The role of storm-generated combined flows in shoreface and inner continental shelf sediment erosion, transport, and deposition

The shoreface is where sand is exchanged between the beach and inner continental shelf under the influence of storm waves. Storm erosion and ravinement redistributes sediment and, over time, affects coastal barrier evolution. This research investigates coastal circulation, sediment dynamics in storms, the ravinement process, shoreface evolution, and the geologic record. Field data collected in Maine for this study include storm currents, cores, grab samples, seismic reflection, side-scan sonar, and fathometer profiles.;A time series of side-scan sonar images on the Kennebec paleodelta, provide the longest and most comprehensive data set to date for any one location. During this study there was a remarkable persistence of sand waves and sand sheet morphology despite major storms and a hurricane in the last decade. Stratigraphy and relief of shoreface and inner-shelf sediment identifies the basal and ravinement unconformities and constrains the age and origin of surficial sand deposits.;Meteorological and oceanographic conditions in the Gulf of Maine produce three types of combined flow that move sand. Storms generate wave-current combined flow that simultaneously produces hummocky cross-stratification in fine sand, and ripples in coarse sand and gravel on the inner shelf. A modified Bagnold equation is used to illustrate how sand moves at 30 m depth. In the absence of local surface wind, incoming swells augment tidal currents to produce wave-tide combined flow about 27 days per year in Saco Bay and at Cape Small. Wind-driven surface currents in northeasters cause downwelling that carries sand from the shoreface to inner shelf for about 11 days per year. Northwesterly gales induce upwelling and sand transport from the inner shelf to the shoreface for about 21 days per year. Prevailing westerlies conserve sand in Maine coastal barriers that would otherwise be lost to the inner shelf by downwelling.;In a new model of ravinement, combined flow reworks coarse-grained ripples that, in turn, scour underlying strata. Contrary to the traditional concept of lag armoring, this study suggests a process of lag excavation of antecedent strata. Storm ravinement supplies sand to inner shelf bedforms and the local sediment budget.