Results of this project included (1) the review of the warning stages of river basins and drainage systems, which were set up in 2010; (2) establishement of the warning stages of gauges installed in 2012 in river basins and drainage systems; (3) study disaster prevent and warning of flash flood; (4) development of stage prediction methods after the stage reaches of the 1st order warning stage; (5) testing and validation of warning rainfall for drainage systems administered by the Fifth River Management Office; (6) drafting the technical manual for the planning of warning stage stations for authority’s river and drainage systems. In which, the review of the warning stages of rivers and drainage systems shows that most of the warning stages stations satisfy the stage under the 10-yr return period of floods, except the following gauge stations: (a) Tiento bridge, Shuanton bridge, Shihcheh bridge, Touchianshui bridge, Jenhong bridge administered by the Fifth River Management are recommended to increase 0.5 m to their 1st and 2nd degree warning stages; (b) Yenpin Station administered by the Eighth River Management Office is also recommended for adjustment the 3rd degree warning stage. Hit by 2011 Nanmadol typhoon and several storms, most of rivers and drainage systems show satisfaction in the current of warning stages.
In Taiwan, the river reaches are short and steep. The times of concentration of upstream reaches of Taiwan rivers are usually less than 2 hours. This project implemented the statistical analysis on the simulation cases to project the river stages in illustrations of the scenarios of 1 and 2 hours. The results can be referred as the flash flood warning announcements. The upstream reaches of Bajhang creek, Puzen creek , and Keelung rivers were taken as the case studies to analyze the flash flood behaviors in their warning areas. In which, the flood plain of Jiunhuei bridge of Bajhang creek can be reached by the flash floods with the events of one hour accumulated rainfall of 60 – 80 mm and two-hour accumulated rainfall of 100 - 110mm. One hour accumulated rainfall of 110 – 130 mm or two hour accumulated rainfall of 160 – 170 mm can cause the river stage overtopping the dike at the Huashing bridge in Puzen creek. Keelung river is identified by the simulation results without flash flood hazards due to the Yuanshanchi diversion weir that starts functioning with the one hour accumulated rainfall of 30 – 35 mm or two hour accumulated rainfall of 50 – 70 mm. Moreover, a further study is suggested to develop the early warning system for the flash flood prone areas.
This project also take into account the need for practical use that many diagrams have been depicted for the fast query on the trends of river stage rise and retreat after the stage reaches its 1st degree warning stage. The application of the diagram simply requires the precedented rainfall, quantitative rainfall prediction to project the river stage, and estimates its overtopping potential. Bajhang creek was taken as the case study and results show that when the stage reaches 19.8 m at the Bajhangshui bridge, the dike overtopping might happen at the Sections 58-1 and 59. Moreover, when the stage reaches 26.69 m at the Chrlanshui bridge, the dike overtopping might happen at the Section 15-1.
After tested and validated with various typhoon and storm events, the rivers and drainage system administered by the Fifth River Management Office, most of the events did not have yet met the standard of 2nd degree warning rainfall. Nevertheless, the 727 and 829 storms in 2010 met the standard of 2nd degree warning rainfall and caused minor inundation on the roads with little loss. The proposed method with the input of warning rainfall can project the inundation map precisely and ready to be used as the reference for evacuated to disaster prevention
The installation of new flood warning stage gauge station can be refered to the “Technical Manual for the Planning of Warning Stage Stations for Authority’s River and Drainage Systems.”