Loading...
| Name: | Description: | Size: | Format: | |
|---|---|---|---|---|
| 1.04 MB | Adobe PDF |
Advisor(s)
Abstract(s)
A erosão no sopé das arribas é, em grande medida, condicionada por duas forças que se opõem: a força de
resistência mecânica das rochas que as compõem e a força erosiva das ondas que as atingem.
Com o objectivo de estimar essas duas forças foi desenvolvido um modelo que combina diversas fórmulas
criadas e testadas por outros autores. Foi seleccionado um sector de arriba localizado na faixa litoral a oeste
de Cascais com características geomorfológicas particulares para exemplificar e testar a aplicação do modelo.
Development of a model to evaluate wave erosion in rock coasts. Erosion at the cliffs base is controlled mainly by two opposite forces: the assailing force of waves that reach the cliff base and the local rock strength. The evaluation of those two forces is essential to understand the present morphology and dynamic of each rock coast sector in order to forecast its evolution. A methodology that enables the calculation of those two forces is proposed. To estimate the wave force at the cliff base we combine formulas developed or stated by several authors. Knowing wavelength, wave height, wave period, slope of the internal continental shelf and the water depth at the cliff base the model give us water depth at the breaking point, the type of wave at the cliff base and above all, the pressure (along with its spatial distribution) applied there by the waves. In the proposed model, the mechanical strength of the cliff base intact rocks is evaluated using its most representative parameter, the uniaxial (or unconfined) compressive strength. Among the factors that can reduce this strength, the existence of discontinuities is generally recognized as being the most important. To calculate the rock mass strength including the influence of discontinuities, the model use the Rock Mass index (RMi) developed by Palmstrøm (1996, a, b). The RMi combines the uniaxial compressive strength of intact rock with the analysis of the joints roughness, alteration, size, termination and density. The model was applied to a rock coast sector located near Cascais (Lisbon). The studied area is exposed to SW and the cliff is cut in cretaceous rocks. Wave data comes from the nearest wave gauge offshore Sines. The results of the model allow us to conclude that deep-water waves with heights above 4,3m had enough erosive force to shape the present local morphology exploring the weakness of a clay marl outcrop. However, the gradual cliff top retreat with the resulting formation of a sub-structural shore platform, enable other processes, mainly sub-aerial, to increase their importance. A storm situation that took place in December 2000 was followed in detail and gave essential elements to complete the results obtained by the model. Therefore, it is concluded that nowadays, the clay marl outcrop suffers a combination of permanent erosion due to subsurface drainage in a high frequency but low magnitude process with a discontinuous attack by the storm waves in a low frequency but high magnitude process.
Development of a model to evaluate wave erosion in rock coasts. Erosion at the cliffs base is controlled mainly by two opposite forces: the assailing force of waves that reach the cliff base and the local rock strength. The evaluation of those two forces is essential to understand the present morphology and dynamic of each rock coast sector in order to forecast its evolution. A methodology that enables the calculation of those two forces is proposed. To estimate the wave force at the cliff base we combine formulas developed or stated by several authors. Knowing wavelength, wave height, wave period, slope of the internal continental shelf and the water depth at the cliff base the model give us water depth at the breaking point, the type of wave at the cliff base and above all, the pressure (along with its spatial distribution) applied there by the waves. In the proposed model, the mechanical strength of the cliff base intact rocks is evaluated using its most representative parameter, the uniaxial (or unconfined) compressive strength. Among the factors that can reduce this strength, the existence of discontinuities is generally recognized as being the most important. To calculate the rock mass strength including the influence of discontinuities, the model use the Rock Mass index (RMi) developed by Palmstrøm (1996, a, b). The RMi combines the uniaxial compressive strength of intact rock with the analysis of the joints roughness, alteration, size, termination and density. The model was applied to a rock coast sector located near Cascais (Lisbon). The studied area is exposed to SW and the cliff is cut in cretaceous rocks. Wave data comes from the nearest wave gauge offshore Sines. The results of the model allow us to conclude that deep-water waves with heights above 4,3m had enough erosive force to shape the present local morphology exploring the weakness of a clay marl outcrop. However, the gradual cliff top retreat with the resulting formation of a sub-structural shore platform, enable other processes, mainly sub-aerial, to increase their importance. A storm situation that took place in December 2000 was followed in detail and gave essential elements to complete the results obtained by the model. Therefore, it is concluded that nowadays, the clay marl outcrop suffers a combination of permanent erosion due to subsurface drainage in a high frequency but low magnitude process with a discontinuous attack by the storm waves in a low frequency but high magnitude process.
Description
Keywords
Litoral rochoso Modelo de erosão Força de resistência das rochas Força erosiva das ondas Estremadura portuguesa Rock coast Erosion model Rock strength Wave assailing force Portuguese Estremadura
Citation
NEVES, M., PEREIRA, A. R. & TRINDADE, J. (2002) Desenvolvimento de um modelo para avaliação da capacidade erosiva da ondulação em litorais rochosos. In Ramos, C., Trindade, J., Neves, M. & Rocha, G. (Eds.) Contribuições para a Dinâmica Geomorfológica. Publicações da Associação Portuguesa de Geomorfólogos, Vol. I, p. 115 – 124. (ISBN: 972-636-137-0)
Publisher
Associação Portuguesa de Geomorfólogos