Experimental Research On Hydraulic Character And Pollutant Diffusion Among The Vegetation Community

The hydraulics characteristic of flow among the plant community that vertical ecological character changes steeply was rarely studied. The natural shrubs which vertical characteristics changes steeply and natural reeds which vertical characteristics changes little were chosen as the experimental objects. The six types of communities were designed. The effect of the ecological character on the vertical change of flow characteristics and pollutant longitudinal diffusion was experimental analyzed. The results show:1) In the vertical direction, it is the positive correlation between the velocity and the flexibility of the plant, however it is negative correlation between the velocity and the plant diameter. While shrubs are fully submerged, the vertical distribution of velocity can be divided into two parts: one is within the range of plant height; the other is above the plant canopy. The vertical velocity distribution of the former can be described as the logarithm curve, and the curve shape of the latter is different along with vegetation types. There are three inflexions in the vertical distribution curve of the turbulence density among the plant community. The first lies in the transition zone from the stem to the canopy; the second is located in the plant deflection point; the third is situated in the top of the plant canopy.2) Based on the balance of the force imposed on the plant, the formula between the drag coefficient due to plant community and Darcy friction factor is deduced, which is combined with Bernoulli equation and Darcy friction formula to calculate drag coefficients. The influence factors of vertical distribution of drag coefficient are analyzed. The vertical tendency of drag coefficient with flow depth, diameter, diameter Reynolds number, flow depth Reynolds number and relative roughness height are a declined curve and could be depicted as the power function or exponent function in the different discharges. The calculation models of drag coefficient are built by use of the multiple nonlinear regression analysis method.3) The pollutant dispersion coefficients in the longitudinal direction are calculated by means of the revised single-station method. Nepf model and Serra model's applicability for the natural vegetation are verified and the results show that the precision of the two models are low. An empirical model is built to calculate longitudinal dispersion coefficient for the natural shrubs using multiple regression analysis method by relating the longitudinal dispersion coefficient to plant diameter, the plan diameter Reynolds number and drag coefficient.4) Chezy coefficient is expressed as the function of the plant ecological character parameter and drag coefficients which are calculated based on the flume experiment result. Drag due to plant was approximately regarded as cylinder-drag or bottom friction, respectively. The two dimensional depth average flow numerical model was built and resolved it by means of TVD-MacCormack scheme. The flume experiment data are validated and show: the calculated result agree well with the measured result among the sparse plants however drag due to vegetation was approximately regarded as the bottom friction or the cylinder-drag; while the drag due to vegetation was approximately regarded as the bottom friction force, the calculated result agree better with the measured result.