Shinji Sassa

Burrowing Criteria and Burrowing Mode Adjustment in Bivalves to Varying Geoenvironmental Conditions in Intertidal Flats and Beaches

The response of bivalves to their abiotic environment has been widely studied in relation to hydroenvironmental conditions, sediment types and sediment grain sizes. However, the possible role of varying geoenvironmental conditions in their habitats remains poorly understood. Here, we show that the hardness of the surficial intertidal sediments varies by a factor of 20–50 due to suction development and suction-induced void state changes in the essentially saturated states of intertidal flats and beaches. We investigated the response of two species of bivalves, Ruditapes philippinarum and Donax semigranosus, in the laboratory by simulating such prevailing geoenvironmental conditions in the field. The experimental results demonstrate that the bivalve responses depended strongly on the varying geoenvironmental conditions. Notably, both bivalves consistently shifted their burrowing modes, reducing the burrowing angle and burial depth, in response to increasing hardness, to compensate for the excessive energy required for burrowing, as explained by a proposed conceptual model. This burrowing mode adjustment was accompanied by two burrowing criteria below or above which the bivalves accomplished vertical burrowing or failed to burrow, respectively. The suitable and fatal conditions differed markedly with species and shell lengths. The acute sensitivities of the observed bivalve responses to geoenvironmental changes revealed two distinctive mechanisms accounting for the adult–juvenile spatial distributions of Ruditapes philippinarum and the behavioral adaptation to a rapidly changing geoenvironment of Donax semigranosus. The present results may provide a rational basis by which to understand the ensuing, and to predict future, bivalve responses to geoenvironmental changes in intertidal zones.

The Dynamics of Liquefied Sediment Flow Undergoing Progressive Solidification

The propagation of solidified zones in the course of subaqueous liquefied sediment flow is theoretically discussed. The coupled systems of Navier-Stokes equations and consolidation equation are numerically solved under moving boundary conditions, with consideration of the concurrent evolutions of the flow surface as well as of the internally formed interface between the fluid and solidified zones. The analyses for collapse of a body of liquefied soil into ambient fluid under gravity show that the liquefied sediment flow manifests itself as a decelerating gravity flow due to the dynamic interaction between the flowing liquefied soil and the progressively solidified zones in the sediment.

Dynamics of Submarine Liquefied Sediment Flows: Theory, Experiments and Analysis of Field Behavior

This paper describes an extension of the computational code LIQSEDFLOW. The salient features of the code lie in the capabilities to describe the multi-phased physics of subaqueous sediment gravity flows. Specifically, it combines Navier-Stokes/continuity equations and equations for advection and hindered settling of grains for a liquefied soil domain, with a consolidation equation for the underlying, progressively solidifying soil domain, via a transition layer that is characterized by zero effective stress and a small yet discernable stiffness. Evolutions of the flow and solidification surfaces are traced as part of solution by using a volume-of-fluid (VOF) technique. The predicted features of gravity flows of initially fluidized sediments with different concentrations conform to the observed performances in two-dimensional flume tests. The submarine flow slide that occurred in 1980 on the Klamath River delta, California is also discussed. The present results demonstrate the crucial role of two-phase physics, particularly solidification, in reproducing the concurrent processes of flow stratification, deceleration, and redeposition, as well as in the formation of scarps and terrace in submarine liquefied sediment flows.

Simplified Liquefaction Prediction and Assessment Method Considering Waveforms and Durations of Earthquakes

AbstractThis paper presents a new simplified liquefaction prediction and assessment method that is capable of considering the influence of the waveforms and durations of earthquakes. The concept of...

Effects of Tide and River Discharge on Mud Transport on Intertidal Flat

Fine-grained sediment budgets based on the monthly bed level and net sediment flux monitoring from October 2006 to October 2007 are estimated to examine the relative contributions of tides and river discharge to mud transport on an intertidal flat adjacent to a river mouth in a semiclosed sound. The estimated sediment budgets are interpreted using the water mass balance equation and the horizontal tidal current pattern measured using high frequency (HF) radar. The intertidal flat accreted during normal discharge conditions are primarily attributable to the alongshore sediment flux toward the river mouth. However, the flat was eroded when the large offshore suspended sediment transport occurred on the flat during the large river discharge. The net alongshore tidal current causes alongshore sediment fluxes toward the river mouth on this intertidal flat adjacent to the river mouth. These findings may be specific to this site, but an adopted comprehensive approach may be applicable to other sites.

Bearing Capacity of Breakwater Mound Under Tsunami-Induced Seepage Flow

Water level difference induced by tsunami generates seepage flow in a mound of caisson-type breakwaters. It has been pointed out that the seepage force weakens a bearing capacity of a mound. The present paper describes bearing capacity properties of a mound under seepage flow, by using centrifuge model tests and finite element method analyses. Two types of model tests were conducted in the study: horizontal load tests and combined tests with horizontal load and seepage flow. According to the model tests, seepage force resulted in decrease of a bearing capacity. In addition, it was confirmed that finite element method analyses could simulate the model tests, and the suitable hypothetical adhesion was shown to simulate the effect of seepage force.

DRUM CENTRIFUGE TESTS ON WAVE-SEABED-OFFSHORE MONOPILE INTERACTION

今後,海洋構造物の建設の増加が予想される中で,洋 上風力発電などの基礎として用いられるモノパイルの安 定性に関わる問題が重要性を増してくる.これまで,地 盤の表層や浅部に着目したモノパイルまわりの洗掘問題 について関心がもたれてきた.しかし,地盤全体を含 めた,波-地盤-モノパイルの相互作用について調べら れた例は少ない. 波-地盤の相互作用問題の一つとして残留間隙圧の 蓄積による地盤液状化があげられる.重要なこととし て,地盤が液状化することで,外部流体と液状化地盤と が2層流体としてふるまうことが理論と実験から示され ている.これによる構造物の不安定化の一つとして, 埋設パイプラインが浮上することが実験で得られている .波-地盤-モノパイル系を考えた場合,地盤が液状 化することで,外部流体/液状化地盤の2層流体がモノパ イルに直接作用するとともに,液状化にともない実質的 な根入れ長さが減少することから,モノパイルが極めて 不安定になることが予想される. 本研究の目的は,ドラム型遠心載荷装置の水路(図-1 参照)を用いて,砂地盤-モノパイル系の波浪実験を行 い,波による残留過剰間隙水圧の上昇による地盤液状化 とそれにともなうモノパイルの不安定化を詳しく調べる ことである.

Sedimentation History of Sandbars in Flood-Tidal Delta Evaluated by Seismic Method in Lake Tofutsu, Japan

ABSTRACT Watabe, Y. and Sassa, S., 2016. Sedimentation history of sandbars in flood-tidal delta evaluated by seismic method in Lake Tofutsu, Japan. An extensive pattern of sandbars within a flood-tidal delta complex can be seen at the tidal inlet of Lake Tofutsu, one of the lagoons located along the Okhotsk Sea in Hokkaido, Japan. The primary aim of this study was to investigate the sequential sedimentation history of these sandbars. The shear-wave velocity structure, which is equivalent to the stiffness variation, of these sandbars was examined using a surface-wave method called the multichannel analysis of surface waves (MASW) to identify and describe the stratigraphy of the flood-tidal delta at the tidal inlet of Lake Tofutsu. The MASW results demonstrated that even though the sandbars appeared to comprise the same sandy material, the sediment under the sandbars had a very varied stratigraphy, comprising the river-borne muddy sediment derived from the upstream side of the lagoon and wave-induced marine sandy sediment derived from the coastal sand of the Okhotsk Sea. The second aim of this study was to verify the sedimentation history described on the basis of the MASW results through laboratory testing of sediment samples collected at characteristic points along the MASW array. The sampling method was modified to be applicable to low-cost sediment investigation at tidal flats and was conducted by human power without the aid of an engine. The collected samples were examined in the laboratory to obtain the depth profiles of grain-size distribution and carbon 14 dating. The results obtained from the sampling are consistent with the sequential sedimentation history evaluated by MASW.

Intertidal Sedimentary Structures and Their Formation Mechanisms in Sandy, Muddy, and Sand-Mud Layered Flats

In the present study, we explored the capability of a surface wave method, called here MASW (Multi-channel Analysis of Surface Waves) in evaluating the cross-shore and along shore sedimentary structures of intertidal flats. Field surveys were carried out at sandy, muddy, and sand-mud layered flats, when these flats were exposed during spring tides in summer. The present method has proved very useful to investigate the sedimentary structures of various types of intertidal flats. The formation processes of the intertidal sedimentary structures can be explained as a consequence of cyclic elastoplastic contraction of soils that are subjected to a variety of suction dynamics under the tide-induced ground water table fluctuations