Fine grain sediment transport and deposition in the Patos Lagoon Cassino beach sedimentary system

Abstract

Extensive mud deposits superimposed on the predominantly sandy inner continental shelf adjacent tothe Patos Lagoon estuary, indicates that the Lagoon is a potential source of fine sediments to the coastal sedimentary system. The lagoon is large and shallow, and the water movement is mainly controlled by wind-driven set-up and set-down. The mean river inflow is around 2000 m3 s1, although peak flowrates exceeding 20,000 m3s 1have been observed during El Nin˜o periods. Though the tidal elevations are small, tidal velocities in the lagoon’s inlet can be significant due to the large extension of the backwaters. Moreover, significant exchange flows can be generated between the estuary and coastal area due to barotropic pressure gradients established as a function of wind and freshwater discharge.The predominant net flow is seawards, but opposite near-bed flows due to pronounced vertical salinity stratification can also be observed. The coastal area is characterized by small tidal effects, large scale ocean circulation, wind-induced residual flows and wave-driven currents, where the waves originate from swell or are locally generated. Fine sediment is brought into the Patos Lagoon by the rivers and its deposits are likely to have long residence times. These fine sediment deposits can be remobilized by locally generated waves, and driven towards the channels and lagoon 0s shallow bays. Suspended particulate matter (SPM) concentrations within the lagoon do not exceed a few 10 mg l1, though higher values have beenmeasured occasionally. In the southern estuarine part of the lagoon, fine sediments may accumulatedue to gravitational circulation effects, yielding SPM concentrations of a few 100 mg l 1. The export of fine sediment from the lagoon to the coastal area occurs predominantly during NE winds. This explains why the majority of the off shore sediment deposits, known as the Patos Facies, are more widespread towards the southern portion of the inner continental shelf. These sediments deposit in the form of fluid and more compacted mud, between the 6 and 20 m isobath, in layers with thickness varying between a few dm to 2 m causing marked lateral differences in grain size. Recent sediment core data, indicates that fluid mud occurrence increases towards the shore and that the mud depocenter remains in the same area as previously mapped two decades before. On a long-term basis, this lateral heterogeneity in sediment properties controls the geomorphology of the inner continental shelf and shoreface, and influences the shoreline accretion rate and beach morphodynamic south of the inlet. Short-term effects are associated with episodic events of mud deposition on the beach during heavy storms that often result in strong gradients in hydrodynamic processes. These gradients in turn influence the morphodynamic behavior on the sectors affected by the mud deposits and can create coastal hazards relating to beach usage.