The Examination of Impacts of Artificial Cutoff on Channel Shift

Artificial cutoffs are short channels excavated across the neck of river bends to improve channel alignment for navigation or as a flood control measure. The purpose of this paper is to analyze the effects of cutoffs on channel shifts simulated for different channel types and various flow conditions. "Linear Theory of River Meanders" [Johannesson and Parker, 1989] is adopted to evaluate the shifts of both original and cutoff channels. Result analysis is based on the responses, especially the perturbed term u1 of the downstream depth-averaged velocity U, of the simulated channels under the hypothetical flow conditions. The limits of width to depth ratio for applying this model is briefly discussed and results of numerical experiments are summarized. Results indicate that cutoffs have little effect on the upstream channel shift, yet rather significant channel alteration in the downstream reach. Also, the reach averaged centerline velocities U for simulated channels with stream power less than 35Wm^(-1) are of the same order of magnitude as the computed excess velocities u1.

關鍵字

Perturbation method ； Cutoff ； Channel shift

並列摘要

河川採取人爲截彎取直（Artificial Cutoff）最主要是以便利航運或防洪爲目的。本文將藉由擾動法（Perturbation Approach）來探討截彎取直對於河川移位（Channel Shift）之影響。文中將比較截彎取直對於不同河川類型、底床坡度、流量及水流功率（Stream Power）所造成之移位。至於截彎取直所造成之移位將利用「河川蜿蜒的線性理論」（Linear Theory of River Meanders, Johanesson and Parke, 1989）配合數學模式加以模擬計算。本文中數值模擬分爲兩部份：（1）截彎取直對於不同類型之河川所造成的移位，以及（2）截彎取直對於相同類型之河川但不同底床坡度、流量及水流功率（Stream Power）所造成的移位。結果分析顯示，截彎取直工程對下游段之河川移位影響顯著，且河川之水流功率值（=35Wm^(-2)）爲估計河川冲刷能力之有效指標。於使用數學模式的可能偏差將於文中一併討論。

並列關鍵字

擾動法；截彎取直；河川移位

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