# 臺灣博碩士論文加值系統

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 本文採用實驗與數值方法，探討水流於彎道動床沖淤平衡後床形於彎 道各處砂面之橫向剖面、縱向剖面以及橫向坡度於縱向上之變化，並與 Odgaard (1981)之彎道橫向坡度計算公式做比較。實驗於成功大學水利及 海洋工程學系一樓雷射實驗室20公分寬之180 度彎道循環水槽中進行，待 彎道底床受水流作用至底床沖淤平衡時，沿彎道縱向上每隔10度量測底床 之高程，分析得底床坡度。另外亦運用二維紊流計算模式DIM-2U，模擬床 形變化並以實驗結果互為比對。 由本文之實驗結果得知，於彎道20度前，底床沖淤變動小，於彎道30度後 底床之橫向剖面才有明顯之改變 ； 由彎道各處之砂面沖淤縱向剖面圖中 可看出距右岸15公分 (b/B=0.75)及10公分公分(b/B=0.5)處，即彎 道內岸及中心處，於彎道50度至120度間底床呈下游高於上游逐漸淤積型 態，最高淤至+0.6h(h為上游均勻流水深)， 彎道內岸區120度至160度間 則呈淤積量漸減之趨勢；距右岸5公分處(b/B=0.25)，即外岸區，於彎 道30度至50度間底床沖刷呈向下游逐漸刷深之型態，最深刷至 -0.9h~1.7 h ， 50度至100度間刷深量漸小，100度至180度間刷深量又由0.2h~0.5h 刷深至-1.0h。 Odgaard（1984）之公 式所得之結果與實驗所得相近，顯示應用Odgaard（1984）之公式以預測 底床橫向坡度有不錯之結果。 由本文中探討得知，二維之底床沖淤模式如DIM-2U，均需考慮橫向坡度及 其對橫向輸砂之影響，才可對180度彎道動床之模擬得到較佳之結果。
 Bed configuration in a 180 alluvial channel bend at steady statecondition was studied experimentally and numerically. The experimentswere conducted in a 20cm wide glass channel. Elevations of the channelbed at steady state were measured transversely at cross-sections of thebend in interval. Topography as well as the longitudinal, transverseslopes were obtained from the measured data. It is observed from the bed configuration that the bed variation in achannel bend could be divided longitudinally into five regions andtransversely into two regions. Transversely, the two regions are the innerbank and the outer bank regions. Bed material tends to be eroded from theouter bank region and deposited at the inner bank region.Longitudinally, the five regions are the entrance(0~20),front(20~50), mid(50~100), rear(100~135) and the exit(135~180)regions. At the entrance, bed is keeping flat and at the front region, thebed at inner bank deposits increasingly from 20~35 and thendecreasingly from 35~50 ; the bed at outer bank however graduallyerodes and reaches the deepest erosion point at 50. This erosion couldreach -1.5 water depth. At the mid region, bed elevations at both innerand outer banks rise and are higher than those at 50. The highest pointsare at 120 at the inner and at 100 at the outer banks. At the rear region,bed elevations at both inner and outer banks decrease and reach thesecond largest erosion hole, -1.0h at 140 . The erosion and depositionpatterns at the exit region are not consistent due to upstream influences. Further analysis show that Odgaard's (1984) formula describes thetransverse slope at the channel bend reasonably well. A 2-D alluvialmodel like DIM-2U should consider the mechanism of the transverse bedslope and it's effect on the transverse sediment transport, such that thebed evolution in a alluvial channel bend can be simulated.
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