河道穩定之疏浚規劃

Abstract

本研究疏浚之目的是為改善河道通洪能力，透過改變底床坡降與主深槽和漫灘的寬度比值，建構疏浚佈置設計。以受疏浚影響之河段內，所有斷面之深槽最低點之高程的平均絕對改變值(Mean Absolute Difference, MAD)為評估準則來設計採穩定性為主的疏浚佈置。MAD值較小表示其所對應的疏浚佈置會令河道產生較具穩定性的反應。本研究將統計學上的試驗設計(Design of Experiment, DOE)與水理輸砂數值模式(Sedimentation and River Hydraulics – One Dimension，SRH-1D)結合，有系統地評估各種疏浚佈置對河道縱剖面的影響。同時，也對訂定疏浚佈置的因子與河道穩定之關係作釐清。接著，再由變異數分析(Analysis of Variance)，分析各因子對河道穩定的相對重要性。

本論文研究河段，為自天輪壩(上游邊界)到大甲溪河口(下游邊界)之間 的主河道。疏浚工址，是位於松鶴橋與裡冷橋之間的四個河道斷面。模擬結果指出對原始河道而言，最輕微改變斷面的操作，並不必然是維持河道穩定性之最理想的疏浚佈置。進而，在較理想(即MAD值最小)的疏浚佈置下，沿河道之時間-平均之單位流功差TAUP之震盪幅度，趨近最小。明確而言，MAD值與TAUP值具高度正相關性。此結果說明基於維持河道穩定性，較理想的疏浚布置，外力加諸於河道的能量增減，較符合河性所需。因此，可同時參考MAD值與TAUP值，選擇其排序皆接近最小所對應的布置，作為較理想的疏浚布置。

In this study, an investigation of dredging effects on longitudinal riverbed profile is made by simulating 64 dredging arrangements with varying bed slope and channel-to-floodplain width ratio. Using MAD (Mean Absolute Difference) of riverbed changes as the stability criterion, this study presents a preliminary investigation using Statistical Design of Experiment (DOE), in conjunction with a numerical sediment transport model (SRH-1D), to systemically evaluate various dredging arrangements for identifying the most desirable one. Through the “Analysis of Variance” of the results from the DOE under different dredging arrangements, relative importance of various dredging factors affecting riverbed stability can be analyzed.

The study river reach extends from the Tianlun Dam to the Tachia River mouth to the Taiwan Strait within the Tachia River Basin. The selected four dredging sites along the study river reach are between Shonghu Bridge and Lilun Bridge. It is interesting to find that the dredging operation with minimal change in existing riverbed profile is not necessarily the best option to maintain channel stability. We also observe that a desirable dredging arrangement with low MAD value generally produces smaller fluctuation amplitude in the difference in time-averaged unit stream power along the channel ( TAUP  ). To maintain channel stability, effective dredging operation is required to consider its influence on river regime. Therefore, the MAD value and the TAUP  value can be used in the similar fashion, and the arrangement in which the MAD value and the TAUP value are arranged close to the minimum is the most desirable dredging arrangement.