Sediment dynamics and morphological evolution in the Tagus Estuary inlet

Abstract

The morphological evolution of the Tagus estuary inlet in the last century is analyzed in order to characterize its dynamics, explain its behavior and anticipate its future evolution. First, the evolution is characterized through a literature review, complemented by new data analyses. This review synthesizes the present understanding of the inlet's dynamics and highlights some key questions that remain unanswered. To address these questions, a 2DH process-based morphodynamic model is implemented and validated, and then used to refine the previous understanding of the inlet's dynamics. A new conceptual model, which highlights a seasonal behavior of the inlet's morphodynamics, is proposed. During winter time, the residual sediment transport is directed seaward along the navigation channel, towards the inlet mouth along the beaches adjacent to the inlet, and towards the estuary over the southern part of the ebb delta (Cachopo Sul). During the maritime summer, residual sediment fluxes are approximately half those occurring during winter. The relative importance of tidal flows increases, leading to sediment fluxes directed towards the southwest over the Cachopo Sul. The sediment deposits formed during the summer can be mobilized under specific wave conditions and transported to the northern stretch of the beaches to the south of the inlet. A positive feedback between the erosion of the Cachopo Sul – or, similarly, sea level rise – and the sediment fluxes over this bank suggests that the evolution of the bank observed during the last decades will continue in the foreseeable future.