A laboratory and field investigation into the discharge characteristics of an experimental flood alleviation scheme on the River Roding in Essex
PublisherUniversity of Bristol
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An investigation was carried out to determine the stage-discharge characteristics of a meandering compound channel with vegetated floodplains. The fundamental interaction mechanism between a main channel and its floodplain was also investigated. Field and model studies provided the data for this thesis. A stretch of the River Roding in Essex was monitored for the field study and a scale model of a short reach of the river was reproduced in the laboratory. A Froudian model was constructed with vertical and horizontal scales of 16: 1 and 50: 1 respectively. Field date over a two year period were used to match the model to the river. The response of the river to proposed changes was then predicted by varying roughness and shaped parameters within the model. To do this, stage discharge curves were determined for the model and scaled up to prototype conditions. Detailed velocity traverses were carried out across selected sections of the model to determine discharge proportions between floodplain and main channel for different depths of floodplain flow. A computerised data collection facility was developed to assist the model study. Vegetation density and distribution on the floodbanks of compound rivers, such as the River Roding investigated here, can have a significant effect on total discharge capacity. Over 40% increase in maximum discharge capacity was realised in the abovementioned scheme between heavily vegetated and cleared floodplains. Sever meanders of the floodplain boundaries produced large form roughness, resulting in flow separation on sonie bends and a reduction in the effective width of the floodplain. Removal of these, requiring relatively small excavation of the floodbanks, could significantly improve the carrying capacity of the river. The discharge in the main channel was reduced at overbank conditions due to the shear interaction between floodplain flow and main channel flow. Manning roughness coefficients were calculated for particular vegetative conditions of the river. A proposal was made to incorporate a correction factor into calculations of discharge in compound rivers with meandering main channels. The correction factor would be applied to the discharge calculated for an equivalent compound channel without meander. It depended on the degree of meander, or sinuosity, of the main channel, roughness ratio between main channel and floodplain, and depth of flow.