Tide–Surge Interactions in Lingdingyang Bay, Pearl River Estuary, China: a Case Study from Typhoon Mangkhut, 2018

There is a need for accurate estimates of extreme sea levels for use in coastal engineering during typhoon seasons. Therefore, a numerical study was carried out to investigate tide–surge interactions induced by Typhoon Mangkhut in the Pearl River Estuary. A nested model was first validated and then used to study the spatial and temporal behavior of the storm surge and tide–surge interactions in the Pearl River Estuary during Typhoon Mangkhut (2018). The numerical results showed that the typhoon could induce nonlinear oscillations with amplitudes of approximately 0.3 m in the bay, which were caused by tide–surge interactions. Thus, we investigated the temporal and spatial features of the nonlinear oscillations and tracked their origin. First, for most stations inside Lingdingyang Bay, the peak of the nonlinear oscillations did not coincide with the highest stage of the surge and instead occurred afterward. Spatially, much larger nonlinear oscillations occurred at the top of the bay than near the mouth. Second, through a series of sensitivity experiments that involved translating the original typhoon landfall process forward and backward relative to the tide within a limited timeframe, the nonlinear oscillations showed tidally influenced characteristics. Specifically, there tend to be positive oscillations at low tide and negative oscillations at high tide, which explains the fact that surge is always enhanced (or weakened) under low (or high) tide. In addition to the phase difference between tide and surge, we also tested the influence of tidal range on the nonlinear oscillations. The results showed that the strength of the nonlinear oscillations in the bay was proportional to the tidal range under the same meteorological conditions. Finally, we conducted additional experiments by altering terms in primitive movement equations to weigh the importance of the origins that produce the nonlinear oscillations in terms of hydrodynamic mechanisms. The results indicated that nonlinear bottom friction was the major factor causing significant nonlinear oscillations, accounting for nearly three-fourths of the total. The next most influential factor was the shallow water effect, accounting for the other one-fourth of the total. The advective term showed little effect on the nonlinear oscillations in the open bay of the Pearl River Estuary.