HIO GRANDE BAR - THE CASE HISTORY OF A LAGOON OUTLET INTO A TIDELESS SEA

摘要

Rio Grande harbour is situated inside the outlet of Lagoa dos Patos (9 910 km2) and Lagoa Mirim (3 770 km2). Most of the rivers in the State of Rio Grande do Sul, that are not tributaries of river Uruguay, flow into the two lagoons and the latter discharge through Canal do Norte into a tideless sea. Flood flows can exceed 20 000 cumecs. The outlet is on a sandy coast with littoral drift in both shoreline directions. The time history of the outlet can be divided into three periods. Before breakwater construction, the bar was extremely unstable in lay-out, position and controlling depth. The latter ranged from 2.5 to 7 m. From 1911 to 1916 two 4 km long rubble mound breakwaters were built. They deepened the entrance channel to 8 - 10 m but a new bar came about further offshore entailing objectionable navigation conditions in the approach channel. No previous dredging was carried out as the breakwater construction proceeded out to sea, and as result a total of Ik million cubic metres was scoured out between the two breakwaters and discharged offshore to build up the new bar. Some improvement by dredging was not achieved until 1962. The improvement of the approach channel lay-out as well as deepening the entrance to 14 m below datum are under consideration. Field and model investigations have been designed for this purpose.