon Public Broadcasting (OPB.org) reported “Almost all of the state [California] has received rainfall totals of 400% to 600% above average since Christmas, with some areas receiving as much as 30 inches of precipitation, causing massive flooding. The severe weather has killed at least 19 people since late December. Since New Year’s Eve, the California Department of Conservation’s landslide mapping team has documented more than 300 landslides.”

Evidence shows that the frequency and hazard of debris flows are increasing, possibly due to changes in climate dynamics (both macro- and microscale). For example, extreme rainfall events increase the erodibility of soil and the driving force of flows. Further, melting permafrost changes soil moisture and permeability. These changes could be linked to dangerous events. Kimberly Hill studies debris flows. Hill is a physicist in the Department of Civil, Environmental, and Geo- Engineering (CEGE) and the St. Anthony Falls Laboratory (SAFL) who studies engineering materials, geomorphology, and granular physics problems. More precisely, Hill studies signatures of particle-scale physics in the behavior of natural and manmade infrastructure materials. Specific applications of her research include debris flows in steep upland regions, sediment transport in rivers, and roadbed materials. Hill focuses her research on understanding the physics of particle-flow processes and applies that understanding to civil and geophysical infrastructure problems.

Colliding pairs of particles follow relatively simple physical laws that make their motions easy to predict. Bulk flow behaviors, however, are much harder to predict. For instance, as the particle density increases, as in granular road beds, hot mix asphalt, or muds flowing down steep inclines, the problem of predicting the motions of the particles in these flows becomes intractable. To predict behaviors of these complex systems from first principles, one must understand the statistical interactions of particles as well as environmental details, such as local rock properties and relevant climatic details.

Hill’s group researches how climate and rock properties contribute to specific hazards in debris flows. They collect data at debris flow field sites around the globe, from California to Taiwan. Recently, her group traveled to the White Mountains in Owens Valley, California, to collect data from four neighboring debris fans with similar climate histories but vastly different flow signatures. In March 2023, Hill traveled to Taiwan to collect data from twin debris flows in the Laonong River Valley that were deposited under warmer, moister climate conditions.

Based on these and other analyses of field data, Hill and her research group develop models and perform basinscale experiments in the St. Anthony Falls Laboratory and benchtop-scale experiments in the Civil Engineering Building. These experiments support the importance of mineralogy and grain size in the pattern of debris flows, which is also indicated in the field. For example, they found that the content of clay impacts the speed of flows and the apparent avulsion frequency and larger particle size distribution on fans. Changing fine particle content (silt and/or clay) alters avulsion frequencies, erosion behavior, and size sorting behaviors in the field and in the laboratory, providing critical predictive insights that can be used to protect human life and infrastructure. Kimberly and her students are then able to combine the results from physical experiments and field observations with computational simulations to understand the physics at play, again, with the goal of improving predictive capability.

Hill points out the importance of crossdisciplinary collaboration with her colleagues in helping advance the field component of her research on natural hazards. “I’m not classically trained in field work and geomorphology, both critical components of this study. My group would be severely challenged to build such a rich field study without experts in geomorphology and field methodology to lean on. I feel lucky to have colleagues who are interested in bringing together the expertise from my group in particle-fluid flows with theirs in field data collection, geomorphology, and civil infrastructure problems.” Planned field work includes a 2024 trip with colleagues at Dartmouth to a remote site in the Aklavik mountain range in the northern part of the Northwest Territories where changing permafrost is an issue. In addition, some of Hill’s Taiwanese colleagues have a specific interest in and concern over the problem of building infrastructure in a region of their country plagued by frequent large-scale debris flows. Hill is currently collaborating with Taiwanese colleagues to develop an international course on natural hazards and civil infrastructure with a field component in Taiwan, which is particularly exciting and builds on her and her collaborators’ expertise.

Hill and her group also collaborate with researchers in areas beyond those focused on natural hazards to study other types of engineered debris flows—drawing physics-based connections between the two. One of these other areas of application involves a collaboration with structures and pavement colleagues at UMN. Computational modeling of various particle-fluid mixtures in vastly different pavement systems has helped inform the dynamics in mixtures relevant to debris flows. Specifically, hot mix asphalt is compacted with materials made of sand- to gravel-sized particles and with binders often mixed with clays and other fine particle materials. Hill’s group investigated the manner in which collaborators in the CEGE structures and pavement groups were able to speed compaction rates of hot mix asphalt by adding graphene nano-platelets. The addition of graphene nano-platelets increased effective viscosity of the fluid-like asphalt binder mix, providing a thicker, more consistent coating on the gravel sized particles to allow them to more easily rearrange and compact under pressure.

Another area of study models granular road bases, including effects of moisture, with colleagues at UMN, along with David Potyondy, Senior Geomechanics and Software Engineer at Itasca Consulting Group, and John Siekmeier, Research Engineer at the Minnesota Department of Transportation. Hill’s group proposed a way of representing soil moisture efficiently in simulations of how these granular road bases flow and settle, and then worked to import this framework into PFC-3D, a commercial code used by the pavement and geomechanics community. Hill was delighted to learn that Potyondy had entitled this part of the code the “Hill model.” By 2018, more than fifty researchers on several projects were using this computational framework, and it was awarded a Research Partnership Award from UMN’s Center for Transportation Studies for “significant impacts on transportation involving teams of individuals.”

Other work by Hill’s group and collaborators on natural and engineered materials include environmental reclamation of oil sands tailings ponds (funded by Canada’s Oil Sands Innovation Alliance), use of dredged material for wetland restoration (funded by Great Lakes Consortium), and sediment transport (funded by the Office of Naval Research).

Hill is now focusing her primary efforts on her long-term goal of building a deeper, physics-based understanding of complex particle-fluid systems in nature while also including considerations of the communities they affect.

Dramatic landslides can capture popular attention. Kimberly Hill’s work helps us understand the physics behind such dramatic slides and, more importantly, how we might improve prevention efforts and mitigate the consequences.