河川之水流屬水文循環中重要環節，其豐枯季節流量變化更為水文觀測之重要項目，不論在水利工程相關之水資源管理調配及工程規劃設計，以及生態保育與水污染防制等之相關學術研究，均需賴準確可靠之流量觀測資料作為基礎參數，方得以進行相關分析作業。本論文主要是以現場觀測試驗分析的方式，透過觀測作業方式改善與實測資料分析，針對實際河道之表面流速與平均流速之關係進行探討，提出颱風豪雨期間如何快速量得渠道平均流速之可能建議，期待藉由觀測時間之縮短，進而降低觀測人員於惡劣天候下可能遭遇之風險。
現場觀測試驗的部分，選定曾文溪流域中下游地區之流量站為試驗位址，分析其表面流速與平均流速之關係，以現場試驗方式進行流速關係推求，所得之成果可做為工作人員實務作業之參考。實測資料以線性迴歸的方式來推求各流量站其平均流速與表面流速的比值，新中(1)站為0.83，二溪大橋站為0.75，左鎮站為0.85，玉田站為0.81，玉豐大橋站為0.80。新中(1)站於全洪程觀測洪水漲退過程期間，河道底床變化之情況顯示，河道主深槽高程於洪水退水階段初期仍有下降之情況。利用體積法完成5個水文測站站址之河床質採樣調查，分析計算各水文站河床粗糙長度值ks，推得其粗糙長度之經驗公式為ks=D30，則ks建議值新中(1)站為1.83mm，左鎮站為0.022mm，玉田站為0.007mm，玉豐大橋站為86.41mm，二溪大橋站為1.23mm。

Flows in river play a crucial role in hydrologic circulation, so the flow variations in dry and wet seasons are even more important in hydrologic observation. Without reliable and accurate flow observation data as basic parameters, neither engineering planning and design and management and regulation of water resources for hydraulic engineering projects nor academic researches in ecological conservation and water pollution control can conduct their analytic works. This paper investigates the correlation between surface velocity and averaged velocity of an actual river course with field observation, experiment and analysis which mean improved method of observation and analysis of measured data. The investigation results are used to provide feasible advices for swift measurement of averaged channel flow velocities in typhoon and torrential rain. The reduction of measurement time will improve the safety of observers under bad weather.
The field observation and analysis choose five stream gauging stations in midstream and downstream of Tsengwen river as experimental sites to analyze the correlations between their surface velocities and averaged velocities. The correlations derived with field experiments provide the operational reference for field workers. The ratio of surface velocity to averaged velocity of each stream gauging station is derived by analyzing measured data with linear regression: the ratio of Hsin-Chung(1) station is 0.83; the ratio of Er-Si station is 0.75; the ratio of Zho-Jhang station is 0.85; the ratio of Yu-Ten station is 0.81; the ratio of Yu-Feng station is 0.80. The measured cross-section water depth shows that channel level decreases during flood recession. Volume method is used to examine the bottom samples of all five stations. A suggestion of the roughness length ks of Tseng-Wen River is ks=D30. It’s estimated that the roughness length ks of each station: the roughness length ks of Hsin-Chung(1) station is 1.83mm; the roughness length ks of Zho-Jhang station is 0.022mm; the roughness length ks of Yu-Ten station is 0.007mm; the roughness length ks of Yu-Feng station is 86.41mm; the roughness length ks of Er-Si station is 1.23mm.

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