本研究將影響天然渠道中流水沖蝕土壤之因子分為水理特性及植生特性。水理特性之六項參數為流速V、流體密度ρ、流體黏滯係數μ、重力加速度g、水深h及過水面積Aw,而植生特性之四項參數為植株特徵面積Ag、曼寧係數n、根密度RD及根長密度RLD。本研究並利用因次分析推導出六項無因次參數,以建立各參數與土壤沖蝕度相互間之關係。試驗以百喜草作為植生材料,設計3種植栽密度及9種植栽天數,進行根物理量及抗拉力試驗與渠槽沖蝕試驗。研究結果顯示根系為影響土壤沖蝕之關鍵參數,至少可保護整體樣本80%,此外,葉片也可保護10%之表土樣本。當百喜草植栽天數達到50天後,在每一立方公尺土體內,根重超過2 g或根長超過100 m,植生保護土壤之效能便開始產生,隨著植栽時間愈久,根抗拉力急速上升,土壤沖蝕度也明顯地驟降。植生特性參數與土壤沖蝕度之關係中,物理和無因次參數均與土壤沖蝕度呈現乘冪之趨勢。最後,利用在不同植生條件下之植生特性參數與土壤沖蝕度建立迴歸方程式。
This study divides the effective factors of soil erosion on the nature channel into two characteristics, hydraulic and vegetation. Hydraulic factors include flow velocity, flow density, flow viscosity, gravity, depth and cross section area, while vegetation factors include characteristic area of plant, manning resistance coefficient, root density and root length density. The study also deduced six dimensionless parameters by dimensional analysis, to build up the relationship between those parameters and soil erodibility. The experimental data collected from a straight flume with bahia grass on channel bed, which was set up by 3 different planted density and 9 different growing period. The results show that roots are the key parameter to affect soil erosion. Roots strength can protect the sample over 80 percent, while leaves can only protect the top soil about 10 percent. Once root density exceeds 2 g/m³ or root length density exceeds 100 m/m³ the soil with bahia grass can protect soil effectively from flow erosion. With the growing day longer, root’s pulling resistance lift rapidly, and soil erodibility decrease obviously. The result also show that the soil erodibility has a power relationship with physics and dimensionless parameters. Finally, the study established the regression equations between vegetation parameters and soil erodibility in different vegetation conditions.
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