Abstract:
The morphology of tidal inlets is very dynamic, due to the combined action of
waves, tides and river flows. The morphological changes of small and shallow
inlets are particularly relevant, as even slight variations in the bathymetry may
induce a dramatic effect on tidal propagation and distortion. Occasionally, these
complex dynamics may lead to the closure of the inlet and thus degrade the
water quality upstream. Numerical morphodynamic models constitute attractive
tools to study these morphological changes, although their application is still
time-consuming and requires a deep insight into the relevant processes. This
study aims at analyzing the morphodynamics of the tidal inlet of a small and
shallow coastal system (the Aljezur coastal stream), through the
implementation, validation and exploitation of the MORSYS2D morphodynamic
modeling system. The Aljezur stream is located in the south-west coast of
Portugal and subject to the north Atlantic waves and winds, and tides along the
Iberian shelf. The stream is about 36 km long, 1-3 m deep and 10-40 m wide.
Five field campaigns were carried out between 2008-2010 to provide
bathymetry, water levels, waves and currents, both in the estuary and the
adjoining beach, for the understanding of the dynamics of the stream and for
the application, calibration and validation of MORSYS2D. This 2D
morphodynamic modeling system simulates the non-cohesive sediment
transport processes and the resulting bathymetric evolution in coastal regions.
The system includes a wave model (SWAN), a circulation model (ELCIRC) and
a sediment transport and bottom update model (SAND2D). The application,
calibration and validation of MORSYS2D were a step-by-step procedure due to
the numerous variable inputs and processes involved (e.g., forcings,
parameters, formulations). The procedure started with simulations forced only
by the tide, and progressively were added the wave and sediment transport
processes. Different data sets (water levels, velocities, wave parameters and
bathymetries) were used to validate each step. Morphodynamic simulations
conducted between consecutive field campaigns provided the final validation.
In order to investigate the effect of the several processes on the
morphodynamic evolution of the inlet (e.g., waves and peak river flows),
synthetic simulations were performed. From the exploitation of the model,
results show that the morphodynamic variability of the beach is dominated
mostly by the waves while river flow dominates the morphodynamics of the inlet
region when peak flows occur. The model system reproduces correctly the
hydrodynamics (waves, levels and velocities), and produces predictions of
bathymetry qualitatively correct. This study brought a new understanding of the
system and its variability.
