In Situ Observations of Ripples on the Surf Zone of a Beach

Abstract

Ripple migration was observed on the surf zone of a beach in the eastern coast of Korea. A 3D sonar sensor and a current meter were placed together on a sand bar in water depth of 3.4 m for about 10 days in the spring of 2017. Bedloads were calculated from the records of ripple geometry and migration and were compared with estimates from a sediment transport model (SEDTRANS96). The ripples observed ranged in height (Hr) and length (Lr) from 0.5 to 8.9 cm and from 11.4 to 125.3 cm, respectively. They were highly dependent on the wave height (Hs) and thus wave-induced current speed (Uc). The largest ripples (Hr = 3.64 cm, Lr = 51.43 cm) occurred during the arrival of waves of 1 m height, the highest waves in the measurement period. At this time, the current and ripple migration speeds (Uc and Ur) were 4.08 cm/s and 17.13 cm/h, respectively. Therefore, the bedload was estimated as 5.785 kg/h/m. However, as Hs decreased to 0.5 m, the size and Ur of the ripples and Uc decreased simultaneously. At the wave height of 0.5 m, the values of Hr, Lr, Uc, and Ur were 1.97 cm, 18.82 cm, 2.72 cm/s, and 0.26 cm/h, respectively. The resulting bedload was estimated as 0.047 kg/h/m. A comparison of the measured and modeled bedloads indicated a model overestimation by a factor of 6-8. The model used the default values of 1 cm and 10 cm for ripple height and length, respectively. The rather large discrepancy may have been derived mainly from inaccuracy in estimating bed shear stress as well as ripple geometry. Therefore, this study emphasizes the importance of in situ ripple observations to place reasonable constraints on modeled bedloads.