Supermoon Drives Beach Morphological Changes in the Swash Zone

Abstract

The effect of the supermoon, which appears much larger than a normal full moon, on the morphological changes of a sandy beach was investigated by analyzing a 25-year daily observation data set of beach morphology in the swash zone. The beach morphology fluctuated in two cycles related to the supermoon: the semisynodic month cycle (from full moon to new moon, and vice versa) and the anomalistic month cycle (from perigee to perigee). The supermoon makes the erosion of the upper swash zone more likely by generating a larger tidal range. The high-water-level contour positions observed during supermoons retreated significantly, although there were no trends in the change for all the observations. Erosion due to high waves coincident with a supermoon can be expected to be more severe in the upper swash zone; including the supermoon effect in beach morphodynamic models can improve coastal management. Plain Language Summary A "supermoon" is a full moon that appears much larger than a normal full moon. It causes an unusually high tide known as a "king tide," and it increases the risk of coastal inundation. However, beach morphological changes induced by supermoons have not been investigated despite the effect of such changes on the vulnerability of coastal regions to inundation. Here, we show the correlation between the morphological changes (erosion and accretion) across the beach and the moon cycles using long-term beach observation data. Our results indicate that the supermoon increases the risk of more severe beach erosion near the shoreline. The erosion is caused by a larger tidal range resulting from a stronger lunar gravitational force. The findings emphasize the importance of understanding the extreme erosion and inundation caused by the supermoon effects, particularly when combined with high waves and storm surges.

Seasonal Variability On Cross-shore Profile in Meso-tidal Settings Due to Lunar Cycle Effects in Kuala Terengganu Coast

“…On Full Moon phase, the beach volume result showed a substantial gain in sediment during premonsoon compared to New Moon phases during pre-monsoon (Oct 20-Mac 21). Our ndings diverged from prior research, which associated beach erosion with Full Moon phase due to rise of wave energy and tidal ranges (Banno & Kuriyama, 2020b;Darsan, 2013). The interplay of these natural physical forces, such as waves, tides, and winds, alongside the seasonal monsoonal variations, underscores the complex and dynamic nature of coastal processes in the studied region.…”

“…Therefore, while intense volume of factors during one season is eroding the surface, inversely it is recovered (accretion) during the other calm season to regain sediment facilitated by wind action (Kaiser & Frihy, 2009;Quartel et al, 2008). Additionally, external factors, especially outside of the earth planetary system, such as lunar cycle is also factored to be accounted for morphology changes during monsoon variations (Banno & Kuriyama, 2020a;Pye & Blott, 2008;Ramos & Ranieri, 2021).…”

Reinterpreting the Bruun Rule in the Context of Equilibrium Shoreline Models

“…Recall that this is a theoretical scenario, in which all Bruun's assumptions are satisfied. In reality, such conditions are not granted, and other processes, such as variability in wave climate, may dominate the evolution of the beach when combined with SLR [46]. Applications of the (Y09 + PF)* to these cases are expected to capture interactions between waves and SLR and produce results that diverge from the Bruun model application.…”