HR: 1340h
AN: OS23C-1333    [Abstracts]
TI: Morphodynamic Modeling of Shoreline Transgression and Regression Driven by Changes in the Frequency and Magnitude of Coastal Storms
AU: * Swenson, J B
EM: jswenso2@d.umn.edu
AF: Department of Geological Sciences and Large Lakes Observatory, University of Minnesota Duluth, 1114 Kirby Dr SE, Duluth, MN 55812 United States
AB: Sequence stratigraphy has long emphasized sea level fluctuation as a causal mechanism for transgressive-regressive cycles preserved in the ancient record. However, the surf zone is also the locus for the interplay of fluvial input with waves and currents, and several field studies have attributed modern shoreline transgression and fluvial degradation to changes in the hydrodynamic regime of the receiving basin. Here I explore quantitatively how fluctuations in the shallow-marine wave-current field might drive large-scale shoreline translation and the generation of transgressive-regressive sequences. I develop a morphodynamic model in which the repeated occurrence of floods and large coastal storms, of specified frequency and magnitude, drives long-term evolution of the sediment surface in the fluvial and shallow-marine environments. The model couples these environments across the surf zone, which collapses to a shock condition at the shoreline. The breaker height of waves associated with large coastal storms sets the height of the shoreface shock; the flux discontinuity across the surf zone drives lateral migration of the shock. I analyze the stratigraphic response to sinusoidal variations in the shallow-marine wave-current field that are superimposed on a background of steady subsidence, eustatic sea level, and fluvial input of water and sediment. For a reasonable range of wave heights and downwelling currents, variations in the magnitude and frequency of large coastal storms generate cycles of shoreline transgression and regression with stratigraphically significant amplitudes. Specifically, more frequent and larger storms increase the fraction of sediment supply partitioned to the shallow-marine environment, thereby driving shoreface transgression and fluvial degradation. Shoreface translation is most sensitive to variations in the strength of the downwelling current. Model results suggest that the phase relation between shoreline and clinoform rollover is the primary stratigraphic indicator for differentiating between sea-level-driven and wave-current-driven transgressive-regressive cycles.
DE: 4211 Benthic boundary layers
DE: 4267 Paleoceanography
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 1719 Hydrology
DE: 1724 Ocean sciences
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting