應用水工模型於溪床土砂災害治理對策之評估研究

2009年8月莫拉克颱風因強降雨引發嚴重坡地崩塌，大量土砂造成林邊溪支流來社溪溪床沖淤變化及下游地區土砂淤積，並直接衝擊位處低位河階的聚落安全。來社溪高變動的土砂，一直以來困擾著主管機關。地方政府自2012年起長期投入溪段清疏及緊急治理工程，亦受到鄰近保全對象對於成效的質疑。本研究以來社溪與內社溪匯流口上下游約2公里溪段，做為水工模型試驗範圍。水工模型試驗之目的，在於克服自2009年8月莫拉克颱風以來來社溪所面臨的整治困難與疑慮，其中包含：(1).清疏工程對於高變動溪床是否有效的質疑、(2).多年以來清疏所產生之土砂於溪岸堆置遭溪水侵襲的問題、(3).東部落上游護岸基腳屢遭溪水沖刷之控制對策、(4).來義鄉-D001大規模崩塌潛勢區坡趾淘刷問題等。本研究除了探討來社溪所面臨高變動溪床土砂沖淤及治理的問題外，並設法找出最合適的治理對策。水工模型試驗的結果發現，河道清淤的作法雖然於100年重現期距的極端降雨事件下無法達到預期功能，但在常時條件下，仍不失為經濟且有效的治理方案。東部落上游護岸基腳屢遭溪水沖刷問題，可藉由沉箱保護工搭配系列丁壩解決；系列丁壩則可解決大規模崩塌潛勢區坡趾淘刷問題，但工程所用經費龐大，是否符合經濟效益仍待主管機關綜合評估。

關鍵字

土砂災害；溪流治理；水工模型

並列摘要

2009 Morakot Typhon caused severe landslide and debris disaster in Tainan Branch Office jurisdiction. Debris and sediment directly induced scour as well as sedimentation in Linbian River. They also indirectly impacted the settlement situated along low-stage river terraces. Highly alternated sedimentation in Lai-Sher Stream has always troubled the corresponding authorities. The local government has invested in river dredging and emergency treatment projects since 2012. However, all these efforts have been questioned by neighboring settlement about the effectiveness. This study takes the 2-km river reach covering the upstream and downstream of the confluence of Lai-Sher and Nei-Sher streams and uses as the scope of hydraulic model tests. The purpose of this study is to overcome the difficulties and doubts in the remediation of Lai-Sher Stream since Typhoon Morakot in August 2009. Questions need to be answered include: (1) the effectiveness of sediment dredging operations, (2) safety of the yards along the Lai-Sher Stream that used to store sediment from dredging operations, (3) countermeasures for the protection of revetment foundation upstream of Easy Village, which has been repeatedly eroded by the stream, (4) scour at the foothill of Lai-Yi Township-D001 Large-scale landslide potential site. In addition to finding answers for the preceding questions, this study also tries to find the most appropriate control measures for future governance. Results of the hydraulic model tests found that, although the method of river dredging cannot achieve the expected function under the extreme rainfall event with a return period of 100 years, it is still an economical and effective treatment plan under normal conditions. The foundation of the bank revetment upstream of East Village has been repeatedly scourged by streams, which can be solved by using caisson protection structures with a series of spur dikes. Series of spur dikes can also solve the problem of scouring at the foothill of D001 Large-scale landslide potential site. However, the project cost is huge and may still need a comprehensive assessment by the corresponding agencies for the economical benefit.