|
Abstract
|
Detailed daily topographic maps of beach and inner nearshore areas indicate a cyclic pattern to processes and responses in this non-tidal environment. This pattern is the result of complex interaction between changes in shoreline configuration, discontinuous nearshore sand bars, and environmental variables such as barometric pressure, wind velocity, breaker height, and longshore currents. Of the 18 variables measured, barometric pressure appears to provide the best index for changes in coastal processes. The results of these variations are morphologic changes in the beach and inner nearshore area.
As a low pressure system approaches the coast there is an increase in wind velocity, breaker height and longshore current velocity as barometric pressure drops. When the low pressure system passes barometric pressure rises and there is a reversal of wind direction with an accompanying reversal of longshore current direction.
This cycle in conditions is accompanied by a pattern of responses in the position and morphology of the shoreline and sand bars. During high pressure and low energy conditions shallow discontinuous sand bars have somewhat regularly spaced rip channels. The shoreline is slightly sinuous with protuberances behind the sand bars. Longshore currents are slow and small waves break on the bars causing their shoreward migration. Shoreline sinuousity increases as protuberances grow and embayments are slightly eroded. An approaching low pressure system causes increase in wind, waves and longshore current velocity. As the pressure system passes there is reversal of longshore current direction at the time of maximum wind velocity and wave height. During this time rapid longshore currents are defl cted by the sinuous shoreline such that strong rip currents are formed. These rip currents pass over sand bar crests and excavate channels. At the same time new bars are forming as sediment accumulates in relatively quiet areas between rip channels. As a result there is apparent migration of the bar form. The return to low energy conditions yields a pattern much like that during the previous time of low energy.
In response to these coastal processes, the bar form oscillates back and forth alongshore rather than migrating down the beach. although the sediment does move alongshore. |