Numerical investigation of the effect of wave diffraction on beach erosion/accretion at the Gangneung Harbor, Korea

Since Gangneung Harbor breakwaters were built in the east coast of Korea from 1992 to 2002, the shoreline inside the harbor had severely changed as it was accreted in the area inside the harbor but eroded further south during the observation period (1979–2005). We investigated this process using Telemac-2DH model to calculate flows, waves, sediments and morphological changes in unstructured grids. The performance of the model was validated using the experiment data by Graven and Wang (2007). The model was run in two cases by turning on/off the diffraction mode using the formula by Holthujisen et al. (2003). For this, we tuned the model by setting a shadow zone behind the breakwaters and by applying the diffraction formula only inside the shadow zone. The model results showed that wave heights significantly increased inside the shadow zone when the diffraction mode was on, compared to the case when the diffraction mode was off. This effect of diffraction was confirmed by the observations as the wave heights measured inside the shadow zone became 10–20% lower than those measured outside, which were nicely simulated by the model. In addition to the wave height, the wave-induced currents became stronger in the innermost area of the shadow zone with the diffraction mode. The model also successfully predicted the observed morphological change pattern because it simulated the shoreline erosion at the southern end of the shadow zone where the currents were bifurcated. Inside the shadow zone, sediment deposition occurred, corresponding to the observation, when the diffraction mode was on, whereas this deposition process was not simulated when the mode was off. The results support that the observed shoreline accretion in the harbor was mainly due to the reduced wave and current energy, which emphasizes the importance of accurate modelling of diffraction effect in the prediction of shoreline evolution.