Resumo

KOERNER, K. F.; OLIVEIRA, U. R.  e  GONCALVES, G.. Effect of coastal erosion contention structures on the coastline: Hermenegildo Beach, Rio Grande do Sul, Brazil. RGCI [online]. 2013, vol.13, n.4, pp.457-471. ISSN 1646-8872.  https://doi.org/10.5894/rgci405.

Hermenegildo beach, located in Santa Vitória do Palmar, Rio Grande do Sul state, in the extreme south of Brazil (fig.1), has been undergoing a significant problem of coastal erosion that started at the end of the 50’s and has gotten worse since the 90’s. It has damaged seashore properties whose owners have faced this situation by armoring their properties with containment structures against the erosion (fig.2). This study aimed at analyzing whether the containment structures along Hermenegildo beach have actually hold coastal erosion and what impact they have on the characteristics of this stretch of beach. In order to carry it out, a geodesic GPS in real time kinematic (RTK) mode was used to map the so-called reference lines: all the upper limit of the beach in the urbanized part (limit of slopes and containment structures on the beach) and the upper limit of the beach along the frontal dunes in the south and the north of the urban area (the basis of the frontal dunes) on a coastal stretch that is about 5 km long; the high waterline along the same stretch (fig.5); an interpolated line was done linking linear regression made with the GPS points from foredunes from north and south of the urbanized part (fig.6). Additionally, 22 topographic profiles which are transversal to the beach were mapped, in order to investigate the morphological differences of the slopes found in this study (fig.7). These data were supported by data collected by other tools, such as aerial images, mosaics of conventional photographs and the identification of containment structures, the use of a Geographic Information System (GIS) and by the literature review itself. Points marked by the GPS in sectors called North Dunes and South Dunes have shown that the upper limit of the beach with these dunes has a relatively straight conformation on the map, similar to what is expected from a natural environment in that place (where most beaches are quite straight or oblong, except for washout channels which generate little recesses). However, the Urbanized Shore sector has a very irregular upper limit on the beach due to different shapes of containment structures or even to their absence on this stretch. The analysis of the differences found between the upper limit of the beach in the urbanized part (structured zone) and the one close to the frontal dunes in the south and the north of the urban area showed that the central part of the urbanized shore - where there is the largest number of containment structures, such as rockfill, which are more resistant to coastal erosion - advances over the shoreline, by comparison with the adjacent areas, and comprises a containment area of 54,100m² in the period under analysis (fig.8). The longest distance between the Urbanized Shore sector and the interpolated dune line was 45.3 meters (fig.8, ‘M’). Therefore, this study concluded that the containment structures on the beach, even though they were built by dwellers that did not have specific knowledge about coastal engineering, have hold, at least partially, the horizontal migration of the shoreline towards the landside, although several houses have already been destroyed and replaced by beach areas over the past 50 years. It must be highlighted that the authors of this study do not stimulate this kind of construction in other urbanized coasts. However, there is evidence that the protection structures, such as rockfill, have been efficient in Hermenegildo since they held the erosive process in that place, at least in the period under investigation. On the other hand, fixing the horizontal mobility of the coastline leads to significant impacts on the beach environment, mainly in the morphology of the part that has been more affected by the structures. Impacts comprise the decrease in the beach width, no berms - a fact that diminishes the declivity on the beach face - and steep slopes on the upper limit of the beach (fig.9); thus, it gets harder to gain access to the beach. During the measurement, the waterline had a concavity in front of the central area (fig.8); it means that sediment has been removed from the areas close to the containment structures and that they interfere with the sediment balance in the horizontal plane besides generating vertical sediment deficit. Such concavity seems to be quite dynamic in terms of space and time; thus, a deeper analysis is needed. The short distance from the coast to the waterline (fig.8, ‘m’) was related to the projection of the containment structures and the concavity of the waterline. The beach area is very small, not only for the dissipation of the wave energy but also for the users’ leisure, i. e., the natural shore has been thoroughly suppressed, a situation that is not considered adequate to that place. In addition, since there is progressive advance of the sea over the projected area and progressive reduction between the swash zone and the containment structures in the urbanized area, it is not known how long these structures are going to keep protecting the urban area. Therefore, it is necessary to reflect on long-term management strategies along with effective environmental planning to account for future constructions. Regarding the material and methods employed in the study, it is worth mentioning that the GPS-RTK is a very efficient and precise tool which enables researchers to collect numerous data quickly. In fact, all positioning data - that referred to the GPS - used in this study could be collected on one day in situ. Even though errors may occur, they can be considerably mitigated.

Palavras-chave : GPS-RTK; Shorelines; Coastal morphodynamics; Coastal Zone Management; GIS.

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