The roles of vegetation, sediment transport, and humans in the evolution of low-lying coastal landforms: Modeling and GIS investigations

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2018

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Abstract

Low-lying coastal landforms such as barrier islands and river deltas are attractive sites for human habitation and infrastructure. They are also highly vulnerable to both climate change impacts such as rising sea levels or increases in storm intensity and anthropogenic impacts such as changes in sediment supply. In this dissertation I aim to improve understanding of some of the primary drivers of the evolution of low-lying coastal landforms over varying space (1-100s km) and time (decadal to millennial) scales. I focus in Chapter 2 on the influence of shoreline curvature and resulting gradients in alongshore sediment transport on shoreline change; in Chapter 3 on the influence of wave-edge erosion on back-barrier marsh resilience; and in Chapters 4 and 5 on the cohesive effects of vegetation on river deltas.

Sandy coastlines, often associated with low-lying barrier islands that are highly vulnerable to sea level rise and storms, can experience high rates of shoreline change. However, they also attract human habitation, recreation, and infrastructure. Previous research to understand and quantify contributions to shoreline erosion has considered factors such as grain size, underlying geology, regional geography, sea level rise, and anthropogenic modifications. Shoreline curvature is often not considered in such analyses, but subtle shoreline curvature (and associated alongshore variation in relative offshore wave angles) can result in gradients in net alongshore transport which can cause significant erosion or accretion. In Chapter 2, we conducted a spatially extensive analysis of the correlation between shoreline curvature and shoreline change rates for the sandy shorelines of the US East and Gulf coasts. For wave-dominated, sandy coasts where nourishment and shoreline stabilization do not dominate the shoreline change signal, we find a significant negative correlation between shoreline curvature and shoreline change rates over decadal to centurial and 1-5 km temporal and spatial scales. This indicates that some of the coastal erosion observed in these areas can be explained by the smoothing of subtle shoreline curvature by gradients in alongshore transport. In other settings, this signal can be obscured by tidal, anthropogenic, or geologic processes which also influence shoreline erosion. While limited in practical application to long, sandy shorelines with limited human stabilization, these results have widespread implications for the inclusion of shoreline curvature as an important variable in modelling and risk assessment of long-term coastal erosion on sandy, wave-dominated coastlines.

The marshes and bays in the back-barrier environment between barrier islands and the mainland can also experience wave-driven erosion, and their dynamics are coupled to those of barrier islands. Previous results show that overwash provides an important sediment source to back-barrier marshes, sustaining a narrow marsh state under conditions in which marsh drowning would otherwise occur. In Chapter 3, I expand the coupled barrier island-marsh evolution model GEOMBEST+ to explore the effects of wind waves on back-barrier marshes. I find that the addition of marsh-edge erosion leads to wider, more resilient marshes and that horizontal erosion of the marsh edge is a more efficient sediment source than vertical erosion of the marsh surface as it drowns. Where marshes and bays are vertically keeping up with sea level, and the net rate of sediment imported to (or exported from) the basin is known, the rate of marsh-edge erosion or progradation can be predicted knowing only the present basin geometry, sea-level rise rate, and the net rate of sediment input (without considering the erosion or progradation mechanisms). If the rate of sediment input/export is known, this relationship applies whether sediment exchange with the open ocean is negligible (as in basins dominated by riverine sediment input), or is significant (including the loss of sediment remobilized by waves in the bay). Analysis of these results reveals that geometry and stratigraphy can exert a first order control on back-barrier marsh evolution and on the marsh-barrier island system as a whole, and provides new insights into the resilience of back-barrier marshes and on the interconnectedness of the barrier-marsh system.

Coastal wetlands such as marshes are also an important component of river deltas. Like barrier islands, these low-lying landscapes are both attractive to human settlement (providing fertile farmland, fisheries, hydrocarbon reserves, and many other services) and prone to hazards such as flooding and land loss. Delta evolution is governed by complex interactions between coastal, marine, and fluvial processes, many of which are still not well understood. In Chapters 4 and 5, I use the delta-building model DeltaRCM to explore the effects of vegetation, specifically its ability to introduce cohesion, on delta morphology and the dynamics of delta distributary networks. The use of this rule-based model allows me to simplify vegetation dynamics and effects in order to enhance the clarity of potential insights into which processes or interactions may be most important in the context of vegetation as a cohesive agent.

Cohesive sediment exerts a significant influence on delta evolution, increasing shoreline rugosity and decreasing channel mobility. Vegetation has been assumed to play a similar role in delta evolution, but its cohesive effects have not been explicitly studied. In Chapter 4, I use DeltaRCM to directly explore two cohesive effects of vegetation: decreasing lateral transport and increasing flow resistance. I find that vegetation and cohesive sediment do alter delta morphology and channel dynamics in similar ways (e.g. more rugose shorelines, deeper, narrower, less mobile channels), but that vegetation may have additional implications for deltaic sediment retention and stratigraphy, by confining flow and sand in channels. My results suggest that sediment composition is a first-order control on delta morphology but vegetation has a stronger influence on channel mobility timescales. To fully understand the cohesive influences acting on a delta, the influence of vegetation should be considered in addition to fine sediment.

In Chapter 5, I explore the cohesive effects of vegetation on delta evolution under different environmental conditions. The dynamics and evolution of deltas and their channel networks are controlled by interactions between a number of factors, including water and sediment discharge, cohesion from fine sediment and vegetation, and sea level rise rates. Vegetation’s influence on the delta is likely to be significantly impacted by other environmental factors. For example, increasing sea level or sediment discharge increases aggradation rates on the delta, and may result in sediment transport processes such as deposition and erosion, both of which can kill vegetation, happening more rapidly than vegetation growth. I conduct two sets of experiments; in the first, I explore the interactions between vegetation and sea level rise rate, and in the second, between vegetation and rate of sediment and water discharge. As expected, I find that sea level rise decreases vegetation’s ability to stabilize channels but that vegetation can still exert a strong influence on the delta at low rates of sea level rise. This limit appears to be higher for channel dynamics than delta morphology, supporting the findings of Chapter 4. In addition, I propose two new insights into delta evolution under different discharge conditions with and without vegetation. First, without vegetation, I observe a shift in avulsion dynamics with increasing water discharge: from a few active channels supplemented by overbank flow and undergoing episodic avulsion (with low discharge) to many active channels experiencing frequent local and partial avulsions (with high discharge). Second, with vegetation, increased sediment discharge and associated aggradation results in more frequent switching of the dominant channels, but also prevents vegetation from establishing in non-dominant channels, resulting in more frequent channel reoccupation and therefore in channel network planform stability. These insights have important implications for understanding the distribution of water, sediment, and nutrients on deltas in the face of future changes in climate, human modifications of fluxes of sediment and water to the coast, and especially for restored or engineered deltas with controlled water or sediment discharges.

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Lauzon, Rebecca (2018). The roles of vegetation, sediment transport, and humans in the evolution of low-lying coastal landforms: Modeling and GIS investigations. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/16896.

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