IDEM - Inlet Dynamics, Evolution and Management in the Ria Formosa

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Organizational Unit

Publications

Hydrodynamic and sediment fluxes through the inlets of the Ria Formosa

Publication . Pacheco, André; Ferreira, Óscar; Williams, Jonathan

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.

2011Doctoral thesis

Open access

Applicability of sediment transport models to evaluate medium term evolution of tidal inlet systems

Publication . Pacheco, A.; Williams, J. J.; Ferreira, Óscar; Garel, Erwan; Reynolds, S.

This paper derives local formulae to estimate bed roughness and suspended transport and present a method to calculate net sediment transport at tidal inlet systems, combining field data and a range of well established empirical formulations. To accomplish this, measurements spanning a spring-tidal cycle of mean water levels, waves, near-bed flow turbulence and bed forms were obtained from the Ancão Inlet, Ria Formosa lagoon system, Portugal. High-resolution hydrodynamic data were gathered using acoustic equipments and by measuring sediment properties (grain-size diameter and bed form dimensions) under fair-weather conditions. The results compared favourably with available direct and indirect field observations of sediment transport rates. The approach appears to be robust and widely applicable and so can be applied to the same conditions in any tidal inlet system. This is of particular importance when attempting to understand sediment transport at inlet mouths, and has practical applications in a range of coastal engineering and coastal management areas concerned with navigation safety, coastal erosion, ecosystem health and water quality. The study discusses the applicability of the method on evaluating system flushing capacity, giving important insights on multiple inlet evolution, particularly with regard to their persistence through time. The methodological framework can be applied to assess the long-term stability of single- and multiple-inlet systems, provided that estimates of sediment storage at ebb-tidal deltas are available and sediment transport estimates during storm events are statistically considered.

2011Journal article

Restricted access

Morphodynamic evolution of fetch-limited beaches

Publication . Carrasco, Ana R.; Ferreira, Óscar; Freire, Paula Maria dos Santos

This thesis examines the morphodynamic evolution of a fetch-limited backbarrier beach located in the Ria Formosa barrier island system. Three different scales of analysis were ascribed: a short-term scale, a medium-term scale, and a long-term scale. Main issues addressed were: (a) the factors governing sediment transport under very limited-fetch conditions; (b) the morphological evolution at medium to long-term, and (c) the role of human interventions in beach morphology. At short-term (over the tidal cycle) sediment transport patterns were locally determined by employing fluorescent tracer techniques, and quantifying morphologic variations and transport rates. The medium- to long-term evolution (between months and from years to decades) was set based in topographic surveys and aerial photograph analysis. Finally, a general framework was developed for flood hazard assessment at fetch-limited coastal environments. This framework embraces the hazard zones identification, risk analysis and the associated most reasonable management options.

2012Doctoral thesis

Open access