Convexity, Concavity, and Human Agency in Large-scale Coastline Evolution
Coherent, large-scale shapes and patterns are evident in many landscapes, and evolve according to climate and hydrological forces. For large-scale, sandy coastlines, these shapes depend on wave climate forcing. The wave climate is influenced by storm patterns, which are expected to change with the warming climate, and the associated changes in coastline shape are likely to increase rates of shoreline change in many places. Humans have historically responded to coastline change by manipulating various coastal processes, consequently affecting long-term, large-scale coastline shape change. Especially in the context of changing climate forcing and increasing human presence on the coast, the interaction of the human and climate-driven components of large-scale coastline evolution are becoming increasingly intertwined.
This dissertation explores how climate shapes coastlines, and how the effects of humans altering the landscape interact with the effects of a changing climate. Because the coastline is a spatially extended, nonlinear system, I use a simple numerical modeling approach to gain a basic theoretical understanding of its dynamics, incorporating simplified representations of the human components of coastline change in a previously developed model for the physical system.
Chapter 1 addresses how local shoreline stabilization affects the large scale morphology of a cuspate-cape type of coastline, and associated large-scale patterns of shoreline change, in the context of changing wave climate, comparing two fundamentally different approaches to shoreline stabilization: beach nourishment (in which sediment is added to a coastline at a long-term rate that counteracts the background erosion), and hard structures (including seawalls and groynes). The results show that although both approaches have surprisingly long-range effects with spatially heterogeneous distributions, the pattern of shoreline changes attributable to a single local stabilization effort contrast greatly, with nourishment producing less erosion when the stabilization-related shoreline change is summed alongshore.
Chapter 2 presents new basic understanding of the dynamics that produce a contrasting coastline type: convex headland-spit systems. Results show that the coastline shapes and spatially-uniform erosion rates emerge from two way influences between the headland and spit components, and how these interactions are mediated by wave climate, and the alongshore scale of the system. Chapter 2 also shows that one type of wave-climate change (altering the proportion of `high-angle' waves) leads to changes in coastline shape, while another type (altering wave-climate asymmetry) tends to reorient a coastline while preserving its shape.
Chapter 3 builds on chapter 2, by adding the effects of human shoreline stabilization along such a convex coastline. Results show that in the context of increasing costs for stabilization, abandonment of shoreline stabilization at one location triggers a cascade of abandonments and associated coastline-shape changes, and that both the qualitative spatial patterns and alongshore speed of the propagating cascades depends on the relationship between patterns of economic heterogeneity and the asymmetry of the wave-climate change--although alterations to the proportion of high-angle waves in the climate only affects the time scales for coupled morphologic/economic cascades.
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