The present study describes the active role of met-ocean parameters on the heavy flooding that occurred during July 2018 in the Kuttanad region, Kerala. The model-derived data as well as observations showed a high anomaly of wave height in July 2018 off Kerala was not correlated with local wind anomaly and indicated possible connection of wave anomalies to swell waves. The high period and high amplitude swell waves suggest a wave setup formation near shore for flooding intensification in the Kuttanad region. The wave observation at an open ocean location (AD09) also confirms the presence of high swell waves during the same period. Anomalous elevated non-tidal sea level was evident in the tide gauge data. The narrow opening at Thottappally, is not able to discharge the outflow of the heavy flood water and it is further complicated by the wave setup at the entrance caused by long period swell waves.

Identifying coastal stretches that are likely to erode under high wave conditions is essential to the coastal management community in addressing beach erosion-related issues. The manuscript presents a case study to predict erosion of a selected beach located on the eastern Indian coast during the 2018 southwest (SW) monsoon season. The pre-monsoon beach topography surveyed using DGPS is used as the initial topographic condition in the morphological model. The model simulates subsequent beach erosion during the SW monsoon season, which is forced by forecast waves. Beach erosion advisories that classify the beach as eroding, accreting and no change are disseminated with a lead period of 10 days. During the SW monsoon period, we have monitored the beach using the dumpy level, and the beach topography during the postmonsoon is generated using DGPS surveyed profiles and are used for model validation. The beach area under erosion observed from the model result is homologous with the in-situ observations. The model shows cross-shore sediment transport to dominate during the SW monsoon, particularly in the central and northern sectors of RK beach. The model estimated shoreline position agrees with the DGPS observations, where the central sector has undergone maximum erosion. The comparison shows the model’s ability to simulate beach morphodynamics like landward shifting of the berm crest under a high wave setup and oscillating water levels due to tides. The study highlights the importance of numerical modelling systems that decipher the beach response to the high monsoon waves.

$\bf{Highlights}$

$\bullet$ Beach erosion forecast system for RK Beach, India.

$\bullet$ Forecast system is developed using a combination of WAVEWATCHIII and Xbeach.

$\bullet$ Beach erosion advisories during monsoon period.

The present study highlights the wave–tide interactions in a tide-dominant coast Versova, along the west coast of India. Versova is a macro tidal area and is home to fisheries. Model simulations are carried out to investigate wave–tide interactions with and without incorporating water level (WL) variations in the model setup. The simulation results are compared with the observed data at Versova. Model comparison with wave observation shows that the simulated significant wave height (Hs) reproduced the observed wave heights with an accuracy of scatter index = 8% and correlation = 0.94 with the inclusion of WL variations. The incorporation of WL variations created the energy modulations in the low-frequency part of the wave spectra, raising the periodical modulations in wave height. This low-frequency wave energy modulation is absent in the without WL simulations, resulting in underestimation of energy density which causes underestimation of Hs by ${\sim}$1 m. Hence this study strongly suggests that water level variations must be incorporated into the wave model to accurately represent wave modulations which are significant during monsoon and extreme events in the tide-dominant coastal areas.

This study assesses the performance of different source term packages of WAVEWATCH III (WWIII, V-6.07) wave model for various wave conditions in the Indian Ocean (IO). Eight simulations of WWIII were made for the year 2017, four using default source term packages (ST2, ST3, ST4, and ST6) and another four by tuning the wind–wave interaction parameter (${\beta}$) in the ST4 and ST6 schemes. The simulated wave outputs are compared with in-situ and altimeter wave fields over a wide range of weather conditions. All wave simulations have significant errors in low wind speeds (e.g., in pre-monsoon season) compared to medium (e.g., post-monsoon) and strong (e.g., monsoon season) winds which is independent of the error in the forecast wind. Overall, the ST4 scheme reproduces well the wave characteristics in all seasons and different conditions of IO, while the ST6 scheme is best suited for cyclonic weather conditions for wave simulation. Changing the WWIII parameterization schemes based on wind conditions is not a practical option for a timely wave prediction for a basin like the IO, where varied wave patterns exist year-round, owing to the full spectrum of wind conditions. Instead, this study advocates selecting a scheme that works well in all conditions, like ST4, and tuning it to suit well in cyclonic conditions.