| 概要 |
In view of the actual status of coastal dune fixation in our country, the present paper mainly reports the basic research result through wind tunnel experiment for the purpose of aiming at rational bl...own sand control. First of all, the actual status of blown sand damage and its prevention works is introduced, and it is pointed out that to illucidate the blown sand phenomenon in sandy plain mainly such as artificial sand dune and also the interrelation of said phenomenon with water content of sand surface layer is of utmost importance for blown sand control. Secondly, as establishment of measuring method is essential in experimental illudiation of these subjects, the measuring device of water content of sand surface layer utilizing light reflection and the device counting the number of blown sand using laser were trially produced to make clear the characteristics and the range of application. In dealing with the blown sand phenomenon, the sand movement in dune sand of wind dry condition as its base and also the relation of the threshold velocity of blown sand with the grain diameter distribution of sand dune are considered to be unclear in the conventional researches, and therefore the blown sand quantity formula in consideration of the grain diameter of sand dune was determined through demensional analysis and wind tunnel experiment. From this formula the writer could present the method of objectively judging the blown sand limit, ascertain its effectiveness, thereby making clear the interrelation between the grain diameter of dune sand and the blown sand limit. Furthermore, from on-the-spot survey and analysis of the relation of geographical features mainly in artificial sand dune with grain diameter distribution, and also from the wind tunnel experiment between errosion and grain diameter change, it was made clear that wind erosion and coarseness of grain diameter of dune sand are conspicuous in the land behind high artificial sand dune and that the land just behind artificial sand dune becomes the starting point of new blown sand toward internal land. As both the sand movement of size-group and the threshold shear velocity of size-group, which are the cause of coarseness of grain diameter, have not been illucidated as to the subject of blown sand, the dimensionless equation for it has been looked for and obtained. In order to clarify the influcence of water content of sand surface layer on the blown sand quantity and the threshold velocity of blown sand regarding the blown sand in the moistured sand surface condition, the influence of water content ratio was grasped as the change of coefficients respectively of the blown sand quantity equation in wind dry condition and of the dimensionless equation in threshold shear velocity, and its relation with water content ratio was obtained through experiments. The influcence of water content ratio on vertical distribution of blown sand was also expressed by the relation of dimensionless coefficient with water content ratio, thereby making it clear that the blown sand quantity, while being uniformed in vertical direction, takes a special reduction configuration in inverse proportion to the increment of water content ratio, which attributes mainly to the saltation of blown sand. The relation of water content ratio with grain diameter of blown sand was clarified by an index called the blown sand ratio of size-group in which both the median diameter of blown sand and the standard deviation tend to get bigger in proportion to water content ratio, and also the sand of larger size-group tends to occupy a large portion of sand composition. Based on the aforementioned respective results, it was discussed as its conclusion that contriving " uniformity of grain diameter and surface uniformity of geographical features " is effective as a conceptive guidance relating to the blown sand control.続きを見る
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