HIGH-FLOW GRAVEL INJECTION FOR CONSTRUCTING DESIGNED IN-CHANNEL FEATURES

A planned gravel bar was dynamically constructed at a river rehabilitation project site through a combination of mechanical channel reconfiguration and high-flow gravel injection. Mechanical alterations intended to promote deposition in a target area were followed by the injection of 1570m3 of gravel and small cobble 400m upstream during a high-flow dam release. This dynamic bar construction proved successful in that bed elevations increased locally by more than 1m in the target area and a new alternate bar sequence was created nearby. Morphodynamic modelling used to plan the injection correctly identified general areas prone to deposition or scour, but did not correctly predict the magnitudes of change or resolve features at the scale of individual alternate bars. A flow perturbation induced by a constructed meander bend just upstream from the injection point may have contributed to alternate bar development. Despite the observed bar development, sediment budget calculations indicate that the quantity of gravel stored in the target reach remained constant, perhaps because of an interruption in the delivery of bed material fromupstream. The estimated gravel load over the release increased steadily in the downstreamdirection and, at the downstreamstudy area boundary, had attained a magnitude 4.6 times larger than the quantity of injected gravel. These results demonstrate that bedload entrainment from the channel perimeter dominated the gravel supply to downstream reaches and suggest that the gravel injection had little effect on geomorphic adjustments downstream from its target area. Copyright (C) 2013 John Wiley & Sons, Ltd.