Óscar Ferreira

The first two years of an inlet: sedimentary dynamics

Abstract A monitoring program, including the acquisition of a series of topo-bathymetric surveys and oblique aerial photos, has been carried out at Ancao Inlet since its artificial opening in June 1997. This paper presents morphological and volumetric results, including inlet channel evolution (in terms of width, depth and cross-sectional area) and tidal delta formation. The results are analysed in relation to the prevailing oceanographic conditions. This allows the development of a conceptual evolutionary model for the natural evolution of artificially opened inlets with four stages: Stage 1 is the artificially opened inlet; Stage 2 represents the inlet after reaching dynamic equilibrium for the channel while ebb- and flood-tidal deltas are still developing; Stage 3 is the “mature” migrating inlet; and Stage PS (stage post-storm) is the inlet situation after high-energy conditions. The transition from Stage 1 to Stage 3 is a continuous process with intermediate sub-stages. Transition to Stage PS is a discrete phenomenon, caused by high-energy events, after which the inlet gradually changes back to the previous stage. Ancao Inlet was in Stage 1 in June 1997. In April 1998 the inlet channel reached dynamic equilibrium, thus beginning Stage 2. One year after the opening (July 1998) the inlet showed fully developed ebb and flood deltas, reaching the end of Stage 2. Transition to Stage 3, with the beginning of the downdrift migration and bypassing processes, took place during the second year. Stage PS characteristics were observed in January 1999 after high-energy storm conditions, with strong evidence of landward sediment transport. Although the conceptual model was developed for Ancao Inlet, it is envisaged that it could be applicable to other mixed-energy migrating inlets opened in an artificial way.

Field observations of sand-mixing depths on steep beaches

Abstract A series of field studies were carried out on three Portuguese beaches (Garra˜o, Faro and Culatra) to assess sand-mixing depths under a range of wave heights (0.34–0.80 m), wave periods (5.1–7.0 s) and mean grain sizes (0.26 and 0.38 mm). All the studied beaches had a reflective profile with a steep upper foreshore (tanβ of 0.10–0.14) and a more gentle low-tide terrace. In all experiments, plunging waves were breaking on the beach face. The study has identified a linear correlation between significant wave height at breaking ( H b ), and average sand-mixing depth ( Z m ), whereby Z m = 0.27H b . The empirical relationship is ten times larger than a previous one proposed by other authors working on gentle-slope dissipative beaches, and confirms previous findings on similar reflective beaches carried out in the USA. It was also confirmed that Z m is related to wave period, but does not seem to be a function of mean sand size.

Implications of Sea-Level Rise for Continental Portugal

Abstract The Portuguese coastline has a high diversity of coastal types, which will react differently to an accelerated sea-level rise. Estuaries and coastal lagoons will be most affected by a rising sea level. Amongst these, the Sado and Tagus estuaries and the Ria de Aveiro and the Ria Formosa coastal lagoons are probably the ones where socioeconomic impacts resulting from accelerated sea-level rise would be greatest. Sandy shores will face increased erosion. However, it is likely that at this type of coast other factors, such as sand deficiency caused by damming river basins, will continue to play a larger role in erosion than accelerated sea-level rise. Hard rocky coasts will be the least affected by accelerated sea-level rise. Specific adaptation policies for accelerated sea-level rise impacts do not presently exist in Portugal. However, existing laws can be used to prevent and/or reduce socioeconomic impacts if they are strictly applied. A strong commitment to coastal management by Portuguese authorities is therefore necessary in order to prevent and minimise future implications of accelerated sea-level rise.

Recent Evolution of the Natural Inlets of a Barrier Island System in Southern Portugal

ABSTRACT Four natural inlets, existing in a multi-inlet, barrier island system, were studied for the period between the 1940s and 1996. Inlet width and position of the inlet channel were determined using a series of vertical aerial photos and charts. The objective of the work was to determine the association between inlet migration patterns and different hydrodynamic conditions, major storms and engineering interventions. Results indicate that natural inlet opening and evolution were mainly affected by three factors: (a) existence of sub-embayments (western and eastern) inside the system, (b) exposure to wave energy, and (c) inlet efficiency. Two distinctive eastward migration patterns were found by analysing the correlation coefficient (r) between inlet width evolution and inlet migration and by the comparison of the shape of the curves fitted to the inlet migration behaviour.Typical migration of the high-energy flank (on the west side of the system) is characterised by an initial stage of readjustment, with low migration rates, followed by a stage of high eastwards migration rates, up to a limiting position. Inlet width remains reasonably constant during the entire migration cycle, thus the correlation between inlet width and position is very low.Typical inlets on the low-energy flank (east side of the system) are formed by barrier breaching during major storms and produce initially very wide inlets. Eastward inlet migration on the low-energy flank follows a natural logarithmic curve where channel migration is accompanied by strong constructional processes on the updrift barrier. Due to subsequent inlet width reductions, the correlation between inlet width and position is significant.

RISC-KIT: Resilience-Increasing Strategies for Coasts - toolKIT

ABSTRACT Van Dongeren, A., Ciavola, P., Viavattene, C., De Kleermaeker, S., Martinez, G., Ferreira, O., Costa C., R. McCall, 2014. RISC-KIT: Resilience-Increasing Strategies for Coasts–toolkit. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 366–371, ISSN 0749-0208. Recent and historic high-impact events have demonstrated the flood risks faced by exposed coastal areas. These risks will increase due to climate change and economic development. This requires a re-evaluation of coastal disaster risk reduction DRR strategies and prevention, mitigation and preparedness PMP measures. To this end, the UN Office for Disaster Risk Reduction formulated the Hyogo Framework for Action, and the EU has issued the Floods Directive. By their nature, neither is specific about the methods to be used to assess coastal risks, particularly those risks resulting from dune and structure overtopping, the non-stationarity of surge and flash flood events, and coastal morphodynamic response. This paper describes a set of open-source and open-access methods, tools and management approaches to fill this gap. A Coastal Risk Assessment Framework will assess coastal risk at a regional scale. Thus critical hotspots can be identified for which an impact-oriented Early Warning System/Decision Support System is developed. This can be applied in dual mode: as a forecast and warning system and as an ex-ante planning tool to evaluate the vulnerability. The tools are demonstrated on case study sites on a range of EU coasts with diverse geomorphic settings, land use, forcing, hazard types and socio-economic, cultural and environmental characteristics. Specific DRR plans will be developed for all sites. A management guide of PMP measures and management approaches is to be developed. The toolkit will benefit forecasting and civil protection agencies, coastal managers, local government, community members, NGOs, the general public and scientists.

Artificial Construction of Dunes in the South of Portugal

Abstract The Cacela Peninsula on the south coast of Portugal was, previous to nourishment operations, extremely vulnerable to overwash events. During the 1995–96 winter, extensive overwashing led to the opening of a new inlet. Simultaneously, the lagoon channel in the backbarrier was silting up with the overwash deposits, inlet flood delta sands, and accretion of fine sediments. Between October 1996 and February 1997 the channel was dredged, and the sediments were deposited on the western 2,000 meters of the Cacela Peninsula, forming an artificial dune ridge. The sediment characteristics of the dredge spoils were in contrast with the natural dune sand. The spoils had higher silt and clay content, lower mean grain size, and poorer sorting. In January 1998, three washover breaches were infilled, with sand removed from the foreshore, on the eastern part of the Cacela Peninsula. These sands were coarser and more poorly sorted than the original dune sediments. A 2-year monitoring program, consisting of beach/dune profiling, topographic surveys in specific areas, and sediment analysis, was established to observe the morphological evolution of these areas. The total eroded volume recorded for the dune nourishment project after 2 years was about 106,500 cubic meters, corresponding to 33% of the total 325,000 cubic meters of sediment that was deposited. The washover infilling showed little or no variation. However, the downdrift beaches and foredunes experienced severe erosion. Both protective measures accomplished their purposes in terms of preventing erosion; however, a natural landscape was only created where dune development by aeolian processes was possible.

Influence of Wave Action and Lithology on Sea Cliff Mass Movements in Central Algarve Coast, Portugal

Abstract The vulnerability of a rocky cliff to direct wave attack is a function of its lithological, structural, and morphological characteristics. The intensity of wave attack at the cliff foot depends on incident wave characteristics, nearshore bathymetry, beach and shore platform topography, coastline orientation, storm surges, and tidal range. The main goal of this paper is to relate the role of wave action as a geomorphic process influencing coastal cliff erosion with the control imposed by lithological and structural characteristics. For that purpose, a numerical wave propagation model (STWAVE) was used to evaluate differences in breaking wave height and energy along the study area (Galé–Olhos de Água, South Portugal) for a set of representative wave conditions and compared with existing mass movement data. As the study area presents wide longshore variation in wave exposition and breaking wave energies, five sectors were defined with contrasting wave action. When the distribution of mass movements along the coast is analyzed without considering the lithological variation, there is no relationship between the number and displaced volumes of mass movements and wave energy for each sector, with the majority of the movements and the greater volumes occurring in the least energetic sector. Therefore, lithology represents the dominant control on mass movement occurrence. However, if lithological variation is controlled by analyzing only the most common lithology in the study area (Miocene carbonate rocks), spatial variations in nearshore wave energy driven by the interaction of wave conditions with coastline orientation are found to influence mass movement occurrence. Resumo Os factores que condicionam a evolução de arribas rochosas são variados, incluindo processos continentais, marinhos e factores intrínsecos ao próprio substrato rochoso. Por exemplo, a vulnerabilidade das rochas ao ataque directo das ondas é igualmente função das suas características litológicas e estruturais. Por sua vez, a intensidade do ataque às arribas rochosas pelas ondas depende de características das ondas, da batimetria, da topografia das praias e plataformas de abrasão adjacentes, da orientação da linha de costa, da existência de sobreelevação do nível do mar e da amplitude das marés. O sector costeiro de arribas rochosas do Algarve central (Galé a Olhos de Água), proposto para estudo, expõe principalmente a Formação Carbonatada de Lagos-Portimão, do Miocénico. Constitui excepção o sector entre as praias de S. Rafael e da Baleeira, onde as arribas litorais expõem margas do Cretácico e calcários do Jurássico. O principal objectivo deste trabalho é definir o papel da acção das ondas, como um processo geomórfico, na erosão de arribas costeiras e comparar com o controlo imposto pelas características litológicas e estruturais das rochas. Para tal, foi utilizado um programa de modelação da agitação marítima (STWAVE) para determinar diferenças nas características das ondas na rebentação, ao longo do troço costeiro em estudo. A altura e energia das ondas na rebentação foram obtidas para cada condição testada e foram comparadas com os dados de movimentos de massa existentes. Uma vez que a linha de costa da área de estudo mostra uma forte variação longilitoral em relação à sua exposição às ondas, foram definidos cinco sectores com diferentes orientações. Dada a orientação diferenciada dos troços e a variabilidade longilitoral nas geoformas que os constituem, foi possível analisar a frequência de ocorrência de uma dada geoforma ao longo de troços com igual litologia mas com diferente orientação à ondulação dominante. Os resultados deste trabalho apontam para uma clara diferenciação energética entre troços costeiros, em função da sua orientação e do rumo de agitação (ex. SW versus SE). Quando se faz uma análise dos movimentos de massa sem considerar as variações litológicas, não se observa nenhuma relação entre o número e volume deslocado dos movimentos de massa e a energia das ondas para cada sector, com a maioria dos movimentos e os maiores volumes a ocorrerem no sector menos energético. Assim, a litologia representa o factor de controlo dominante na ocorrência de movimentos de massa. Contudo, ao analisar apenas o tipo litológico mais comum na área de estudo (rochas carbonatadas do Miocénico), verifica-se que as variações longilitorais de energia das ondas derivadas da interacção das condições de onda com a orientação da linha de costa são determinantes na ocorrência de movimentos de massa nesta litologia.

A Precise and Efficient Methodology to Analyse the Shoreline Displacement Rate

Abstract The shoreline change rate is one of the most significant parameters in analysing sandy shore behaviour with time. This parameter can be monitored by means of low- and high-resolution survey methods, depending on the objectives of the monitoring programme. Survey efficiency is also very important for achieving high resolution in both space and time. Another important aspect is the precision and significance of the obtained results, not only from the survey method itself but also from the comparative analysis used to process the data from several surveys. The survey method and the processing algorithms are the basis of shoreline analysis. This paper presents an evaluation of two proposed high-resolution methods that are simultaneously highly accurate and very efficient. These methods are based on a global positioning system (GPS) in differential mode for surveying and on novel algorithms for assessing the spatial change rate of the shoreline. The most significant difference of the two presented survey systems is the physical support: whereas one uses a land vehicle (motor-quad) to delineate the shoreline in wide straight coastal stretches, the other considers an on-foot simplified version to survey small, more irregular stretches. The analysis of the error associated with the proposed methodologies is thoroughly described in this paper. In both modes of operation, system-inherent errors are within the centimetre level—in general lower than 0.05 m. Operation-specific errors can remain within the centimetre level, but if instrument handling is careless in the on-foot survey method, they can reach decimetre levels. If successive monitoring surveys are not carried out under similar field morphological conditions, when the frontal dune baseline is adopted as a shoreline indicator, rough errors can be introduced. Two case studies of the application of these methods, evaluating the shoreline evolution of two distinct coastal regions in Portugal, are presented.

Evaluation of Cliff Retreat and Beach Nourishment in Southern Portugal Using Photogrammetric Techniques

Abstract This paper develops an evolutionary model of shoreline retreat along the cliffed coast of Forte Novo–Garrão (Algarve, Portugal) and investigates the relationship of the retreat pattern to coastal protection works, to the Quarteira fishing harbor, and to artificial beach nourishment at Vale do Lobo. The amounts and rates of cliff retreat for six coastal cliff sectors were measured using high-precision digital photogrammetric techniques utilizing data from three aerial photograph surveys conducted in 1991, 1999, and 2001. A geographic information system was used to integrate photogrammetric data and to quantify the temporal and spatial patterns of retreat. Forte Novo was the cliff sector with the highest amount of retreat, 22.7 m between 1991 and 2001. The other coastal sectors recorded retreats of less than 10 m for the same period. Retreat rates ranged from 2.27 m/y−1 at Forte Novo to a minimum retreat rate of 0.14 m/y−1 at Garrão. The replenishment measures at the Vale do Lobo beachfront have been largely successful, as there was negligible erosion between 1999 and 2001 along the cliff sectors protected by beach enlargement. These coastal management measures have been significant in reducing the coastal hazard to people and property located near, and downdrift of, the replenishment.

Coastal barrier stratigraphy for Holocene high-resolution sea-level reconstruction

The uncertainties surrounding present and future sea-level rise have revived the debate around sea-level changes through the deglaciation and mid- to late Holocene, from which arises a need for high-quality reconstructions of regional sea level. Here, we explore the stratigraphy of a sandy barrier to identify the best sea-level indicators and provide a new sea-level reconstruction for the central Portuguese coast over the past 6.5 ka. The selected indicators represent morphological features extracted from coastal barrier stratigraphy, beach berm and dune-beach contact. These features were mapped from high-resolution ground penetrating radar images of the subsurface and transformed into sea-level indicators through comparison with modern analogs and a chronology based on optically stimulated luminescence ages. Our reconstructions document a continuous but slow sea-level rise after 6.5 ka with an accumulated change in elevation of about 2 m. In the context of SW Europe, our results show good agreement with previous studies, including the Tagus isostatic model, with minor discrepancies that demand further improvement of regional models. This work reinforces the potential of barrier indicators to accurately reconstruct high-resolution mid- to late Holocene sea-level changes through simple approaches.