SIMPLE SHEAR STRESS APPROXIMATIONS AND A FIRST STEP IN MODELING MECHANICAL EROSION IN CAVES

Opposing assumptions have been made in modeling the evolution of stream and cave channels. Models of surface channel evolution have typically neglected dissolution, and models of cave formation have not considered mechanical erosion. After early development of cave channels, sediment can be transported by turbulent flow. This sediment can mechanically erode the cave channel by saltation of the bed load, abrasion, and plucking. Deposited sediment can also affect the development of caves by armoring the soluble floor of the channel and forcing dissolution upward, a process called paragenesis.

Approaches to modeling stream channel cross-section evolution typically calculate mechanical erosion as a function of boundary shear stress (τ_b). Boundary shear stress can be calculated using computationally expensive methods, such as computational fluid dynamics (CFD), or through simpler approximations. As a first step in modeling mechanical erosion in caves a simple method for approximating τ_b is used, modified from previous work on open bedrock channels and applied to full conduits within caves. The results of boundary shear stress calculations are compared to less simplified methods for verification.

Mechanical erosion rates are calculated from τ_b, discharge, sediment size, sediment supply, and properties of the bedrock. The simple methods for calculating τ_b allow modeling of these rates through many time steps without large computational expense. Using this model the dynamics of paragenesis can also be explored, as sediment transport capacity can be calculated from τ_b.