The development of lagoon sandy spits is dependent on sediment supply (mainly through longshore transport) and is often related to erosion ofupdrift beaches under high-angle waves. This paper provides an understanding of the short-to medium-term shoreline evolution of three symmetrical lagoon sandy spits and the related erosion of the adjacent bay beaches of Lagoa dos Patos, Brazil. To do so, two of the most widely used methods to quantify shoreline changes were applied: the transects-from-baseline method and the change polygon method. A historical analysis of the shoreline movement of approximately three decades (1984-2013), was carried out, aiming to track the source of sediments to the spits. Alternate phases of shoreline stability, deposition, and recession (less frequent) in short time intervals (years) showed correlation with the El Nino-Southern Oscillation and Southern Annular Mode indexes. Shoreline stability along beaches in the medium term (60.4% of Arambare and 73.9% of Graxaim beaches, by the linear regression rate calculation), together with the very low average shoreline change rate of +o.7 m y-1 found using both methods, demonstrates that the sediment supply to the spits has been significantly reduced in the past three decades. Furthermore, the large-scale subaqueous portions of the spits are under wave-induced sedimentary reworking accompanied by the up-building of sandbanks from the submerged bars, followed by its landward migration and merging to the spit's shoreline. These results suggest that the spits of Lagoa dos Patos have their growth limited once the amount of sediment supply has been significantly reduced.
This research uses a process-based model (Delft3D), validated with measured wave data, to investigate the controls that a cuspate shoreline and its associated submerged morphology have on wave generation, propagation and attenuation within a large elongated lagoon (Lagoa dos Patos, Brazil). A method based on the Energy-Flux-Method was applied to the historical wind dataset to define representative wind cases to be used as forcing conditions in the model. The results show that, during extreme wind conditions, the spits dissipate wave energy in the lagoon. This (wave attenuation and the wave sheltering effect) controls the stability of the adjacent coastline. The wave attenuation varies between 18 and 46% along the submerged spit depending on the crest width and the amplitude of the incident waves. Waves are mainly attenuated in the proximal and central portions of the spits where the spits are wider, resulting in a reduced transmitted energy to the adjacent coastline, while larger waves are also attenuated on the distal end of the spits. The degree of attenuation depends on the direction of wave generation, the respective fetch, the spit width, and the water depth. A strong relationship of mutual co-adjustment between the morphology and the wave field results in a very low occurance of oblique wave angles of incidence, especially for waves propagating accross the long lagoon axis. Furthermore, the wave attenuation over the spits is also responsible for its progressive erosion, which from decades to centuries, may lead to an increase of changes on the lagoon shorelines.
The coastal sediment budget is an essential tool to understand erosion/accretion patterns along coastlines and to predict future changes. This work is one of the first approaches of a coastal sediment budget study in a cuspate shoreline covering the whole embayment-spit system. Taking this into consideration, this study aims to estimate the the main longshore sediment transport patterns and propose a conceptual model of the regional sediment budget of the lagoon cuspate shoreline (Lagoa dos Patos-Brazil), while also analyzing its influence on the spit development. To do so, representative wind conditions (direction and speed) were used to force a validated lagoon wave model utilizing the process-based modelling suite Delft3D. The potential longshore sediment transport was computed along widely distributed cross-sections, based on representative wave cases, and the annual sediment budget was estimated. The results showed a pattern of inter-related source and storage areas along the bay beaches and the occurrence of short-term nodal zones (convergent and divergent), which have an important control on the regional sediment budget of the coast. The central littoral cells of the embayments behave as temporary sediment storage areas, while the southern littoral cells act as sources of sediments to the spits. The findings of this study demonstrate the important control that short-term nodal zones have on the annual coastal sediment budget of complex coastline shapes and, the importance of the budget of the adjacent updrift cell to the sediment supply for the spits.
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