Strength matters: Resisting erosion across upland landscapes

Abstract

Soil‐covered upland landscapes comprise a critical part of the habitable world and our understanding of their evolution as a function of different climatic, tectonic, and geologic regimes is important across a wide range of disciplines. Soil production and transport play essential roles in controlling the spatial variation of soil depth and therefore hillslope hydrological processes, distribution of vegetation, and soil biological activity. Field‐based confirmation of the hypothesized relationship between soil thickness and soil production is relatively recent, however, and here we quantify a direct, material strength‐based influence on variable soil production across landscapes. We report clear empirical linkages between the shear strength of the parent material (its erodibility) and the overlying soil thickness. Specifically, we use a cone penetrometer and a shear vane to determine saprolite resistance to shear. We find that saprolite shear strength increases systematically with overlying soil thickness across three very different field sites where we previously quantified soil production rates. At these sites, soil production rates, determined from in situ produced beryllium‐10 (10Be) and aluminum‐26 (26Al), decrease with overlying soil thickness and we therefore infer that the efficiency of soil production must decrease with increasing parent material shear strength. We use our field‐based data to help explain the linkages between biogenic processes, chemical weathering, hillslope hydrology, and the evolution of the Earth's surface. © 2019 John Wiley & Sons, Ltd.

Profiles of Near‐Surface Rock Mass Strength Across Gradients in Burial, Erosion, and Time

“…This observation may be indicative of a negative feedback, wherein an increase in rock mass strength due to increased erosion rates acts to resist further erosion, which in turn provides another mechanism to grow topographic relief. Our results apply to fresh and slightly weathered rock masses, but recent work on soil and saprolite strength demonstrates an increase in strength with increased weathering extent (Heimsath & Whipple, 2019). The relationship between erosion rate and soil/saprolite strength is opposite the relationship we document with rock masses, and suggests that as the strength of rock masses increases with decreased weathering, the strength of the overlying soil actually decreases.…”

Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya

“…The GSI observations also nearly encompass the full range of conceivable values, ranging from 0 to 95, with a mean value of 50 and a standard deviation of 21. The large range in GSI values results in shear strength estimates that range from 0.2 to 8.5 MPa at 5 m depth, characteristic of materials ranging from stiff soils to minimally fractured hard rock (Heimsath & Whipple, 2019;Sutcliffe et al, 2004).…”

“…We observe a systematic increase in mechanical competence from ridges to hillslopes to channels, which is driven by weathering conditions (Figure 6; Table 1). Excluding sites above tree line elevations (3.9 km; Schickhoff, 2005) because of the abrupt change in weathering and vegetation associated with high altitudes unrelated to the topography itself (Riebe et al, 2004), no ridge sites have weathering observations exceeding "fair" conditions (mean GSI = 32 ± 16), and the median ridge V s profile and predicted shear strength values are typical of highly weathered rock and saprolite (∼180 m/s and 0.01 MPa at the surface to 500 m/s and 0.2 MPa at 20 m depth; FEMA, 2004;Heimsath & Whipple, 2019;Sutcliffe et al, 2004). Weathering conditions among hillslope sites span the full range from "very poor" to "very good" (mean GSI = 48 ± 19) with the median V s profile ranging from 280 to 800 m/s from the surface to 20 m depth, and shear strength values of 0.2-1.0 MPa from the surface to 20 m depth.…”

Urban soils in Brazil: A review

“…Erosion forms (sheet erosion, linear erosion) are not the focus of these specific papers, but the relationship between the exposition of saprolites at the surface, high annual precipitation means and erosion susceptibility may be a subject to be investigated in tropical and subtropical zones, since saprolites tend to have low shear strength, commonly attributed to lack of structure, low amounts of clay and/or high amounts of silt and very fine sand (Scholten, 1997;Morais et al, 2004;Heimsath and Whipple, 2019).…”