水庫異重流排砂效率及運移行為之數值模擬與模型試驗

Abstract

當水庫集水區水流進入水庫後，因流速減緩使得輸砂能力降低，其挾帶入庫的泥砂可能沉降庫底或形成水庫異重流，若入庫的泥砂在水庫形成異重流且運移至壩體時，其運動行為將受到下游壩體形狀之影響。本研究進行水槽試驗，用以探討壩體附近無排砂操作情況下，水庫異重流運移至壩前，受阻於不同水庫壩體坡度後產生的現象及行為。由試驗結果可知，在相同單寬入流量及濃度下，異重流在壩體斜坡之爬升速度以及最大爬升高度與壩體坡度成正比關係；但在相同壩體坡度與單寬入流流量下，異重流最大爬升高度與異重流壅高高度反比於入流濃度。 此外，試驗結果分析得知，異重流正湧波之波速主要受理查遜數(Richardson number)、異重流正湧波所行之相對距離及壩體坡度之影響。本研究異重流正湧波波速之經驗式，可供不同壩體坡度的水庫概算出異重流正湧波迴溯上游之距離及速度，有助於估算形成異重流水庫所需的時間。 本研究利用計算流體力學模擬模式(CFX)，以及水槽試驗的試驗結果，模擬異重流於試驗水槽中的運移過程，其中包含異重流頭部與本體的運移、異重流沿著下游尾水板爬升與異重流正湧波的形成；在不考慮泥砂與清水間之動量交換條件下，採用代數滑移數值方法 (Algebraic Slip Model, ASM)來模擬異重流濃度與速度隨著時間的變化，以及異重流正湧波的沿程運移。本研究採用高嶺土試驗與鹽水試驗所量測的數據，進行數值模式的檢定驗證與適用性探討。並將檢定驗證完成的參數與數值方法，作為模擬鹽水異重流受到下游尾水板傾斜角90度與45度影響的依據。由模擬結果可知，模式模擬高嶺土異重流與鹽水異重流受到下游尾水板傾斜角90度的影響前，到達下游尾水板前的沿程速度剖面與濃度剖面，其相對誤差約為5%，而受到下游尾水板傾斜角90度的影響後，異重流正湧波波速相對誤差則約為10%；模擬鹽水異重流受到下游尾水板傾斜角45度的應用例時，除了異重流平均爬升速度約有6.98%的相對誤差、異重流爬升高度有約5.94%的相對誤差，以及異重流正湧波的模擬速度有約6%的相對誤差外，CFX模式模擬的異重流頭速、異重流厚度及異重流正湧波厚度平均相對誤差皆小於5%，因此，經由試驗案例的模擬分析，採用CFX模式中的ASM數值方法求解異重流各試驗案例，其模擬結果相當符合試驗值。 本研究經由石門水庫之潛入點位置分析後發現，利用潛入點經驗公式以及理論公式所推求之潛入點發生位置，以密度福祿數為1 ( )時最為接近。此外，利用霞雲站迴歸公式及用羅浮站實際量測數據所推估之潛入點位置約有2個斷面的差異性。若以霞雲站迴歸公式與羅浮站實際量測數據所推估之密度福祿數，在尖峰流量過後則約有1.5倍的差異性。因此若以霞雲站迴歸公式所推估之密度福祿數用以判斷潛入點發生位置，則密度福祿數建議採用2.0，即可約略等同於用羅浮站實際量測數據所求得之實際異重流潛入點位置。 最後本研究根據在曾文水庫在壩前所觀測之分層泥砂濃度值，所推估之發電取水口(PPI)及永久河道放水口(PRO)之排砂濃度及排砂效率可知，不論是出流泥砂濃度值或是出流泥砂濃度之逐時變化，本研究校正過後之方程式最能符合實際量測之出水泥砂濃度及排砂效率，且由壩前數值模擬泥砂濃度及實測數據所推估之排砂效率，率定出排砂效率參數K值約等於0.2。且根據所計算之排砂效率相對誤差值可知，本研究所採用之公式具有相對準確性，且由數值模擬結果可知設置減淤通道前雖然各出水工(PRO、發電取水口及溢洪道)之排砂比較高。但相對將泥砂帶至壩前的量也越多，越容易導致泥砂沉積在壩前而影響日後之出水工操作。當減淤通道設置後，大部分之泥砂會被減淤通道帶離水庫，因此相對降低了各出水工之排砂效率，此現象對於出水工之持續運作有所幫助。且由模擬之減淤通道排砂比可知，規劃之減淤通道之排砂比皆大於各出水工之效能，對於水庫防淤有更進一步的助益。

When turbidity current flows into a reservoir, the main cause of sedimentation problem is the sediment transport ability. The sediment transport ability decays by means of velocity when turbidity current flows into a reservoir. After that, the inflow sediment may deposit on the bottom of the reservoir or develops to the density current. When density current develops and moves to the downstream, its flow mechanism is affected by dam. Therefore, this study focus on the behavior and character of density current during density current is affected by different dam slopes. The experimental results present the density current climbing velocity and the maximum climbing height are directly proportional to the dam slopes. But, the maximum density current climbing height and its thickness are inverse proportional to the inflow concentration. On the other hand, the positive surge is mainly affected by Richardson number, the relative distance traveled by positive surge and dam slopes. Based on this research, the experimental formulas can utilize to compute positive surge velocity under variant slopes of the dam. It can be helpfully to estimate the timing of reservoir turning into turbidity. A laboratory study was conducted using a flume which combined CFX, fluid simulation software with the experimental results to simulate the density urrent movement. Algebraic Slip Model (ASM) scheme of CFX was adopted to simulate the hydraulic properties of density current including the head velocity, concentration and velocity distribution of body movement, climbing velocity and climbing height along the dam slope, thickness of density current and positive surge velocity. Since particle fall velocity was the major physical parameter in ASM scheme. It was taken into account for sediment transport between sediment and clear water. The kaolin and saline water were used as density current fluid materials. The results of the experiment were employed for numerical calibration and verification. The CFX was then employed to simulate the movement of density current movement with downstream shape of 900 and 450. The calibration and verification results showed that relative deviation (5%) in concentration and velocity distribution was satisfied. The simulation results indicate that relative deviation in climbing velocity and climbing height along the slope were 6.98% and 5.94%, respectively. Other hydraulic properties mentioned above also had less than 6% relative deviations. These experimental values agree well with the results of numerical simulation. In the result, this numerical tool is suitable for simulation the movement of the density current. This study based on empirical formula, theoretical derivation and field observation to investigate plunge point variation in Shihmen reservoir. A successful estimation of plunge area was been used to analysis the possibility position of plunge point. Based on the investigation results, the plunge point is approximated at . In addition, the plunge area have 2 section difference due to the inflow sediment concentration is from regression formula and field measurement. According to the estimation of , there are 4 times difference and 1.5 times difference before peak discharge and recession duration, respectively. Therefore, the is suggested to estimate plunge point when inflow sediment concentration is used from regression formula. The research results of Tsengwen reservoir indicate that proposed equation is relatively accuracy to estimate venting efficiency of power plant intake and permanent river outlet. The simulated hydrograph of venting efficiency using modified equation is much better than others. And, the venting coefficient K of modified equation is calibrated to 0.2 that was using Typhoon event in TsengWen reservoir. Besides, although lower structures can drainage higher sediment concentration and reach higher venting efficiency, but, large sediment would be carried to near the dam to increase deposition elevation. Over a period of time, deposition elevation would higher than intake elevation and affect its operation. Therefore, if bypass tunnel and desiltation passage can be established at upstream the deposition delta can be controlled and deposition elevation of dam site also could be maintained. Moreover, if drainage facility could be set more upstream of reservoir, coarse material could be flushing out by bypass tunnel and fine sediment could be vented by desiltation passage. In addition, such planed concept can prevent reservoir sedimentation and keep more water resources.