In situ observations of ripples on the surf zone of a beach

Abstract

The ripple migration was observed on the surf zone of a beach in the eastern coast of Korea. A sonar sensor logging ripples and a current meter were placed together on a sand bar in water of 3.4 m for about 10 days in the spring of 2017. Bedloads were calculated from the records of the ripple geometry and migration and were compared with the estimates of a sediment transport model, SEDTRANS96. The ripples observed ranged in height (Hr) and length (Lr) from 0.5 and 8.9 cm and from 11.4 and 125.3 cm, respectively. They were highly dependent upon the wave height (Hs) and thus wave-induced currents (Uc). The largest ripples (Hr = 3.64, Lr = 51.43 cm) occurred during waves of 1 m, the highest waves in the measurement period. In this spell, the current and ripple migration speeds (Uc and Ur) were 4.08 cm/s and 17.13 cm/h, respectively. Therefore, the bedloads were estimated at 5.785 kg/h/m. However, as Hs decreases to 0.5 m, the size and Ur of the ripples as well as Uc all together decreased simultaneously. At waves of 0.5 m, the values of Hr, Lr, Uc and Ur were 1.97, 18.82 cm, 2.72 cm/s, and 0.26 cm/h, respectively. The resulting bedloads were estimated at 0.047 kg/h/m. The comparison of the measured and modelled bedloads indicates an overestimation of the model by a factor of 6-8. The model used the default values of 1 and 10 cm for the ripple height and length. This rather large discrepancy may be derived mainly from