二維不恆定河川水流數值模式之研究-以員山子分洪道為例

Abstract

本研究主要目的係以基隆河員山子分洪段為研究標的，藉此發展出一套良好的二維不恆定數值計算模式，以作為日後不管在學理研究或實務操作上的選擇考量。本研究針對員山子分洪段之水理特性來做探討，特別是為滿足颱洪期間高洪水位流量觀測之需求，利用本數值模式來推估該分洪段各分洪設施的流量，以期能更準確掌握包括：由基隆河上游進入員山子分洪段之入流量、經右岸側溢流堰分洪進入靜水沉砂池、經由分洪堰進入分洪隧道之洪水量，以及主河道經由攔河堰孔口排放至下游河道之洪水量，以提供未來在洪水預報、分洪管理及相關設施維護改善等利用之參考。 數值模擬方面，本研究採用二維MOC不恆定數值方法，藉由此河段上游集水區之降雨逕流分析之流量檢驗，河段上游進水口均屬於亞臨界流況，但本研究蒐集研究河段之水位邊界條件，僅有二場洪水歷線符合入水口水位高於出水口水位之物理邊界條件，取其一場做為率定二維區域之河槽粗糙係數，再以另一場之洪水歷線水位做為檢驗。計算結果大致符合量測結果且全區域均能滿足質量守恆之物理原則。本研究先將數值模式進行參數率定，且率定時注重不恆定流之水位-流量關係，並檢驗研究區域內的質量守恆。於河川不恆定流的水位與流量關係並非一對一關係，而恆定流為一對一之水位-流量率定曲線。河川之二維區域河段亦需以不恆定流加以正確模擬。其他場次之洪水歷線，因下游水位站水位高於上游水位站水位，但流況為亞臨界流，故判斷此一特別現象為該處之排沙閘門之操作所造成之影響，建議本研究之二維MOC模式必須修改時變性之邊界條件模擬。

The objective of this study is to apply 2D numerical method of characteristics to investigate relation of flow and stage elevation at the Yuansantze Diversion Section of Keelung River. It is intended to develop such a relation for practical engineering purposes to predict inflow and discharges through all of the outlets so that a better flood control and facility maintenance can be achieved. Numerically, a two dimension model of Characteristics is introduced and repeated to observe it performance properly. After applying a rainfall-runoff model for the upstream basin which has a outlet at the inflow cross-section and check the flow conditions, it is found that a hydraulic jump is impossible to occur at the downstream of the inlet under sub-critical flow condition. However, it was found that water elevations of field stages in this area indicate water elevations of downstream stages are higher than those of upstream in most cases. Rather, there are only two cases in which water elevations of the upstream gage stations are higher than those of downstream stations. One case is use for model calibration and the other for model verification. The computed results are compared with field measurements. Reasonable comparisons are observed. The difference may due to the effects of sudden closure of the sand gates. Mass conservation is checked as required. It is well known that there is no unique rating cure between discharge and elevation for unsteady flows. It is risky to extend rating cure based on steady assumption. An accurate numerical 2D model to simulate flows through a complicate river sections is strongly in need. A capability of the model to simulate the controlling behavior of sand gates is recommended to be incorporated into the numerical model for better simulations in the future.