Desenvolvimento de um microerosómetro do tipo TMEM para aplicação em plataformas rochosas litorais

Abstract

Resumo -A determinação da taxa de erosão das plataformas rochosas litorais necessita de equipamento específico, adaptado às condições ambientais em que se desenvolvem estas plataformas. Neste trabalho, descreve-se a concepção de equipamento adaptado a esse fim -um microerosómetro do tipo TMEM (Traversing Micro-Erosion Meter) -ao qual foram introduzidas algumas modificações, que visam melhorar quantitativa e qualitativamente os dados a obter. São também indicados, para além das vantagens e limitações deste equipamento, os procedimentos principais seguidos nas fases de instalação e medição com o TMEM. Referem-se ainda os primeiros resultados obtidos num ensaio experimental levado a cabo numa plataforma calcária situada nos Tombadoiros, a norte da Ericeira, entre os níveis mínimo da preiamar morta e o nível máximo da preiamar viva. Os resultados correspondem a 96 dias de evolução entre duas observações no período de Outono/Inverno de 2000/2001 e apontam para uma taxa de rebaixamento médio da plataforma rochosa de 0,153mm/ano. A cobertura pelo líquene Verrucaria maura de mais de metade dos pontos de medição condicionou, no entanto, os resultados obtidos, tendo a separação destes em dois grupos permitido concluir que o sector de rocha limpa apresentou uma taxa de erosão 2,5 superior à do sector coberto por aquele líquene.Palavras-chave: plataforma rochosa litoral, microerosómetro, taxas de rebaixamento, Estremadura portuguesa

Downwearing rates on shore platforms of different calcareous lithotypes

“…The value for biocalcarenite downwearing measured in our work is similar to that observed by several other authors for limestone in shore platforms between 0.034 and 0.400 mm year − 1 (Neves et al, 2001, Cucchi et al, 2006, Swantesson et al, 2006a, Furlani et al, 2010 whereas lowering rates in calcarenite and siltstone fall within values reported by other works for detrital rocks: (i) siltstone: 1.8 mm/year − 1 (Gill and Lang, 1983), (ii) sandstone: 1.254 mm/year − 1 (Porter et al, 2010), and (iii) greywacke: 0.300-0.405 mm/year − 1 (Gill and Lang, 1983). That difference between carbonate and more detrital rocks is likely to be related to mass properties like fabric and porosity.…”

“…To further investigate the importance of porosity, we parameterized data from our study and from other published data (Gill and Lang, 1983;Spencer, 1985;Stephenson and Kirk, 1998;Neves et al, 2001;Andrade et al, 2002;Swantesson et al, 2006a;Blanco-Chao et al, 2007;Stephenson et al, 2010) concerning rock texture, instead of grouping them in the conventional way by lithological composition. Our parameter attribution was based on general concepts such as: (i) crystalline rocks are less porous than sedimentary rocks; (ii) finegrained rocks are less permeable than coarse-grained sedimentary rocks; and (iii) the texture of rocks with large fossils is more heterogeneous (i.e., more porous) than those containing microfossils.…”

“…Four triangular sections designated here as 'stations,' were chosen to quantify the rate of downwearing of surface shore platforms by using a TMEM which allows monitoring of 255 points following the methodology developed by Neves et al (2001). Two TMEM stations were positioned at Olhos de Água (OA1A and OA1B) and two at Galé (G1A and G1B).…”

Decadal scale micro erosion rates on shore platforms

“…There are numerous studies reporting platform surface lowering, measured using the micro-erosion meter (MEM) or traversing microerosion meter (TMEM) (Trudgill, 1976;Kirk, 1977;Robinson, 1977a,b, c;Spencer, 1981;Gill and Lang, 1983;Spencer, 1985;Shakesby and Walsh, 1986;Kirk, 1998, 2001;Neves et al, 2001;Inkpen et al, 2004;Cucchi et al, 2006;Foote et al, 2006;Gomez-Pujol et al, 2006). These studies usually measured erosion over periods of about two years.…”

Analysis of relationships between micro-topography and short- and long-term erosion rates on shore platforms at Kaikoura Peninsula, South Island, New Zealand

“…A common method used to measure changes in the surface of the shore platform has been the micro-erosion meter (MEM) or the traversing micro-erosion meter (TMEM) (Trudgill, 1976;Kirk, 1977;Robinson, 1977a,b,c;Spencer, 1981Spencer, , 1985Gill and Lang, 1983;Shakesby and Walsh, 1986;Kirk, 1998, 2001;Neves et al, 2001;Inkpen et al, 2004;Cucchi et al, 2006;Foote et al, 2006;Gomez-Pujol et al, 2006;Inkpen and Stephenson, 2006). Usually these studies measure rates over a two-year period, although recent studies have also used these instruments to monitor rates of surface change over a few days or a few hours (e.g.…”