應用小波法分析橫向水利構造物對河川流態之影響

Abstract

興建大壩或堰等橫向水利建造物可提供水資源、水力發電、防洪、娛樂等功能，在人類社會高度發展過程中扮演重要角色。然而，大部分在興建之初都未考慮對生態系統的衝擊效應，包括可能大幅度改變了河川的流態狀況，及降低對水生生物覓食、產卵等棲地品質，並進一步導致棲地單一化而降低生物多樣性。本研究旨在應用小波分析法量化橫向水利建造物對河川流態的影響程度，找出河川流量中受影響程度最大的關鍵時間尺度，同時加入環境流量和相關的棲地保護進行討論，並以臺灣北部大漢溪中游的鳶山堰為例，分析其興建前後為主研究案例。為了更好的識別多重尺度臨界值，小波方法和時間序列超越機率的應用起到關鍵作用，例如日尺度，周尺度，月尺度和年尺度等等。考慮到在汛期與非汛期之間的長期影響以及不同的流量機制，本研究對1970年至1995年下游的歷史流量資料進行蒐集作為研究資料。此外，借助水平二維模式SRH-2D用於計算最具影響力流量下的流態特徵，以確定相關棲地模式的水文變化。結果表明，興建啓用堰之後不論在本研究中哪種時間尺度，其對下游的流量振幅變化都比較劇烈，即下游流態變得比較不穩定，且影響主要表現在2日尺度至1星期尺度週期內。對低頻率高能量事件（即汛期），其年尺度影響並不明顯；然而，較低流量事件相應的臨界環境流量在非汛期受到顯著影響。另外，棲地模式的模擬結果發現淺流（glide）為主要棲地類型，且攔河堰運作後，下游河道水域總面積減小、棲地單一化，棲地品質亦有下降趨勢，可見其對下游生態系統的確有負面影響，尤其對深流（run）及淺瀨（riffle）影響最大。本研究建議攔河堰在規劃取水時機及取水量時若能增加自然流態的考量，或營造淺瀨及深流的棲地型態，將有助於降低對下游環境流量及生態系統的衝擊。

Dams and weirs contribute significantly to human society, such as water resources supply, power generation, flood control, and recreation. However, most of them were built with little consideration on ecological conservation, altering the river flow regime dramatically and leading to severe effects on the endemic aquatic organisms. This study aims at quantifying the difference of flow regime before and after the construction of Yuanshan weir, located in the middle reaches of Dahan Creek, northern Taiwan. The environment flow and related habitat conservation are also discussed. Wavelet methods and time-series transcendence probability were used to identify the critical time scales, such as daily, weekly, monthly, and yearly. Considering the long-term effects and the variant flow regimes between flood period and non-flood period, the historical flow discharge data of the downstream from 1970 to 1995 was collected. In addition, a horizontal two-dimensional model, SRH-2D, was used to calculate the flow characteristics of the most influential flow for determining the hydrological alterations of related habitat pattern. The results show that no matter which time scale in this study, the amplitude of the downstream flow changes drastically after the weir construction, indicating that the downstream flow regime becomes unstable. The influence is mainly manifested in the 2-day scale to the 1-week scale period. For low-frequency with high-energy events, usually representing flood period, the annual scale impact is not significant; however, the corresponding critical environmental flows of lower-flow events are significantly affected during non-flood periods. The simulation results of the habitat suitability model found that the dominant habitat type was glide. We also found that after the weir operation, the total water area declined. Besides, both of the aquatic habitat diversity and habitat quality declined, particularly for the impact of run and riffle. The results reveal that the weir has significant negative effects on the downstream ecosystem. This study helps to provide suggestions for the construction and effective use of water conservancy projects in the future, as well as the management of the water environment in the downstream reaches. We suggest that dams and weirs can better exert their water conservancy functions while considering natural flow regime for maintaining the ecological benefits of downstream reaches.