277-5 Breaking the Runaway Feedback in Humped Soil Production: A Slope-Dependent Weathering Law Based on Rock Strength

Session: Critical Zone Science: Intersection of Processes Linked to Geomorphology, Ecology, Fire and Climate

Presenting Author:

Authors:

Abstract:

Landscapes with patchy soil present an important test to models of soil production. On the one hand, isolated bedrock outcrops in otherwise soil-mantled landscapes are prima facie evidence of ‘humped’ soil production, in which soil production rates are highest at some non-zero soil depth. On the other hand, humped functions exhibit runaway positive feedback when coupled to slope-dependent sediment transport (e.g., soil creep). Once an outcrop emerges, it continues to grow in perpetuity as bedrock weathering rates are unable to keep up with erosion on steepening outcrops. A general solution to this unrealistic model behavior is to add negative feedback that limits the height of bedrock features. Rock mass strength and mass wasting (e.g., due to rockfall) provide a physical basis for simulating limits to relief. As such, we build a new model in Landlab that incorporates three process components: (1) a humped soil production function to simulate bedrock weathering; (2) a nonlinear, depth dependent soil transport law to simulate creep; and (3) an additional probabilistic weathering and transport rule intended to represent mass wasting as outcrops steepen.

To construct and test the utility of this new model, we use airborne lidar from a tor-rich landscape—the Rampart Range, CO—to constrain rock mass strength for bedrock outcrops. Lidar slope data is first segmented into individual bedrock objects. Using drone-based mapping as ground truth, we find that convex hulls around high slope thresholds in the lidar data (32°- 45°) do well at mapping bedrock objects (Matthews Correlation Coefficient > 0.6). The calibrated slope-hull method reveals >200 outcrops in the landscape with greater than 50 meters of relief. Outcrop boundaries are then used to extract the raw point cloud data for each feature, which are used to build strength envelopes that describe the maximum relief maintained at a given slope gradient. Empirical strength envelopes are converted to an effective cohesion based on Culmann limit-equilibrium slope stability. The probability distributions for effective cohesion are the basis for the rockfall rule used in numerical simulations. This new model exhibits three useful characteristics: (1) Provides negative feedback to the growth of bedrock outcrops when implementing humped soil production; (2) Honors limits to the maximum relief observed in bedrock features; (3) Produces a large range of outcrop properties that better mimic natural landscapes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9340

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