特低頻電磁波遙測技術於伏流水帶劃分之研究－以高屏溪流域為例

Abstract

台灣年平均降雨量約 2510 mm，為世界平均值的2.5倍，但降雨時空分布不均，南部區域豐枯比達9：1為甚，又因山高流短使得水源儲存不易，以致於枯水期河川逕流量大幅減少，為能穩定南部地區水資源之供應，伏流水（hyporheic flow）的開發具有其必要性。然而，伏流水的研究大多侷限於生物以及生態領域，研究焦點為伏流水帶（hyporheic zone）與生態環境之間的相互關係，對於伏流水帶擷取利用的評估相對缺乏。再者，實務工程上，同樣缺乏掌握伏流水帶邊界的有效分析方法，也難以做到伏流水開發的精準評估。 為解決以上問題，本研究目的為伏流水帶的劃分、評估與結果驗證。本研究透過SYT及DGT特低頻電磁波物性探勘儀（SYT/DGT ultra low frequency remote sensing techniques ），進行伏流水帶探測工作，參照研究區的水文、地質與岩芯等地真資料（ground truth）建立水文地質概念模型，並透過三階段判識率定各岩層電阻率（resistivity）的值閾，進而劃分伏流水帶邊界並評估最佳取水目標區。本研究亦在高屏堰上分別於乾、溼季施測，以勾勒出豐、枯水期伏流水之變化情形。 依照本研究結果，水文地質概念模型與電性響應一致，推估伏流水帶介於河床底部至第一阻水層之間（約為-4m~-38m），並以第一含水層中之礫石層（約為-24m~-34m）為最佳取水目標區。乾、旱季之測量則呈現明顯季節性差異，兩者有相同的電阻率轉換深度，惟電阻率變化趨勢不同，呈現溼季時表層相較「富水」、底層相較「乾涸」，乾季時則表層相較「乾涸」、底層相較「富水」。 根據以上所述的特低頻電磁波遙測法，希望以此探勘方法增加現今伏流水帶分析的快速性與準確性，以達到更完善的伏流水資源利用。

With growing instability of spatial and temporal precipitation variability and water supply in Taiwan, utilizing alternative water sources is one of the important trends in water resources development. Understanding that hyporheic water is one of the resources that has the characteristics of slow flow velocity, low turbidity, high water quality and low ecological impact, it has a great potential to be utilized. However, the researches about hyporheic water are mainly focused on the interrelationships between the hyporheic zone and its biological ecosystem. The researches about utilizing hyporheic water are relatively insufficient and the potential water content can’t be effectively estimated. Therefore, the purpose of this study is to propose an innovative procedure to investigate distribution of the hyporheic zone and to verify the results by comparing with ground truth data. We also made measurements in wet and dry seasons to see the time series change. The results show that by applying Ultra Low Frequency Electromagnetic Wave Remote Sensing techniques, in most cases the stratigraphic section identified in the study successfully matched the ground true well data and hydrogeological conceptual model. The hyporheic zone lies between riverbed and aquitard 1 (-4m~-38m) and the optimal water intake area lies in gravel layer of aquifer 1 (-24m~-34m). The resistivity profiles of wet and dry seasons show significant difference owing to precipitation variability. In conclusion, this study provides an effective way to evaluate hyporheic zone in southern Taiwan. We hope that this study can contribute to practical hyporheic water resources development project.