Paulo A. Silva

Observations of velocities, sand concentrations, and fluxes under velocity‐asymmetric oscillatory flows

[1] U‐tube measurements of instantaneous velocities, concentrations, and fluxes for a well‐sorted, medium‐sized sand in oscillatory sheet flow are analyzed. The experiments involved two velocity‐asymmetric flows, the same two flows with an opposing current of 0.4 m/s, and a mixed skewed‐asymmetric flow, all with a velocity amplitude of 1.2 m/s and flow period of 7 s. We find that the net positive transport rate beneath velocity‐ asymmetric oscillatory flow results from large, but opposing sand fluxes during the positive and negative flow phase. With an increase in velocity asymmetry and, in particular, velocity skewness, the difference in the magnitude of the fluxes in the two half cycles increases, leading to larger net transport rates. This trend is consistent with the observed increase in skewness of the oscillatory bed shear stress. Phase‐lag effects, whereby sand stirred during the negative flow phase has not settled by the time of the negative‐to‐positive flow reversal and is subsequently transported during the positive flow phase, are notable but of minor importance to the net transport rate compared to earlier experiments with finer sands. In the vertical, the oscillatory flux is positive above the no‐ flow bed. Within the sheet flow pick‐up layer, the oscillatory flux is negative and similar in magnitude to the positive flux induced by the residual flow. The 0.4 m/s opposing current causes more sand to be picked up during the negative than during the positive flow phase. Above the no‐flow bed the resulting negative oscillatory flux is comparable in magnitude to the current‐related flux.

Sediment transport in nonlinear skewed oscillatory flows: Transkew experiments

New experiments under sheet flow conditions were conducted in an oscillating water tunnel to study the effects of flow acceleration on sand transport. The simulated hydrodynamic conditions considered flow patterns that drive cross-shore sediment transport in the nearshore zone: the wave nonlinearities associated with velocity and acceleration skewness and a negative mean current, the undertow. Net transport rates were evaluated from the sediment balance equation and show that (1) the acceleration skewness in an oscillatory flow produces a net sediment transport in the direction of the highest acceleration; (2) the net transport in the presence of an opposing current is negative, against the direction of the highest acceleration, and reduces with an increase in flow acceleration; and (3) velocity skewness increases the values of the net onshore transport rates.

Establishing the wave climate influence on the morphodynamics of a coastal lagoon inlet

The morphologic changes in estuaries and coastal lagoons are very complex and constitute a challenging task in coastal research. The bathymetric changes result from the combined action of tides, waves, rivers discharge and wind stress in the area of interest. Additionally, an accurate knowledge of the sediment transport is essential to achieve a good morphological characterization. This work establishes the influence of the wave climate on the morphodynamics of the Ria de Aveiro lagoon inlet by analysing the numerical results of the morphodynamic modelling system MORSYS2D. The numerical simulations considered a realistic coupled forcing of tidal currents and waves. The computed sediment fluxes and bathymetric changes are analysed and compared with the erosion and accretion trends obtained from the numerical simulations forced only by tidal currents, in order to establish the wave climate influence. The final bathymetry and the corresponding changes are compared with bathymetric data collected through surveys. It is concluded that: (a) the morphodynamics of the study area is dominated by the wave regime in the lagoon inlet and nearshore areas, while in the inner areas is tidally dominated; and (b) the inclusion of the wave regime forcing constitutes an improvement in order to accurately reproduce the local morphodynamics.

Tactical urbanism: Towards an evolutionary cities’ approach?:

Tactical urbanism initiatives have been interpreted as an alternative and a challenge to formal spatial planning tools to the need for a more responsive planning system. Short-term implementation, scarce resources and citizens’ involvement are said to be the key characteristics of this emerging movement in urbanism. In tactical urbanism, everything seems focussed on one thing: action. This paper analyses tactical urbanism initiatives in the United States considering three main aspects: the process, its interaction with planning institutions and the respective urban design outcomes. For this, the relation between tactical urbanism and complexity theory (in which self-organisation and evolution play an important role) is addressed. Findings suggest some contributions that tactical urbanism can make to urban design and spatial planning, in evolutionary terms and possible role for tactical urbanism in alternative to traditional division between plan making and plan implementation.

Mean Circulation, Seasonal Cycle, and Eddy Interactions in the Eastern Brazilian Margin, a Nested ROMS Model

Abstract A nested configuration of the Regional Ocean Modeling System (ROMS) is used to study the seasonal circulation patterns and mesoscale activity of the Eastern Brazilian Margin (EBM). The EBM encompass an oligotrophic and bathymetrically complex zone in the NW South Atlantic from 8°S to 20°S. Sea-level anomaly data are used to validate the model. Analysis of the mean circulation reveals that the EBM is dominated by seasonal and spatial dynamics of the southward Brazil Current (BC) and the northward North Brazil Undercurrent (NBUC), as well as their connection to the South Equatorial Current (SEC) dynamics. The EBM can be divided in three dynamic provinces, which are seasonally connected either by the permanent main flow or by mesoscale process. In the northern province, from 8°S to 13°S, the NBUC is the major permanent feature and the BC is just a thin flow, confined to the top few meters. As it moves southward, the BC gets deeper and stronger. In the middle province, from 13°S to 16°S, the dominance of the top 0–100-m circulation is seasonally alternated between the southward BC flow and the northward NBUC flow. In the southern province, from 16°S to 20°S, the BC appears as a dominating surface feature. While on the top (0–100 m) the main current presents a pronounced seasonal and spatial variability, on subsurface waters (100–500 m) the NBUC connects the EBM continuously. Finally, analysis of the regional simulation reveals well-defined cyclonic and anticyclonic eddies. They detach from the main flow and translate along the domain throughout the year. The translation patterns are associated with the seasonal variability of the main EBM flow, with anticyclonic mesoscale features translating southward and mesoscale cyclonic features translating northward. On their pathway, these features may come very close to the margin, interacting with the near-shelf flow.

Monitorization of sea sand transport in coastal areas using optical fiber sensors

Novel optical fiber sensors for monitoring the dynamic variations of the sea bed level in the nearshore were developed. The sensors are based on fiber Bragg gratings embedded in a polyurethane resin. For this solution sensitivities of 0.50±0.02 nm/m were achieved. The sensor was tested under controlled laboratory conditions and during an observational survey on the Algarve (Portugal) coastline.

Parameterizations of wave nonlinearity from local wave parameters: a comparison with field data

ABSTRACT Rocha, M.V.L., Silva, P.A., Michallet, H., Abreu, T., Moura, D., Fortes, C.J., 2013. Parameterizations of wave nonlinearity from local wave parameters: a comparison with field data Three different parameterizations of wave nonlinearity from local wave parameters are applied to field data (free-surface elevation and flow velocities) gathered in different beaches along the Portuguese coast, under diverse wave conditions. The hydrodynamics of the various sites are analyzed, together with the performance of each formulation. The data considered extend the range of the data previously used by the authors of the parameterizations, including longer wave lengths and higher Ursell numbers. A comparison is done, based on skill and agreement index values, to understand the performance and applicability of each parameterization and what could be changed to improve such formulations.

From Informal to Formal: What Can Be Learned from Reviewing 50 Years of Portuguese Models, Policies and Politics

This chapter considers the formalization of informal settlements in mainland Portugal. The country experienced a rise of illegal settlements during a period of economic growth (corresponding to the 1960s and 1970s). Informal developments culminated in an exploding social and environmental problem with hundreds of thousands of citizens living in informal settlements, mainly around the Portuguese capital. The Portuguese example evolved from the late 1970s “critical stage” to our days in which informal settlements are residual and about to be solved. The general aim of this review is to discuss how different types of informal settlements influence the way public policies are designed, in four complementary perspectives: informal settlements’ characteristics, including physical, social-economic and livelihood; local public policies initiatives and models of action; national public policies and paradigms; political involvement and the bridging role of political leaders. In this context, the success of public policies depends on its capacity to co-evolve with local organization’s initiatives involving informal settlement dwellers. The scope of this review concentrates on two kinds of informal settlements: those in spaces not belonging to the residents; and those in spaces involving land ownership. The methodology adopted will follow those four perspectives along the last five decades (since the early 1960s). By demonstrating the benefits of long-term analysis of public policies in highly complex, hierarchical and dynamic environments as are informal settlements, the authors hope to stimulate more studies of similar nature.

Longshore Transport Estimation on Ofir Beach in Northwest Portugal: Sand-Tracer Experiment

AbstractThis work aims to shed some light on longshore sediment transport (LST) in the highly energetic northwest coast of Portugal. Data achieved through a sand-tracer experiment are compared with data obtained from the original and the new re-evaluated longshore sediment transport formulas (USACE Waterways Experiment Station’s Coastal Engineering and Research Center, Kamphuis, and Bayram bulk formulas) to assess their performance. The field experiment with dyed sand was held at Ofir Beach during one tidal cycle under medium wave-energy conditions. Local hydrodynamic conditions and beach topography were recorded. The tracer was driven southward in response to the local swell and wind- and wave-induced currents (Hsb=0.75m, Tp=11.5s, θb=8−12°). The LST was estimated by using a linear sediment transport flux approach. The obtained value (2.3×10−3m3⋅s−1) approached the estimation provided by the original Bayram formula (2.5×10−3m3⋅s−1). The other formulas overestimated the transport, but the estimations resu...

A contribution to climate change assessment of storm surges along the coast of Mozambique

ABSTRACT Aramuge, A., Rocha, A., Silva, P.A, 2014. A contribution to climate change assessment of storm surge along the coast of Mozambique. 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. 253–258, ISSN 0749-0208. About two-third of total population of Mozambique lives along the coast which is 2700 km long. Mozambique is affected by tropical cyclones which are formed in the Indian Ocean, most of them inducing storm surges along the coast. This study presents an analytical model to estimate sea level changes, particularly extreme events (storm surges) from atmospheric pressure and wind data. The meteorological tides, for Maputo tide gauge station, were obtained by applying a low pass filter, with a cut off frequency of 33 hours. The analytical model was validated by comparing the generated meteorological residuals, trough inverted barometer and wind forcing, against the meteorological residual obtained from the tidal gauge. The analysis of the distribution curves of relative and cumulative frequencies, allowed the definition of three classes of storm surges namely: significant, very significant and highly significant for the percentile of 95, 99 and 99.9 respectively. There is a good agreement with a high correlation between the results obtained from the two methods. The alongshore wind, mainly from south or southeast, represents the main contribution for storm surge generation. The analytical model will be used to evaluate changes in the statistical properties of storm surges for future climate change scenarios along the Mozambican coast.