Brazilian Coastal Processes: Wind, Wave Climate and Sea Level

Abstract

The coast of Brazil has substantial physical and environmental diversity, constituting a constant challenge for coastal management. This diversity is characterized by heterogeneity regarding the morphology of the coast and its hydrodynamic components, such as waves, tides, sea level changes and atmospheric pressure gradients. In this chapter an historical review regarding the existing observed data is presented.

Also a detailed description of the wave climate, astronomical tide and storm surge along the Brazilian coast is provided, based on the SMC-Brasil databases: Downscaled Ocean Waves (DOW), Global Ocean Tides (GOT) and Global Ocean Surges (GOS). Finally a briefly description of the SMC-Brasil is provided focusing on it is principal characteristics and an example of it is application to the Massaguaçu beach is shown on Appendix.

Appendix 1: Massaguaçu Beach SMC-Brasil Case Study

Massaguaçu beach is an embayed beach located on the north coast of Sao Paulo in lee of several islands that affect the wave propagation toward the coast and, consequently, the beach morphology. It has an erosion problem along its central part (Fig. 2.13).

Fig. 2.13

Erosion problems in central part of Massaguaçu Beach

Bathymetry and wave climate data needed to carry out this study were obtained from SMC-Brasil database, which contains offshore and local bathymetry (Fig. 2.14) and wave climate information (Fig. 2.15).

Fig. 2.14

Study area bathymetry obtained from SMC-Brasil (Database generated with General Bathymetric Chart of the Oceans, Brazilian nautical charts and some local bathymetries)

Fig. 2.15

Downscaled Ocean Waves (DOW) points near the study site are shown. Wave climate direction of three points are shown to indicate the variations of wave direction in the area of study

The SMC-Brasil wave climate database, predicts 85 % of the waves arriving from the east-south, with the most energetic waves coming from the south to south-southeast.

Once the offshore wave climate is characterized, it is possible to propagate the wave climate toward the coast using SMC-Brasil. Figure 2.16 shows a southerly storm propagation. As can be seen in this figure, although southern waves are very energetic offshore, the islands provide considerable shelter to Massaguaçu Beach and significantly reduce the wave energy that reaches the beach (approximately 70 % in this case).

Fig. 2.16

Height and wave direction maps obtained for a southern storm (H_s = 5 m and T_p = 15 s, approximately). Massaguaçu Beach insert to right

The analysis of current patterns for all the wave directions in the study area revealed that there are three main zones along the beach (Fig. 2.17):

• In the south, currents are irregular, with direction depending on the wave direction as well as transverse currents.

• In the center, there is a reduction in current magnitude and a change in direction.

• In the north, currents generally increase toward the northeast, except at the end of the beach, where an offshore rip current is generated for some wave directions.

These wave dynamics and currents generate a net sediment transport from the central part of the beach toward the extremes, with a net longshore sand transport toward the north, resulting in an erosional “hot spot” in the center of the beach, where historically the beach has had erosion problems. In order to check the beach stability, the equilibrium planform of Massaguaçu was obtained by using SMC-Brasil. This system fits different equilibrium planform models based on the wave climate at the control point and the energy flux direction. Figure 2.18 shows the long-term equilibrium planform in blue and the shoreline in 2006 in black, and confirms sediment transport toward the north is responsible for shoreline retreat in central part of the beach.

Fig. 2.17

Analysis of current patterns in the three zones of the study area using SMC-Brasil

Fig. 2.18

Massaguaçu equilibrium planform

Once the wave-beach morphodynamic are analyzed, coastal works can be proposed to reduce the erosion problem in the study area. For example, one of the proposed solutions in this study was the construction of a detached breakwater in the north (Fig. 2.19). This solution generates a static equilibrium planform in the central-north zone that could reduce the present littoral drift toward the north. The proposed detached breakwater could generate a 60 m width dry beach in the north and predicts a 40 m shoreline advance at central part. However, it requires a large amount of sand nourishment (approximately 1,400,000 m³) and there is a lack of natural sand sources near the study site. In fact, there is a minimum sand volume required to stabilize the beach, which depends on the dynamics, their variability and the nourishment sand size; but the rest of the refill material can come from other sources, even artificial. In order to reduce the sand volume, it was proposed to nourish the active profile (beach profile affected by dynamics and their variability) with a sediment that permit stabilizing the beach in the middle and long-term (D₅₀ = 0.25 mm), with the non-active profile filled with another material (D₅₀ < 0.25 mm) because it is not affected by the dynamics and their variability. Figure 2.20 shows the associated equilibrium profile.

Fig. 2.19

Location of the dettached breakwater and predicted equilibrium planform proposed to solve the central beach erosion

Fig. 2.20

Future equilibrium profile proposed to solve the erosion problem at central part

Finally, the SMC-Brasil can assess the impacts of global climate change impacts in future solutions and can take into account measures to mitigate negative impacts in the present design.