Synthesizing Theory and Observations to Understand Sediment Deposition Patterns: Sand Ripples and Salt Marshes

Abstract

Sediment deposition patterns occur across many Earth and planetary environments. The complex, coupled and nonlinear dynamics of sediment transport across a range of length and time scales makes these patterns difficult to understand. Models of these emergent systems typically require parameterizing the effects of small/fast-scale interactions to describe the large/slow-scale dynamics and resulting patterns. In many cases, well-established parameterizations are not available, and scientific guess work is required. This dissertation takes the approach of deriving large-scale emergent sediment transport dynamics directly from small-scale interactions. The methods of this framework are based on detailed modeling of small-scale interactions derived from experimental observations and direct simulations. Through these analyses, the large-scale variables and resulting sediment deposition patterns consistently emerge from the sub-pattern-scale dynamics. I apply this approach to study wind ripples and salt marshes. I derive an analytical model of wind ripple emergence influenced and supported by direct numerical simulations of grain-scale sediment transport and ripple formation. I show that wind ripple emerge through the dynamics of grain-bed impacts and their wavelengths are thus set by the grain size, and therefore don't vary much across different planetary conditions. Yet ripple speeds are set explicitly by the transport conditions and thus vary greatly---even propagating against the wind in some instances (a novel result predicted from the analytical framework). And from the well-tested interactions of turbulent flow on vegetation, I derive a large-scale understanding of sediment deposition across marsh basins. Detailed modeling shows that, while the dense vegetation that occupies salt marshes controls small-scale flow and sediment transport dynamics, large-scale flow and sediment deposition patterns are independent of marsh vegetation. The methods I develop in this dissertation, although applied to the case studies of wind ripples and salt marshes, can be used to describe a wide verity of large-scale sediment deposition patterns.