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Pocket Beach Nourishment Design

Mangaratiba, Rio de Janeiro, Brazil

A beach nourishment design was developed for a condominium in Mangaratiba, Rio de Janeiro, Brazil, using the equilibrium beach profile theory (Dean, 1990; 1977). The pocket beach—Turtles Beach at Ponta do Cação—is located in the western side of Mangaratiba Bay, Angra dos Reis, among several rock outcrops. The site was undergoing erosion during moderate and high storms, as localized currents and breaking waves removed sand material away from the site sub-cell. Beach nourishment construction was carried out in 2000. Using appropriate technology, sand deposits offshore of the site were dredged and placed as beach fill. The technical aspects of the beach nourishment design and the construction method were carried-out by Acqua personnel. Figures 1 and 2 show site photographs before and after beach nourishment works.

A coastal study was carried out to define design nearshore waves and currents as well as the necessary fill volume to stabilize the beach. The equilibrium beach profile theory was used as model for beach nourishment design. Equilibrium beach profiles tend to be concave upward with milder slopes when formed by finer sediments (Dean, 1977). A geometrical equilibrium beach profile method was developed for design of the dry beach width, where profile variability and depth of closure during design storm events were tested for alternative foreign sand characteristics. Both inland and offshore sand sources were considered, i.e., nourishment scenarios with both coarser and finer sand than native material. The geometrical method developed mimics the changes in beach profile as result of cross-shore transport processes during design events. In this way, the dry beach width was designed for a specified net beach volumetric change such that the predicted final equilibrium beach profile would be located a certain distance seaward of the initial profile.

For engineering applications, it is important to note that different definitions for beach face slope and beach profile would yield different design slope values. Engineering judgement must be exercised when choosing the appropriate value of beach slope for littoral processes prediction and beach nourishment design. Factors that should be considered include the site beach geometry, native sand sizes and ongoing surf zone processes. Sayao and Graham (1991) developed an equation for the prediction of beach profiles, based on the work of Sunamura (1984). The movable bed beach slope m, which develops under wave action inside the breaking zone, was defined as:

formula

where (Hb/Lo) is the breaker steepness and (Hb/D) is the relative grain size parameter. This equation was used in the development of the geometrical equilibrium beach profile method.

Several construction methods were considered including the utilization of sand from both inland and offshore sources. Littoral transport of sand was estimated and construction drawings for the project fill and a terminal groin structure at the east side of the beach were prepared. The terminal groin proved to be an important part of the beach nourishment design and helped to retain beach material in place. No detrimental impacts on neighboring properties were detected. The coastal study included details of concept development, site data collection, coastal engineering analyses, geometrical equilibrium beach profile method, final design of beach nourishment, construction oversight and a limited 5-year site-monitoring program, which included periodic site photographs and collection of beach samples. No noticeable change of beach characteristics since the 2000 post construction record were reported during the post-construction monitoring.

Fig 1. Turtles Beach before nourishment (1998)
Fig 2. Turtles Beach after nourishment (2000)

References

  1. Dean, R.G. 1990. Equilibrium Beach Profiles: Characteristics and Applications. University of Florida Report UFL/COEL-90/001, Coastal and Oceanographic Engineering Department, Jan, 69 p.
  2. Dean, R.G. 1977. Equilibrium Beach Profiles: US Atlantic and the Gulf Coasts. Ocean Engineering Report No. 12, Civil Engineering Department, University of Delaware, Newark, DE.
  3. Sayao, O. and Graham, J. 1991. On the Prediction of Beach Profiles. ACOS Bulletin No. 5, Vol. 1, National Research Council Canada, Ottawa, Ontario, pp. 14-23.
  4. Sunamura, T. 1984. Quantitative Predictions of Beach-face Slopes. Geological Society of America Bulletin, Vol. 95, pp. 242-245.

Offshore Breakwaters for Port Development

Summary of paper published in Ports 2013 © ASCE 2013

Introduction

This summary project presents examples of offshore breakwaters for port development along the Brazilian coastline, describing case studies with site-specific methodologies used for breakwater design and construction. Most of the breakwaters described are of the berm concept, where the initial constructed berm width is allowed to reshape under wave action to a modified sea-side profile, which is more stable during the structure lifetime.

Four offshore breakwater sites are discussed here, from the Northern coast and progressing South: Port of Pecém, in Ceará State (CE), Salgema Terminal, in Alagoas State (AL), Sergipe Marine Terminal, in Sergipe State (SE), and São Sebastião Oil Terminal, in São Paulo State (SP).

Wave Climate

In Brazil, wave climate field campaigns usually cover short periods of time making the data difficult to use in reliable determinations of design conditions along the Brazilian coastline. Even in sites where measurements are longer than a couple of years, instruments problems occurred just when they are mostly needed: during storms and extreme wave conditions. Thus, the use of numerical models of wave generation to synthesize a long enough database for design predictions are a viable alternative.

A WW3 model forced with winds from NOAA´s database was used to reconstruct sea states in some locations off the port sites (depths of about 100 m) for periods of 10 to 30 years. The data was used as input parameters for wave transformation modelling (using SWAN) and to identify extreme events for breakwater designs.

From available information of past wave climate measurements in intermediate water depths of about 20-25 m, an estimation of the wave height variation along the Brazilian coastline is presented in Figure 1. It is customary to accept that the wave climate along the Brazilian coastline is stronger in the South near the Rio Grande Jetties site (Melo et al., 2010; see point RS in Figure 1) and decreases up the coastline towards Northeast Brazil in Ceará State (Sayao et al., 2006; point CE in Figure 1).

Selected Marine Terminals

The offshore Port of Pecém is located about 60 km north of Fortaleza, CE, at a sandy beach and dune shoreline. The port is protected by a berm breakwater, about 1700 m long, in the shape of an ‘L’, located in 17 m of water below datum. Sayao et al. (2006) presented design and construction monitoring details of the berm breakwater, which was constructed from 1998 to 2001. Presently, 1 km of breakwater expansion works are underway, with completion expected for 2011.

Salgema Terminal is located 4 km away from the Port of Maceió, and has a 230 m pier located in 10 m of water depth. It was designed in the 1980’s, with the aid of physical modeling, carried out in DHI, Denmark.

The Sergipe Marine Terminal, was constructed in Barra dos Coqueiros, some 25 km north of Aracaju, SE. Today, it is known as the Ignacio Barbosa Marine Terminal (TMIB). The terminal consists of a pier located in 10 m of water (below chart datum), connected to the shoreline and the port area by an access trestle 2,400 m long and protected by a 550 m long offshore breakwater (Murray and Sayao, 1990).

Construction of TMIB started in 1988 and the breakwater was completed in 1992. The offshore breakwater was built on a sea bed of soft soil. In 1989 the first stretch of the rubble mound construction sank due to the low sheer strength of the soft foundation – consisting of an 8 m extremely soft layer of marine clay deposit underlying 4 m of surficial sand of varying compactness – as well as an artesian condition below (Ortigao, 1991).

Following extensive geotechnical studies (Sandroni, 1997), a berm breakwater was adapted to this unfavorable subsoil conditions, designed with the aid of physical modeling, including a 5 m thick geotechnical berm over the seabed to compensate for the poor sub-soil conditions and balance the forces due to the rubblemound mass.

São Sebastião Oil Terminal, SP, is located in protected waters and does not require a breakwater. Several other marine terminals are also located in protected waters, such as the ones inside Sepetiba Bay (Rio de Janeiro State) and inside Port of Rio Grande (in Rio Grande do Sul).

Newer offshore terminals are present along the coastline of Brazil. Among these new projects, the Port of Açu is in operation, in the North of the Rio de Janeiro State coastline. Port of Açu incorporates an onshore area of about 100 km2 where an industrial complex was developed, and future plans include a steel plant, cement plant, a thermoelectric power station and several iron ore pellet power stations. Proposed depths of water in front of the offshore berths are 18,5 m (below chart datum) in the initial phase, with deepening to 21 m (below chart datum) proposed for the expansion phase.

View of Ignacio Barbosa Marine Terminal (TMIB) (source: Google Earth, 2010)
Graph showing Estimate of Significant Wave Height along Brazil Coastline.

References

  1. Murray, M., and Sayao O. 1990. Offshore breakwater for the Sergipe Marine Terminal, Brazil. Proc., 22nd International Conf. Coastal Engineering, ASCE, Delft, Holland, Vol.3, pp. 3207-3221.
  2. Melo Filho, E., Hammes, G.R., Franco, D., Romeu, M.A.R. 2008. Aferição de desempenho do modelo WW3 em Santa Catarina. In Portuguese. Presented in 3rd SEMENGO (Seminar in Ocean Engineering), FURG, Rio Grande/RS, Brazil.
  3. Ortigão J A R (1991) Embankment failures on soft clay in Brazil, Geotechnical News, December, Vol 9, no 4, Bitech Publishers, Vancouver, pp 68-70.
  4. Sandroni,S.S, Brugger,P.J., Almeida, M.S, and Lacerda, W.A. 1997. Geotechnical properties of Sergipe Clay. Proceedings of Recent Developments in Soil and Pavement Mechanics, Rio de Janeiro, Brazil, A.A. Balkema, Rotterdam pp. 271-278.
  5. Sayao, O., Accetta, D., Pires, V., Pinto, Antonio P., Freire, Paulo C., and Silva, D. (2006). Port of Pecem Berm Breakwater, Design and Post-construction Monitoring. Proc., 30th Int. Coastal Engineering Conference, ICCE 2006, San Diego, CA, USA, W. Sc., pp. 5034-5045.

Dike Vulnerability due to Sea Level Rise

It is estimated that about 40% of the world’s population lives within 100 km of a coastline. This coastal population and the coastal infrastructure it relies on for economic viability is highly vulnerable in the face of global sea level rise projections. This vulnerability will be exacerbated by projected increases in the frequency of the most intense tropical storms. Sea level rise along with more intense storm events could mean more frequent damage to sea dikes and increased coastal flooding.

In British Columbia (BC) there are more than 200 regulated dikes, with a total length of over 1100 km, protecting 160,000 ha of valuable land. The BC Ministry of Environment provides guidelines for the design of sea dikes to protect low lying lands that are exposed to coastal flood hazards arising from their exposure to the sea and to expected sea level rise due to climate change (Province of BC, 2018). These guidelines are used to define the upgraded sea dikes flood construction levels that are now necessary to raise dike elevations so that they continue to exert their protective function in the future, towards a lifetime up to the year 2100. In some cases, the crest elevation of dikes needs to be raised by about 2.5 m to 3 m to be able to protect lands and residences in its lee.

The vulnerability of sea dikes may be evaluated using the PIEVC (Public Infrastructure Engineering Vulnerability Committee) Protocol method (Engineers Canada, 2016), which has been applied since 2007 to over 50 infrastructure risks assessments in Canada and in other international sites. The PIEVC method is used to assess the vulnerability of built assets (infrastructure, buildings, facilities) to the impacts of climate changes. Using the PIEVC protocol one may identify sea dike components that are especially vulnerable to climate - and weather-related impacts and evaluate the risks to these infrastructure elements, as well as propose typical mitigation measures.

As shown in Fig. 1, the Province of BC recommends that a 1.0 m sea level rise (SLR) be used by the year 2100, for all coastal defense (flood protection) projects with a design life of 50 to 100 years, while recognizing the uncertainties of all SLR predictions. Province of BC (2018) recommendations are in line with the Intergovernmental Panel on Climate Change (IPCC) AR5 Summary for Policymakers (IPCC, 2013), which reports that global mean sea level rise (SLR) for RCP8.5 scenario by the year 2100 is 0.52 to 0.98 m. For most locations, RCP8.5, which is the scenario giving the largest amount of global SLR, provides the largest projection of relative sea-level rise.

Fig. 1. Recommended global SLR curve for planning and design (Province of BC, 2018).

Fig. 2 gives an example of a typical section of an existing dike design protecting reclaimed land, today. To reflect future design including the recommended SLR allowance, a new engineered section is necessary to resist the 50-year and 100-year return period extreme high-water levels and overtopping due to waves. Following guidance defined in Province of BC (2018), an example of adaptation of existing dike protection structures to account for future SLR is shown in Fig. 3, an upgraded dike protection for operating in future years. It may be necessary to define detailed engineering design features, such as elevating the fill at the back area of the dike (Fig. 3), further designing the crest of the revetment with a wave wall in required locations, to add geotextiles or drainage canals at the lee of the dike, etc.

Fig. 2. Typical dike protection section, today (all elevations and dimensions are in meters).
Fig. 3. Typical dike protection section, future (all elevations and dimensions are in meters).

References

  1. Province of BC (2018). Flood Hazard Area Land Use Management Guidelines; originally dated May 2004 by the Ministry of Water, Land and Air Protection of British Columbia and amended by the Ministry of Forests, Lands, Natural Resource Operations and Rural Development in January 18, 2018.
  2. Engineers Canada (2016). PIEVC Engineering Protocol for Infrastructure Vulnerability Assessment and Adaptation to a Changing Climate. Principles and Guidelines, Version PG-10.1, dated June 2016.
  3. IPCC (2013). Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F. et al, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA.

Coastal Engineering Review, Niemeyer Bike-path Damage

Paper published in PIANC-COPEDEC IX, 2016, Rio de Janeiro, Brazil

A bike-path along the Niemeyer rocky coast in Rio de Janeiro, Brazil, was designed and constructed to connect the neighborhoods of Leblon and São Conrado. Construction works began in September 2014 with the intent to open the lane to the public before the 2016 Olympic Games in Rio (August 2016). Approximately half of the bike-path extension (total length of 3.8 km) was designed as an independent structure on the rocky hillside at the sea side of Niemeyer Avenue. The remaining portion was built overhanging or as landfill, but attached to the roadway.

On April 21, 2016 a 50 m section of the bike-path, designed as an independent structure, collapsed and was destroyed by storm wave action about three months after inauguration. Figure 1 shows the damaged section of the bike-path at the sea side of the existing Av. Niemeyer after the accident.

Fig. 1. Damaged Bike-path, April 21, 2016 (Source: G1.globo.com)

A coastal engineering review of the bike-path design was carried-out where some of the causes for the bike-path accident are discussed. The coastal study approach was to define the wave climate in front of the bike-path using a numerical model (SWAN), and to estimate wave run-up and overtopping on the smooth rocky ledge, as well as wave reflection, and wave slamming forces. The risk during storms to pedestrians and vehicles against overtopping events was evaluated, as the Av. Niemeyer roadway is already about 100 years old.

Because of vehicle traffic restrictions on Av. Niemeyer during the construction phase, the bike-path structure was built in pre-molded parts, in spans of 6 m and 12 m at the rocky ledge, following the winding path of the century-old route. Along the way, rock slope containment works were performed, such as soil nailing, cable-stayed curtains, etc.

Nearshore site data availability is very scarce at this particular exposed location, for coastal engineering analyses. Calibration of SWAN runs was obtained using existing measurements from a wave buoy inside Guanabara Bay, about 12 km from the accident site. A video taken through a bus window during the wave jet impact showed that the wave period was 18 s, which is longer than average (about 12 s), when compared with the site wave climate during storm events. Significant wave heights (Hs) during the April 21, 2016 event were estimated at 3 to 4 m, before run-up and reflection.

Wave run-up (R) in smooth slopes is a function of the Iribarren number ξ (Battjes, 1974): R/Hs ≈ ξ. Assuming incident wave conditions as described above, wave run-up levels during the storm event of the April 21, 2016 accident may reach elevations of about 20+ m above MSL. Other run-up methods described in the CEM (2003) and in CIRIA (2007) also yield similar predictions.

As the elevation of the bike-path deck is estimated at about +25 m above MSL, wave run-up levels do not reach the Av. Niemeyer elevation at this site; however, the presence of the vertical walls of the structure called ‘Castelinho’, located at an elevation of about +15 m above MSL, as seen in Figure 1, caused the accident and the damage of the bike-path due to impulsive overtopping conditions. The estimation of violent, impulsive wave overtopping at these vertical walls was carried-out, using the method of Allsop et al., (2005), also given in Besley et al., (1998), which predicted an upward jet velocity so high that the bike-path deck structure was completely destroyed by the single wave action. Limits of wave overtopping volumes for vehicles and pedestrians were also estimated and shown to be higher than allowable limits (CIRIA, 2007). These analyses determined that traffic during similar storms is compromised and lead to the recommendation that protocols be in place for the interdiction of both the Av. Niemeyer bike-path and its parallel roadway during the intervals of such extreme events.

References

  1. ALLSOP, N.W.H., BRUCE, T., PEARSON, J. and BESLEY, P. (2005). Wave Overtopping at Vertical and Steep Seawalls. Proc., ICE, Maritime Engineering 158, Sep. 2005 Issue MA3, pp. 103–114.
  2. BATTJES, J.A., (1974). Surf Similarity. Proc., 14th Conference of Coastal Eng. ASCE, pp. 466–480.
  3. BESLEY, P., STEWART T., and ALLSOP, N.W.H. (1998) Overtopping of Vertical Structures: new Methods to Account for Shallow Water Conditions. Proc. Int. Conf. on Coastlines, Structures & Breakwaters'98, pp 46-57, March 1998, ICE/Thomas Telford, London.
  4. CEM (2003). Coastal Engineering Manual. Engineer Manual 1110-2-1100, U.S. Army Corps of Engineers, Washington, D.C.
  5. CIRIA (2007). The Rock Manual. The use of Rock in Hydraulic Engineering (2nd Ed.). C683 Report.