Height of the Upper Limit of the Backshore on the Sea of Japan Coasts(<Special issue>Depositional process and Sedimentary environments)

The upper limit of the backshore is given by the base of sea cliff, the seaward end of coastal dunes, or the seaward limit of vegetation colonization. These locations coincide with the upper limit of the swash of storm waves. This paper reports that (1) the position of the upper limit of the backshore is stable, though the magnitude of waves during each storm event differs considerably, (2) the height of the upper limit of the backshore, BH, increases with increasing grain size of beach material, D, and (3) BH on the Sea of Japan coasts is smaller than that on the Pacific coasts. Storm waves transport beach material offshore and form an inner bar in the surfzone as a temporal sediment reservoir. The inner bar gradually migrates onshore under post-storm wave conditions, and welds onto the beach face forming a berm. When the inner bar exists in the surfzone, waves break on the seaward slope of it and flow shoreward as a bore to lose much of their energy; therefore the shore break height, H_b is reduced. That is, the inner bar has a wave filtering effect by acting as a submerged breakwater. Since higher storm waves break further offshore on the seaward slope of the inner bar, they must travel a long distance as bores to reach the shore, losing much more of their energy. Thus the inner bar acts to reduce the temporal variability in H_b. The water depth at a step base also influences H_b. The step-base depth, h_s, increases with increasing wave dimension as long as the waves fall in the formative condition for step morphology. If the wave magnitude exceeds this condition, the step is destroyed. Therefore, the maximum h_s appears just before step destruction, at which the maximum H_b occurs. It is reasonable to surmize that the maximum possible H_b for a given beach is constant, even if each storm has different incident wave heights in the offshore region. Because of less significant temporal fluctuations in H_b, there is no considerable difference in the uprush limit of waves for each storm event. This position is the landward limit of the backshore, which is fairly stable. The morphology of the landward zone from this position rarely changes, and the zone is covered by coastal plants which have a high salt tolerance, e.g. Elymus mollis, Carex kobomugi, and Vitex rotundifolia. The colonization of the vegetation assists in stabilizing the zone. On a fine sand beach, (1) an inner bar is likely to develop in the surfzone, (2) h_s is small, and (3) the step readily diappears in the early stages of storm wave attack; consequently, H_b during storm events is greatly reduced. On a coarse sand beach with a deeper step base, on the other hand, an inner bar is not likely to be formed, and the step is maintained until higher storm waves occur. As the result, the maximum possible H_b on a coarse sand beach is higher than that on a fine sand beach, and BH thus increases with increasing D. Since outer bars also work as a wave filter during storm events, BH decreases with increasing the number of outer bars. Most Japanese beaches facing the Sea of Japan have two or three outer bars. On the other hand, most of the Pacific beaches in Japan have only one outer bar. Consequently, there is a tendency that BH on the Sea of Japan coasts is smaller than that on the Pacific coasts.