The influence of bed roughness on partial alluviation in bedrock channels

Abstract

The extent of alluvial cover on a bedrock channel bed strongly influences the efficiency cover is commonly modeled as a simple function of sediment supply relative to the transport capacity of the stream, although other factors are likely to be important, particularly the roughness of the underlying bedrock surface. Here I report results of laboratory experiments investigating the influence of bedrock channel topography on extent of alluvial deposition. The results are compared to observations made of conditions in natural partially alluviated bedrock channels. The experiments were conducted in a model river with concrete beds of varying roughness conditions. Sets of experiments were conducted varying sediment supply rate for different roughness and shear stress conditions. I used a laser microtopography scanner to measure the bed topography and quantified bedrock roughness as the standard deviation of bed elevation relative to bed slope. Maps of alluvial cover were used to calculate percent cover. A comparison of sediment flux out of the channel to sediment supply into the channel was used to determine partial cover state of equilibrium. I found that low-roughness beds require a relatively high sediment supply before alluvial patches form, and as supply increases, can accommodate only low levels of partial alluvial cover before runaway alluviation rapidly converts the bed to an aggrading alluvial condition. In contrast, highly rough bedrock surfaces partially alluviate at very low sediment supplies and allow stable high fractional bed coverage. All else equal, rougher beds accumulate higher percentages of alluvial cover. Comparing these results to natural channel settings will involve scaling standard deviation of elevations by some factor, such as sediment grain size or channel width.