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Waterfront and Marine Geohazards

This laboratory promotes studies for better understanding and managing risks from a wide spectrum of hydro-geoenvironmental hazards, which include sediment yield, debris flow, liquefaction, piping, scouring, erosion, siltation, seawater intrusion and submarine mass movements.

The research efforts have been directed toward establishing an integrated framework by which to predict the complex behaviour of fluid-sediment systems under dynamic environmental loading, with consideration of their inherently multi-scale nature.

Academic Staff

Tetsuya HIRAISHI

HiraishiProfessor (Disaster Prevention Research Institute)

Research Topics

Coastal hazard due to tsunami, storm surge, stormy wave and long period wave is mainly studied. Sedimentation and environment in waterfront is also the important subject to protect the human activity area boundarying water. Their characteristics are investigated mainly by the large-sized experimental study.

Contacts

Ujigawa Open Laboratory Disaster Prevention Research Institute
TEL: +81-75-611-0520
FAX: +81-75-611-0530
E-mail: hiraishi.tetsuya.2c@kyoto-u.ac.jp

Yasuyuki BABA

baba2Associate Professor (Disaster Prevention Research Institute)

Research Topics

Contacts

Shirahama Oceanographic Observatory, Disaster Prevention Research Institute
TEL: +81-739-42-553
FAX: +81-739-42-5532
E-mail: baba@uh31.dpri.kyoto-u.ac.jp

Hideaki MIZUTANI

mizutani2Assistant Professor (Disaster Prevention Research Institute)

Research Topics

My research topics are clarifying mechanism of flow structure and topographical changes that occur in coastal and river zone by field observations and numerical simulations, and improvement of those numerical prediction techniques for prevention and mitigation of disasters.

Contacts

Ujigawa Open Laboratory Disaster Prevention Research Institute
TEL: +81-75-611-5220
FAX: +81-75-611-0530
E-mail: mizutani@uh31.dpri.kyoto-u.ac.jp

Research Topics

Integrated analysis of sediment transport and deposition processes in waterfront and marine environments

The delivery of sediments from rivers to coastal oceans has profound implications in estuarine and marine environments. The sediments deposited nearshore are exposed to a wide spectrum of dynamic loading, exhibiting diverse forms of hydro-geoenvironmental phenomena.

On the shorter end of time scale, for instance, deposits of loosely packed granular sediments can undergo liquefaction due to earthquake shaking or storm-wave loading, resulting in flow slides or submarine landslides. On the longer end of time scale, the effects of sediment transport under moderate environmental forcing over a prolonged period of time can add up to serious consequences such as retreat of shorelines.

It is important herein to appreciate the fact that natural sedimentary landforms, such as sand beaches and tidal flats, are products that have come out of exquisite balance of forces, and that they are effectively in dynamic equilibrium. Indeed, this reasoning has led us to exploring the multi-scale nature of the fluid-sediment systems for better understanding the processes involved and for enhancing the predictive capability pertaining to the system responses (refer to Fig. 1).

image : Diagram illustrating the multi-scale nature of sediment dynamics in waterfront and marine environments
Figure 1: Diagram illustrating the multi-scale nature of sediment dynamics in waterfront and marine environments

The behaviour of saturated and unsaturated granular fills behind seawalls subjected to water-level fluctuations

The amenity and safety of waterfront areas has received increasing attention from many sectors of the society. This particular research project aims at establishing a theoretical framework by which to predict, with high accuracy, the behaviour of granular fills behind seawalls under water-level fluctuations.

Specifically, we have developed an analysis procedure by which to consistently model the elastoplasticity, water retention and seepage characteristics of unsaturated granular soil. The capability has favourably been tested against the measured performance of suction, degree of saturation and soil moisture changes in an artificial sand beach on the Ohkura coast.

The related ongoing research projects include laboratory experiments that are designed for looking at the processes of wash out in the submerged part of a granular fill under water-level fluctuations and for looking at the ensuing process of cavity formation in the unsaturated part of the granular fill (refer to Figs. 2 and 3).

The research project of this sort will contribute to developing a risk management strategy pertaining to artificial sand beaches.

photo : Flow out of sand and cavity formation
Figure 2: Flow out of sand and cavity formation

photo : Cave-in due to collapse of cavity
Figure 3: Cave-in due to collapse of cavity

The dynamics of underwater liquefied sediment flow and its implications in seabed processes

The purpose of this research project was to develop a numerical model for predicting the flow dynamics of liquefied granular soil in coastal oceans. Specifically, it proposes a workable analysis procedure that can describe the mobility and internal flow structure of the liquefied soil with consideration of the process of solidification that should occur in the flowing sediments.

In order to describe the propagation of solidified zones in the course of subaqueous liquefied sediment flow, we combined a system of Navier-Stokes equations with a consolidation equation in a consistent manner, and solved the entire equations using the method of finite differences under moving boundary conditions. Due consideration was given to 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 (Fig. 4).

graph : Predicted internal flow structure of a body of liquefied granular sediment
Figure 4: Predicted internal flow structure of a body of liquefied granular sediment

Laboratory Website

http://rcfcd.dpri.kyoto-u.ac.jp/cse/index.html