Santo André Lagoon
Background">Background
In Portugal, 80 km south of Lisbon, in the southern half of the Tróia-Sines littoral arc, the Santo André Lagoon is situated. It is separated from the Atlantic Ocean by a sand barrier, around 5 km-long and with an orientation of N15°E. The barrier is formed by an active beach-foredune system including numerous washovers in the northern tip and its morphological complexity increases southward, where beach and foredune weld and lean against an older, robust and vegetated aeolian ridge (Cruces, Freitas, & Andrade, 2009).
Since at least 17th century, the sand barrier is artificially breached regularly (annual) in spring tide conditions to maintain the water quality (promoting water exchange, prevent eutrophication and drain the alluvial deposits, reclaimed for agriculture (Cruces, Freitas, & Andrade, 2009)). To maximize the lifespan of the inlet, breaching is performed in March-April (once in February, 2001) at the end of winter, when the large storms already have been occurred and the wave energy is less, thus less sediment transport. Besides the moment of breaching – at the end of winter - the lifespan of the inlet also depends on other factors, among which waves, tidal amplitude and lagoonal water level in the moment of breaching are the most important (Cruces, Freitas, & Andrade, 2009).
The barrier is not only breached artificially, but it is also occasionally breached during storms (Cruces, Freitas, & Andrade, 2009). In this case, the barrier will breach in the northern tip.
The barrier will be breached during spring tide, when the maximum gradient between the lagoon water level and the level of the ocean occurs (Pires, et al., 2011). During that period the head difference between the water level in the lagoon and the sea water level has its maximum. But also the ebb and flood velocities are at its maximum. The inlet is ephemeral and evolves naturally until complete silting up in a matter of weeks, essentially through wave-driven cross-shore sediment transfer (Freitas, et al., 2003).
A movie of the opening of the lagoon can be found on http://www.youtube.com/watch?v=PA-ag4srq84
. In this movie the opening of March 7, 2002 can be seen.
Problem description">Problem description
Lagoon is not breached naturally, or not significant often, but occasionally. Therefor artificial breaching is necessary to refresh the water. After opening of the inlet, it will be closed within weeks (depending on dry or rainy, thus level of lagoonal basin (Cruces, Freitas, & Andrade, 2009)). Closure will take place due to a combination of an energetic wave regime and meso-tidal range. This tidal inlet was investigated in the scope of a research project funded by the Portuguese National Agency for Research, which allowed elaborating a database of bathymetric and topographic surveys and water level data inside the lagoon over the period where the inlet was open in 2009.
Previous modelling is performed with the process-based modelling system MORSYS2D, which resulted in overall good predictions of the inlet development after its opening, but failed to reproduce its closure.
Objective">Objective
The main objective of this master thesis is:
Understanding of the physical processes involved in closure of the artificial breached inlet. With the understanding of the processes, modelling of the evolution of the Santo Andre Lagoon can be improved, in order to reproduce its closure by using XBeach.
A better understanding of the physical processes (by modelling and testing for this case) is not only important for this particular case, but could also contribute to the understanding of other cases and closure of inlets in general.
Observed bathymetry (left) and modelled bathymetry (right) of the opening in 2009
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