Physically-based models of bedrock incision processes in mountain streams

Many important insights into the dynamic coupling among climate, erosion, and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision. However, many questions regarding incision processes and morphology of bedrock streams still remain unanswered. A more mechanistically-based incision model is needed to study landscape evolution. Major bedrock incision processes include abrasion by bedload, plucking, and macroabrasion. The purpose of this dissertation is to develop a physically-based model of bedrock incision that includes all processes mentioned above by means of theoretical analysis, numerical modeling, flume experiment, and field observation. To build the model, we start by developing a theory of abrasion, plucking, and macroabrasion mechanisms. We then incorporate hydrology, the evaluation of boundary shear stress, capacity transport, an entrainment relation for pluckable particles, a routing model linking in-stream sediment and hillslope, a formulation for alluvial coverage (investigated here experimentally), a channel width relation, Hack's law, and Exner equation into the model so that the evolution of bedrock channels can be simulated. The model successfully simulates various features of bed elevation profiles of bedrock streams commonly found in natural rivers. We also aim at testing the model quantitatively with field data. The model is tested in two separate regimes; an abrasion-dominated regime and a plucking-macroabrasion-dominated regime. The abrasion submodel is tested quantitatively with the Clearwater River in Washington State, USA, and the plucking-macroabrasion submodel is tested quantitatively with the Unnamed Drainage #1 in Southern Indiana, USA, For the Clearwater River, the abrasion model successfully simulates the evolution of a bed elevation profile that finally evolves to a steady state, concave-upward profile matching the present-day field data regardless of the input of initial bed profile millions of years ago. For the Unnamed Drainage #1, the plucking-macroabrasion model successfully creates a concave-upward profile with knickpoints similar to the field condition, and quantitatively simulates and explains the recorded distance of knickpoint migration and incision rates in the past 19 years. It is hoped that the present work represents a step closer to modeling landscape evolution with more detailed physics underpinning the bedrock incision processes.