VARIATIONS IN CROSS-CHANNEL BEDROCK ERODIBILITY: A PHYSICAL EXPRESSION OF ABRASION PROCESSES AND BEDROCK PROPERTIES

Erosion processes influence river morphology and the surrounding landscape. The interaction between weathering and erosion within river channels determines how erodibility varies across a channel, which strongly influences channel morphology. Here we consider how sediment impacts reduce bedrock erodibility over a variety of lithologies. Sediment impacts of sufficient energy help propagate surface fractures and weaken the bedrock surface thereby enhancing the efficiency of other processes to remove material. We investigate this hypothesis by measuring the competency of different channel surfaces (stoss/lee sides of bedrock bedforms and channel margins at a height above bankfull) that are subject to different energies and weathering regimes. Streams in Prescott National Forest, AZ expose bedrock through a variety of lithologies, which provides a natural testing ground. We present a novel method to detect variations in bedrock erodibility that correspond to different erosion processes. Cross-channel differences in bedrock erodibility are estimated through the use of decimeter scale P-wave velocity, Schmidt hammer rebound, and uniaxial tensile strength tests. Sediment impact energies are estimated from grain size distributions of channel sediment and 10^-3 m resolution DEMs of the channel bed acquired with photogrammetry. Observations reveal weaker surfaces on stoss sides relative to lee sides of bed protrusions. In some cases the strength of channel bed is nearly that of the channel margin. Preliminary results show that the P-wave velocity of surface rock is up to 60% slower than rock from as little as 5 cm depth and that the degree to which surface rock is damaged beyond the conditions of non-damaged rock from depth is sensitive to the size of sediment in the channel. We use the findings to propose a new damage based framework for understanding how rock properties influence erosional resistance.