Contrib. of Rock-Mass Strength to Topographic Form and Post-Fire Eros.
Rock-mass strength is typically assumed to influence geomorphic processes by setting the erodibility of landscapes. However, the contribution of rock-mass strength to topographic form is often overlooked, largely because rock-mass strength is challenging to quantify over the broad spatial scales relevant to geomorphology. Although laboratory tests can be easily conducted on small, intact rock pieces, the integration of discontinuities (fractures, bedding planes, weathering) ultimately limit the strength of rock masses at outcrop scales to values orders of magnitude lower than what is quantified by laboratory techniques. Due to a lack of approaches to quantifying scale-dependent rock mass strength, the interdependencies of rock strength, erosion, and topography remain largely unresolved. Here we present a novel approach to assessing the mechanical properties of rock masses that combines a range of spatial scales and surface to subsurface techniques, including subsurface S-wave velocities, Schmidt hammer hardness values, and Geological Strength Index (GSI) observations of outcrop fracture characteristics. To aggregate the contributions of the intact (unfractured) strength, and the outcrop-scale fracture structure and surface weathering, we adapt the Hoek & Brown criterion to quantify outcrop-scale shear strength as a function of depth. We apply these approaches to the Western Transverse Ranges of southern California, USA, where we quantify strong landscape-scale gradients in rock-mass strength associated with the original burial depth of clastic sedimentary rocks. We then assess the contribution of this rock-mass strength gradient to topographic form and event-driven erosion caused by the 2018 Woolsey Fire. We quantify post-wildfire erosion with repeat airborne-LiDAR surveys, and we document positive relationships between area-normalized erosion, rock-mass strength, and channel slope. We suggest that rock-mass strength is expressed as differences in slope that help set the erosional capacity of channels to transport sediment. Although near-surface materials have some intrinsic erodibility, in natural landscapes rock strength and slope become coupled, with slope dominating the erosional response following wildfire.