A study of flow structures in a two-stage channel using field data, a physical model and numerical modelling

This thesis consists of two main elements: the analysis of field measurements of velocity and resistance in a river, undertaken over a three year period, and numerical modelling of open channel flow. An Acoustic Doppler Current Profiler (ADCP) was used to measure the spatial distribution of velocity in two cross sections of a small meandering river (River Blackwater, Hampshire) during inbank, bankfull and overbank flow conditions. The same reach of the river had been previously studied over a number of years, as well as investigated on a 1:5 scale physical model, making it possible to compare the flow structure in the river and in the physical model. A new measurement procedure and data processing methodology were developed for ADCP measurements, suitable for use in times of flood. Methods for orientating the measured velocity data and reducing the velocity fluctuations in the data, due to their instantaneous random nature, are described. The post processed data has been verified against 300s time-averaged velocity data at several locations along the cross sections, and also against ADV measurements obtained under the same flow conditions. The approach of averaging several transect data together has successfully reduced the noise in the ADCP velocity data. The field data indicates incremental rises in Ud with rising water level, while the opposite is true for the physical model data. Key similarities and differences between the secondary flow patterns in the river and model have been identified. The discharge capacity of the main river channel is significantly reduced during summer months, due to seasonal growth in vegetation, reaching a minimum in August. The gradient of the stagedischarge rating curve for summer months can be seven times larger than that for the winter months. The falling limbs of the rating curve have a higher discharge capacity than the rising limbs for summer months (June/July to October/November). An attempt was made to predict the stagedischarge relationship for overbank flow conditions using a quasi 2D RANS model, SKM (Shiono and Knight Method). SKM is shown to be capable of simulating the lateral distribution of the depthaveraged streamwise velocity in the river and physical model with reasonable accuracy, subject to appropriate choice of three calibration parameters. The research shows that the ADCP has a great potential for obtaining accurate 3D velocity data in rivers during flood events, and that the SKM is a useful modelling tool. The importance of taking into account the effect of vegetation when undertaking engineering design has been demonstrated