Dynamics and mechanics of bed-load tracer particles
Understanding the mechanics of bed load at the flood scale is necessary to link hydrology to landscape evolution. Here we report on observations of the transport of coarse sediment tracer particles in a cobble-bedded alluvial river and a step-pool bedrock tributary, at the individual flood a...
| Main Authors: | , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-12-01
|
| Series: | Earth Surface Dynamics |
| Online Access: | http://www.earth-surf-dynam.net/2/513/2014/esurf-2-513-2014.pdf |
| Summary: | Understanding the mechanics of bed load at the flood scale is
necessary to link hydrology to landscape evolution. Here we report
on observations of the transport of coarse sediment tracer particles
in a cobble-bedded alluvial river and a step-pool bedrock tributary, at the
individual flood and multi-annual timescales. Tracer particle data
for each survey are composed of measured displacement lengths for
individual particles, and the number of tagged particles
mobilized. For single floods we find that measured tracer particle
displacement lengths are exponentially distributed; the number of
mobile particles increases linearly with peak flood Shields stress,
indicating partial bed load transport for all observed floods; and
modal displacement distances scale linearly with excess shear
velocity. These findings provide quantitative field support for
a recently proposed modeling framework based on momentum
conservation at the grain scale. Tracer displacement is weakly
negatively correlated with particle size at the individual flood scale;
however cumulative travel distance begins to show a stronger inverse
relation to grain size when measured over many transport events. The
observed spatial sorting of tracers approaches that of the river
bed, and is consistent with size-selective deposition models and
laboratory experiments. Tracer displacement data for the bedrock
and alluvial channels collapse onto a single curve – despite more
than an order of magnitude difference in channel slope – when
variations of critical Shields stress and flow resistance between
the two are accounted for. Results show how bed load dynamics may be
predicted from a record of river stage, providing a direct link
between climate and sediment transport. |
|---|---|
| ISSN: | 2196-6311 2196-632X |