Hillslope Evolution in Block-Controlled Landscapes

Rocky hillslopes dotted with boulder-sized blocks and covered by a thin, non-uniform soil are common in both steep landscapes and arid environments on Earth, as well as on other planets. We have long known how to read geologic structure from aerial imagery; for example, folds in layered rock generate trains of hogbacks. Yet, while the evolution of soil-mantled, convex-upward hillslopes in uniform lithology is reasonably well understood, the influence of heterogeneous lithology and geologic structure on hillslope form and evolution has yet to be properly addressed at a process level. Landscapes developed in layered sedimentary rocks feature sharp-edged landforms such as mesas and hogbacks that exhibit steep, linear to concave- upward ramps with scattered blocks calved from resistant rock layers overlying softer strata. Here I pose the question: What roles do these blocks play in landscape evolution? Using a combination of numerical modeling, fieldwork, and mathematical analysis, I demonstrate that blocks profoundly alter the style and pace of hillslope evolution in rocky landscapes. First, in a numerical model of hillslope evolution I show that the presence of discrete blocks and their interactions with the production and transport of soil can explain the characteristic concave-up hillslope profiles observed in landscapes developed in layered rock on Earth. The presence of blocks increases both the relief and the persistence of topography in these settings. I use these numerical results to develop an analytical framework that characterizes the steady-state form of layered hillslopes in horizontal, tilted, and vertical rock. I find that hillslope weathering and transport processes in the presence of blocks lead to self-organization that allows hillslopes to maintain a steady relief and form through time. Next, I present the first process-based 2D numerical model of river canyon evolution that incorporates the roles of blocks in both hillslope and channel processes. The model reveals that channel-hillslope feedbacks driven by the delivery of large blocks from hillslopes to the channel are necessary and sufficient to develop the cross- sectional and planview morphologies of river canyons observed on Earth. Block feedbacks lead to persistent unsteadiness in the landscape, strongly modifying erosion rates over long periods of time, even under steady forcing conditions. Finally, I explore the common triangular and scalloped mapview patterns developed in tilted rocks when incised by dip-parallel streams. The work presented here demonstrates the importance of large blocks of rock in governing hillslope processes and rates, and advances our understanding of landscape evolution in layered rocks

Terrainbento 1.0: a Python package for multi-model analysis in long-term drainage basin evolution

Models of landscape evolution provide insight into the geomorphic history of specific field areas, create testable predictions of landform development, demonstrate the consequences of current geomorphic process theory, and spark imagination through hypothetical scenarios. While the last 4 decades have brought the proliferation of many alternative formulations for the redistribution of mass by Earth surface processes, relatively few studies have systematically compared and tested these alternative equations. We present a new Python package, terrainbento 1.0, that enables multi-model comparison, sensitivity analysis, and calibration of Earth surface process models. Terrainbento provides a set of 28 model programs that implement alternative transport laws related to four process elements: hillslope processes, surface-water hydrology, erosion by flowing water, and material properties. The 28 model programs are a systematic subset of the 2048 possible numerical models associated with 11 binary choices. Each binary choice is related to one of these four elements &ndash; for example, the use of linear or nonlinear hillslope diffusion. Terrainbento is an extensible framework: base classes that treat the elements common to all numerical models (such as input/output and boundary conditions) make it possible to create a new numerical model without reinventing these common methods. Terrainbento is built on top of the Landlab framework such that new Landlab components directly support the creation of new terrainbento model programs. Terrainbento is fully documented, has 100 % unit test coverage including numerical comparison with analytical solutions for process models, and continuous integration testing. We support future users and developers with introductory Jupyter notebooks and a template for creating new terrainbento model programs. In this paper, we describe the package structure, process theory, and software implementation of terrainbento. Finally, we illustrate the utility of terrainbento with a benchmark example highlighting the differences in steady-state topography between five different numerical models.</p

The role of infrequently mobile boulders in modulating landscape evolution and geomorphic hazards

A landscape’s sediment grain size distribution is the product of, and an important influence on, earth surface processes and landscape evolution. Grains can be large enough that the motion of a single grain, infrequently mobile in size-selective transport systems, constitutes or triggers significant geomorphic change. We define these grains as boulders. Boulders affect landscape evolution; their dynamics and effects on landscape form have been the focus of substantial recent community effort. We review progress on five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution? 2) How do boulders influence hillslope processes and long-term hillslope evolution? 3) How do boulders influence fluvial processes and river channel shape? 4) How do boulder-mantled channels and hillslopes interact to set the long-term form and evolution of boulder-influenced landscapes? 5) How do boulders contribute to geomorphic hazards, and how might improved understanding of boulder dynamics be used for geohazard mitigation? Boulders are produced on eroding hillslopes by landsliding, rockfall, and/or exhumation through the critical zone. On hillslopes dominated by local sediment transport, boulders affect hillslope soil production and transport processes such that the downslope boulder size distribution sets the form of steady-state hillslopes. Hillslopes dominated by nonlocal sediment transport are less likely to exhibit boulder controls on hillslope morphology as boulders are rapidly transported to the hillslope toe. Downslope transport delivers boulders to eroding rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, river channels adjust their geometry to accommodate the boulders supplied from adjacent hillslopes such that rivers can erode at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels, paired with the channel response to boulder delivery, drives channel-hillslope feedbacks that affect the transient evolution and steady-state form of boulder-influenced landscapes. At the event scale, boulder dynamics in eroding landscapes represent a component of geomorphic hazards that can be mitigated with an improved understanding of the rates and processes associated with boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in landscape boundary conditions (tectonics, climate, and lithology) with the goal of understanding boulder dynamics as one component of landscape self-organization