In response to reductions in sediment supply, gravel-bed rivers undergo a coarsening of the sediments that comprise the river’s bed and, over some longer time scale, a river’s grade may also be reduced as sediments are depleted from upstream reaches.  Coarse, degraded river reaches are commonly observed downstream of dams across the Western United States.  Following dam closure, these riverbeds become immobile under the altered flow and sediment supply regimes, leading to a reduction in the available salmon spawning and rearing habitat.  Gravel augmentation to these streams is now common practice.  This augmentation is typically seen as resurfacing the static coarse bed.  As an alternative, we propose that the addition of appropriately finer gravels to these channels may be capable of mobilizing an otherwise immobile coarse surface layer, creating the potential to release fine material trapped beneath the surface.  A series of laboratory experiments were recently completed at the University of California to test this hypothesis in a 30 m long and 0.86 m wide gravel-bedded flume channel.  These experiments were designed to suppress the development of lateral topography and allows us to focus on grain-to-grain interactions.  Results demonstrate that fine bedload pulses are capable of mobilizing all grain sizes in the bed surface armor (including the largest grains) as finer bedload fills the interstices of the coarse surface layer.  This suggests that gravel augmentation using fine gravel may provide an effective means of improving bed mobility conditions.  New experiments were conducted in May-June 2006 as part of StreamLab06 to determine how lateral topography modulates the mobilization process. Analysis is underway.

Major accomplishments:

Papers
Venditti, J.G., W.E. Dietrich, P.A Nelson, M.A. Wydzga, J. Fadde, and L. Sklar, Can coarse surface layers in gravel-bedded rivers be mobilized by finer gravel bedload?,  EOS Trans.,  AGU, 86:52, Fall Meet. Suppl., Abstract H51H-05.

Fadde, J., J. G. Venditti, L. Sklar, M.A. Wydzga, P.A Nelson, and W.E. Dietrich, Propagation of sediment pulses in flume experiments simulating gravel augmentation in armored channels downstream of dams, EOS Trans., AGU, 86:52, Fall Meet. Suppl., Abstract H53D-0487.

Venditti, J.G., W.E. Dietrich, P.A Nelson, M.A. Wydzga, J. Fadde, and L. Sklar, Mobilization of coarse surface layers by finer gravel bedload: A tool for use in comprehensive coarse sediment management plans. Workshop on Physical Modeling Experiments to Guide River Restoration Projects: Implications for Restoring Dam-Impacted Rivers, December 13, 2005, University of California, Richmond Field Station, Richmond, California.