| 英文摘要
| 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. |
