Mud deposits at Cassino beach: role of dredging

Abstract

Mud deposits on Cassino beach are associated with waves generated from storms during cold front passages and extratropical cyclones from the southern quadrant. During such episodes fluid mud deposited at the shoreface and surf zone is transported towards the surf zone and the sandy beach prism. The first mention on the origin of mud has always attributed a source exclusively natural for the deposits. However, new interpretations and recent data indicate that the increase in dredged volumes over the years increased the frequency of mud depositions. In order to verify this association, records of dredged and discarded volumes in the Port of Rio Grande and Cassino beach shoreface were analyzed. Maps of bottom types, silt and clay ratio, and fluid mud thickness obtained from superficial samples and sediment cores between 4 and 15 m water depth at the time of the 1998 impact were also reanalyzed. Current conditions near the dumping sites in the estuary and inner shelf in 1998 were reconstructed by hindcast based on reanalysis data and an ADCP anchored 15 m off the Rio Grande coast. In order to verify the effect of the passage of a cold front on the bottom near the ocean dumping site, data from two mooring stations made on the crest and swale of a sand ridge field were considered. To further understand the disposal of the dredged material, a particle transport model was created for two periods related to the depositional events. In the period of 1998, it was simulated the disposal of dredged material on two locations, one with the disposal of 640,000 m3 of sediment inside the estuary at 8 m of depth and, the other, with 2.3 million m3 of disposal material, at 13 m depth in the shoreface. The simulation at depth of 19 m, for the dumping site of 2014 considered the disposal of 1,000.000 m3 at 19 m water depth. The occurrence of massive concentrations of an estuarine gastropod associated with the deposits at the beach led to a literature search on their occurrence in the area after researches carried out for more than three decades on the fauna ecology of sandy beaches. From the 1970s onwards, dredged volumes intensify and were deposited into the estuary and ocean using Hopper dredges. There was an increase in episodes of mud deposition at CB. The sedimentation patterns after dredging operations indicate that natural sedimentary processes are altered in a transient way. Currents generated by winds and storms in both quadrants north and south provide conditions of instability of fine sediments in the disposal sites with flow components parallel and also direct to the coast regardless of wave action. Hydrodynamic modeling indicates that sediments disposed into both the estuary and the shoreface during northeast and southern winds have the potential to be driven towards the beach. The massive deposition of millions of organisms after dredging events is not a natural process and is an indication of the dredging activities. Dredging can be an important part of the environmental impact because the natural accumulation process is enhanced by intense pulses of fine sediments in short time favoring more frequent depositions since the historical and current disposal sites are not appropriate. Negative environmental impacts are related to beach usage since it affects sunbathers, surfers, vehicle traffic, morphodynamic changes, and massive mortality of the benthic fauna.