Morphodynamics of Praia do Cassino, RS

Abstract

Aburacl . The C.ssmo beach mot'phodynamic osc illates between the dissip.:nive and intennedi;nc SUites. accumulating sedi ments in the primary dunes and in long shore bars of combat to South Atlantic Western coasUlI storms. Appro:"'mately five years (1991-1996) of dail y linornl cnvironmcntal observations of wind, waves, long shore currents and beach width allied to coastlincs transvcrse topographical profilcs promoted a qual itative methodology which detennined thc physica l agent's regime and its influences in the sedimentary beaeh stock and morphodynamic states. Three main ineidenee angles, South. Southeast (perpendicular to the coastl ine) and East were characterised in relation 10 Ihe follOWing factors: wave regime. omega parameler (W .. IlblTp.Ws) and Ihe mean sea level oscillations. The meteorological tides (5tonn surges) condition the destruction of the close bars to the beach face Dnd occasionally the frontal dunes. These high wave energy events (hangover) Dfe frequent for the South quadrnnt (12% a year). The Southeast quadrant (42% occurrences) presentS conditions of swell which detennmc times of dyn ami c balance of the morphology. These conditions facilitate the development of an intennediary state with long shore bars troughts. sma ll er variat ions in the sedimenlilry stock but significanl alterations in the bars fonn. The East quadrant (4 6~. a year) constantly governed by moderale winds and local WDves with rare events of swell favours the enla rgement to the backshore and coalition of the first bar with the beach face, typical intcmlediate state of beach in construction. The cons tructive intcmlcdiary SIlI1C facilitates th e largest beach sedimentary varia1ion volume as i1 becomes more suscep1ible to strong stonns (autumn stonns). The dissipative state presents small er mobIl ity trought the soft s lope. leaving the eoneavc beach face. concentrating the stonn effect in the surf zone and front al dunes. We charactensed periods or constructi"e and destructive beach profil es. when:: the omega parameter eyidcnced oscillations among the dissipative modal Slate, of larger rrequency in the autumn and winter. and secondary;nlennetiiatc states in the spring anti summer.