Position and Height of the Landward Limit of the Backshore on Naka Beach, Ibaraki, Japan

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  • 茨城県那珂海岸における後浜上限の位置と高度
  • イバラキケン ナカ カイガン ニ オケル ゴ ハマジョウゲン ノ イチ ト コ

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Abstract

The landward limit of the backshore is given by the base of sea cliffs, the seaward end of coastal dunes, or the seaward limit of vegetation colonization. It is considered that these locations coincide with the upper limit of the swash of storm waves. Since the magnitude of waves during each storm event differs considerably, it is anticipated that the stability of the backshore limit is not secure and that its height can vary. Using data on beach profiles and waves obtained from a Pacific beach in Japan, this study examines these points.<br> The field investigation was carried out at Naka beach, Ibaraki (Fig. 1). Vegetated sand dunes 200 to 300m wide develop behind the beach. An outer bar is always present, but an inner bar only develops under certain wave conditions in the nearshore zone. Three monitoring sites were selected: N site (mean grain size of beach sediment D=0.76mm); C site (D=0.66mm); and S site (D=0.26mm) (Fig. 2). Beach profile surveys were conducted at each site for a period of two years beginning August 28, 1980. Daily averages of deep-water significant wave height and period are shown in Fig. 3. No significant along-shore difference in incident wave characteristics was observed. The coast is situated in a microtidal environment with a mean tidal range of 1m.<br> At all 3 monitoring sites, the seaward limit of dune grass colonies was characterized by a specific topographic break in the beach profile. It was found that this position rarely shifted during the two-year survey period, although many storms of different magnitude occurred (Fig. 4), and that the position corresponded almost completely to the landward limit of beach change due to waves (Fig. 4). Therefore the position can be used as the landward limit of the backshore, which was fairly stable. This stability suggests that there is no significant difference in the uprush limit of waves in each storm event. This is probably due to the lack of significant temporal fluctuations in breaker height at the shoreline during storm events, as mentioned below.<br> Waves break on the seaward slope of an inner bar and flow shoreward as a bore, dissipating their energy by viscous damping and bottom friction. As a result, the shore wave-break height is lower than in cases where there is no inner bar occurrence. That is, the inner bar has a wave filtering effect by acting as a submerged breakwater. During storm events (Fig. 5), an outer bar works as an effective wave filter along with the inner bar, because the water depth of the outer bar relative to the breaker depth of larger storm waves becomes smaller. Storm waves are therefore affected by the doublewave filtering effect until they reach the shoreline. Since higher storm waves break further offshore on the seaward slopes of the outer and inner bars, they must travel a longer distance as bores to reach the shore. Consequently, higher storm waves lose much more of their energy in comparison with lower storm waves; therefore the breaking height at the shoreline of the storm waves becomes much smaller. Thus the nearshore bars act to reduce the temporal variability in the shore break height during storm events.<br> The shore break height is influenced by the water depth at a step base. The step-base depth 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 possible step-base depth appears just before step destruction, after which the maximum shore' break height occurs. Since the wave condition for step formation does not vary unless the grain size of beach material changes, it is reasonable to surmise that the maximum possible shore break height for a given beach is constant, even if each storm has different incident wave heights in the offshore region.

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