Research On The Influence Of Aquatic Vegetation On Manning Coefficient In Open Channel Flow

In measures to promote sediment transportation and deposition,reduce non-point source pollution,and control soil erosion,vegetation filter belts or vegetation hedges are widely used.Compared with boundary friction resistance,vegetation resistance is stronger.The presence of vegetation will affect the flow velocity,significantly increase the flow resistance,reduce the flow capacity,and then change the flow distribution and sediment separation process.Manning coefficient is a key indicator to characterize water flow resistance,and it is also an indispensable parameter for studying flow hydrodynamics and establishing hydraulic models.Although a large number of Manning coefficient studies have been carried out,the role and proportion of vegetation resistance in the total Manning coefficient Still unclear.Therefore,it is necessary and urgent to develop the role of vegetation in water flow resistance.The research results can expand the theoretical connotation of the Manning coefficient of open channel hydraulics,and have practical significance for the engineering application of the Manning coefficient in floodplains,ecological ditches,natural ditches,etc.This experiment was carried out in the circulating flume of the Soil and Water Conservation Base of Huazhong Agricultural University.The real vegetation-Typha(Typha orientalis Presl)was selected as the experimental material.Based on hydraulic theory,deduce the sum of squares relationship between total manning coefficient and vegetation component manning coefficient and boundary component manning coefficient.Five vegetation patterns(uniform,front dense and back sparse,front sparse and back dense,cluster,random),two submergence ratios(h _W/h _V=0.75,h _W/h _V=1.5)and three channel cross section forms(Rectangle,trapezoid,wedge).A 3×41 water level measurement matrix is arranged along the channel,and the water depth at each point is measured with an automatic water level meter;In the vertical direction,three velocity measurement sections are set at the entrance,middle,and exit of the vegetation area,using Acoustic Doppler Flow Meter(ADV)to measure velocity.The results are used to analyze the contribution of vegetation components to the total Manning coefficient in different vegetation configuration patterns,submergence ratios and channel cross section forms.The following main results are obtained:(1)Along the channel,vegetation can significantly increase the water depth by 5-9 cm and change the flow state,transition from the M₁ curve to the M₂ curve.The changing trend of the Reynolds number and Froude number curves is opposite to the water depth curve.The total Manning coefficient and the vegetation component Manning coefficient show a fluctuating downward trend along the flow direction,and the trend is the same,(R2=0.51?0.87),the total Manning coefficient and the vegetation component Manning coefficient are both the largest under the rectangular cross section,and the trapezoidal cross section is in the middle,The wedge cross section is the smallest.The vegetation component Manning coefficient accounts for more than 90%of the total Manning coefficient.The linear fitting effect of the total Manning coefficient and the vegetation component Manning coefficient is the best,and the fitting effect of the total Manning coefficient is better than that of the vegetation component Manning coefficient.(2)In the vertical direction,the entire curve is S-shaped.The Manning coefficient curve has two obvious inflection points.The minimum inflection point appears at half the height of the vegetation,the maximum inflection point is close to the top of the vegetation,and the value of the maximum inflection point is close to the value at the bottom of the channel.In the trapezoidal and wedge cross section form,the Manning coefficient at the bank site is greater than the bank slope,and the cluster pattern are opposite,and the difference between bank sites and bank slope in a cluster pattern is the largest.(3)The Manning coefficients along the channel and the vertical direction have similar results.For the rectangular cross section,the random pattern has the largest Manning coefficient;for the trapezoidal cross section and wedge cross section,the manning coefficient of the front sparse and back dense pattern is the largest.The Manning coefficient under the rectangular cross section is significantly larger than the trapezoidal cross section and larger than the wedge cross section,and the difference between the trapezoidal cross section and the wedge cross section is not significant.(4)Under constant discharge and the same vegetation density,the submergence ratio has the greatest influence on Manning coefficient,followed by the channel section shape,and the vegetation pattern is the smallest.The rectangular cross section has the largest Manning coefficient among the three channel cross section forms,and the difference between the five patterns is more obvious.Under the same channel cross section form,the Manning coefficient of submerged is greater than emerged,and the difference between the five patterns is more significant.Therefore,when the Manning coefficient is the largest,the difference between other conditions is the most obvious.