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Coincidence of storm surges and river discharges due to typhoons in the Pampanga delta

Rot, H.J. aan het (2018) Coincidence of storm surges and river discharges due to typhoons in the Pampanga delta.

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Abstract:The Pampanga delta (Philippines) is due to its geographical location prone to typhoons which can result in extreme discharges and storm surges in Manila Bay. In most flood risk studies, river discharges and storm surges are considered independent, but if there exists dependence between storm surges and river discharges, this might have a significant influence on design levels and expected inundations. Previous studies showed the importance of taking into account the joint occurrence of storm surges and high discharges for different regions in the world, but the importance differs per catchment. In this study, the importance of taking into account the coincidence of storm surges and discharges in exposure and risk studies in the Pampanga delta has been explored. This study shows that there is an average time lag of 36 hours between the occurrence of storm surge and discharge peaks in the Pampanga delta, which seems to decrease when both peaks are more extreme. There is also a clear shift in the probability distribution of the storm surges during extreme discharge events in comparison with independent events, resulting in significantly higher storm surges during extreme discharge events. It was also shown that there is an increased probability of joint occurrence of extreme discharges and extreme storm surges in comparison with the independent probability. The effect of the joint occurrence of extreme storm surges and extreme discharges on inundations is investigated based on inundation maps of hypothetical scenarios with different combinations of storm surge, tide and discharge. With these scenarios, the importance of storm surge, river discharge, tide and the timing of those components relative to each other were investigated. The inundation maps are simulated by the hydrodynamic model Delft3D-FLOW. The forcing data that is used in Delft3D-FLOW consists of river discharges and wind and pressure fields that are derived from historical typhoon tracks from the Joint Typhoon Warning Centre (JTWC). The discharge input for the rivers is determined by hydrological simulations in wflow, which is a hydrological model developed by Deltares based on the PCRaster Python framework. The wflow model for the Pampanga has been calibrated using water level measurements by the Pampanga River basin Flood Forecasting and Warning Centre (PRFFWC) and a rating curve for the measurement station at Mount Arayat. The results of the hydrodynamic simulations in Delft3D-FLOW show that the inundation extent and depth are dominated by the discharge. But neglecting the joint occurrence of storm surges and high discharges (with both an estimated return period of five years) results in an underestimation of the inundations over a large area. The underestimation of the inundation depth reaches up to 30 cm in the north-western part of the Pampanga delta and more than 50 cm on a local scale in the surroundings of Manila Bay. Without the extreme river discharges, the simulated inundations are restricted to some parts in the surroundings of Manila Bay and some parts in north-western Manila. Furthermore, the results show that the timing of the tide with respect to the storm surge has a significant influence on the inundation depth over a large area in the Pampanga delta. Due to the uncertainties in the hydrological simulation, the Digital Elevation Model, and the wind and pressure fields that are used to force Delft3D-FLOW, the conclusions about the exact inundation depth and inundation extent are uncertain. Nevertheless, it can be concluded that the inundations are dominated by the river discharges. Furthermore, based on the significant differences in the simulated inundations with and without storm surge, it can be concluded that neglecting the joint occurrence of storm surges, discharge peaks and high tides results in an underestimation of the inundation depth over a large area and the inundation extent on a local scale. Based on the conclusions of this research, it is recommended to take into account the joint occurrence of storm surges, discharge peaks and high tides in exposure and risk studies in the Pampanga delta. To mitigate flooding, it is recommended to explore the possibilities to increase the time lag between the storm surge and discharge peaks and that cut-off the discharge peaks itself. It is also highly recommended to take into account the extraordinary land subsidence and sea level rise in exposure and risk studies in the surroundings of Manila Bay since it will probably result in more severe inundations due to storm surges in the future.
Item Type:Essay (Master)
Clients:
Deltares
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/76852
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