Beach processes

Abstract

What may be viewed as a relatively simple system, the beach, underlain mainly by sand, can in fact frequently manifest a variable and spatially complex system. In an estuary, the beach, as a shoreline deposit, spans the range of environments from the river entrance to the marine estuarine mouth. Estuarine beaches, whether as a long continuous shoreline or as a discontinuous set of pocket beaches, traverse three major environments in terms of hydrodynamic setting, hydrochemistry, macroscopic biological setting, microscopic biological setting, and sediment provenance. As such, the beach in estuaries is subject to five major environmental gradients (Figure 9). In terms of hydrodynamic setting, there is the part of the estuary located at/near the marine environment that is dominated by ocean waves and, to a lesser extent, by intra-estuarine wind waves, tides, and onshore winds; there is the central estuary dominated by intra-estuarine wind waves, wind, and lesser effects from tides, river current, and floods. There is the riverine part that is dominated by river currents, wind waves, wind, and, to a lesser extent, tides. In terms of hydrochemistry, there is the marine part that is dominated by marine salinities and the attendant effects on biota and their biological processes and marine authigenesis/diagenesis. There is the central estuary dominated by fluctuating salinities or brackish waters, the attendant effects on biota and their biological processes, and estuarine authigenesis/diagenesis. There is the riverine part that is dominated by freshwater and its attendant biological and authigenesis/diagenesis products. In terms of biological setting, there is the marine part that is dominated by marine assemblages. There is the central estuary dominated by euryhaline biota specialized for estuarine conditions. There is the riverine part that is dominated by freshwater biota. In terms of sediment provenance, the tripartite subdivision of estuaries is reflected in the exogenic sedimentary particles (those derived outside of the estuarine basin) in that there is a marine component dominantly towards the estuarine mouth, a mixed component in the central estuary, and a riverine component towards the river mouth. The tripartite subdivision of estuaries also is reflected in the composition of endogenic sedimentary particles and sediment types (those derived inside the estuarine basin) in that peat and bioclasts (shells) are diagnostic of freshwater parts of the estuary (though peat is not within the beach environment; its presence leeward of beaches adds hydrochemical complexity to freshwater seepages). As such, the beaches of the estuary provide a framework to viewing and studying beach processes across the longitudinal range of estuarine environmental variability. This variability of beach setting within the estuary and the beach processes relative to beach setting is expressed geomorphologically, stratigraphically, lithologically, biologically, and authigenically/diagenetically. At smaller scales, the physical, biological, and chemical processes operating on beaches result in environment-specific features such as sedimentary structures, specific suites of lithology such as laminated sand, or concentrations of shell and rock gravel, shell lenses, burrow structures, bioturbation, and chemical products. These environment-specific processes and products are preserved geohistorically in the evolving stratigraphy under the estuarine beach (Figure 10).