HR: 0800h
AN: H41C-0318    [Abstracts]
TI: Geologic Framework Versus Surficial Processes: Call it a Draw? (in a Model Under Some Circumstances)
AU: * Valvo, L M
EM: lisa.valvo@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Center for Nonlinear and Complex Systems, Center on Global Change , Durham, NC 27708 United States
AU: Murray, A
EM: abmurray@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Center for Nonlinear and Complex Systems, Center on Global Change , Durham, NC 27708 United States
AU: Ashton, A
EM: ada@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Center for Nonlinear and Complex Systems, Center on Global Change , Durham, NC 27708 United States
AB: Models often involve the implicit assumption that changes to the shape of a sedimentary coastline, such as the Southeast Coast of the United States, are due to wave-driven alongshore sediment transport. This assumption is most valid along coastlines where the shoreface, or the nearshore area strongly influenced by waves, remains covered with a veneer of sediment, as weathering of the shoreface produces sand at a rate at least commensurate with local erosion rates. However, if the rocks or semi-lithified deposits composing the shoreface weather slowly, so that the sediment supply is locally or regionally insufficient to keep up with the combined demands of a divergence of alongshore transport and cross-shore losses, the geologic framework can exert a significant influence on coastline evolution. A more complete analysis requires incorporating the effects of underlying geology into models treating nearshore sediment transport. We start with a model recently developed to investigate large-scale interactions over long time periods. This preliminary model is similar to "one-line" models used in many coastal engineering applications although it can treat arbitrarily complex shoreline shapes and wave climates. We incorporate weathering of the underlying geology along a lithologically heterogeneous coast into the preliminary model. We do not explicitly simulate the complex set of physical, chemical, and biological processes involved in shoreface weathering. Instead, we include the effects of weathering in a simple modeling framework that relates weathering rate to the thickness of sediment covering the underlying material. We treat thickness as a proxy for how often the underlying material is exposed as a possibly patchy sand cover shifts around. Since fine-grained sediment is lost offshore and is unavailable to the nearshore system for transport, the amount of material delivered to the nearshore system from shoreface weathering is also modulated by the fraction of coarse material composing the rock. Analyses of coastline evolution based solely on lithological heterogeneities can implicitly lead to an apparent paradox. It has been suggested that where a shoreface is locally composed of material that is more mobile or contains a larger fraction of fine material than surrounding shoreline segments, long-term shoreline retreat rates are higher. Subtle divots in the coastline corresponding to such locations seem to support that idea. However, in the long term, such spatially heterogeneous shoreline retreat rates would tend to produce a progressively more crenulated coastline, in conflict with the observation that sandy coastlines tend to be fairly smooth on the regional scale. Our groundbreaking model combining lithologic influences and sediment-transport processes offers a resolution to this apparent paradox. Shoreline divots do tend to form in the model where the shoreface weathers more rapidly or is composed of finer material. However, alongshore transport tends to partially fill in such divots, slowing the weathering rates there while increasing them in adjacent areas where mobile sediment is preferentially removed. The long-term result is an alongshore-uniform erosion rate. Subtle shoreline undulations reflecting the heterogeneous geologic framework can exist in the model, but they their amplitude reaches a steady state rather than increasing with time. In this model, when the regional erosion rate is greater than the rate that the slowest-weathering rocks along a shoreline can transform into mobile material, bays flanked by rocky headlands develop.
DE: 4546 Nearshore processes
DE: 3022 Marine sediments--processes and transport
DE: 3210 Modeling
DE: 3220 Nonlinear dynamics
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting