| Summary: | 碩士 === 國立臺灣海洋大學 === 河海工程學系 === 94 === In this thesis, a system for modeling coastal flooding due to typhoon-induced wave overtopping flows over seawalls is developed based on the physiographic flood forecasting system by Young (2000). The effective rain-intensity over the calculation domain and GIS are included and the wave overtopping flows are taken as the boundary conditions along the seawalls. The system is particularly evaluated by the severe flood event at National Taiwan Ocean University (NTOU) with partial failures of its vertical seawalls in the attacking periods of Typhoon Herb in 1996. Given typhoon-induced time series of water level variation and wave height and period by Tsai et al., (2005), overtopping flows at three sites of the seawalls were first calculated by different empirical formulae. Comparisons among the formulae demonstrates that those by Allsop et al., (1995) and Besley et al., (1998) are more reasonably applicable than, especially, Goda’s (1985) for monolithic and composite vertical seawalls, respectively. By applying the wave overtopping conditions to the flood modeling system, the results further illustrate that the maximum flood height on NTOU campus by adopting Besley et al.’s formula (5.74m) is smaller than that by Allsop et al.’s (6.02) and even smaller than the field record height of 6.44m.
By selecting the formulae by Allsop et al. the modeling has clearly shown that in three most low-lying cell area of the campus nearby hills, flood height due to exclusive wave overtopping could amount to about 80% of those by combined effects of precipitations and wave overtopping. The ratio could become even higher to 100% in the campus nearby seawalls. In the end, by neglecting the partial failure of the seawalls flood heights could be reduced by 35% to 50% in the three low-lying areas of campus nearby hills, almost by 100% at three high-rising areas of the campus nearby coastal water and by 9 to 28% in the other areas. The case study clearly illustrates that wave overtopping over seawalls could potentially play an important role in the coastal flood due to typhoons and partial failures of the seawall structures could significantly escalate the flood heights.
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