Beach Nourishment and Artificial Surf Reef

Abstract

Balneário Camboriú is a very touristy city in southern Brazil, situated in the five kilometer wide Camboriú Bay. Its main tourist attraction is the beach, which is 5800 m long and rather narrow with a dry width of 10 to 20 m. The city is facing several problems regarding the beach that have a negative impact on tourism. The main problems are: the beach is overcrowded in summer and becomes entirely shaded in the mid-afternoon due to the many high-rise buildings along the beach. To resolve these problems the municipality of Balneário Camboriú wants to nourish the beach to a width of 100 m. The preliminary design of the beach nourishment was made by the Danish Hydraulic Institute DHI, who based their design on data measured by the Brazilian Hydraulic Institute INPH. The Vale do Itajaí University (UNIVALI) in Itajaí, eight kilometer north of Balneário Camboriú, thinks the design of DHI is incomplete and based on inappropriate data and fears that the municipality of Balneário Camboriú will carry out the proposed beach nourishment any without further investigation. In this project the design of DHI has been analysed and a new design of the nourishment has been made. First the morphodynamics of Camboriú Bay have been studied. The bay can be defined as a headland-bay with a parabolically shaped beach between two protruding headlands, formed by energetic swell waves from the southeast. The beach planform is in dynamic and close to static equilibrium, which means it is stable as long as sediment supply is maintained. Because the bay is rather indented, there is no sediment exchange with the surrounding area. The tide in Camboriú Bay has a small range and little influence, inducing very small currents. Therefore the beach of Balneário Carnboriú is wave dominated, except for the south, which is more sheltered from incoming waves. The beach has a very gentle slope and can be classified as dissipative. After the study of the morphodynamics the data needed for the design of the nourishment have been investigated. The DHI design is based on data from INPH, which has done measurements of the bathymetry, grain sizes, cross-shore beach profiles and waves in Camboriú Bay. UNIVALI has also done measurements of these parameters and, except for the bathymetry, both data show several differences. Considering the grain size, the results of INPH show a larger and more constant median sand diameter than the results of UNIVALI. The results of UNIVALI seem to be more reliable, because they took more samples, they are more experienced with measurements in Camboriú Bay and they measured a significantly smaller grain size in the south of the bay, with is plausible because in that area a small river, Rio Camboriú, flows into the bay. The cross-shore beach profiles measured by UNIVALI extend much further seawards than the ones measured by INPH, which only take a small part of the area that is influenced by the waves into account. Since for a beach nourishment the whole area where waves have influence on the cross-shore profile is important, again the data of UNIVALI are more useful. With regard to the wave data, both the measurements of INPH and of UNIVALI have drawbacks. UNVALI did wave measurements for almost a year but on Brava Beach, six km north of Balneário Camboriú, not in Camboriú Bay itself. INPH did do measurements in Camboriú Bay, but only for a period of two weeks, which is not sufficient for reliable wave measurements, as they admit themselves. This means there are no good wave data available for Camboriú Bay. In the design of the nourishment this problem can be circumvented by deriving the closure depth (the depth where the waves have no impact anymore on the beach profile) from the cross-shore profiles instead of from the wave data. Unlike INPH, UNIVALI has also done research on a possible borrow area that serves as a sand source for the nourishment. This area is located 20 km east of Balneário Camboriú and has sand characteristics that are comparable to those of the beach in Balneário Camboriú. After the investigation of the relevant data, the nourishment design of DHI has been analysed, DHI used the method of Dean, a well-known method for designing beach nourishments. In their design the beach is widened with 100 m and the initial slope of the nourished beach is 1:20. This results in a final beach width after the profile equilibration (due to wave action) of about 40 to 100 m at MSL +0,7 m, depending on the location and the size of the borrow sand. The total amount of sand necessary for this design is 1,8 million m3. Since the design of DHI is based on data from INPH that are not very reliable or accurate, their design contains several uncertainties. Besides they used a constant closure depth of MSL -3 m along the entire beach, which is strange because the wave activity varies between the north and the south. Therefore the design of DHI has been verified with the use of the data from UNIVALI. It turned out that the constant closure depth assumed by DHI (instead of a variable closure depth) coincidentally has no significant impact on the necessary amount of sand; only the distribution of sand along the beach changes. Also, it turned out that if the DHI design were based on the beach profiles measured by UNIVALI, the total fill volume would be almost 30% larger, which is probably caused by the fact that the original beach width measured by UNIVALI is smaller than the one measured by INPH. The reason for the latter is unclear. Finally, it turned out that the final equilibrated beach width would be very different if DHI used the grain sizes obtained by UNIVALI instead of by INPH. Because of the several uncertainties in both the data measured by INPH and the nourishment design of DHI, a new design of the nourishment has been made based on the data from UNIVALI, which are more appropriate. In the new design the beach is extended to a final equilibrated width of 100 m. To maintain a gentle slope, which induces a wide surf zone with good conditions for wave surfing (this will be discussed below), the beach is nourished with compatible sand, which means the grain size distributions of the borrow sand and the native sand are comparable. Since it is not known at what height the municipality of Balneário Camboriú wants to have a beach of 100 m wide, two alternatives have been investigated: one with a final beach width of 100 m at MSL +0,7 m and one with a final beach width of 100 m at MSL +2,5 m. In both alternatives the initial slope of the nourished beach is 1:39. The amount of sand necessary for the first alternative is 2,33 million m3, which is 0,5 million m3 more than in the design of DHI, but with the result that the (equilibrated) beach will be wider and the slope much more gentle. The stability of the planform of the nourished beach will be influenced by the presence of the jetty constructed in the south of Balneário Camboriú in 2004. It will block sediment supply from Rio Camboriú to the beach, which will probably result in erosion, since the beach is for the most part in dynamic equilibrium. To prevent erosion to the new beach, the concept of headland control has been applied. By relocating the diffraction point in the north and the south of the bay, the predicted shoreline in static equilibrium, corresponding with the new diffraction points, will coincide with the shoreline of the nourished beach. This can be achieved by the construction of a 230 m long jetty in the south, or the extension of the existing jetty in the south, and the construction of a 150 m long jetty in the north. As mentioned before, the gentle beach slope in Balneário Camboriú induces a wide surf zone with good surfing conditions, especially for beginning surfers. However, the nourishment design of DHI will result in a much steeper beach, leading to a narrower surf zone. To improve the surfing conditions after the nourishment, it is possible to construct an artificial surf reef on which optimal surfing conditions can be created. In this project a design of such a reef in Balneário Camboriú has been made as well. Since a surf reef is a relatively unknown subject, a literature study on surf reefs was carried out first. This has resulted in an extensive description of the creation of good surfing rides and the wave breaking types favoured by surfers. The main parameters in describing a surfing wave are the wave breaker height, the peel angle, the breaking intensity and the wave section length. "Short boarders", who make up the majority of surfers in Balneário Camboriú, favour high waves with an extreme breaking intensity and a long surfing ride. For the creation of good surfing waves, it is necessary to alter the existing bathymetry. This is done by the various components of a surf reef, each in a different way and on a different scale. By adjusting the configuration of the components, the four surfing wave parameters can be influenced such that the desired surfing conditions are created. The most common construction method for artificial surf reefs is the use of sand filled geotextile bags. The main reasons why this method is often preferred to other methods are the lower construction costs and the possibility to modify and optimise the reef after construction. An example of an artificial surf reef constructed with sand filled geotextile bags is the Narrowneck surf reef in Australia. This reef forms the basis for the design of the artificial surf reef in Balneário Camboriú. With the knowledge obtained in the literature study, a design of the surf reef in Balneário Camboriú has been made. Two locations were found to be the most suitable for the construction of the surf reef. The first location is on a rocky outcrop in the north of Camboriú Bay, where the water depth is small and the waves are the highest thanks to convergence and shoaling. The second location is situated between the first location and the beach. Two alternative surf reefs, alternative A and alternative B (one on each location), have been investigated. With the help of MIKE 21 the wave propagation on both alternatives has been modelled. To determine the best alternative they were judged on five criteria. Alternative B on the rocky outcrop proved to be the best option. This alternative is cheaper and creates better surfing conditions than alternative A. Besides,for alternative B the probability of the formation of a tombolo behind the surf reef is negligible, while for alternative A, which is much closer to the coast, this probability is very large. Finally, the costs of the new design of the beach nourishment and the artificial surf reef in Balneário Camboriú have been estimated. The beach nourishment will cost approximately 13 million euros and the construction of the surf reef approximately 1 million euros.