Research On The Hydraulic Mechanisms Of Gradually Varied Flow In Vegetated Channels

Vegetation is the important basis for the formulating and maintaining of the gradually varied flow in river channels. Research on the hydraulic mechanisms of vegetated gradually varied flow is the frontier of eco-hydraulics. A series of experiments on the gradually varied flow in vegetated channel were conducted. Based on theoretical and experimental analyses, the flow field structure, energy dissipation, Manning’s coefficient and turbulent structure affected by rigid vegetation in open channel were investigated. The following conclusions can be obtained.1nd, The theoretical and experimental analyses on the flow field structure indicated that:1) vegetation can make the gradually varied flow state transforming between M1 and M2 types, the water head of the raised water level increases with the vegetation density, vegetation section length and submerged vegetation height; 2) the relative submergence can be used to describe the effects of rigid submerged vegetation on the flow field structure in open channel, especially for the raised water head; 3) the gradually varied flow in vegetated channel can be determined with the conditions including:flow in river is under subcritical flow condition; the river is positive open channel with gentle slope; the gradually varied flow state is changed by the vegetation block; 4) vegetation block can reduce the local flow velocity and the reduction amplitude increases with the vegetation density, vegetation section length and submerged vegetation height; 5) the irregularity coefficient can be utilized to describe the irregularity of vertical velocity affected by rigid vegetation block.2rd, The experimental results on energy dissipation showed that:1) under different vegetation parameters, the cross-sectional unit energy increases with vegetation density, vegetation section length and submerged vegetation height, while overlaps within certain flow depth; 2) the cross-sectional unit potential energy is dominating when the water depth is relatively high, while the curve for cross-sectional unit energy is bended and the rate of kinetic energy increases if the water depth is lower; 3) vegetation block can raise the total energy before and within vegetation section in comparison with non-vegetated case, the longitudinal distributions of total energy can be described as:basically un-changed before the vegetation section, decreasing within the vegetation section, similar to the non-vegetated case when water flows out of the vegetation section; 4) the increase of vegetation density, vegetation section length and vegetation height/decrease of relative submergence can reduce the kinetic energy within the vegetation section.3th, The theoretical and experimental analyses on the Manning’s coefficient indicated that:1) with the coefficients containing vegetal parameters, there are sum of squares relationships between the total Manning’s coefficient with its boundary and vegetal components for rigid un-submerged and submerged vegetation, and the computational methods for vegetal Manning’s coefficient can be obtained; 2) the total Manning’s coefficient rapidly increases at the entrance of vegetation block, gradually decreases within the vegetation section and recovers to the level before vegetation section when water flows out; 3) there are linear relationships between the total/vegetal Manning’s coefficient with vegetation density/submerged vegetation height and power functional relationships with the ratio of vegetation coverage area to bottom area, vegetation section length and relative submergence, respectively.4th, The experimental results on turbulent structure showed that:1) vegetation parameters have obvious effects on the distribution of relative turbulent intensity in vegetated channel, the relative turbulent intensities can be generally ranked as longitudinal> transverse>vertical; 2) with medium/high vegetation densities, the relative turbulent intensities within vegetation section are higher, and the averaged values for low/medium densities are lower; 3) relative turbulent intensities are higher within the vegetation section of different lengths, and the average value increases with vegetation section length with slight fluctuations; 4) short vegetation sections can make the water flow before the vegetation section perturbative, and such effects will decrease with the increase of vegetation section length; 5) with the increase of submerged vegetation height, the curves for relative turbulent intensity tend to similar tendencies, the average values slightly increase with vegetation heights.5th, Based on the theoretical and experimental analyses, the following innovative results can be obtained.1) The relative submergence which is more general in comparison with vegetation height was proposed to describe the rigid vegetation effects on open channel flow.2) The conditions for gradually varied flow unvegetated channel were summarized in expecting to understand the mechanisms of flow state transitions in vegetated channels.3) An irregulation coefficient was proposed to judge the applicability of the traditional power functional law for the vertical flow velocity distribution in vegetated channel.4) The computational methods for the vegetal Manning’s coefficient of rigid un-submerged and submerged vegetation are proposed which can provide some reference values for the applications of Manning equation in gradually varied flow in vegetated channel.The research on the hydraulic mechanisms in vegetated channel is the frontier of eco-hydraulics and eco-engineering science. The results of the research may demonstrate some theoretical significance to the development of eco-hydraulics and some guidance to the ecological restorations of rivers and constructions of wetland.