Laboratory experiments using a fiber-optic LDV system and a small pressure transducer have been made to reveal detailed characteristics of the velocity field in the surf zone and its relationship to the sediment suspension with special reference to the three-dimensional large scale eddies referred to as "obliquely descending eddies", the existence of which was recently revealed by Nadaoka (1986). A conditional sampling technique has been used to find that the obliquely descending eddies bring highly intermittent intensive turbulence to the bottom with the large onshoreward momentum at the upper layer of the water and thus essentially characterize the turbulent flow field in the surf zone. Visual observation and concentration measurements, especially a coherence analysis of two data sets of concentration close to the bottom, have shown that the sediment suspension is mostly governed by such large scale eddies in a wide extent of the surf zone; i.e., the eddies hit the bottom and then lift up the sediment into suspension, yielding the spot-like sediment cloud in accordance with the three-dimensional eddy structure.
As a water quality purification measure for areas such as lakes, a sand capping method by sand-upwelling system was developed. This method involves welling up clean sand from below contaminated sludge by using a water jet. The disposal of sludge and the acquirement of pure sand are not necessary like dredging and conventional sand capping works. Besides, the ecosystem will hardly be disturbed by this method. A scale model test and field tests at Lake Suwa were carried out to study sediment purification effects, sand capping conditions and turbidity range. As a result, it was demonstrated that sand capping was succeeded by this method. Some characteristics of this method were revealed, the thickness and the area of the sand capping depended on the jet discharge rate and the sand diameter, the turbidity range was very small caused by this method. After seven months later, the field observation was carried out in Lake Suwa. The thickness of the sand capping layer and the aerobic environment were maintained. Also, many Limnodius appeared in the sand capping area.
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