The behaviour of beds of fine-grained sand under fluid wave trains was investigated using centrifuge modelling. Three sets of centrifuge wave tank tests with viscous scaling were performed, such that time-scaling laws for wave propagation as well as consolidation were matched. The test programme consisted of either progressive- or standing-wave tests on loosely packed, fresh deposits of soil, as well as repeated travelling-wave tests with intervening periods of consolidation. The results from the progressive- and standing-wave tests indicate that for each of the wave-loading regimes, there exists a critical cyclic stress ratio below which liquefaction does not occur. The critical value for the progressive-wave-loading series was found to be considerably smaller than that for the standing-wave-loading series. Moreover, the wave-induced liquefaction of the sand beds was of a progressive nature. In fact, under the action of severe travelling waves, liquefaction was first induced in the uppermost layer of the sand bed, and then the liquefaction front advanced downward in the course of wave loading, eventually bringing the entire soil bed into a state of complete liquefaction. It is also shown from the repeated wave tests that the liquefied beds of soil underwent significant densification in the consolidation processes that followed. In each of the subsequent wave-reloading stages, re-liquefaction was observed to occur in such a way that the final depth of the liquefaction front became shallower. Nous avons étudié le comportement ďun banc de sable fin soumis à des trains ďondes fluides en utilisant une maquette centrifuge. Nous avons conduit trois lots ďessais en bac centrifuge avec échelle de viscosité, de facon à lui faire correspondre les lois ďéacchelle de temps šappliquant à la propagation des ondes tout comme à la consolidation. Le programme ďessais consistait de tests ďondes progressives et d'ondes stationnaires passant sur des dépôts de sol frais, non compacté ; le programme comportait aussi des essais répétés ď'ondes progressives avec des périodes intermittentes de consolidation. Les résul- tats des essais ďondes progressives et ďondes stationnaires indiquent que pour chacun des régimes de charges ďondes, il existe un taux de contrainte cyclique critique en dessous duquel la liquéfaction ne se produit pas. On a constaté que la valeur critique pour la série de charges ďondes progressives était bien inférieure à celle de la série de charges ďondes stationnaires. De plus, la liquéfaction des bancs de sable causée par les ondes était de nature progressive. En fait, sous ľaction ďondes fortement progressives, la liquéfaction se produisait ďabord dans la couche supérieure du banc de sable, puis le front de liquéfaction avancait au cours de la charge ďondes, mettant finalement le banc de sol ender dans un éat de liquéfaction totale. Nous montrons également, en nous basant sur les essais ďondes répétés que les bancs de sol liquéfiés subissent une densification significative au cours des processus de consolidation qui solvent. Dans chacun des stades suivants de recharges ďondes, nous avons observé qcune nouvelle liquéfaction se produisait de telle manière que la profondeur finale du front de liquéfaction diminuait.
On 28 September 2018, a strong earthquake with a moment magnitude of 7.5 occurred on the island of Sulawesi, Indonesia. This earthquake caused extensive liquefaction and liquefaction-induced flow slides inland. Despite a strike-slip fault, which typically displaces land horizontally, being unlikely to produce significant tsunamis, the earthquake in fact caused devastating tsunamis. Our field investigations showed that there was an occurrence of extensive liquefaction in coastal areas. Significant coastal liquefaction can result in a gravity flow of liquefied soil mass that can cause a tsunami. A comparison with a past disaster of the strike-slip fault Haiti earthquake tsunami indicated that essentially the same occurred at the Palu coast of Central Sulawesi. Namely, liquefaction-induced total collapse of coastal land caused liquefied sediment flows, resulting in a tsunami. An important difference between this time and Haiti was that such total collapses and flows of coastal land due to liquefaction occurred at several (at least nine) places, resulting in multiple tsunamis. Analysis of the tidal data implied that less than 20% of the tsunami height was related to tectonic processes, and the majority was caused by the coastal and submarine landslides as characterized by liquefied gravity flows.
[1] In this paper we present and discuss the role of the dynamics of suction, that is, negative pore water pressure relative to atmospheric air pressure, in the evolution of intertidal sandy flats. This is done through the combined use of field evidence, laboratory experiments, and a theoretical model. Field observations were performed in the estuary of Obitsu River located in the east coast of Tokyo Bay, Japan. Laboratory experiments were performed in a calibration chamber as well as in a laterally scaled model flat subjected to water/groundwater level variations. The present study demonstrates that the suction dynamics associated with the tide-induced groundwater level variations play a substantial role in the temporospatial evolutions of voids, stiffness, and surface shear strength in cyclically exposed and submerged soil. The suction-induced void state changes are a consequence of cyclic elastoplastic contraction of the soil and are accompanied by distinct morphological changes, despite the lack of significant surface shear stresses acting on the soil. Such soil behavior occurs in regions where suction develops above the groundwater level during low tides and strongly depends on the way in which such suction develops. Discussions are made on how these suction dynamics effects could contribute, via feedback involved in surface transport processes, to the overall morphological evolution of cross-shore intertidal flat soils.
Predators may have a series of alternative foraging modes. Under the food resource maximization hypothesis, predators are expected to shift between foraging modes such that they attain the highest intake rate in response to prey availability and constraints varying with environmental conditions. To test this hypothesis, we measured foraging action rate (actions per unit time), capture rate (captures per unit time), and intake rate (amount of energy and nutrients per unit time) for 2 foraging modes, pecking (feeding on epifauna at the sediment surface) and probing (feeding on infauna by inserting the bill into the sediment), in dunlin Calidris alpina on an intertidal sandflat. The birds chose their foraging mode to attain higher feeding success, i.e. individuals that obtained higher capture and intake rates by pecking allocated a higher proportion of foraging effort to pecking, and vice versa. The birds shifted foraging mode from probing to pecking with increased time after emersion. The shift may be related to decreasing efficiency of probing due to increases in the costs of energy and time caused by decreasing sediment penetrability (increasing hardness) with time after emersion. Our in situ study empirically suggests that, while environmental constraints reduce the predators' foraging mode flexibility, the birds show individual-based appropriate adjustments in their foraging mode to attain a higher intake rate at a given time and patch. This extends the ideal forager model for patch choice into foraging mode choice. KEY WORDS: Decision making · Feeding ecology · Foraging behavior · Intertidal ecosystems · ShorebirdsResale or republication not permitted without written consent of the publisher OPEN PEN ACCESS CCESS
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