Flow Characteristics of Partially Vegetated Trapezoidal Channel Cross-section with Flexible Vegetation

An important feature of most rivers and canals is their habitats for a rich of aquatic vegetation which is extremely important from an environmental and ecological point of view. These variations in habitat mean that it is rare to find a cross-section of a natural channel which has the same roughness along its perimeter. In these channels, flow conditions lead to a complex flow situation with intensive mass and momentum exchange between main channel and banks, and secondary currents. Therefore the flow structures that occur in these channels are extremely complex. In this research, a method for predicting the total conveyance of trapezoidal channel cross-section with varying roughness in perimeter due to growth of flexible vegetation is developed based on the 2D velocity distribution in open channel. Where, numerical hydraulic model is coupled with physically based flexible vegetation flow-resistance equation. The study covers three patterns of flexible vegetation growth, the first is vegetation growth on bed and side banks with equal height on both, the second is vegetation growth on bed only and sediment roughness on side banks and the third is vegetation growth on side banks and alluvial on the bed. The resulting model is capable of simulating the total conveyance of trapezoidal channels with growth of vegetation on bed and/or side banks, depth average velocity distribution and bed shear stress distribution along the channel cross-section. From the numerical results, series of regression relationships were developed for computing the equivalent Manning coefficient for vegetated channels and another series were developed for computing the vegetated channel flow rate. These can be used as an alternative solution for channel designs; relieving the practitioner of the need to run the numerical program. A comparison between the present method and traditional empirical approaches proved that the model is superior on the other methods. The flow capacity results obtained are shown to be in good agreement with actual field data, as are the equivalent Manning's roughness coefficient.