National Institute for Land and Infrastructure Management, Ministry of Land, Infrastructure and Transport Japanese Page
River Department

Research themes of the Coast Division
[1] Beach process

2. Reversible and irreversible sand beach variation

1. Reversible sand beach variation

As shown in Figure 1, a sand beach undergoes a repeated process of longitudinal cross-sectional variation. At a natural sand beach, a berm is often formed in the foreshore due to the effects of calm waves (Figure 1(a)). However, when waves gradually become higher as an atmospheric low pressure or a typhoon approaches, the berm is eroded and the bottom sediment is transported offshore where a bar is formed (Figure 1(b)). As the waves become even higher, the berm erosion progresses, and the bar develops and moves offshore (Figure 2(c)).

When developed to the level corresponding to the wave properties, the bar breaks high waves and the topography from the bar to the coast becomes stable (Figure 1(d)). Subsequently, when the waves gradually become lower after the low pressure system or typhoon passes, a berm starts to be formed in the foreshore and the bar moves coastward (Figures 1(e) and 1(f)). Then, when the waves calm down, the bar moves further coastward with the head bent forward, and a shoal is formed slightly offshore from the shoreline (Figure 1(g)). Finally, a berm develops as the bottom sediment is washed up from the shoal to the foreshore due to the action of calm waves (Figure 1(h)).

A natural sand beach undergoes repeated longitudinal cross-sectional variation as shown in Figure 1, corresponding to the scale of waves (reversible sand beach variation). Foreshore erosion and bar formation are considered to occur over a short time, whereas bar destruction and foreshore restoration occur over a relatively long time. Such reversible sand beach variation can actually be observed at ordinary coasts.

Figure 1 Conceptual drawing of reversible sand beach variation

2. Reversible and irreversible sand beach variation-a case example of Kaike Beach in Tottori Prefecture

In the area where a natural sand beach remains at Kaike Beach, the longitudinal cross section in July 1995 was a gently sloping configuration with almost uniform gradient as shown in Figure 2(a). Subsequently, two bars were formed in March 1996 due to the action of wintertime waves. Then, the bars were destroyed in July 1996, troughs then were filled back, and the longitudinal cross section returned to almost the same configuration as that in July 1995. In addition, reversible variation of longitudinal cross section took place from July 1996 to July 1997 as shown in Figure 2(b).

Nevertheless, in the period between July 1997 and March 1998 as shown in Figure 2(c), the section between P1 and P2 was greatly eroded, from where sediment was transported offshore, and a large bar was formed in the section between P2 and P3. Almost no variation of the longitudinal cross section was observed until July 1998, and the large bar remained until July 1999. Thus, repeated reversible variation of the longitudinal cross section was observed from July 1995 to July 1997, but irreversible variation took place from July 1997 to July 1999.

Observation data of Kaike Beach show that stormy waves exceeding H1/3 = 3.0 m significant wave height and T1/3 = 9.0 s significant wave period hit the area three times in February 1998, which was within the period from July 1997 to March 1998 (almost all the data for 1997 are missing). In particular, stormy waves of H1/3 = 3.68 m significant wave height and T1/3 = 9.4 s significant wave period hit the area on February 21, 1998.

Thus, when a bar that has formed due to the action of stormy waves remains constantly, waves of a height that do not break on the bar directly act on the foreshore unless the trough on the coast side becomes shallower. Should this condition continue for a long time, erosion at the foreshore section will progress, eventually damaging the sand beach. Accordingly, when the foreshore is eroded and a large-scale bar is formed offshore due to the action of stormy waves, it is crucial for evaluating the stability of the sand beach as a coastal conservation facility to judge whether the variation is reversible or irreversible.

Figure 2 Reversible and irreversible sand beach variations at Kaike Beach

The Coast Division researches methods for evaluating the stability of sand beaches by conducting experiments using large-scale two-dimensional water channels to reproduce the actual sites as exactly as possible.