Case study Piçarras Beach

Abstract

Master project report. Piçarras is one of the touristic beaches of Santa Catarina state in Brazil. Piçarras beach is a headland bay beach. In the bay irregular features like an island, rocky outcrops and shoals are present influencing wave propagation. In the south Piçarras is bounded by Piçarras river. The river mouth has been fixated in 1970, after which erosion started. The part just a few hundred meters north of the river jetty has the most severe erosion. The erosion gradually decreases towards the north, where even some accretion has been measured. When the situation became critical a nourishment was executed in 1999, which has disappeared totally on some places. The decrease in beach width causes a devaluation of the houses and a decrease in tourism which consequently leads to a decrease of employment. It is therefore necessary to investigate the causes and the amount of the erosion and to generate measures to counteract the negative impact of the erosion. Prosul, a Brazilian engineering company, has designed a nourishment of which execution started in July 2008. The main goal of this study is investigate erosion at Piçarras beach and to design a nourishment to counteract the effects of the current erosion. A model has been built to represent the situation at Piçarras beach. With the model the evolution of the nourishment and the evolution of the existing plan of Prosul could be evaluated. The bathymetry has been composed of recent profile measurements and old nautical maps. They have all been related to the reference level of IBGE. To investigate the erosion at Piçarras beach the wave climate has been schematised. The available wave data was given for four direction (NE, E, SE, S) in the form of wave heights and periods. To be able to compare what the results of the incoming wave energy from these four directions were on the erosion and accretion on the beach, a schematisation has been made. A representative average wave per direction has been determined, that supplied the same energy input from that direction as did all the different waves from that direction. Headland bay beaches are historically formed in such a way that the incoming waves and thereby the wave energy, arrive perpendicular at the beach, thus absorbing the incoming wave energy in the most efficient way. This theory formed the basis of this schematisation. The mean tidal variation is 0.6m, at spring tide this is 0.9m. Storm surges lead to a set-up in water level of approximately 1.0m. Currents and wind are not taken into account. The sediment present at the beach has a D50 of 0.285mm. The sand used in the nourishment of 1999 was coarser than the native sand, which had a D50 of 0.260mm. Erosion processes can take the sand either in cross-shore direction or in longshore direction. Without looking at the underlying process, just to get a realistic idea of the erosion and accretion patterns, the amount of eroded sediment has been calculated with shoreline changes and deduced erosion rates [2]. The erosion of the past nine years is calculated to be 395,000 m3. To find out where the sediment is transported to at Piçarras beach a model has been build. First the nearshore wave conditions have been modelled with Delft3D (D3D) for the four wave scenarios. These conditions serve as input for Unibest (UB). This program is applied to model the shoreline changes.