開口式防砂壩流量推估模式理論發展與試驗分析

Theoretical and Experimental Analyses of Discharge Estimation for Open Check Dams

詹勳全(Hsun-Chuan Chan) ； 楊信凱(Hsin-Kai Yang) ； 林柏瑋(Po-Wei Lin) ； 雷子毅(Zih-Yi Lei)

《中華水土保持學報》 50卷1期 (2019/03) Pp. 11-21

https://doi.org/10.29417/JCSWC.201903_50(1).0002

摘要

本研究旨在建立通過開口式防砂壩流量推估模式，提供分析通過防砂壩流量之參考。模式分別估算防砂壩梯形溢洪口及其下方矩形開口之流量，以線性方式進行流量疊加，由於水流通過防砂壩之流況會因水深變化而有所差異，本研究將防砂壩流況依據壩上水深與壩頂長之比值區分為寬頂堰及銳緣堰流況，並考慮防砂壩上游河道通水斷面積與防砂壩開口斷面積兩者差異之影響，利用無因次化之通水斷面積比修正流量推估模式中的流量係數，使模式能有效反映壩體開口狹小產生之束縮效應，提升模式推估流量之準確性。透過不同梯形溢洪口及矩形開口組合之五種開口式防砂壩模型進行110組試驗與分析後得知，當水流僅於矩形開口內流動時，近似於寬頂堰流況，此時因壩體開口較小導致束縮效應明顯，使流量係數與通水斷面積比兩者關係成一次線性遞增，且決定係數R^2為0.870；當水流漫淹至梯形溢洪口時，流況近似於水流通過銳緣堰之情況，梯形上寬下窄的幾何特性導致束縮效應隨流量增加而減小，流量係數與通水斷面積比兩者關係成一次線性遞減，而決定係數R^2為0.848；在不同流況下流量係數均與通水斷面積比呈高度相關，推估流量與實際流量之平均誤差百分比皆小於3%。另外經比較本文模式及以往模式推估本研究實驗成果可知，以往模式推估流量之平均誤差介於4%至25%，本文模式推估流量的平均誤差遠小於以往模式，由於本研究模式僅需以防砂壩開口斷面尺寸及壩上水深即可推估流量，可兼顧準確性及便利性。

關鍵字

開口式防砂壩 ； 流量係數 ； 流量

並列摘要

This study established a model for estimating the flow rate of discharge from an open check dam. The model separately estimated flow rates at the trapezoidal spillway and rectangular opening below the spillway. A linear combination of both flow rates was used to estimate the total flow rate of the dam. Considering that flow conditions varied with the water depth and dam length, flow conditions of the check dam were grouped into broad-crested weir condition and sharp-crested weir condition based on the aforementioned ratio. Moreover, a dimensionless factor water area ratio, A_r, corresponding to the influence of the difference between the flow area of the channel and the open check dam, was used to estimate the flow rate coefficient. This was expected to effectively reflect the contracting effect due to the narrow opening of the check dam in the model and improve estimation accuracy. A total of 110 experiments were performed on five opening check dams with different combinations of trapezoidal spillway and rectangular openings. The results revealed that flow conditions were similar to those of the broad-crested weir when water flowed only through the rectangular opening. At this point, the contracting effect was significant because of the small opening of the dam, which increased the flow rate coefficient linearly with increasing A_r. The coefficient of determination, R^2, was equal to 0.870. In case of flow through the trapezoidal spillway, flow conditions were similar to those of the sharpcrested weir. The geometry of the wide top and narrow bottom in a trapezoid reduced the contracting effect as the flow rate increased. The flow rate coefficient decreased linearly as A_r increased. R^2 was equal to 0.848. The flow rate coefficient was highly correlated to A_r under different flow conditions. When estimated and measured flow rates were compared, averaged error decreased below 3%. In addition, estimation results from the present model and previous models revealed that averaged errors produced by the previous models ranged between 5% and 30%, which were higher than those produced by the present model. The present model requires only the dimensions and water depth of the check dam for flow rate estimation and provides both accurate and convenient estimation of flow rates for an open check dam.

並列關鍵字

Open check dam ； Discharge coefficient ； Discharge