Investigation of improving policy on mountain area inundated problem-Toushe Living Basin in Yuchih Township Nantou County as an example

Abstract

Toushe Living Basin at Yuchih Township in Nantou County is famous for its peat soil basin (soil is soft and loose). The area of the watershed at the upstream of Toushe Bridge covers approximately 495 hectares. We can enters the Toushe Village from the northwest by driving on the 21st Provincial expressway . The expressway is then connected to a one-way road 62st Provincial expressway. It goes along a seemingly ellipse road and exit to 21st Provincial expressway from the south of Toushe Bridge. Alongside the road there are one and two-story houses and the slope of the ground on the upper side of the way is rather steep. The Shueiwei Creek, one branche,of the Shueili Creek is the mainstream in the watershed and its drainage channels flows through it. The rest of branches congregate the drainage of every farm and flow into drainage channels. The circularity ratio of the watershed is high as 0.61. Moreover, the area is silted by ancient lakes,the layers contain peat beds and lots of organic clay, so its drainage capability is quite poor. Because the compound cross-sectional channel used for temporary drainage at the exit of the watershed is short, and the effectiveness of channel bottleneck will form back water. Therefore, the flood always occurs when typhoons bring heavy rainfall. The flood area covers over 60 hectares. The objective of this research is to probe into the overall plans of drainage system by hydrologic model analysis in order to put forward ways to effectively ameliorate the problems of inundation. Through boring survey and the utilization of the one-dimensional model of HEC-RAS developed by Hydrologic Engineering Center, U.S. Army Corps of Engineers, the flux capability of current drainage channels were calculated. The two-dimensional values of FLO-2D was then employed to simulate the inundation mode and cross-referenced the data of inundation brought by Typhoon Mindulle on July the 2nd in 2004 and heavy rain caused by Typhoon Sinlaku in 2007. The results showed that there are four effective methods to solve the problems of inundation. Firstly, Shueiwei drainage widen the 2m complicated cross-sectional channel to 15m. Secondly, Shueiwei drainage-1 widen the 2m complicated of cross-section channel to 15m. Shueiwei drainage -4 and Shueiwei drainage -5 deepen the 1m in depth of cross-section channel to 2.5m. Thirdly, drainage widen the 8m of cross-section channel to 15m. Fourthly, flooding area will reduce 76.25%(45.75 hectares) after drainage improvement. The research suggest the lowest area in the basin as a detention pond and drive wooden piles around it. At the same time, use the special quality of the peat soil basin to widely plant the special indigenous plants such as dawn redwood in order to cultivate a special ecology and coservation area, to ensure the sustainable use of water resources, to enhance local sightseeing value, and, in turn, to give a brand-new appearance of the countryside of this area.

南投縣魚池鄉頭社盆地之土質為遠古湖泊淤積而成之泥炭土，因其土層鬆軟而以活盆地著稱，盆地出口之頭社橋，其上游集水區面積約495公頃，進入本試區可自台21甲線由集水區之西北方進入頭社村後即連接單車道之投62線，繞一近似橢圓形後即可由頭社橋南邊銜接台21線而出，投62線之上邊坡多為1～2層樓之聚落分佈，坡度較陡。集水區之主流為水里溪之支流水尾溪排水幹渠，其餘各支流則匯集各農地之5條排水入於幹渠。因集水區之圓比值高達0.61，且其土層均由泥炭層及大量有機質的黏土組成，排水能力甚差，每逢颱風豪雨時由集水區流出之洪流在排入集流點頭社橋下之複式斷面渠道時，因通水斷面縮減而形成排水瓶頸所產生之迴水常造成多達約60公頃之淹水災情。因早期盆地種植水稻影響較少，近10年因稻作收益降低改種絲瓜、敏豆、茄子、蕃茄等旱作為主，甚不耐淹浸，常造成盆地內農作物重大損失。本研究旨在以水文模式分析探討其排水系統並做整體規劃，以提出有效改善淹水問題之方法。 本研究透過地質鑽探和測量調查並運用美國陸軍工兵團水文工程中心所發展之HEC-RAS(一維模式)演算現況排水路各重現期距之通水能力，再以FLO-2D(二維數值淹水模式)進行淹水模擬，利用2004年敏督利颱風豪雨所造成嚴重之七二水災與2008年辛樂克颱風豪大雨的資料進行驗證後，結果提出可有效改善淹水問題之四種對策：(一)排水出口改善：以加強即時排水為目標，可將原來0.90%之縱向渠道坡度改為1.51%，並將原寬2 m圳渠之橫斷面拓寬為15m。(二)坡地排水改善：分別將水尾溪排水幹線之1由原為4m之渠道挖寬為15m、水尾溪排水幹線之4和之5由原為1m之渠道挖深為2.5m以截排西南方和東北方之坡地逕流，讓逕流安全地排至水尾溪排水幹線。 (三)排水路改善：針對未達10年重現期距之洪峰流量且25年不溢堤保護標準之渠道斷面，由原來之8m拓寬至15m。(四)上述三項改善方案完成後，淹水情形可減少76.25%(45.75ha)，但仍有25.75%(14.25ha)之面積會淹到水，故建議將該區域朝多目標利用之濕地發展，除了可利用活盆地泥炭土之特殊地質，以營造特殊生態保育景觀、確保水資源之永續利用及提昇當地之觀光收入之外，更可展現另一種全新的鄉村風貌。