Hazard and risk assessment of watershed in South Taiwan.

INTRODUCTION

In 2009, the typhoon Morakot caused many multimodal sediment disasters in Taiwan. The Soil and Water Conservation Bureau invested a lot of resources in the reconstruction project. To accelerate the stability of soil in the catchment area, and reduce the possibility of secondary disasters. After a period of time, appropriate review and governance benefit assessment should be carried out. In this study, 2 major areas with heavy sediment disasters within the jurisdiction of the Tainan Branch, Soil and Water Conservation Bureau were chose to do hazard and risk assessment.

METHODS

This study collected the documents of the erosion and sediment control engineering over the years from 2009. Then, matched with the field survey, digital elevation model analysis, and using the evaluation matrix to assess the level of hazard and risk of selected major disaster areas. The row and column of the evaluation matrix including “function of structures” and “environmental condition (EC)”. Function of structures are divided into 4 levels: Nice, Good, Poor, and Bad. Environmental condition is assessed by four factors “landslide rate of watershed (%)”, “upstream channel slope (degree)”, “river erosion or siltation change (m)”, “preservation factor”. Landslide rate of watershed (LA) means the percent of landslide in the watershed. Upstream channel slope (US) means the slope of the channel from the middle to the top of watershed. River erosion or siltation change (CD) means the maximum vertical height change of river bed. Preservation Factor considered the protected targets and the preservation distance. According to the individual scores of the four factors, the weighted average is taken and divided EC into 4 levels. The hazard and risk assessment work can be done according to the evaluation results of “function of structures” and “situation of environment”.

RESULTS AND DISCUSSION

In this study, we chose two sites, i.e., Cianghuangkeng (Tainan City), Henansiang (Kaohsiung), to practice hazard and risk assessment, in 2018. 2009, typhoon Morakot caused 1.96 ha of landslide, and brought about 160,000 m³ of sediment at Cianghuangkeng. From 2009 to 2018, the Soil and Water Conservation Bureau practiced 6 erosion and sediment control engineering. According to the results of assessment, the level of function of environment is good, and the level of environmental condition is A. Therefore, the result of hazard and risk assessment is low. Cianghuangkeng has low potential for hazard and risk. In this way, the evaluation result of hazard and risk assessment in Henansiang is also low.

According to the results of evaluation matrix, the potential of hazard and risk could divide into three levels: high (H), middle (M), low (L).

CONCLUSIONS

This study use evaluation matrix method to assess the hazard and risk of the major disaster areas caused by the typhoon Morakot event. According to the assessment results, we can review whether the remediation strategies and directions of key disaster areas need to be revised. It will help improve related technologies, provide reference for future related governance planning strategies, and effectively promote the improvement of soil and water conservation.