Assessing Nearshore Bar Movements during Storms Using Time-Averaged X-Band Radar Images

The study examines temporal and spatial changes in nearshore morphology using time-averaged images obtained by X-band radar along a 3-km long section of Cassino beach, southern Brazil. A GIS platform has been used to quantify the mean position arid magnitudes of cross-shore movements of the three nearshore bars identified on the radar images in a time period between April 15 and May 31, 2005. Analysis of the time-averaged images show that increasing wave energy results in straight bars while reducing wave energy gives rise to crescentic features. The first bar responds rapidly to changes in wave energy while the two other bars respond only to extended periods of hydrodynamic forcing and generally exhibit a lagged response time in relation to the first bar. A storm with significant wave height of 1.1 m, period of 8 s and currents reaching 1.5 m/s resulted in migration rates of about 30 m/day for the first bar and 13 m/day for the second and third bars that lasted two to four days.