Depth of Activity, Sediment Flux, and Morphological Change in a Barred Nearshore Environment

Sediment flux and associated morphological change in a continuously submerged, nearshore, crescentic bar system is documented for discrete storm events using an array of depth of disturbance rods in conjunction with structural indices recorded in epoxy peels of box cores from the active layer. Steel rods (=1 or 2 m in length, 0.5 cm in diameter) buried to a depth of 55 cm with a free sliding washer (=0.6 cm internal diameter) accurately record the spatial variability of: (1) maximum depth of sediment activation; (2) net bed elevation changes; and (3) degradation-aggradation cycles. The depth of activity is related to the magnitude of the incident wave -(70 cm for deepwater waves of 2 m height, 6 sec period versus 23 cm for a height of 1.5 m and period of 5 sec)- and location within the bar system. Maximum values occur on the seaward side of the bar crest where wave breaking, asymmetric oscillatory motion and/or rip current flow would be a maximum during the storm. A secondary maximum is associated with longshore currents in the landward trough. Depths of activity minima occur on the landward slope in response to height loss due to breaking and increase in water depth. In general storms erode the bar profile with scour maxima on the seaward side of the crest and near the toe of the landward slope. Aggradation occurs on the upper landward and upper seaward slopes steepening both slopes and producing a seaward displacement of the bar crest over the crescent area. Structural indices suggest increasing rates of landward transport (through either ripple or lunate mega ripple migration and sheet flow) as water depth decreases up the seaward slope. Landward transport across the bar crest and down the landward slope is also indicated. On the upper landward slope and bar crest of the crescent, however, small- to medium-scale seaward-dipping cross-stratification indicates a distinct seaward flux of sediment, which is interpreted as resulting from rip-type flow. This is not found on the shoal areas and this differentiation is instrumental in maintaining the on-offshore sediment balance as well as the crescentic form