Abstract
The purpose of this paper is to measure daily changes of the longitudinal profiles of the beach. The investigation was done at Hiratsuka Beach in Kanagawa prefecture during the period from Aug. 22, 1968, to Aug. 29, 1968. The seven measuring lines of the longitudinal profiles were set up at the interval of 150m. to 290m. along the shoreline.
Along the measuring lines, the highest points reached by sea-water in Aug. 21 were determined, and timbers (3.7×4.4×180.0cm) were hammered into the ground to a depth of about 1.5m. at the interval of 2m. from each highest point to the sea. These stakes were called measuring points, and the heights of these stakes were measured at 8, 12, and 16 hrs.
The result of the measurement is as follows:
The vertical change (cm/4h) of the measuring points tends to increase from land side to sea side. Thus, it is shown that difference of vertical change at the measuring points is caused by the different scalc of tractive force which exerts on the beach-face. The tractive force (τ0) is given by the following formula:
τ0=wRI where R is hydraulic radius and I is gradient.
Then, the domain of R and I which determines τ0 is examined at the measuring points. As a result of the examination, gradient (I) has no differences among the measuring points. Accordingly, it is supposed that the difference of the vertical change (cm/4h) at the measuring points is determined by the difference of the hydraulic radius (R).
