Mud-induced wave damping and wave-induced liquefaction

详细信息	查看全文 | 推荐本文 |

• 作者：Johan C. Winterwerp^a ; ^b ; ^{han.winterwerp@deltares.nl} ; Gerben J. de Boer^a ; ^b ; Gert Greeuw^c ; Dirk S. van Maren^a ; ^b
• 关键词：Fluid mud ; Liquefaction of mud ; Cyclical triaxial tests ; Wave damping ; Wadden Sea
• 刊名：Coastal Engineering
• 出版年：2012
• 期刊代码：10_03783839
• 类别：civ
• 出版时间：June, 2012
• 卷：64
• 期：Complete
• 页码：102-112
• 文件大小：2080 K

摘要

This paper describes the results of a study on potential wave damping in the Dutch Wadden Sea by viscous wave energy dissipation within soft and/or liquefied mud layers. The results are relevant for wave height boundary conditions for levee design along the mainland coast.

From a site inspection, we observed that the mud fields along the Wadden Sea coast are quite dynamic, and seem to migrate in alongshore direction during storm conditions. A site along the Wadden Sea coast with extensive soft mud deposits, known as Zwarte Haan, was selected for more detailed laboratory and model analyses. Both field and laboratory measurements on sediment cores indicated strengths of a few kPa, i.e. slightly above the Liquid Limit. To assess the sensitivity of the mud deposits to liquefaction induced by incoming waves, we carried out cyclical triaxial tests in the laboratory on a series of samples taken from the field. The stresses prescribed were established from the wave heights and periods expected during storm conditions. However, even at stresses well beyond these values, the triaxial tests did not indicate full liquefaction of the mud samples; only some frictional losses were monitored.

These results are in contradiction with laboratory studies, where liquefaction has been measured frequently. However, our observations corroborate with other recent observations in literature, indicating that wave damping does not occur during a storm, but afterwards. It is hypothesized that fluid mud, dissipating wave energy, is formed by so-called auto-saturation and/or from deposition of mud eroded during that storm, and not from liquefaction during the storm. Wave damping then occurs through viscous dissipation in the fluid mud layers until these mud layers regain their initial strength through consolidation.

Next, we assessed the degree of wave damping to be expected at the beginning of a storm, when the bed is still unaffected and during/after storm conditions, when fluid mud is formed. We used the measured mud properties and those typical for fluid mud, as found in the literature. This assessment was done through simulations with a new version of SWAN, referred to as SWAN-mud. These simulations suggest significant wave damping under fluid mud conditions, but only 10%damping at the beginning of the storm; moreover, the degree of wave damping over a muddy bed may decrease during the storm, as the soft mud deposits may be eroded by the waves.

Our analyses suggest that wave cannot liquefy soft mud beds under natural conditions, and that fluid mud layers, damping incoming waves by viscous dissipation, are formed from mud eroded by the shear flow induced by the waves' orbital motion. Note that in-situ pore water pressure measurements are required to corroborate this hypothesis.