Numerical investigation of excessive surge induced by wave overtopping in an inlet-bay system

During Hurricane Sandy, wave overtopping occurred along the barrier beach in front of the Rehoboth Bay, one of the Delaware Inland Bays, as indicated by field observations and the morphological profile model CSHORE. CSHORE estimated a total overtopping volume of about 9.7 ×107m3 which is close to the storm surge-induced water volume entering the bays through the inlet (about 1.3 ×108m3 ). In this study, we incorporated the wave overtopping result from CSHORE into the nearshore community model, NearCoM-TVD, to investigate the wave overtopping-induced excessive surge (OIS) in the Delaware Inland Bays. With the wave overtopping applied in the flux boundary condition, the model successfully reproduced the high water level measured at the station located in the Rehoboth Bay. Additional numerical experiments for the separate effects of wave forcing and local winds are also carried out. The results show that wave forcing has a minimal effect on the water level inside the bays. The adjustment of local wind forcing does not qualitatively improve the agreement between the model and data. The model with included wave overtopping predicted an excessive surge of about 20 cm in the Rehoboth Bay, agreeing better with the measured data. Finally, the responses of OIS to different overtopping conditions were performed using an idealized model setup. A strong asymmetry, represented by a much longer draining period versus a shorter period of overtopping increase to the OIS peak, was found in the OIS process for a given temporally symmetric time series of overtopping flux. The asymmetric feature is caused by the imbalance between the overtopping flux and the discharge through the inter-connecting channel inside the Inland Bays. Nonlinearity plays an important role in the process of OIS as demonstrated in modeling OIS-tide-surge interaction.