濕地曼寧n值之研究

Abstract

植物為濕地生態系主要生產者，不同的植生可營造不同的生態功能及棲地條件，並影響該生態系統中的水動力行為。植生在水動力模式中常以曼寧n值表示，一旦曼寧n值決定後即可進一步解算流速、水深等流場特徵，故濕地水理中的n值決定極為重要。本研究以考慮摩擦因子f(Darcy-Weisbach friction factor)之單層渠流與上層為均質水、下層為植物之模型的渠流比較，以求含摩擦因子f的流速，且以曼寧公式換算求得曼寧n值。 延續江(1995)之假設，含植物之上層均質水層以混合長度紊流模式控制、下層植物層以多孔介質來表示，至於單層均質流吾人試以一動量方程式加入摩擦項代表整個流場之控制式。首先，簡化控制式並以Runge-Kutta法來估算無因次參數α、β之初始猜測值，再利用擾動法迭代求解α、β和摩擦因子f與流速，接著與含植物之雙層流解析解以連續方程式連結，以求得f值最後再以曼寧公式換算求得曼寧n值。 驗證之部分，以(Freeman 2000)實驗數據做為比較，結果顯示其曼寧n值誤差幅度雖然不一，但仍在一範圍內。本研究亦對設計流場之控制式做敏感度分析。最後將挖子尾地區之觀測數據代入本研究模式求得之曼寧n值，代入SRH-2D模式中作為應用，由流速分布圖和底床剪力圖分布顯示，結果顯示，紅樹林疏伐後，原紅樹林區域的平均流速由0.001(m/s)增至0.013(m/s)、平均底床剪應力由0.0001(N/m^2)增加至0.002(N/m^2)，顯示紅樹林在該濕地中扮演重要的邊岸保護功能（bank erosion protection），對於減緩水流衝擊效果顯著，亦表示本文所計算之紅樹林曼寧n值有助於反應此現象。

In ecological research, it is often necessary to estimate the flow rate, flow velocity, etc. of natural open channels. However, the bottom of the general channel is usually densely covered by vegetation. Ecologists often use Manning`s n value to represent the resistance of the whole flow field. Manning`s n value changes While the resistance differs from vegetation to vegetation. In this study, a single-layered open channel model with a friction factor f (Darcy-Weisbach friction factor) is compared with the open channel flow of which the upper layer and the lower layer are homogeneous water and vegetation respectively, in order to obtain the flow velocity with the friction factor f. Then, Manning`s n value can be converted by Manning formula. Refer to the assumption of Jiang (1995), the homogeneous water layer in the upper layer of the plant is controlled by the mixed-length turbulence mode, and the lower layer is represented by the porous media. As for the single-layer homogeneous flow, we add a friction term into the momentum equation to govern the whole flow field. First, simplify the governing equation and use Runge-Kutta method to estimate the initial guesses of the dimensionless parameters, α and β, and then use perturbation method to iteratively obtain the α、β、friction factors f and flow velocity. By connecting the analytic solution of the two-layer flow with continuity equation. We thus obtain the f-value and Manning n value by Manning formula. As the verification, the result is compared with the experimental data of Freeman (2000).Although the result shows that margin of error of Manning n value is not the same, it is still in a certain range. This study also works on a sensitivity analysis of the design flow field. Finally, the model of the research is applied to observed data of the Wazihwei area to obtain the Manning`s n value, and substituted it into the SRH-2D model. In this application, the flow velocity distribution and the bed shear stress distribution show that the effect of water pen-area on relenting the impact of tidal is obvious. Furthermore, this application indicates that the estimation of Manning's n value above achieves good results in this study is good.