臺東野溪集水區土砂治理策略之研究

近年來受到氣候變遷與極端降雨的影響，每逢颱風豪雨事件常誘發山區土石崩塌而引起土砂災害，然而土砂發生機制過於複雜，若能藉由模式有效預測土砂運移過程，將有助研擬土砂管理及治理策略。本研究以過去相關研究模式為基礎，選定臺東野溪樣區，根據其土砂主要來源坡地崩塌與地表沖蝕，分析其土砂生產量與運移狀況，進而採滾動式評估管理方式，針對淤積嚴重河道進行清疏，以恢復河道安全通洪功能。鑑於2009年莫拉克颱風造成臺東南部產生大量土砂災害，選定臺東沿海河系之太平溪、太麻里溪、金崙溪、大竹溪、大鳥沿海及達仁沿海等七個主要次集水區作為研究樣區，比對莫拉克颱風前後各重點樣區之土砂變遷空間分布，及莫拉克颱風前衛星影像判釋，顯示研究樣區在颱風前之總崩塌面積約1131.8ha，而颱風後衛星影像判釋之崩塌地有明顯增加，面積總計達約5292.5ha，總土砂量體約為原來之4.68倍，此為造成臺東縣多處野溪嚴重土砂淤積之主因。又2010-2012年期間，臺東歷經凡那比、南瑪都及天秤等颱風，災害已部份轉移至臺東北部之新武呂溪，因此2012年另增加新武呂溪作為研究樣區，而原本臺東沿海河系中僅保留五個重點集水區加上新武呂溪，共以六個集水區作為2012年天平颱風後之研究樣區，經2012年天秤颱風後衛星影像判釋，比對各重點集水區之土砂變遷空間分布，結果顯莫拉克颱風初期六個重點集水區崩塌面積總計約6443.1ha，而天秤颱風後六個重點集水區崩塌面積總計約5,724.9ha，顯示從2010-2012年期間，崩塌已趨於穩定或復育良好。本研究亦發現臺東土砂清疏採滾動式管理檢討結果，可依野溪集水面積大小及其地形特性與土砂淤積程度不同，所應用策略也各有不同。而野溪治理策略除須改變既有傳統思維外，並可參考時間與空間等因素，依據時間長短劃分為三階段，分別為短期(臨時性防災措施)、中期(野溪)護岸(修復)及長期(集水區整體性規劃)。在空間與治理工法之應用，可依據土壤地質條件劃分為三個區域，分別為縱谷線(中央山脈)、海線(海岸山脈)及南迴線。對於未來治理策略與應用，應該從以下十大面向考慮：1.臺東地區野溪特性與問題；2.臺東野溪治理工法之演進；3.改變舊思維與導入新科技；4.野溪清疏時機與空間考量；5.參考地域特性及工法創新；6.增加滯洪保水與減災避災；7.設計階段之施工風險評估；8.落實生態檢核與友善措施；9.規劃預警系統及自主防災；10.加強設施構造物維護管理。本研究分析臺東地區2009~2012年期間，集水區土砂空間分布，並進行現地調查，綜合理論與實務經驗，提出災害管理與治理策略建議，期望可提供未來集水區土砂治理策略之參考。

關鍵字

颱風事件；土砂災害；野溪清疏；土砂治理；治理策略

並列摘要

Due to the climate change and extreme rainfall, sediment disasters characterized by landslides triggered by typhoons with extremely heavy rain are getting quite common and serious in Taiwan. The causes of sediment are very complicated; however, with the help of the models to effectively predict the processes of sediment transportation, it will be beneficial to working on proper sediment management strategies. This study, based on the previous related research on models, aimed to explore the problems of landslides of main slopelands and soil erosion caused by the extreme rainfall of Typhoon Morakot in the wild stream watersheds in Taitung County. By analyzing sediment yield and its transportation, the study provided the rolling evaluation management to carry out river dredging for sediment deposition in order to restore the safe river transport capacity. Typhoon Morakot in 2009 with characteristics of heavy rainfall intensity and long duration caused many severe landslides in Taitung County. This study selected seven wild stream watersheds in Taitung County as the study sample and compared the spatial distribution of sediment changes in the above-mentioned areas before and after Typhoon Morakot. According to the interpretation of satellite images before Typhoon Morakot, it was found that there was an obvious increase in the area of landslides from 1131.8 hectares to 5292.5 hectares and the total amount of sediment was about 4.68 times that of the original sediment. The increase in landslides was the main cause of sediment of many wild stream watersheds in Taitung County. During post-Typhoon Morakot periods with three typhoons hitting Taitung County, including Typhoon Fanapi (2010), Typhoon Nanmadol (2011), and Typhoon Tembin (2012), Hsinwulu River Basin also suffered from sediment disasters, so this area was added to the study sample along with the previous five hotspots of watersheds for the purpose of the study. The interpretation results of satellite images after Typhoon Tembin showed the total area of landslides in six hotspots of watersheds was 6443.1 hectares after Typhoon Morakot and 5724.9 hectares after Typhoon Tembin respectively, indicating that the conditions of landslides were getting stable or well restored from 2009 to 2012. Based on the results of rolling evaluation management of sediment dredging in Taitung County, this study also found the size of wild stream watersheds, the geographical characteristics, and the extent of sediment deposition were factors in adopting different strategies for river dredging. Besides, it was suggested that wild stream watershed management should take time and spatial factors into consideration. In terms of time, it was better to divide management time into three phases, including the short-term period (focusing on temporary preventative measures), the mid period (emphasizing restoration of bank protection work of wild streams), and the long-term period (centering on integrated planning of watersheds). As for spatial consideration in management strategies, it was suggested, on the basis of geographical conditions of soil, three areas could be divided, including those in Central Range, Coastal Range, and South-Link Line. This study suggested that future management strategies of sediment in Taitung County should take into account the following ten aspects, including the characteristics and problems of wild streams, the development of wild stream management and construction methods , the incorporation of new technology, time and spatial factors in river dredging, geographical characteristics and innovation of construction methods, the improvement of detention and water retention as well as prevention measures of disasters, the construction risk assessment at the engineering stage, the implementation of ecological checks and environmentally-friendly measures, the planning of early warning systems and disaster resistant policies, and strengthening maintenance and management of engineering structures. This study explored the spatial distribution of sediment in the wild stream watersheds of Taitung County from 2009 to 2012, conducted the field surveys, combined academic theories and practical engineering experiences, and provided findings on proper management and prevention strategies of sediment disasters caused by the climate change and extreme rainfall. It is also hoped that the results of this study could be used for the reference of future management strategies of sediment in wild stream watersheds.