Morphology, Vegetation and Sand Fence Influence on Sand Mobility of the Foredune System of Atlântida Sul Beach, Rio Grande do Sul, Brazil

Journal of Coastal Research SI 39 616-621 ICS 2004 (Proceedings) Brazil ISSN 0749-0208 Morphology, Vegetation and Sand Fence Influence on Sand Mobility of the Foredune System of Atlântida Sul Beach, Rio Grande do Sul, Brazil L. L. Tabajara, S. R. Dillenburg and E. G. Barboza Instituto do Milênio/ Projeto RECOS CECO-IG-UFRGS, Porto Alegre, Brasil luiztaba@vortex.ufrgs.br Centro de Estudos de Geologia Costeira e Oceânica CECO-IG-UFRGS, Porto Alegre 91509-900, Brasil. sergio.dillenburg@ufrgs.br eduardo.barboza@ufrgs.br ABSTRACT TABAJARA, L.L.; DILLENBURG, S.R. and BARBOZA, E.G., 2006. Morphology, vegetation and sand fence influence on sand mobility of the foredune system of Atlântida Sul beach, Rio Grande do Sul, Brazil. Journal of Coastal Research, SI 39 (Proccendigs of the 8th International Coastal Symposium), 616-621. Itajaí, SC, Brazil, ISSN 0749-0208. TheAtlântida Sul beach is a wave and wind dominated open beach, showing a NE/SW orientation and located in the northern coast of Rio Grande do Sul, southern Brazil. In this paper the sand mobility of its foredune system was studied through a periodically survey of the terrain altitude by reading a set of 1 m-long rules fixed along the terrain surface. To evaluate the parameters controlling sand mobility thematic maps of the terrain morphology, vegetation coverture and sand fence distribution were compared. The obtained results show that the mobility of sands and the wind flux at the foredune system depends of the pre-existent morphology and of the degree of vegetation coverture. Management experiments showed that the construction of fences in a transverse orientation to the dominant NE wind is more effective to retain sands being transported to the SW, and also is more effective to the control of blowouts formed near washouts. ADDITIONALINDEX WORDS: Eolian transport, blowouts, washout INTRODUCTION The Atlântida Sul beach (Figure 1) is a wave and wind dominated open beach, showing a NE/SW orientation and located in the northern coast of Rio Grande do Sul, southern Brazil. Beach sediments are formed by quartzose unimodal fine sands. The onshore winds (NE, ENE and SE ) produce dune sediments showing a positive skewness, an enrichment on heavy minerals and an increase on quartz grains rounded (M ARTINS, 1967). The humid and temperate climate of the coast has determined the presence of strong native vegetation species whose Figure 1. Study area showing an aerial view of experiments site at Atlântida beach.

Morphology, Vegetation and Sand Fence Influence on Sand Mobility of the Foredune System of Atlântida Sul Beach 617 degree of coverture was recorded. Coverture was defined as the percentage of the terrain surface that is covered by the vertical projection of the vegetation inside the square. Five degrees were used: (1) no coverture, (2) 1 to 25%, (3) 25 to 50%, (4) 50 a 75%, and (5) 75 to 100%. The experiments for construction and stabilization of the foredunes followed the ones applied in a pilot project (T ABAJARA et al., 2000): 1- construction of fences in a transverse orientation to the dominant ne winds; 2- construction of double fences parallel to the shoreline; and 3- spreading of a death vegetation coverture over the terrain (figure 2). RESULTS Figure 2. Experiments for construction and stabilization of the foredunes in a pilot project: a- construction of fences in a transverse orientation to the dominant NE winds; b- construction of double fences parallel to the shoreline; and c- spreading of a death vegetation coverture over the terrain. Interaction with sand and wind could stabilize big areas of dunes (S EELIGER, 1992, CORDAZZO and S EELIGER, 1995). The morphology and evolution of the foredunes are also controlled by secondary factors such as accretion rates, deflation, wave and washout erosion and also human activities (H ESP, 1999). The purpose of this paper is to evaluate the transport of eolian sediments and their interaction with the coastal morphology, vegetation and sand fences built during an experimental foredune management (1 km stretch of coast) performed at Atlântida Sul beach from September 1998 to February 1999. Usually during this period, the NE wind has a great drift potential responsible by a landward (to the SW) dune migration (T OMAZELLI, 1993). METHODS Sand mobility was studied through a periodically survey of the terrain altitude (each two weeks) by reading a set of 1 m-long rules fixed along the terrain surface of three experimental sites (T ABAJARA et al., 2000). To evaluate the parameters controlling sand mobility thematic maps of the terrain morphology, vegetation coverture and sand fence distribution were compared. The obtained results allowed the identification of time intervals of dominant erosion, stability and deposition. The study of vegetation coverture was done on 1 m2 squares associated to the rules location. For each vegetation unit the During the first days of spring (September/October) deflation by the dominant NE wind removes the sand from the backshore leaving shell debris that promote changes in the boundary layer that close the sand input. The deflation promotes the lowering of the beach up to the water table increasing the cohesion of sand grains and also the threshold velocity to eolian transport. This spring period is characterized by a great dispersion of sand, a lowering of the dunes crest, landward migration of the slip faces and maximum blowout activity (Table 1). From November on foredune growth rates increase due to bar accretion at the beach face which in turn increase the availability of sand to the eolian transport (TABAJARA and A LMEIDA, 2003). Under this condition the northernmost fences close to the windward NE have rapidly retained more than 40 cm of sand. At these sectors, wedge-shaped sand deposits were fixed by Panicum racemosum, the most important species fixing sand to the foredunes. Moderate deposition (less than 40 cm) occurred at the lee side of the experiment (Figures 3c and 4c). This general pattern of sand deposition from NE to SW is disturbed by washouts during heavy rain falls that eroded and segmented the foredunes. After the spreading of death vegetation coverage at the entrance of a blowout in experiment 3, a great amount of sand transported by the NE wind was retained. However, erosion continued inside the blowout producing the fall and retreat of its lateral borders. The eroded sand was transported to the SW (Figure 5). Experiment number 1 (construction of fences in a transverse orientation to the dominant NE winds) was the most effective management system, accumulating 2.2 m3/m of sand during the period. DISCUSSION The growth rate of a foredune is directly related with the sand volume transported from the beach (DAVIDSON-ARNOTT and L AW, 1990). However, the formation of erosive and depositional eolian features depends on the interactions between eolian fluxes and sediments with the pre-existent morphology and vegetation coverture (GARES, 1988), and on this study depends also on the management structures (sand fences) fixed at the foredune system. Foredune erosive features are primarily formed by wave action along areas dominated by important washouts (Experiment 1) or connected with semi-permanent rip currents in the surf zone (Experiment 3). The wind flow interaction with the morphology of blowouts increase wind speed in a SW direction forming deflation surfaces, shell pavements and slopes erosion. The zonation of vegetation species in the foredune system is related with individual or group adaptation capacity under different levels of environmental stress. Low and flat incipient foredunes show the association of Blutaparon portulacoides and Paspalum varginatum; isolated and elongated shadow dunes are formed by the deposition of sand inside tuffs of Spartina ciliata at the upper backshore; stabilized foredunes showing lateral uniformity are formed due to sand retention produced by the association of Panicum racemosum and

618 Tabajara et al. Figure 3. Site of experiment 1: from top to bottom are presented the morphology, rules location, sedimentary mobility and coverture of vegetation ( 1- no coverture, 2-1 to 25%, 3-25 to 50%, 4-50 to 75%, 5-75 to 100%).

Morphology, Vegetation and Sand Fence Influence on Sand Mobility of the Foredune System of Atlântida Sul Beach 619 Figure 4. Site of experiment 2: from top to bottom are presented the morphology, rules location, sedimentary mobility and coverture of vegetation ( 1- no coverture, 2-1 to 25%, 3-25 to 50%, 4-50 to 75%, 5-75 to 100%).

620 Tabajara et al. Figure 5. Site of experiment 3: from top to bottom are presented the morphology, rules location, sedimentary mobility and coverture of vegetation ( 1- no coverture, 2-1 to 25%, 3-25 to 50%, 4-50 to 75%, 5-75 to 100%).

Morphology, Vegetation and Sand Fence Influence on Sand Mobility of the Foredune System of Atlântida Sul Beach 621 Senecio crassiflorus; and hummock dunes are formed by Senecio crassiflorus. CONCLUSION The mobility of sands and the wind flux at the foredune system depends on the pre-existent morphology and on the degree of vegetation coverture. Management experiments on foredune systems under erosion could promote a quick dune stabilization. The construction of fences in a transverse orientation to the dominant NE wind is more effective to retain sands being transported to the SW, and is also more effective to the control of blowouts formed close to washouts. Panicum racemosum (South America beach grass) is the vegetation species with the higher capacity to fix and to promote the upward growth of foredunes showing a relatively lateral uniformity. LITERATURE CITED CORDAZZO, C.V.; SEELIGER, U. 1995. Guia ilustrado da vegetação costeira no extremo sul do Brasil. Rio Grande: editora da FURG. 275p. DAVIDSON-ARNOTT, R.G.D; LAW, M.N. 1990. Seasonal patterns and control on sediment supply to coastal foredunes, Long Point, Lake Erie. In: NORDSTROM, K.F.; PSUTY, N.P. and CARTER, R.W.G. Coastal Dunes: Form and Process. Chichester: J. Wiley. cap.9, p.177-200. GARES, P.A., 1988. Factors affecting eolian sediment transport in beach and dune environments. Journal of Coastal Research, Special Issue 3, 121-126. HESP, P.A. 1999. The Beach Backshore and Beyond. In: SHORT, A.D. Handbook of beach and shoreface morphodynamics. Chichester: J.Wiley. p.145-169. MARTINS, L.R. 1967. Aspectos texturais e deposicionais dos sedimentos praiais e eólicos da planície costeira do Rio Grande do Sul. Publicação Especial. Escola de Geologia/UFRGS, 13. 102p. SEELINGER, U. 1992. Coastal foredunes of southern Brazil: Physiography, habitats and vegetation. In: SEELINGER, U. Coastal Plant Communities of Latin America. New York: Academic Press. p.367-381. TABAJARA, L.L. and ALMEIDA, L.E.S.B, 2003. Morfodinâmica dos bancos na praia de Atlântida Sul, RS. IX Congresso da Associação Brasileira de Estudos do Quaternário. Recife:Abequa. Resumo expandido CD Ron, no. 102. TABAJARA, L.L., MARTINS, L.R. and FERREIRA, E.R., 2000. Efetividade de Métodos Estruturais na Construção de Dunas Frontais. Pesquisas em Geociências, 27(1), 97-112. TOMAZELLI, L.J.,1993. O Regime de Ventos e a Taxa de Migração das Dunas Eólicas Costeiras do Rio Grande do Sul, Brasil. Pesquisas, v.20, n.1, p.18-26.