Abstract: The morphology of underwater sand dams on nearshore sandy beaches under the action of waves is discussed, and the viewpoint that underwater sand dams are formed by wave breaking and can develop above the water surface in previous literature is questioned. The different locations of underwater sand dams can form three basic forms of sandy coastal profiles. As for the process of scouring sand beaches and underwater sand dams emerging from the water surface, the elliptical cosine wave theory is used to analyze and explain. Due to the asymmetry of the elliptical cosine wave, especially the asymmetry of the bottom flow velocity, underwater sand dams are gradually pushed upward from a distance away from the shore, approaching the shore, and eventually developing into coastal sand dams emerging from the water surface. By comparing the calculated and experimental values of the elliptical cosine wave with the field measured values, it is proved that the elliptical cosine wave theory is the most consistent wave theory with the long-period surge in the coastal zone after a storm.