Exploring river response to tectonic perturbations with the open source, 2-D SPACE model

Abstract:

Understanding landscape response to tectonic perturbation requires models that can evolve both sediment and bedrock. However, most models of river channel evolution only treat erosion into a single substrate, or use substrate layering systems that preclude co-evolution of a sediment layer and in-channel bedrock. For example, basic sediment-flux-dependent incision models parameterize the effects of sediment flux on bedrock incision, but do not actually compute sediment transport dynamics explicitly. In these models, high sediment fluxes result in cessation of bedrock incision, but cannot cause aggradation of sediment. We present a new algorithm (the SPACE model) for modeling the simultaneous evolution of sediment and bedrock in river channels in 2-D. SPACE explicitly incorporates sediment entrainment, transport, and deposition as well as bedrock erosion, rather than parameterizing the effects of sediment on bedrock erosion into a flux-dependent function. The model tracks sediment thickness and bedrock elevation, and allows self-organization of sediment flux, channel slope, and sediment thickness in response to model forcings. SPACE can therefore transition freely between, and match known analytical solutions for, both detachment-limited and transport-limited behavior. We develop steady state analytical solutions for channel slope, sediment thickness, and sediment flux in the more complex case of a mixed bedrock-alluvial river, and show that a 2-D numerical implementation of SPACE matches the predictions and need not be constrained by detachment-limited or transport-limited assumptions. SPACE is one of few models equipped to embrace the reality that most channel systems on earth transition between bedrock, bedrock-alluvial, and fully alluviated states over geologic time in response to tectonics. The model is particularly useful for modeling landscape response to tectonic forcing as it can treat the storage and evacuation of sediment as well as bedrock incision. We present an example in which SPACE is coupled with a hillslope diffusion model to explore the dynamics of erosion and sedimentation resulting from topographic growth and decay. As a component of the open source Landlab modeling toolkit, SPACE is easily customized and coupled with other models.