圖書典藏及影音數位平台

民國97年士文水庫可行性規劃(7)執行期間，辦理提送環境影響說明書前之公開會議及拜會水庫下游枋寮鄉公所與鄉代表會座談會等士文水庫計畫資訊公開說明後，其中水庫營運後對下游地下水之影響為地方亟為關切之課題，宜進一步辦理相關調查研究並量化分析以澄清民眾之疑慮。

The development of Shi-Wen Reservoir will require a construction of cut-off wall to stop water seepage. This will reduce water seepage to alluvium downstream from the dam. In addition, through controlled release of reservoir discharge, the amount of flow rate at the downstream side of the dam shall decrease and so will be the source of ground water. The purpose of this study is to evaluate the impact of dam cut-off on ground-water condition downstream from the dam, to clarify concerns raised by general public. The results of study are summarized below: 1. A total of 30 channel cross-sections were surveyed. In comparison with data obtained in 2007, it was found that appreciable siltation had occurred on Shi-Wen Hsi, especially near the No.3 Shi-Wen Bridge. 2. Results of ground water observation indicated that there is a direct positive correlation between ground water level and rainfall. The ground water level before and after rainfall can vary several meters, indicating the rate of seepage is high. 3. Gaging station was installed on Naihsi Canal of Pingtong Irrigation Association to evaluate actual water drawn by the canal. It was found the amount of water intake in June and December are appreciably higher than those specified by water right. The situation also occurred in other months. The amount of water withdrawn is affected by the operation of the intake gate. The Association should maintain a gate operation record and monitor its water intake data, to avoid causing adverse impact on the river ecology. 4. The observed ground water level data were also simulated by MODFLOW. It was determined that the permeability of the river sediment is in the range of 1.46~3.34×10-5m/s. 5. The effective storage of the Shi-Wen Reservoir is 65.59×106m3. The construction of the reservoir will result in a reduction of annual river runoff of about 76.17×106m3. This mounts to about 40% of the total annual runoff. 6. The construction of dam cut off is estimated to reduce a subsurface seepage volume of 2.3×105m3. 7. Analysis of hydrologic data from 1962 to 2010 revealed that average annual seepage volume is about 7.34×106m3. After the dam construction, the seepage volume would be 4.56×10-3m3, a reduction of 2.78×106m3. For the year with the highest rainfall (1972), the seepage volume would be 76.5% of that prior to the dam construction. For the year with the least rainfall (1980), the seepage volume is 63.6%. 8. The geological condition at the dam site is similar to that at Chatong Primary School. Therefore the ground subsidence data observed at the school was used to estimate possible degree of land subsidence at the dam. At Chatong the relationship between land subsidence and ground water level reduction is 4.59cm per m. From this data, it is estimated that the maximum subsidence is 5.65cm and the annual subsidence is only 0.19cm. This indicates that after dam construction the amount of ground subsidence would be small.