Monitoring of Mega-Cusp Formation During the Northeast Monsoon Period on My Khe Beach, Vietnam

My Khe Beach, Da Nang, Vietnam, has experienced a dramatic transition from accretion (pre-2015) to severe erosion since 2016,
with shoreline retreat reaching −42.98 m during the 2024–2025 Northeast (NE) monsoon. This study investigated mega beach cusp formation
and its role in accelerating erosional processes. This study analyzed 57 unmanned aerial vehicle surveys (2022–2025) and 385 satellite images,
integrated with wave reanalysis and tidal data. Empirical orthogonal function (EOF) analysis revealed dominant shoreline variability patterns
and their hydrodynamic drivers. The results showed pronounced seasonal oscillation: erosion during the NE monsoon (October–April) and
accretion during the Southwest (SW) monsoon (May–September). EOF Mode 1 (81.1% variance) captured this large-scale seasonal signal. Mega
beach cusps, the primary erosional feature during NE monsoons, exhibit exceptional dimensions: 200–500 m spacing and amplitudes reaching
57.6 m. EOF Modes 2+3 (~7% variance) isolated these cusp structures (~400 m wavelength). Threshold analysis identified critical cusp
initiation conditions: instantaneous wave height exceeding 0.810 m or 7-day cumulative wave power exceeding 29,466 W·day/m, indicating a 5–
7 days morphological response timescale. The erosion of My Khe reflects seasonal oscillation superimposed on erosion trend of −0.39 m/year
(2016–2025), contrasting with progressive sediment deficit at nearby beaches (e.g., Cua Dai), underscoring the influence of high-energy
mega-cusp morphology on seasonal dynamics.