雨水下水道多以在設計流量下為等速渠流的條件進行設計，但亞臨界流流經渠道突擴段會使水位抬升，並影響至上游，因此各渠段之迴水影響可能導致雨水下水道的排水能力無法達到設計要求。本研究依據明渠水力學原理，建立能評估匯流井及人孔水位抬升影響之雨水下水道設計模式，以及雨水下水道流量演算模式等二數學模式，並以水工模型試驗進行驗證。

　　雨水下水道中之流況，可以是明渠流、管流、閘流或混合型的流動型態。設計時，一般要求為明渠流，故本研究只進行明渠亞臨界流實驗。由實驗結果可看出匯流井突擴段的水位抬升所導致的迴水影響，使得上游水面明顯抬升，而數學模式模擬之流量及水位亦與實驗值頗為一致。

　　由實驗及演算結果之比較與討論，顯示在無外水影響之情況下，本研究所建立之雨水下水道設計模式及雨水下水道流量演算模式均具有實用性，可提供雨水下水道設計之參考。

　The size of storm sewer is decided by steady uniform flow for designed discharge. The water surface would be raised when subcritical flow passed through a sudden expansion. The ability of conveyance is not up to designed standard induced by effect of backwater, because of distance is not long enough between each junction chamber or each manhole. According to open channel flow theory, the paper establishes two storm sewer models. One can estimate the influence of rise in water stage at junction chamber or manhole, and the other can calculate the discharge of storm sewer. Also, both of storm sewer models were verified by the experiment.

　Flow types in sewers can be open channel flow, pipe flow, gate flow or mixed flow. Open channel flow is usually requested for the design of sewers. In this paper, subcritical flow is discussed. The results of experiment indicate that the water surface of upstream was raised cause of the expansion of junction chamber. The comparison for simulating and experiment results included discharge and water stage is similar. For designing storm sewer with no lateral flow, two storm sewer models established in the paper are applicable to refer to.

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