1996 Fiscal Year Final Research Report Summary
A STUDY ON WAVE-INDUCED LIQUEFACTION OF SANDY SEA BED AND SEDIMENT TRANSPORT
| Project/Area Number |
06650577
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
水工水理学
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| Research Institution | NIHON UNIVERSITY JUNIOR COLLEGE |
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Principal Investigator |
MAENO Yoshihiko NIHON UNIV.JUNIOR COLL., PROFESSOR, 教授 (90110139)
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| Project Period (FY) |
1994 – 1996
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| Keywords | Liquefaction / Sediment transport / Seabed / Coastal Environment / Measurement / Drifted Sand / Effective Stress / Pore Pressure |
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| Research Abstract |
The effective stress and pore pressure in the surf zone were directly measured to study the relation with the stress, and the wave condition and other factors, and to evaluate the bottom elevation change from the effective stress data. From the careful observation and analysis of the measured data during this period the followings are found.
(1) The pore pressure and effective stress appears to respond to the water surface elevation without any time lag. The measured area is always attacked by breaking waves and the sand there is anticipated to be loose. Then, the pressure on the bottom transmits almost simultaneously to the pore presssure inside the soil. Both the pore pressure and effective stress show larger variation when the short waves with large wave steepness pass.
(2) The mechanism of liquefaction in the surf zone seems to be different from that outside the surf zone. The large effective stress drop and strong flow in the onshore direction when the wave crest passes are the possible cause for the sand movement in the surf zone.
(3) The water depth change estimated from the effective stress records agrees well with the measured depth change. This leads to the following three conclusions (i) The effective stress in sea bed is closely related to liquidization of sand and then to sand movement. (ii) The sea bottom elevation change can be evaluated from the effective stress in the sand. (iii) The effective stress sensor was proved to be useful in estimating local and temporal sediment transport.
(4) The currents going offshore were observed when the sea level was low. The effective stress varied corresponding to these currents. The averaged effective stress over 50 minutes drops sharply at low tide. The shallower water depth may be a possible cause, and the exact reason for this is still being studied. The ground water level near the coast is likely to affected by the long waves and tidal changes.
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