The geometric relationships of salients and tombolos along a mesotidal tropical coast

Abstract

Salients and tombolo landforms provide sinuous shapes to shorelines as a result of depositional conditions behind offshore obstacles. Many formulas considering dimensionless ratios between variables involving feature-obstacle geometries, such as obstacle length (B), the distance from the original shoreline to the obstacle (S), and the protrusion of the obstacle (X) have been established to determine their limiting formation. Based on this approach, we carried out an identification and analysis of the geometric parameters of salients and tombolos behind reefs and breakwaters in a tropical mesotidal environment. Multiple reef lines on the inner shelf are responsible for generating mega-salients, which are most influenced by older offshore lines, and smaller salients behind younger reefs closer to the shore. Salients and tombolos along the coast of Pernambuco State, Brazil, occupy more than 50% of the shore, with approximately 30% being found behind breakwaters. The dimensionless relationships B/S and X/B for the 77 features analyzed showed higher R2 values for salients behind breakwaters (0.907) than behind reefs (0.596), indicating that obstruction geometry plays an important role in the salient-tombolo model. We propose here two novel equations that quantify intermittent salients/tombolos geometries in the study area. The new relationships between obstacle water depth and wave transformation were found to be good predictors of salient size and the periodic conditions of tombolos along a mesotidal coast.