INTRODUCTION

Ocean engineering has been developing with exploration and utilization of ocean resources. In the last 20 years, ocean engineering has move forward on an unprecedented scale with the development of the offshore oil and gas industry. In most cases seafloor sediments support much more loading than on-land soils because of the dynamic environment of the ocean and the special requirement of seafloor structures. In ocean engineering construction serious accidents often happen as the result of foundation failure. Based on data for the bathymetry, shallow seismic profiling and soil sampling with laboratory investigations, we report briefly here on the instability of the seabed and substrate in this area.

Moveable And Variable Bedforms

The development and variation of bedforms are closely relevant to environmental protection and various aspect of ocean engineering. Moveable and variable bedforms result in sand accumulation or erosion and can cause seabed pipelines to lose their bottom support and threaten the safety if shallow foundation structures on the seafloor.

Fig 1 Distribution of the bedform morphologies I, well-developed megaripples; II, partially developed megaripples; III, sandwaves (available in full paper)

Fig 2 Sidescan sonar record showing well-developed megraipples on the seafloor (available in full paper)

Fig 3. Sidescan sonar record showing sandwaves on the seafloor (available in full paper)

It has been detected that the bedform morphologies occupy and area of about 20 000 km2 in the northern East China Sea shelf (Fig 1) (Ye et al., 1983). The transverse bedforms found on the shelf can be divided into megaripples and sandwaves (Fig 2 and 3). The megaripples are most widespread and have waveheights less than 1 m, commonly 0.3-0.6 m, and waveheights vary from 2.3 to 13.6 m. the sandwaves are only developed in the relict sediment area off the Changiang River mouth, with heights from 0.5 to 2.6 m, generally over 1 m, and wavelengths from 70 to 1265m.

From the measurements in different years and analyses of the asymmetrical profiles of sandwaves, it is concluded that the sandwaves are moveable and variable. As an example, it has been estimated that the sandwave around the area of 123°E, 32° N have migrated seaward 6.88 km in a five-year period. The migration rate is 21 times greater than the migration rates in Chesapeake Bay (Swift et al., 1972). Relief less than 1 to 2 m, such as the sandwaves and megaripples. Nevertheless for moveable and variable bedforms it is important to investigate in more detail their shapes and sizes, their movement and the environmental conditions, prevailing diving formation, and to find ways to deal with such situations.

Open And Buried Channels

The most characteristic feature of the shallow stratigraphy in the East China Sea is the series of paleochannels, presumably representing the Late Pleistocene to Early Holocene drainage system. The paleochannels found in the continental shelf can be divided into relict and buried paleochannels. The former retain and buried paleochannels. The former retain their initial V-shape valley on the seafloor and can be delineated mainly by the data from echosounding and sedimentology.

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