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Flood control and sediment transport study of the Vedder River McLean, David George

Abstract

The Chilliwack River flows through the Cascade Mountains until reaching a narrow gorge near Vedder Crossing where it flows onto the Fraser Lowlands and eventually meets the Fraser River. Below Vedder Crossing, the river is actively building an alluvial fan by depositing its sediment load of gravel and sand. This deposition has resulted in frequent channel shifts over the fan surface with the most recent migration occurring around 1894 when the river shifted down Vedder Creek. Over the last century the Vedder River has been undergoing very complex changes in response to changes in the incidence of severe floods, changes in sediment supply and interference from river training. Extensive channelization works carried out in the 1960's induced temporary degradation over part of the channel which was accompanied by rapid aggradation in the reach immediately downstream. This rapid channel adjustment ceased in less than 10 years. In 1975 a flood having a return period of about 10 years deposited 260,000 cubic yards of sediment onto the fan which increased the mean bedlevel by nearly 1 foot. By comparison, the average annual deposition rate was estimated to be 72,000 cubic yards per year. Based on bedload transport calculations, approximately 700,000 cubic yards of sediment could be deposited by a 50 year rainstorm flood. In order to provide long term flood control, either the upstream sediment supply will have to be reduced or dredging will have to be carried out on the lower river. It is not feasible to eliminate aggradation by transporting the incoming bedload through the system and into the Fraser River. Some strategies are considered which, by controlled dredging and training would maintain the channel permanently in its present position. More severe floods would be contained by set-back dikes. It is thought that, with care, these measures could be consistent with salmon habitat requirements.

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