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- Sediment transport measurements with tracers in very low-energy beachesPublication . Carrasco, Ana Rita; Ferreira, Óscar; Matias, Ana; Freire, P.; Bertin, X.; Dias, J. A.This study investigates sediment transport at a very low-energy backbarrier beach in southern Portugal, from a spring-to-neap tide period, during fair-weather conditions. Rates and directions of transport were determined based on the application of fluorescent tracer techniques. Wind and currents were collected locally, whereas the dominant small and short-period wind waves were characterized using a morphodynamic modelling system coupling a circulation model, a spectral wave model, and a bottom evolution model, well validated over the study area. For the recorded conditions sediment transport was small and ebb oriented, with daily transport rates below 0.02 m3 day-1. Tidal currents (mainly ebb velocities) were found to be the main causative forcing controlling sediment displacements. Transport rates were higher during spring tides, tending towards very small values at neap tides. Results herein reported points towards the distinction between tracer advection and tracer dispersion in this type of environment. Transport by advection was low as a consequence of the prevailing hydrodynamic conditions (Hs < 0.1 m, and max. current velocity of 0.5 m s-1) and the tracer adjustment to the transport layer, whereas dispersion was relatively high (few metres per day). Tracer techniques allowed distinguishing the broad picture of transport, but revealed the need for refinement in this type of environments (bi-directional forcing by ebb and flood cycles).
- An evolutionary categorisation model for backbarrier environmentsPublication . Carrasco, Ana Rita; Ferreira, Oscar; Davidson, M.; Matias, Ana; Dias, J. A.Shorelines occurring along restricted fetch environments, such as the backsides of barrier islands, are extremely diverse with respect to their morphologic characteristics and evolution. In order to better understand the morphodynamic evolution of backbarriers and the associated implications for entire barrier systems, this study proposes a backbarrier evolutionary categorisation model based on the development of two types of index: backbarrier evolution tendency and backbarrier maturation condition. The proposed characterisation system is applied to the Ria Formosa backbarrier (located in southern Portugal) for the period 1947 to 2001. Cross-shore and longshore backbarrier trends in Ria Formosa suggest a shrinking of the lagoon system as a consequence of a decrease in the coastal length of the backbarrier coastline and a landward displacement of it. Even though some of the backbarriers examined were found to be in an immature state, the results obtained illustrate a maturing trend for the system overall. Barriers in Ria Formosa fall into two main evolutionary categories: backbarrier reduction and backbarrier growth. This means that neither smoothing nor branching has been significant and therefore that backbarrier recent evolution is closely related to barrier coast length. Application of the proposed characterisation to the Ria Formosa case study has helped reveal backbarrier evolutionary trends and therefore should be of use in the management of backbarrier systems.