Coastline change and offshore suspended sediment dynamics in a naturally developing delta (Parnaíba Delta, NE Brazil)

Continuous coastal erosion has been observed on the shoreline of major river deltas worldwide as a result of relative sea level rise and anthropogenic impacts. However, the relative contribution of sea-level rise on deltaic coastline retreat (i.e. separately from anthropogenic interference) has not been clearly defined, due to a lack of research on naturally developing deltas. The Parnaíba River Delta, located on the NE Brazilian coast is one of the few examples of a river delta that is still developing in almost natural conditions in contrast to many heavily impacted deltas worldwide. The wave- and tide-dominated delta is subjected to delta switching processes with two active distributaries in the east and remnant tidal channels in the west. To address the decadal evolution of the delta in terms of suspended sediment transport and shoreline changes remote sensing techniques applied on orbital sensors were used. Satellite images from LANDSAT missions 2 to 8 were used to assess shoreline change between 1972 and 2016. Moreover, maps of suspended sediment concentration calculated from water leaving reflectance revealed the importance of Parnaíba River as major sediment supplier for the adjacent coast. Sufficient sediment supply from the Parnaíba River (PR) maintains shoreline stability and progradation of the delta in the region of the distributaries despite of a reservoir located 700 km upstream, which is supplied by only 25% of the drainage basin. Westward longshore drift of the sediment yielded by the Parnaíba River is an important transporting agent distributing river sediment along the coast. During ebb-phase, the PR discharge pushes the suspended sediment plume as far as 10 km offshore and the runoff forms a hydrodynamic barrier preventing alongshore transport of sediment from the east to the west part of the delta shoreline. During flood-phase and high tide, the river sediment plume is pushed shoreward by tidal forces and westward by longshore currents. A high turbidity zone extends from the river-mouth plume of the main distributary to the western tidal channels. Turbid water enters the tidal channels during flood-phase and leaves during ebb-phase. Coastal accretion or stability characterizes the eastern active part of the delta while moderate erosion prevails in the western abandoned part damped by the westward directed longshore sediment transport from the PR mouth.