遮蔽效應對混合粒徑輸砂量之影響探討

Analysis of the Armoring Effect to Mixed-Grain Sediment Discharge

吳榮瑜(Zong-Yu Wu)

指導教授 ： 黃宏斌

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/碩士(2007年)

https://doi.org/10.6342/NTU.2007.00704

摘要

本研究以混合粒徑進行輸砂量試驗並觀察試驗過程中泥砂運移現象，發現混合粒徑渠床對於不同水流作用會產生不同程度之分選或遮蔽效應。
在輸砂量試驗中，由於因遮蔽效應所造成之影響尚待進一步之探討，故本研究進行渠槽試驗，蒐集前人研究之試驗數據資料，增加粒徑及流量組數，擴大試驗範圍，並記錄輸砂量隨時間之變化情形，找出輸砂量遽增之時刻以及前後輸砂量之差異，分析遮蔽效應之運作模式，藉以修正目前之輸砂公式。
為分析混合粒徑所造成遮蔽效應之影響，採用Einstein（1950）所提出之序率型輸砂模式，利用無因次輸砂強度 及無因次水流參數ψ進行分析，並與定率觀點之Shields’ Diagram相互對照，發現兩個轉折點，其中一個轉折點為ψ =18.07， ，且泥砂顆粒之邊界雷諾數（Boundary Reynolds number） ；另一個轉折點則為ψ =2.78， ， 。因此，Shields’ Diagram曲線上之兩個轉折點可將輸砂過程區分為三個階段。亦即當ψ >18.07時，表示水流之淘刷力小而未能起動河床質顆粒運動，無論均勻或混合粒徑都呈現低度之輸砂率；隨著水流強度增大，18.07 ≥ ψ > 2.78，河床質開始分選，粗顆粒造成護甲層，對細顆粒產生遮蔽效應，抑制細顆粒流失；至水流強度大到帶動粗顆粒，2.78 ≥ ψ時輸砂量大增。而且各階段間之連接都具有轉折現象表示，於不同之水流強度階段，輸砂率受泥砂顆粒混合粒徑之影響與水流強度間呈現不同之關係。
為了推求輸砂量與水流強度間之關係式，採用Einstein之無因次參數，以D50為代表粒徑，將所收集到之前人研究試驗資料798組，加上本研究所從事之30組，共828組，進行回歸分析，可得下列輸砂量模式。
與其他各家輸砂模式比較，本研究所推導之模式由於將遮蔽效應所可能產生之作用納入考慮，故與考慮泥砂粒徑分佈之標準偏差（σ）謝孟荃模式較為吻合；並以序率觀點之考慮泥砂起動之機率對輸砂量之估算之影響，參照Einstein之無因次參數進行分析，結果與Einstein之結果符合；另外，以無因次之水流強度將河床質泥砂顆粒運動情形分隔出不同之區段，可以發現遮蔽效應及水流對不同粒徑大小之泥砂顆粒分選所造成對輸砂量之影響。

關鍵字

混合粒徑 ； 渠槽試驗 ； 輸砂量 ； 遮蔽效應 ； 序率

並列摘要

This study used mixed-grain as the materials to do sediment transport flume experiment and observed the motion of sediment in the process of the experiment. We could find that mixed-grain in the channel-bed was occurred different sorting or hiding phenomenon in the different flow discharges.
Because the difference caused by hiding phenomenon during experiment haven’t been further discovered in the experiment of sediment discharge, therefore we proceed channel experiments in this study, collect experimental data of existing study, add the number of combination of sediment sizes and flow discharge, and expand experimental data boundary. Besides, we record the transformation of sediment discharge as time passes to discover the time of rapid increase and the difference between before and after the time in sediment discharge to assay operating model of hiding phenomenon.
In order to analyze hiding phenomenon caused by mixed-grain, this study use stochastic type model of sediment discharge proposed by Einstein in 1950.To analyze with using dimensionless sediment transport discharge parameter and dimensionless flow parameter ψ.
This research also compared with Shields’ Diagram which is deterministic. Two turning points are found. One of them are
ψ =18.07, and Boundary Reynolds number .Another turning point is ψ =2.78, ， .Therefore, these two the turning points located on the curve of Shields’ Diagram can divine the process of sediment discharge into three sections. Such as when ψ >18.07, it means small flow discharge can’t motivate the moment of particles of bed material with little scouting toward particles on the river bed ,low sediment discharge is found in both homogenous and mixed-grain. With the increasing of flow discharge as 18.07 >ψ > 2.78,particles on river bed start sorting. Armoring layers are caused by bigger particles to form hiding phenomenon and prevent sediment discharge of smaller particles. Till flow discharge is rapid enough to motivate the moment of bigger particles when 2.78 > ψ, sediment discharge increases rapidly. And the connections of each section all appearances turning ,which tells sediment discharge shows different relationships with flow discharge between each different section because of the influence of mixed-grain sediment distribution in different flow discharge sections.
In order to find out the equation between sediment discharge and flow discharge, Einstein’s dimensionless parameters are adopted and D50 is used as the character grain parameter of sediment particles. Totally 828 sets of experimental data including 798 which have been collected from the literature reviews with adding 30 which have been done in this research are taken to process the multiple regression analysis. The models for sediment transport which is listed below can be found.
By Comparing to each everyone of those other models for sediment transport, the model derived by this research matches with the model of Meng-Chyung Shieh because both models consider standard deviation of particle grained distribution（σ）.But also the Stochastic point of view is taken to consider the effects to sediment discharge caused by the possibility of sediment motivation. To process the analysis according to Einstein’s dimensionless parameters, the results match with Einstein’s. Besides, the moment of sediment particles in bed material load is divined into different sections by dimensionless flow discharge. It can be found that hiding phenomenon and different sorting caused by flow to different grain sizes of sediment particles cause the effects toward sediment discharge.

並列關鍵字

Mixed Grain Size ； Flume Experiment ； Sediment Discharge ； Hiding Phenomenon ； Stochastic

參考文獻

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