...「莫拉克」前日正面吹襲台灣時，南部受災嚴重，高雄錄得二千多毫米的驚人雨量（比香港黑雨警告大二三十倍），多處發生泥石流。其中甲仙鄉小林村災情慘重，全村一千三百多人，據了解僅有四十多人逃出，另百多人獲救，初步估計還有約千人可能被活埋。附近的那瑪夏、桃源、六龜、茂林等山地鄉約十多村莊亦失去聯絡，數千鄉民受困待援。...
根據台灣 "中央氣象局" 8/8及9/8逐日雨量資料:
高雄每小時最高降水量52mm/hr (8/8 6:00)，兩天降水量216.5(7/8)+507 (8/8)=723.5mm
阿里山每小時最高降水量 123mm/hr (9/8 2:00)，兩天降水量1161.5(8/8)+1165.5 (9/8)=2327mm
By Horner's equation, the design intensity i = a / (td+b)^c where parameters a,b & c subject to different districts in Taiwan (水資源局”水文設計應用手冊”)
For return period (T) =200 years and duration (td) =30 min, 高雄 i = 164.3 mm/hr and 阿里山 i = 145.3 mm/hr.
For T=50 years and td =30 min, 高雄 i = 124.8 mm/hr and 阿里山 i = 113 mm/hr.
How about in HK? According to the Flood Protection Standards in Hong Kong, the design return period for urban drainage truck system is 200 years and for urban branch system is 50 years.The design intensity i may be calculated according to DSD's manual 4.3.2 and such parameters a,b & c based on Table 3 by 'Gumbel solution'. Design intensity i = a / (td+b)^c.
For T=200 years and td=30min. , i = 187mm/hr; and T=50 years, i = 156mm/hr
Design intensity (i) both in HK and Taiwan seem to be marginally satisfied with the extreme hourly rainfall. How about the long duration of rainfall?
According to DSD's Technical Manuals Table 4, the extreme depth x (mm) for various return years (T) based on hourly records measured at HKO (1884 – 1939 ; 1947 – 1990) and as follows:-
i) For T=200 years, x = 622mm/day or 729mm (2 days) <2327mm (阿里山) or 723mm (高雄)ii) For T=50 years, x = 507mm/day or 595mm (2 days)
The flooding in Taiwan is obviously as a result of the extreme duration of rainfall and accumulated surface runoff. (The discussion of design duration of rainfall may be referred to DSD's flood prevention Q&A and administration response to Legco on projects 22CD & 81CD.)
In HKO's record, the highest daily rainfall is 534.1mm/day at 19 Jul 1926 and the highest hourly rainfall is 109.9mm/hr at 8 May 1992. Black rainstorm warning when rainfall >70mm/hr.
HKO study found that under the influence of global warming, annual rainfall in Hong Kong would increase at a rate of about 1% each decade in the 21st century - about the same rate as in the past 120 years.
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