Wind Hazard Analysis in Hurricane-Prone Regions

Typhoons are one of the most catastrophic natural phenomena on earth. Thus, communities that are located within the hurricane striking zone are at risk. To reduce the risk and mitigate the related hazards, engineers need to: (1) estimate the parameters such as pressure gradient and rainfall intensity within a hurricane; and (2) assess the hurricane induced hazards, which include extreme winds, storm surges, river floods, and landslides, that a coastal structure will experience during its lifetime. In this thesis, the methodology used in hurricane risk analysis is reviewed and computer algorithms are developed to quantify the hurricane induced wind hazard. The discussion will emphasize on the recognition of hurricane striking zone, identification of hurricane-induced hazards, and assessment of wind risk for hurricanes.

The multiple pathway model is an ideal model to conduct a complete hurricane risk analysis. In this thesis, the need for applying the multi-pathway model to assess the potential human and property exposure to hurricane hazards such as extreme winds are addressed. .In addition to the multiple pathway model, a wind hazard model used to study the wind hazard caused by hurricanes at the site of interest is reviewed. The wind hazard model includes two sub-models. They are: (1) a recurrence model; and (2) an extreme wind model. The recurrence model can be either a homogeneous Poisson process or a non-homogeneous Poisson process. The mean rate of hurricane occurrences for the homogeneous Poisson process is estimated by a constant mean model; on the other hand, the mean rate of hurricane occurrences for the non-homogeneous Poisson process is estimated by a globally constant seasonal model. The extreme wind model can be described by either a Gumbel or a Weibulldistribution. Before the extreme wind model can be applied, the extreme gradient winds induced by hurricanes have to be evaluated. The extreme gradient winds can be assessed by the wind field model of a hurricane.

Furthermore, a simulation model, which is used to assess the hurricane- induced wind hazard at the site of interest, is reviewed. The simulation model is employed because of insufficient data. In simulation, the random variates for each of the following five random variables are generated from the selected theoretical distribution with a random number generator. The required random variables are: (1) the latitude of a hurricane center; (2) the longitude of a hurricane center; (3) the minimum pressure within a hurricane; (4) the translation speed of a hurricane; and (5) the moving direction of a hurricane. Random numbers are generated by the linear congruential generator. Lognormal distribution is the chosen theoretical distribution for each of the five random variables in the case of Hong Kong.

Finally, the wind hazard model with the observed data and the simulation model with the simulated data are applied to calculate the wind hazard curves at Hong Kong. The purpose of applying the wind hazard model is to validate the simulation model. Validation is to find out whether the actual system can be closely resembled by the simulation model. The validation procedure has to be performed because existing data are insufficient. Two Pascal programs called "WindHazard" and "Simulation" are developed. "WindHazard" is written based on the wind hazard model. The input data for this program is the observed data supplied by the Royal Observatory of Hong Kong.

"Simulation" is written based on the simulation model. This program used the internally generated data as the input data. Since the terminating criterion of a simulation is usually governed by the duration of the available data, a 30-year forecast window is used as the terminating criterion in the case of Hong Kong. In this case, hurricane data spans over a period of 30 years from January 1961 to December 1990. This terminating criterion is applied here rather than the criterion based on the number of generated hurricanes because the later one is based on an arbitrary number. With this terminating criterion, the simulation can be classified as a terminating simulation. Thus, the sequential procedure can be applied to analyze the output of the terminating simulation. The procedure estimates the performance of the mean hazard curve for hurricane- induced wind speeds by constructing a 100(1 - α) percent confidence interval for the curve with a relative error γ ~ 0.15. The advantage of the sequential procedure is the simulation terminates as soon as the desired precision has been reached.

Based on the results of the analyses for Hong Kong, the following conclusions can be made:

1. The Gumbel distribution fits the historical observations for hurricane- induced extreme winds better than the Weibull distribution.
2. The non-homogeneous Poisson process fits the existing data for hurricane occurrences better than the homogeneous Poisson process.
3. One hundred and seventy six simulation runs are required to achieve the desired precision, i.e., a 90% confidence interval with a relative error γ ~ 0.15 for estimating the performance of the mean wind hazard curve for hurricanes.
4. The developed simulation model is a valid model for simulating the characteristics of hurricanes. This model can, then, be used to forecast the hurricane-induced wind hazard at the site of interest.