Landscapes encode valuable information about tectonics and climate, as well as the frequency, size, and types of processes that shape them. Reliable extraction of this information, however, requires that we understand the relationships between the processes that transport sediment over the Earth?s surface and the landscapes that they create. The overarching goal of this project is to understand the role of debris flows in shaping steep landscapes. Debris flows are mixtures of mud, rocks, and other incidental debris (such as trees) that can travel at high speeds down valley networks. Their large size, rapid motion, and episodic behavior pose hazards to life and infrastructure. They also erode the valleys they travel through, contributing to the shape of steep landscapes. Debris flows are thought to be a dominant mechanism for transporting sediment and eroding bedrock in steep terrain, but we currently lack a mathematical framework to describe how landscapes respond to erosion by debris flows. In this project, the investigators will develop a model to predict how debris flows erode landscapes and apply this model to understand how changes in tectonics, climate, and land-use influence topography and the sediment volumes passed on to downstream rivers and reservoirs. The project will improve our ability to extract information about tectonics and climate from topographic data and will contribute to resolving a long-standing debate regarding the relative importance of debris flows and water-dominated flows in sculpting bedrock channel networks. This project will train one postdoctoral researcher, one PhD student, one MS student, and three undergraduate students. Two of the undergraduate students will be recruited through the University of Arizona ASEMS (Arizona?s Science Engineering and Math Scholars) program, which is designed to support underrepresented students in STEM fields. The investigators will also produce online community learning tools, lead in-person clinics that support students and researchers in using the model and analysis tools developed through the course of the project, and give guest lectures at a public high school.
Debris flows are thought to be a primary driver of erosion in many steeplands but we currently lack a generalizable, mechanistic approach for quantifying debris flow erosion within landscape evolution models. As such, our ability to predict the dynamics of steeplands, including their geomorphic responses to tectonic, climatic, and anthropogenic forcing, is limited. This project will address three fundamental questions related to the geomorphic role of debris flow erosion: 1) How can we quantify erosion by episodic debris flows over geologic timescales?, 2) How does debris flow erosion influence the morphology of quasi-steady state landscapes?, and 3) How does debris flow erosion influence transient signals (e.g. knickpoints) propagating through the channel network? To address these questions, the investigators propose to analyze debris flow dominated landscapes in the Oregon Coast Range (OCR) and the San Gabriel Mountains (SGM), conduct a field campaign designed to relate changes in landscape morphology to tectonic forcing and environmental factors, and develop a novel landscape evolution model that incorporates debris flow erosion. The debris flow erosion model will employ process-based equations for debris flow motion that honor the discrete nature, transient flow dynamics, and limited mobility of debris flows that differentiates them from water-dominated flows. From this detailed picture of nonlocal debris-flow dynamics, the investigators will then calculate debris flow erosion within bedrock channels integrated over geologic time. The model will be validated through comparisons with topographic analyses of channel network properties and millennial-scale erosion rates in debris-flow-dominated valleys in the OCR and SGM. They will apply their process-based modeling approach in combination with data and observations from our study sites to derive a geomorphic transport law for debris flow erosion, quantify the impact of debris flow erosion on the form of steady state landscapes, and explore how debris flows influence the propagation of transient signals through the channel network.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
