Field observations were conducted in the swash zone at a dissipative barred beach to investigate the characteristics of the water content in the swash zone sand layer and its relationship with ground level change. The datasets were separated based on tidal motion, and selected high tide cases were analyzed in detail. In the analysis, each tidal cycle was divided into four phases based on the water content in the upper sediment layer (approximately 5 cm below the ground surface). Regarding the correlation characteristics between the tide and water content of the upper sand layer, the correlations were negative from low tide until before the rapid increase of water content (Phase 1). Once water reached the sensor, water content significantly increased, and the correlation became positive (Phase 2). During the high tide, the water content remained constant (saturated), therefore, no relationship was observed between the two (Phase 3). During the ebb tide, the water content gradually decreased, and again a positive correlation was observed (Phase 4). The correlation characteristics between the water content of the upper sand layer and the vertical water content gradient were positively correlated for Phase 1 and negatively correlated for Phases 3 and 4. The factors that caused the ground level changes for Phases 2 and 3 were examined. The results revealed that for Phase 2 when the water content rapidly increases, the ground level change was affected by the initial ground level and wave energy flux in addition to the water content change in the sand layer. For Phase 3 when the water content remained constant, the ground level change was affected by the wave energy flux and the initial ground level. It became clear that the external forces contributing to topographic changes vary with the tidal phase.
Suzuki et al. (2026) studied this question.