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Abstract

Figueira da Foz coastal region (W Portugal), that comprises sandy beaches and the Mondego estuary-inlet, is exposed to the high energetic wave climate of the North Atlantic Ocean, which induces important local morphological changes. These changes entail shoaling problems as the formation of a submerse sandbar at the inlet which can block navigation towards local harbor. Therefore, coastal dredging is a non-structural temporal solution for this problem. Nevertheless, dredging activities have an impact on local hydrodynamic processes that should be known beforehand. The aim of this study is to clarify this issue using modeling tools dedicated to help stakeholders in decision making process. For that purpose, Delft3D-WAVE module propagates the most representative regional wave conditions under four dredging scenarios including the respective dump areas. Additionally, Delft3D-FLOW module simulates tidal flows along spring and neap tides. The hydrodynamic parameters, significant wave height, orbital velocity, wave-induced currents, tidal current velocity and total sediment transport are analyzed in comparison with numerical solutions obtained in reference scenario (no dredging and no dumping). The results highlight changes in these hydrodynamic parameters in dredging and dumping areas as well as in surrounded areas. Different scenarios draw a similar change pattern for different parameters, namely a reduction of flow velocity intensity within dredged areas and their increase in adjacent areas. Furthermore, hydrodynamic alterations intensify in response to the increase in significant wave height and wave direction considered. These findings are the basis for understanding the influence of dredging and dumping operations on the hydrodynamic of this coast.

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... These results are consistent with other observational and modeling studies under storm conditions (e.g., in [3,50] and references therein). The maximum tidal current velocity values at Figueira da Foz inlet ranged from 0.8 to 1.2 m.s -1 for spring tides and from 0.4 to 0.7 m.s -1 for neap tides [51]. Previous work [51] revealed a decrease of the residual currents of about 10% between dredging and reference scenarios. ...
... The maximum tidal current velocity values at Figueira da Foz inlet ranged from 0.8 to 1.2 m.s -1 for spring tides and from 0.4 to 0.7 m.s -1 for neap tides [51]. Previous work [51] revealed a decrease of the residual currents of about 10% between dredging and reference scenarios. The tidal current does not promote significant bathymetric changes at the inlet as it is observed during summer (i.e., under low-energy wave conditions) [32]. ...
... These results are consistent with other observational and modeling studies under storm conditions (e.g., in [3,50] and references therein). The maximum tidal current velocity values at Figueira da Foz inlet ranged from 0.8 to 1.2 m·s −1 for spring tides and from 0.4 to 0.7 m·s −1 for neap tides [51]. Previous work [51] revealed a decrease of the residual currents of about 10% between dredging and reference scenarios. ...
Article
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The high energetic wave climate of the North Atlantic Ocean causes important morphological changes at Figueira da Foz coastal system (W Portugal), which is comprised of sandy beaches and the Mondego estuary-inlet. The submerged sandbar at the inlet mouth is highly dynamic inducing short waves shoaling and breaking processes that can entail navigation problems towards the local harbor. Therefore, coastal dredging operations are performed to guarantee safe navigation. Nevertheless, these operations have a limited temporal effectiveness and require a high annual budget to be accomplished. The goal of this research is to seek long-life dredging alternatives using modeling tools (i.e., Delft3D model suite). Delft3D model is used to simulate the morphological evolution of five dredging scenarios during a three-month winter period under three wave climate scenarios. The bed level differences at the dredged area and at the inlet mouth for each scenario are analyzed in comparison with numerical solutions obtained in a reference scenario (i.e., no-dredging). Results highlight morphological changes at the dredged inlet and surrounding areas and their effectiveness in extending the operational lifetime of inlet dredged operations on dredging configuration and wave climate conditions. These findings are the basis for selecting the most suitable dredging scenario to this coastal region under current wave climate conditions.
Conference Paper
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Tidal inlets such as Figueira da Foz inlet (W Portugal) are a source of socioeconomic richness since they act as navigation channels towards harbors. Nevertheless, to maintain their operational capacity and safe navigation requires high management efforts because of the complex inlet hydromorphodynamics. In spite of numerous researches about possible solutions (e.g. jetties construction and sediment dredging), there is no consensus about the best strategy to guarantee navigability. For instance, jetties’ construction entails large changes in sediment transport patterns but, in some cases, sediment accretion areas may be generated at the inlet and therefore consequent navigation hazardous can persist. Numerical models are important tools to predict the impact of proposed solutions, but they need to be validated and calibrated to the study area as each coastal region has its own special features. The goal of this work is to setup a process-based model (Delft3D) to Figueira da Foz inlet. Three Delft3D modules (Delft3D-FLOW, Delft3D-WAVE and Delft3D-MOR) were calibrated and validated against hydrodynamic and bathymetric measurements from field monitoring. Results highlighted the model accuracy in reproducing tidal and wave propagation, as well as morphodynamic evolution after dredging operations. This pioneer study establishes the basis to further researches about dredging scenarios proposed to keep open the Figueira da Foz inlet entrance for longer periods.
Thesis
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The Matakana Banks ebb-tidal delta is located offshore from the Tauranga Entrance to Tauranga Harbour, Bay of Plenty New Zealand. In order to access the Port of Tauranga, Matakana Banks has been dredged to provide a shipping channel with sufficient depth for navigational purposes. In 1968 the first capital-dredging programme included an Entrance Channel through the delta, which was further enlarged in 1992. A biennial maintenance dredging programme has been required to deal with subsequent infilling of the Channel. Recently, the Port has obtained resource consent to further widen and deepen the Entrance Channel. However, while granting the consent, the Environment Court accepted that dredging could result in adverse impacts to the stability of Matakana Banks and the adjacent Panepane Point on the western side of the tidal inlet. Therefore, this study was undertaken to assess the response of the ebb tidal delta to dredging, and determine possible mitigation measures if dredging has adverse effects. The study analysed bathymetric data from single beam echo sounder (SBES) surveys obtained from 1998 to 2011. These data indicate that the main body of the ebb tidal delta is stable and undergoes little change. However, the swash bars located on the swash platform are very mobile, and probably account for the changes reported by the earlier assessments of changes between bathymetric surveys. In order to better track swash bars and assess variations in ebb-tidal delta morphology, multibeam echo sounder (MBES) surveys have been instituted. Comparison of morphological changes with the dredging volume and the wave climate during 1998 to 2011 indicated that the storm events may trigger erosion of the ebb-tidal delta, particularly after a maintenance dredging campaign. However, the ebb-tidal delta volume recovered quickly between dredging campaigns, suggesting that the sediment was redistibuted within the system and not permanently removed. To provide insights into the processes affecting the ebb-tidal delta, and provide the necessary data for the calibration and verification of numerical models, and a major 27-day field programme was undertaken. Sediment traps were used to assess sedimentaion rates, and provide sediment samples for characterising the I sediment grain size and bed roughness distribution for numerical modelling. Concentric arcs of wave and current recorders were used to measure hydrodynamic processes within and around the ebb-tidal delta and tidal inlet, and along the Matakana Island shoreface. The field and historical bathymetric data showed that the ebb-tidal delta can be divided into 3 sub-regions according to the dominant hydrodynamic regime; (1) close to the Entrance Channel and ebb-jet where tidal currents dominate; (2) the central area of the swash platform, where the influence of tidal currents is still present but waves also play important role; and (3) the margins of the swash platform where waves are dominant and tidal influence is minimal. Overall the wave influence becomes more dominant as the distance from the Entrance Channel (main ebb jet) increases, and vice versa for tidal processes. The short- and long-term impacts of dredging on the Matakana Banks ebbtidal delta were investigated by numerical modelling using Delft3D. The model covered dredging locations inside Tauranga Harbour and the offshore areas around the Matakana Banks ebb-tidal delta, and was calibrated by the field measurement data. A month-long time series of wave conditions were used to force a wave model coupled with a hydrodynamic model for the 2013 bathymetry to simulate the present day situation. The modelling results showed that the sediment volume of the ebb-tidal delta fluctuates with tidal range; accretion occurred during neap tides; and erosion during spring tides. To assess the long term impact, the morphological factor (morfac) tool in Delft3D was used. A morfac of 60 was applied to 12-days simulations to predict 2year morphological changes, corresponding to the approximate time interval between maintenance dredging campaigns. The impacts of dredging were then investigated by modelling three different conditions: (1) before dredging started in 1968, using 1967 bathymetry; (2) the present situation using 2013 bathymetry with existing dredging and dumping activities; and (3) future scenarios using the 2013 bathymetry with alternative offshore disposal locations. Conditions of average waves (no storm) and with storm waves were also simulated. Before dredging commenced, the ebb-tidal delta had a continuous terminal lobe from the north to southeast with a minimum depth of 5 m. Sediment transport II modelling indicated that bar by-passing transported sediment past the inlet via mobile bedforms on the terminal lobe, and suggested that the ebb-tidal delta was getting shallower and broader. Simulations incorporating dredging revealed that the tidal currents in the channels of the ebb-tidal delta became more asymmetric (stronger ebb-current), and the ebb-tidal delta became bifurcated and more complex in its morphology. The system changed to inlet bypassing, although only small quantities of sediment appear to be transported past the tidal inlet (most sediment recirculates within the tidal inlet system). Overall dredging does not appear to have affected the stability of the Matakana Banks ebb-tidal delta, but there may have been an increase in the morphologic variability as bedforms circulate over the swash platform, and additional bands of sand bars have formed on the seaward margin of the swash platform. Modelling of alternative spoil disposal sites indicated that shallow nearshore spoil disposal sites were more rapidly dispersed than the offshore spoil disposal site in 20 m depth, particularly during storm events. However, the volumetric differences between the models were small (less than 0.2%). The smallest volumetric changes compared to an initial ebb-tidal delta volume are associated with spoil disposal located northeast of the ebb-tidal delta (DaD New), and the most effective site for contributing sediment to mitigate erosion of Matakana Island is on the shoreface to the west of the ebb-tidal delta.
Article
Full-text available
Many small wave-dominated inlets are naturally unstable and require regular dredging. To mitigate the costs of these operations, the dredged channels should be designed to bring the inlet close to equilibrium and minimise flood dominance. However, it is often unclear how to optimise the configuration of the channels. This study focuses on a small lagoon on the western Portuguese coast that has been subject to frequent and diverse interventions. A process-based morphodynamic model is applied to compare the dredging plan that has been followed for the last 15 years with two new alternatives. These alternative dredging plans increase the tidal prism and reduce flood dominance, mainly due to the higher channel cross-section. Secondary transverse channels do not affect the morphodynamics significantly. For future dredging operations, it is suggested that the bottom depth of the main natural channel be dredged to chart datum and a secondary channel be added to the previous channel with the same bottom depth and half of the channel width.
Article
Small tidal inlets are found to be more sensitive to anthropogenic alteration than their larger counterparts. Such alterations, although typically supported by technical design reports, sometimes require amendments or modification. One of the most suitable tools to conduct the necessary studies in this regard is numerical modelling, since the behaviour of the inlet system in response to proposed remedial actions, can easily be identified. In this paper, various alternative proposals are investigated to determine the most practical and viable option to mitigate the need for ongoing maintenance at a typical small, jettied tidal inlet. The main tool to investigate the alternatives is the hydro-sedimentological modelling of the inlet system, which was performed using the Delft3D software package. The proposed alternative entrance modifications were based upon structural alterations of the inlet system (such as a jetty extension or submerged weir) and non-structural scenarios (such as a change of the time of the dredging campaign or the deposition location of the dredged material). It was concluded that whilst a detailed study is inevitable in order to achieve a comprehensive design plan, based upon the results of this study the construction of a submerged weir at the entrance channel can satisfy the needs of most of the stakeholders, with justifiable costs over a longer period.
Selecção de ondas representativas da agitação marítima para efeito da avaliação do transporte litoral na costa de Aveiro
  • M G O A Barata
  • M J B S Teles
  • J A R Vieira
Barata, M.G.O.A.; Teles, M.J.B.S., and Vieira, J.A.R. 1996. Selecção de ondas representativas da agitação marítima para efeito da avaliação do transporte litoral na costa de Aveiro. Recursos Hídricos, 17 (1), 43-74.
Description of TRANSPOR2004 and implementation in Delft3D-ONLINE
  • L Van Rijn
  • D R Walstra
  • M Ormondt
Van Rijn, L.; Walstra, D. R., and v. Ormondt, M, 2004. Description of TRANSPOR2004 and implementation in Delft3D-ONLINE. Tech. Rep. Z3748.10, WL | Delft Hydraulics, Delft, The Netherlands. 69, 85, 237, 239.