Changes in Bar Morphology in an Aggrading Gravel Bed River

Abstract:

The River Wharfe, UK, is an aggrading gravel bed river, with frequent gravel bars. Management of the river system requires information about the rate and processes of change occurring to the gravel storage within the bars. From a scientific perspective, there are questions about how bar morphology changes as bars are deposited and eroded in this single thread system, about the extent to which flow conditions drive morphological change, and about the extent to which morphological changes can be predicted.

Morphological changes of ten bars along the River Wharfe are reported between early 2012 and late 2014. The bars span a 6 km long length of river, downstream of the point where the river emerges from a confined valley. The bars range in length from 25 to 135 m. Bar grain size decreases downstream as a consequence of strong downstream fining. Bar morphology was surveyed using Terrestrial Laser Scanning at four time periods between early 2012 and late 2014. Each bar was surveyed from at least two scan positions, and georeferenced using a network of permanent survey markers. After initial processing to register the point clouds and remove vegetation, the change detection algorithm M3C2 was used to identify areas of significant volumetric change.

The measured morphological changes between 2012 and 2013 indicate predominantly depositional changes on the bars, with an overall downstream decrease in the volume of change. However, there are local variations superimposed on this pattern. The mechanisms by which the bars change vary between bars, and include downstream progression of an avalanche face and gravel sheet infilling of local hollows. The measured changes are compared to flow data over the study period to identify the extent to which they are driven by flow.