坡地社區淹水問題改善策略之探討 -以台南縣大內地區排水為例

Abstract

台南縣大內鄉位於曾文水庫下游約40公里處，其西區之南181號公路以西之河灘地，因地勢低窪、排水斷面不足、土地利用改變、加上氣候變遷造成暴雨量大增等因素，早年即被劃為曾文溪之洪氾區，每逢大雨輒生嚴重水患。經政府積極努力，堤防雖已於2008年以前陸續完成，雖勉可抵擋曾文溪之外水，然因其內水無法有效排出反而造成本地區之另ㄧ水患問題。 早期排水治理工法多只著重排水路之整治，如輸浚河道以增加通洪能力、截彎取直以縮短排洪時間、天然水路渠道化以加快流速、增加堤防高度以防止洪水漫溢等，其等均只考慮以排水設施將暴雨產生之地表逕流排放至下游地區，而忽略逕流會造成下游生態環境之衝擊。近年來，因生態保育意識高漲，已成工程界難以迴避而須重視之問題，運用非工程方法如：定量降雨預報與淹水警報、緊急應變與防洪社區、土地開發暴雨逕流管理、集水區雨水貯留及入滲措施、洪氾區之劃設與管理、國土復育及國土規劃等方法已成必然趨勢。因此，在治理排水設施時如何避免不當的整治及選擇適當的治水措施與方法，以減少淹水災害及其對生態環境的衝擊，已成區域排水治理上重要的工作。 本研究係利用SOBEK二維淹水模式進行坡地淹水範圍分析，由不同重現期距之降雨條件來模擬大內地區之淹水情形，並探討坡地排水所衍生之淹水原因與問題癥結所在。本研究發現最能根本解決之作法為：須將現有之北(石子瀨)、中(大內)、南(內江) 3條排水路先予拓寬以利排水，而為避免山洪流經市區之箱涵時，因斷面瓶頸而造成水患，則以截流方式將水截排至北邊之石子瀨排水，並於集水區出口處設置滯洪池，同時配合小型抽排方式以處理周邊低窪之內水，待外水退後再自然排除，必可有效改善本區之淹水問題。本研究之結果可提供相關單位在區域排水治理時決策之參考。

Ta-Nei Township is located at the downstream of 40 kilometer of Tseng-Wen Reservoir. Due to the low-lay terrain, narrow cross sections, lands development, and climate change, the area located at the western of Tainan Road No. 181 was zoned as floodplain of the Tseng-Wen River. The floods occurred frequently during the periods of high precipitation. Although the dikes were implemented by government to prevent from the outer water of Tseng-Wen River, the local inner water which can not be drained efficiently became another flooding problem. The traditional flood control measure is channel harnessing, such as dredging, truncation of curviform channel, channelized, and levees rising. These measures only consider that drain the discharge to the downstream, but ignored the impacts to the ecology. Because of the development of ecological conservation in recently years, the nonstructural measures, such as quantity rainfall predict, floods alarm, emergency response, disaster resilient communities, land use control, rainwater harvesting, floodplain zoning and management, and land restoration and reclamation are mentioned gradually for flood mitigation. Avoiding unsuitable flood control implements and selecting appreciate ones to reduce the impacts the ecological environment has become an important task. This study applied SOBEK, a 2-D flooding simulation model, to simulate the flooding area according to the rainfall intensities of different recurrence intervals. This study suggests several measures to mitigate the flood disaster efficiently and will be a reference for decide making. The measurements include increasing the cross sections of north (Shih-Tzu-Kai), middle (Ta-Nei), south (Nei-Jiang) drainages, the diversion of the upstream discharge from mountain area to the Shih-Tzu-Kai drainages, and the construction of retention basin in the downstream collocating with small-scale pumping to deal with inner water.