Medium-term morphodynamic behavior of a multiple sand bar beach

Abstract

Cassino beach, located in southern Brazil, is a very fine sand beach with morphodynamic stages ranging from dissipative to intermediate. This beach can display up to three sand bars which are active during high energy events. Since the beach is located in a microtidal area, flood events are mainly caused by storm surges which can reach 1.8 m. During such events the highest rates of beach erosion are observed. A series of six years (from March 2005 to February 2011) of data from video images have been used to study the nearshore morphodynamic behavior. Daily data of sandbars and waterline position were obtained from these images. The generated data was analyzed in order to recognize flood events patterns as well as patterns of beach morphodynamic both during flood events and during normal conditions. Forty seven flood events were identified, 53% during winter, 24% during autumn, 17% during spring, and 6% in the summer. The average duration of events is 1.5 days. During this period, the whole system is migrating offshore in a rate of 9 m/year. Wavelet analysis showed a gradation of frequencies that govern the variability of the surf zone features. Closer to the beach high frequencies processes (associated with tides and meteorological forcing) have larger influence than further offshore, where longer processes gain importance.