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Morphodynamics and sediment transport in a wandering gravel-bed channel : Fraser River, British Columbia Ham, Darren Gary


This study investigates the relation between sediment transport and channel deformation on a 70 km long wandering reach of lower Fraser River, British Columbia. The reach is characterized by an irregularly sinuous single-thread channel split around large bar and island complexes. An extensive network of secondary channels, produced from channel shifting and abandonment, is found within these complexes and along the adjacent floodplain. Most material accumulates within wide 'sedimentation zones' where deposited sand and gravel create an obstruction to downstream flow conveyance, and in areas of flow expansion where shear stress declines. These sites correspond to the location of prominant diagonal riffles. Changes to reach morphology are dominated by the transfer of coarse alluvial sediments. Small gravel sheets are attached to existing lateral and mid-channel bars, forcing compensating erosion across the channel and propagating instability downstream. Over periods of several decades, entire bars and bar / island complexes migrate downstream in association with the riffles. Sediment budgets constructed for several periods between 1952 and 2003 show that the transport rate into the reach is highly variable over time and does not correlate well with the magnitude or frequency of large floods. Influx into the reach has apparently increased over time, from 180,000 m³/yr from 1952 to 1984 to 494,000 m³/yr from 1999 to 2003. The present high rates are associated with channel alignment upstream of Agassiz-Rosedale Bridge that causes extensive erosion at lower Herrling Island. This sequence is apt to be disrupted within the next several years as the island becomes bissected, and transport rates should subsequently decline. The long-term (1952-1999) rate of 212,000 m³/yr appears to be the more conservative figure for applied gravel management strategies. This budget also indicates that channel reaches upstream of the bridge have degraded. It is probable that sedimentation in the gravel reach during most of the 20th century was influenced by placer mining in the Fraser Canyon which introduced significant (though unknown) quantities of bed material to the river. More recent factors such as development of the Coquihalla Highway and numerous pipeline and transmission lines may also have led to higher inputs than would be expected in a natural, undeveloped system (Church et al., 2001). However, it is difficult to confirm that these activities continue to impact the gravel budget, or that recent degradation is in fact a response to an exhaustion of upstream sediment supply. Although the gravel reach remains a relatively pristine environment relative to most large navigable rivers in populated regions, the isolation of several large sloughs and backchannels, lateral confinement through bank hardening, and removal of sand and gravel from the channel have cumulatively reduced the complexity of channel morphology and reduced the channel gradient. Despite evidence that recent morphologic evolution is trending towards a more stable single-thread meadering habit, considerable downtream instability is expected to continue for several decades as this pattern continues to develop. The relations between sediment transport and channel deformation developed in this thesis should provide better tools to facilitate sound riverine management during this period, but additional survey is required to evaluate such change.

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