SEASONAL BEHAVIOR OF MAR-CHIQUITA TIDAL INLET IN RELATION TO ADJACENTBEACHES, ARGENTINA

Mar Chiquita microtidal inlet is subject to seasonal periods of summer sand availability and winter scarcity. In the long-term trend the coa st is erosive although these environments were deposited in the last 5 ,000 years due to a sealevel fluctuation of 2 m. In the past, the inle t used to migrate northwards and episodically to obstruct. There are s ediment interchanges between the inlet and adjacent beaches. During th e summer, sand is stored as a flood-tidal delta covered by flood-orien ted megaripples. The ebb channel is narrow, shallow and oriented to th e southeast. It's bottom is floored by dunes composed of fine sand and oriented seawards. During the autumn and caused by an increase in pre cipitation, sand is transferred offshore to form an assymetric tidal i nlet. The channel bottom becomes floored by lag beach-rock gravels lyi ng over cohesive muds related to the former and extended coastal lagoo n. During winter, this ebb-tidal delta bypasses sand to the northern b each. The ebb channel is then oriented to the northeast increasing the discharge along the longshore troughs of the northern beach. Sand is therefore transported northwards along this beach and diminishing wave -induced losses downdrift of the hydraulic jetty. Averaging Spring (Oc tober), drift reversed and sand is trasported southwards shoaling the inlet, progressively oriented to the southeast again. Although Mar Chi quita is a typical coastal lagoon, its degree of sediment infill preve nts tidal effects within the entire basin. Tidal amplitude diminishes in a short distance along the channel. The inlet flow area is in a fir st approach in relation to the potential tidal prism, but its main cha nges are in relation to precipitations within the basin. The minimum f low area can vary 10 times in relation to rainfall within the catchmen t basin.