A Dispersive Long-Wave Model for Predicting Coastal Flooding Due to Storm Surges and Surface Waves in Manila Bay

Abstract

Manila Bay is a large shallow coastal water encompassing the urban areas of Metro Manila and various cities of sub-urban provinces in the Philippines. It is a relatively shallow semi-enclosed basin with an average depth of 20m whose coastal areas are crowded with residential, industrial, agricultural and aquaculture production. Its shallow depths imply that the effect of wind stress on sea level becomes appreciable in driving storm surges even during enhanced Southwest Monsoon and the passage of moderate storms. Using a dispersive long wave model coupled with the significant wave model of the Coastal Engineering Research Center (CERC), the occurrence of potentially devastating storm surge flooding around Manila Bay was numerically simulated. A unique characteristic of the new model is the inclusion of the dispersive terms in the associated momentum balance equations. Deep water gravity waves are always dispersive and inclusion of the dispersive terms is expected to provide more accurate modelling results.The predictive capability of the model was verified using observations during the passage of several storms including Typhoon Milenyo (2006) and Typhoon Pedring (2011). The occurrence of the anomalously high storm surge of about 2.5 meters during the passage of 2011 Typhoon Pedring far north of the area was correctly simulated. Numerical integration of the dispersive long wave model with the addition of higher order terms in the momentum balance, appears to give accurate predictions of the coastal flooding due to storm surges and waves. The hydrodynamic set-down which occurs in many coastal areas during strong typhoons can be simulated well by the model. A new empirical model for the hydrodynamic force exerted by the combined action of storm surges, waves and extreme currents is also presented. Initial calculations of hydrodynamic forces generated by an actual typhoon crossing Manila Bay are discussed.