Mixed sediment beach processes: Kachemak Bay, AK

Numerous studies have documented the rates of sediment transport along well-sorted, sand-rich beaches relating sediment transport to forcing by waves and currents. The dynamics of mixed-grain size beaches, however, have received relatively little attention in the scientific literature in spite of the estimate that these systems represent approximately 80 percent of the world's non-rocky coastlines. The tectonically active, megatidal, sea dominated beaches of Kachemak Bay, AK exhibit a wide range of interesting characteristics. In particular, the surficial sediments have a bimodal size distribution, in which self- organized bedforms composed of fine-to-medium sand migrate as coherent packages over a cobble substrate which is also dynamic. Our goal is to characterize and quantify, over various time scales, the coupled sediment transport dynamics and morphological development of this prototypical mixed sediment system. In February 2003 we installed an Argus Beach Monitoring System at the study site consisting of 8 cameras spanning a field of view of approximately 220 degrees. Utilizing a tidal contouring algorithm and over three years of hourly images, we are quantifying the rates, direction, and form of sediment transport taken by both the sand size and cobble fractions of the bi-modal distribution. Annual topographic surveys provide ground- truthing for the image derived data. Hydrodynamic forcing data are being collected with a wave/tide gage deployed in approximately 3m (MLLW) of water about 1.5 km (in the cross-shore) from the ARGUS station. In addition, we are applying new digital imaging technologies to map the spatial distribution of surficial grain size (both modes) along the field of view of the Argus cameras. Alongshore migration rates of intertidal sand bedforms, 1-2 m amplitude and ~200m wave length, over the cobble substrate have been documented to be approximately 250 m/yr. Strong seasonality in migration rates is evident with the majority of sand body movement, up to weekly-averaged rates of movement exceeding 6 m/day, occurring during large wave events. The cobble substrate, over which the organized sand fraction is transported, is dynamic as well. Both video measurements and ground surveys have documented the cross- shore and longshore migration of a large cobble bedform over its life cycle of more than a year. The cobble bedform was formed during a recent winter and by the following spring the bedform had organized into a mobile cobble berm with a landward slipface approximately 1.5 m high and a cross-shore length of about 50 m. During the following year, this feature increased in volume and migrated 50 m in the cross-shore direction and over 100 m in the alongshore. While the organized sand bedform migration rates are well correlated with the larger winter storm season, initial observations indicate that the cobble berm is significantly active during the relatively mild spring and summer months. The long time series of images allows us to not only quantify the transport rates and form of the two distinct sediment size modes but also to examine feedback processes within the overall coupled system.