Morphology and Sediment Transport in Semi-alluvial Channels

English Abstract:

Definitions of semi-alluvial channels differ in detail but the term has been applied to describe those channels in which alluvial deposits are either thin or discontinuous but the underlying non-alluvial (usually cohesive) material in the bed, and /or exposed in the banks, is readily eroded by prevailing flows. The consequence is that river morphology and channel adjustment, e.g. cross-section dimensions and in-channel bars, may be very similar to that of alluvial channels with similar flows. Neither the geographical occurrence nor the controlling conditions of these types of channels are well known, but they appear to be common in glaciated regions with extensive and thick deposits into which rivers are eroded. In the Great Lakes region examples range from small headwater streams, through main-stem rivers and include some of the Great Lakes connecting channels. In some cases the primary depositional cover over the non-alluvial material is gravel-cobble. While these channel types may have some morphological and process characteristics of partially-alluvial bedrock channels, the extent to which concepts of erosion, cover formation-loss, and bedload transport developed for bedrock channels are applicable to semi-alluvial channels is an open question. It is also likely that process analyses such as bedload-transport formulations for fully-alluvial rivers are not directly transferrable to semi-alluvial cases. A systematic program of research is needed to expand knowledge of semi-alluvial river morphology and processes.