圖書典藏及影音數位平台

後龍溪水系支流汶水溪原為苗栗縣縣管河川，自虎山吊橋下游100公尺起至與後龍溪匯流處止，於民國88年由苗栗縣政府完成治理基本計畫並奉前台灣省政府核定公告。民國89年時改列為中央管河川，水利署水利規劃試驗所於95年辦理虎山吊橋以上規劃，惟治理計畫並未核定公告。 本溪自民國88年規劃迄今已歷時15載，期間歷經民國88年921大地震及數十次颱洪，河道地形及周邊環境均有變化，為配合河道地形環境變化及社會發展需求，辦理後龍溪水系支流汶水溪治理規劃檢討。

It has been more than 20 years since the Wen-shui River improvement plan had been announced by 1998. The governance and management issues caused by the river map that is incompatible with the current terrain. Moreover, it taking potential risks of extreme rainfall events and flow changes into account by comprehensive flood defense idea. Therefore, the River Management Office deemed that it necessary to review the Wen-shui River of overall planning as the basis for engineering implementation in the future. It expected to protect people’s lives and property and sustainable use of river resources. The Wen-shui River basin is mainly located in the Townships at Miaoli County. It is the branch of the Houlong River. The Mainstream length is 24.83 km .The area of the basin is 105.52 km2 that all mountainous region. Average slope of the river since the cradle to the confluence of Houlong River is 1/23. There are also several faults in the basin. The Wen-shui River basin that belongs to subtropical climate, and retained the original forest characteristics. The ecological environment is rich and diverse. Most of the land use within the Wen-shui River basin are forestry use. The major industrys are primary industries, too. However, the Wen-shui River is a famous tourist attraction because of beautiful scenery and abundant hot springs. The protected places include the visitor center of Shei-Pa National Park, the region of tofu street, Hushan hot springs, hotels and villages along the river. Over the years, the disaster type in upper region usually were mudslide disaster, the slope collapse, the bank erosion by flood. the disaster type in mainstream were the bank erosion by flood and bottom phase submerged. It adopted rainfall data of Song-an, Heng-long-shan, Da-hu (1), and He-Sin rainfall gauge stations for rainfall analysis. The hydrological rainfall frequency analysis via Log-Pearson Type III (LPT3) Distribution to analyze the storm of yearly maximum 24 and 48 hours in every recurrence (return period, TR). Then, according to Triangular unit hydrograph Method, Dimensionless unit hydrograph Method with the 24 and 48 hours rainfall type to analyze the peak-flow of yearly maximum 24 and 48 hours in every recurrence (return period, TR). After comprehensive comparison, in view of increased rainfall and security, This project adopted the peak flow of Dimensionless unit hydrograph Method with the 48 hours rainfall type. Take junction of Wen-shui River as a example, the peak flow in 25-years recurrence (return period, TR) is 1,690 cms. The river course slope of Wen-shui River is steep and flow speed is rapid. The width of channels vary with mountains and terraces. According to the riverbed elevation data Over the years, most of the river presenting erosion trends for a long time. However, section 13.1~18 are all deposition trends for short or long term. The Hydrocomp Simulation analysis result shows both sides heights along the river all higher than the flood stage of the Wen-shui River in TR=25 yr except section 4.1~4.2 right bank. The channel and environment of Wen-shui River have been improved based on various units management for a long time.According to results of the review, it’s no longer increased new construction on Wen-shui River, but section 4.1~4.2 right bank must be heightened 150 meters. It ensure the safety of channels through management, such as strengthen the basic of constructions to prevent flood erosion, section 0~4.1 and section 13.1~18 must regular dredging because it is easy deposition area in the river. Part of the watercourses that erosion potential is large, it must strengthened the basic of levees. However, the structural measures are not only way to solve flood problems. We should consider the possible disaster in the future, the engineering and non-engineering measures have be carried out at the same time. The total engineering cost is about 7.5 million NT. dollars. The annual benefit of whole plan is 0.2218 million NT. dollars and annual cost is 0.563 million NT. dollars. The ratio of benefit to cost is 0.39 that less then 1. However, it has benefit and the positive influence in many aspects after this project completion, including natural disasters prevention, land utilization, landscape, human outdoors activity, ecology, education, water quality improvement, and so on. It also increase residents’ support to the government and the development . Therefore the whole project is quite value and immediate emergency of the investment.