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Hydrodynamic and sediment fluxes through the inlets of the Ria Formosa
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Abstract(s)
Research into the consequences of engineering works on the long-term behaviour of
inlet systems has been hindered by the absence of suitable datasets, the shortcomings of
existing formulae when applied to different inlets, and the difficulties particular to
multi-inlet situations. Cross-sectional channel areas adjust to hydrodynamics and
sediment transport conditions. The impacts of a new inlet on the adjacent shoreline can
be unpredictable and are difficult to quantify. The coupling of morphology and
hydrodynamics analysis should therefore be extended to all inlets in order to infer the
stability of the overall system based on the distribution of the tidal prism through time
and the patterns of inlet circulation and sediment transport.
Within this thesis both historical and present-day hydrodynamic data collected using
high frequency equipment are combined to analyse the recent evolution and equilibrium
of a multiple-inlet system. Methods for calculating sediment budgets, analysing the
evolution of inlet parameters and determining present hydrodynamic and sediment
transport patterns are coupled together, with the aim of improving understanding and
prediction of tidal inlet morphodynamics. The results augment existing knowledge
about multiple-inlet systems and improve understanding of their short- to medium-term
stability.
The findings indicate that combining sediment budget calculation and inlet
parameter analysis is useful for understanding historical sediment pathways and magnitudes, as well as for analysing the evolution of an inlet, or multiple-inlet system,
towards equilibrium. Existing formulae for inferring inlet stability that relate crosssectional
area to tidal prism should be revised with a view to including other external
variables (e.g. stratigraphic controls) and making their application more flexible to cope
with the range of different inlet conditions.
Although it is generally acknowledged that most multiple inlets are unstable and
cannot co-exist, detailed measurements of hydrodynamic variables obtained over
complete spring and neap tidal cycles in a multiple-inlet system in Southern Portugal
(Ria Formosa) indicate that the two main inter-connected inlets servicing an embayment
(Faro-Olhão and Arrmona inlets) can coexist, at least over a time scale of several
decades. Their coexistence cannot be explained simply using empirical equilibrium
relations or inlet hydraulics.
Residual flow between the inlets appears to play an important role in enhancing
their stability. The morphology of the inner channels connecting the inlets can play an
important role in inlet stability by controlling the interconnections between the inlets.
The capacity to exchange large portions of the tidal prism, while maintaining
independent behaviour for the majority of the neap-spring tidal cycle, can contribute to
the stability of multiple inlets by altering residual flow and, consequently, transport
capacity. This can be particularly important for tidal conditions in which inlets can drain
each sub-basin independently.
However, it is the availability of sediment stored in the ebb-tidal deltas (and its
capacity to be carried into the inlets during storm events) that ultimately dictates the
overall equilibrium, independently of hydraulic flushing capacity. If littoral drift is
strong, ebb shoals trap significant quantities of sand. During periods of increased wave
activity, the tide may not have the capacity to transport material away from the inlet
mouth area, and the inlet will accumulate sand due to the shoreward migration of the
ebb shoal. Such movement of sediment leads to the obstruction of the inlet channels,
thereby affecting the hydraulic efficiency and eventually leading to inlet closure over
the long term. The long-term equilibrium of sediment storage in the ebb-tidal deltas
must therefore be considered when analysing the possible equilibrium of multiple-inlet
systems.
Regarding the techniques used, a statistical evaluation of the best method for
extrapolating velocity measurements to the unmeasured areas of a vertical profile is
presented. To calculate cross-sectional discharge, mean velocities and bed-friction
velocities, custom Acoustic Doppler Current Profiler (ADCP) software makes use of
theoretical models (e.g. 1/6 power-law and logarithmic law) to reconstruct velocity
profiles based on models calibrated and tested for fixed current-meters The aim of this
work was to define the best method to extrapolate velocities when using boat-mounted
ADCPs, allowing the accurate calculation of the tidal prism, as well as increasing the
reliability of the sediment transport estimations. The results revealed the logarithmic
law to be more robust across different velocities and channel morphologies.
Quantification of sediment transport in tidal inlets remains a fundamental
requirement for developing both conceptual and numerical modelling of tidal inlet
function and evolution. Well-established empirical formulae were used to estimate
bedload, suspended load and total sediment transport rates. The results compared
favourably with direct and indirect field observations of sediment transport rates.
Although subject to a range of errors, the methods adopted have helped to quantify net
accretion/erosion and enabled evaluation of both the flushing and bypassing capacity of
tidal inlets in the system. The work has increased knowledge of sediment dynamics in
multiple inlet systems and identified appropriate approaches for the prediction of
sediment transport in these environments. They may therefore assist in identifying
evolutionary trends for a single tidal inlet or a multi-inlet system.
The approach and methodology followed in this study could be applied to other
multiple-inlet systems around the world in an attempt to “design with nature”,
combining an understanding of sediment movement in a region, development projects and regional sediment-management actions.
Description
Tese de dout., Ciências do Mar, da Terra e do Ambiente (Ciências do Mar-Oceanografia Física), Faculdade de Ciências e Tecnologia, Univ. do Algarve, 2011
Keywords
Hidrodinâmica Zonas costeiras Lagunas Sedimentologia Ria Formosa