Study on the Sand Stabilizing Effects of the Windbreak Arrangemens in Wind Tunnel Experiment

台灣西部沿海地區飛砂問題嚴重，雖防風林帶已廣泛種植以防杜風害與砂害，然防風林帶樹種組合之不同，對於定砂效果必然相異，且近年來國內廣泛推廣混交林以強化更新林相之防護機能。本研究針對低木、高木與混交林等三種林況，分別設計十組不同林型，每種林型厚度皆為4H，分別為純高木組成之A-8型、純低木組成之B-8型與林型1～8之混交林型等共十組，利用風洞設備進行吹砂試驗，探討定砂功效之異同，低木樹種選擇黃槿(Hibiscus tiliaceus L.)，高木樹種則選擇木麻黃(Casuarina equisetifolia L.)作為代表，實驗用砂取自大安溪與大甲溪出海口處飛砂嚴重區域，並配合砂粒物性分析以了解砂粒之特性。 物性分析結果：大安溪與大甲溪現地含水率皆在1.2%以下，平均粒徑為0.14mm與0.22mm，平均密度為2.72g/cm3與2.66 g/cm3，質地皆為砂土，大甲溪之砂粒較為接近球型。 風洞試驗結果：砂粒粒徑範圍0.25～0.84mm時，起動風速介於5.0～8.0m/s之間，砂粒粒徑小於0.25mm時起動風速為4.0m/s；防風林帶最佳攔砂厚度約為2.5H～3H，不同持續時間對林後飛砂量變化影響不大，空流場量測結果邊界層厚度約25cm。 B-8型（黃槿純林）林帶後方堆砂量最多，砂丘主要產生在林帶後方，定砂效果最差，故配有高木之林型定砂效果優於純低木之林型；林型4（前高後低）定砂效果高達90%，然林帶內砂丘有漫延至林帶後方之現象，若考慮堆砂緩慢，且對林帶後方防護較佳而定砂效果次之林型1（低高單排交錯）為最佳之定砂林型。The coastal areas at the western Taiwan have serious problems of aeolian sand. Although the windbreak forest belt has been extensively planted to prevent wind damage and sand damage, however, the different sand fixation effect between difference combination of tree species of windbreak forest belt will be dissimilar inevitable. Moreover, in recent years, mixed layered forest has been widely promoted in Taiwan to enhance the protection function of renewal forest. The research aimed at three kinds of forests conditions of low woods, high woods, and mixed forest to design ten different forest types. The thickness of each forest type was 4H, and they were: Type A-8 consisted of pure high wood, Type B-8 consisted of pure low wood, and eight mixed forest types of Type 1~8. The sand blowing test in wind tunnel was utilized to explore the difference of sand fixation effect. The tree species of low wood adopted Linden hibiscus (Hibiscus tiliaceus L.), and Beef Wood (Casuarina equisetifolia L.) was chosen as the representative of the tree species of high woods. The experiment sands were got from the districts of serious aeolian sand at the mouths of Da-an River and Dajia River, and the research cooperated with the physical property analysis to understand the features of sands. The results of physical property analysis: The on-site moisture content of Da-an River and Dajia River was both under 1.2%; the average grain diameter was 0.14mm and 0.22mm; the average density was 2.72g/cm3 and 2.66 g/cm3; the texture was both sandy soil, but the sands of Dajia River was closer to ball shape. The results of wind tunnel test: The start-up wind speed was between 5.0 and 8.0m/s when the range of sand grain diameter was 0.25～0.84mm; the start-up wind speed was 4.0m/s when the sands diameter was smaller than 0.25mm; the best sand fixation thickness was 2.5H~3H; different duration did not have much influence on the variation of aeolian sand quantity behind the forest; the air flow field measuring results: the boundary layer thickness was 25cm approximately. Type B-8 of pure forest of Linden hibiscus had the most sand accumulation quantity behind the forest belt; the sand hill accumulated behind the forest primarily, and its sand fixation effect was the worst. Therefore, the sand fixation effects of forest types with high woods were better than the forest type with pure low wood. Forest Type 4’s sand fixation effect achieved 90%; however, the sand hill within the forest belt had the phenomenon of stretching to the place behind the forest belt. If considering the forest type of slow sand accumulation, better protective property to the place behind the forest belt, and second sand fixation effect, Type 1 (low wood and high wood staggered by single row) was the best sand fixation forest type.摘要 I Abstract II 目錄 IV 圖目錄 VI 表目錄 X 符號說明表 XI 第一章 緒論 1 一、前言 1 二、研究動機 3 三、研究目的 5 四、研究流程 6 第二章 前人研究 7 一、大氣紊流模擬 7 二、風洞實驗之相關研究 8 三、飛砂之相關研究 10 四、防風林之相關研究 11 第三章 採樣地點、材料與方法 13 一、採樣點介紹 13 (一)大安溪 13 (二)大甲溪 15 二、材料與方法 17 (一)物性分析 17 1.砂粒含水率 17 2.粒徑分析 18 3.砂粒密度 19 4.砂粒質地 21 5. 砂粒形狀係數 23 (二)風洞試驗 24 1.風洞設備 24 2.模型設計 27 3.風洞預備試驗 28 4.風洞主軸試驗 29 第四章 結果與討論 33 一、砂粒物理性質分析 33 (一)砂粒含水率分析 33 (二)砂粒粒徑分析 33 (三)砂粒密度分析 34 (四)砂粒質地分析 34 (五)砂粒形狀係數分析 35 二、風洞實驗 37 (一)不同砂粒起動風速 37 (二)不同厚度吹砂觀測 38 (三)不同吹砂時間觀測 39 (四)空流場量測 40 (五)不同林型配置之防砂效果 41 1.地形分析 41 2.飛砂量探討 59 3.砂丘運移變化 59 第五章 結論與建議 64 一、結論 64 二、建議 64 參考文獻 65 附 錄 6

The Study on the Sand Fixation Effects of the Windbreak Arrangements

本研究為瞭解防風林林冠組成對飛砂防制之效果，選定大安溪與大甲溪鄰近出海口處，飛砂及揚塵危害嚴重地點設立樣區，採集試驗所需之砂粒樣本，再於室內進行砂粒物性分析，並於風洞從事砂粒起動風速、不同林帶厚度、不同吹風時間及林冠組成對飛砂量影響之觀測；試驗結果顯示，大安溪與大甲溪土壤質地皆為級配不良的砂土，密度為2.72g/cm^3與2.66g/cm^3，平均粒徑與形狀係數分別為0.27mm、0.951及0.32mm、0.940，而砂粒含水率皆在1％以下；再者，粒徑2～0.84mm之砂粒其起動風速為13.5～14.5m/s；粒徑0.84～0.42mm之砂粒其起動風速為7.5～8.5m/s；粒徑0.42～0.25mm之砂粒其起動風速為5.5～6.5m/s；粒徑0.25mm以下之砂粒其起動風速低於4.5m/s。又防風林帶最佳攔砂厚度為2.5～3.0倍木麻黃模型高，而不同吹風時間對林後飛砂量變化影響不大，在實驗個案中，吹風10分鐘後飛砂量皆漸趨穩定。至於林冠組成對定砂效果方面，本研究中選用木麻黃及黃槿兩種樹種，組合成10種不同林型配置，結果以前段高後段低之混交林，具有最佳之定砂效果；而低矮之單純林，定砂效果最差。For investigating the sand fixation effects with the tree species combinations of the windbreaks, we chose the estuaries that the locations were with serious harms such as blown sands and dust emissions near Daan and Dajia Rivers to set up the sampling spots for collecting the required sand samples for tests, then performed the sand property analysis indoors and observed the results caused by different wind speeds, sand particle sizes, woodland belt thickness, wind durations, canopy compositions and clown sand quantities in the wind tunnel. The test results indicated that the soil textures of Daan and Dajia Rivers were poorly graded sands with the densities 2.72 and 2.66g/cm^3, average particle sizes and form coefficients 0.27mm, 0.951 and 0.32mm, 0.940, and that the sand moisture contents were all below 1%. Furthermore, the ranges of the start-up wind speeds for different particle sizes 2~0.84, 0.84~0.42, 0.42~0.25mm and below 0.25mm were 13.5~14.5, 7.5~8.5, 5.5~6.5 and 4.5m/s, respectively. The best sand fixation thickness of the windbreak forest belt was 2.5~3.0 times model of beefwood. The different wind durations did not have much influence on the variation of the aeolian sand quality behind the forest and the sand accumulation quantities of the experiment cases after 10-minute wind duration were all stabilized gradually. As far as the sand fixation effects of the forest types, we combined two tree species beefwood and linden hibiscus to 10 different tree arrangements and found that the mixed forest with high woods in the front and low woods behind had the best sand fixation effects. However, the pure forest of the low woods had the worst fixation effects

防風網編織形式對減風效果之風洞試驗研究

Windbreak Net is an extremely important facility of prevention of wind damage; however, windbreak net's function varies with different types; hence, the research's main purpose was to compare the differences of shelter effect with diverse windbreak net weaving types. The research adopted three types of windbreak nets of higher market share in Taiwan: plain weave, knitted weave, which had two different mesh shapes, and ribbed weave, which had two varied weaving methods. The wind tunnel test was conducted with these five sets of windbreak net, and relative wind velocity, wind velocity reduction coefficient, protection index, and economic benefit were used respectively to do comparison and make analysis to explore the influence of weaving type, mesh number and shape on wind velocity reduction effect.The test results showed that when all windbreaks had the same ventilation rate, the windbreak net of plain weave had the best shelter effect, the secondary was ribbed weave, and the worst was knitted weave; however, if both the shading and anti-insect functions were required, the windbreak net of knitted weave type was relatively more ideal to choose; yet if taking economic benefit into consideration, the windbreak net of ribbed weave was the cheapest. As for the mesh shape, triangle had better shelter effect than rectangle, and in this research case, mesh number had no significant difference in wind velocity reduction.防風網是風害防治極為重要之設施，然而防風網之功能因其形式不同而異；因此，本試驗主要目的即在於比較防風網編織形式對減風功效之差異，研究中採用國內市占率較高之平織、針織及羅紋織三種形式防風網，其中針織有兩種不同網目形狀，而羅紋織則有兩種不同編織方式，將此五組防風網，進行風洞試驗，分別以相對風速、減風係數、保護指數及經濟效益比較分析，探討編織形式、網目數量及形狀對減風效果之影響。經試驗結果顯示，當透風率相同時，在減風功效方面，以平織形式之防風網最佳，羅紋織次之、針織最差；然而若需兼具遮陽及防蟲功效，則應選用針織形式之防風網較為理想；但若考量經濟效益，則又以羅紋織防風網最為價廉。至於網目形狀以三角形比矩形有較佳之減風功效，而網目數量在本試驗個案中，對於減風並無明顯差異