Short-term coastal morphodynamics simulation using wave phase-resolving and phase-averaged models.
Authors: Gumbira Gugum , Li Ming 
Published: March 2024

Understanding coastal morphodynamics is crucial due to the growing human activity in the coastal regions. This study compared wave phase-averaged and phase-resolving models to simulate short-term morphological changes. Both models were validated against the LIP1C experiment and then applied to simulate bar movement on the microtidal coast of Indonesia. The wave phase-averaged model used in this study was developed based on the FVCOM model, while the phase-resolving model was built by combining FUNWAVE and 1DV models. Both models were modified to take into account the wave skewness and asymmetry effects on the near-bed sediment transport.In the LIP1C experiment, the models were used to simulate a 201 m long open channel flow with 4.10 m depth. Irregular waves with a height of 0.60 m and an 8 s period were imposed at the offshore boundary, representing calm to moderate conditions. The simulation was conducted for 18 hours to identify model performance in producing onshore bar movement. The wave height, wave set-up/set-down, and currents magnitude produced by both models showed satisfactory results with a Normalised Root Mean Square (NRMS) value less than 0.10 for nine points measurement. Onshore bar movement was reproduced by the wave phase-averaged and phase-resolving models. The former model under-predicted the total transport rate values, while the latter model showed a similar order of magnitude against the experiment, leading to better agreement of bar growth. Two crucial factors in the onshore sediment transport were wave skewness and asymmetry. The wave skewness and asymmetry effects in the phase-averaged model were obtained by using the empirical formula. The phase-resolving model obtained better results because these factors were directly solved in every time step. The Brier Skill Score (BSS) were 0.78 and 0.83 for the wave phase-averaged and phase-resolving models.The models were used to simulate wave-induced sediment transport on the microtidal coast of Indonesia. It was a semi-enclosed bay in the northern part of Java Island with waves and currents entering from the North. Bathymetry data analysis showed a bar position located around 45 m from the coastline. A bottom-mounted Acoustic Doppler Currents Profiler (ADCP) was placed around 500 m from the coastline. The instrument was used to collect waves and currents data for ten days and to validate model results. Model results, such as significant wave height, surface elevation, and currents magnitude, showed good agreement against measurement. The NRMS values were less than 0.15 and 0.10 for phase-averaged and phase-resolving models. During the ten-day measurement, the hydrodynamics condition at the surface level was dominated by 0.25 m/s south-directed currents that moved sediment materials from offshore to onshore. As a result, the total transport rate was dominated by near-bed transport rather than suspended load transport. The bar moved near the shore, around 35 m from the coastline. The bar movement in this simulation was reproduced by both models, with the phase-resolving model showing better results.