Streambed Structure, Stream Power, and Bedload Transport: A Unified Outlook for Gravel-bed and Bedrock Streams

Abstract:

Interactions among streambed structure, stream power, and sediment transport in rivers have been widely observed and documented. Perennial gravel-bed streams typically possess a surface bed layer that is coarser than the subsurface material. This coarser surface layer is, however, absent from some ephemeral gravel-bed streams and in some cases the reverse phenomenon occurs. Ephemeral streams also exhibit considerably higher efficiency in transporting sediment. In steep bedrock rivers, the hydraulic-rock interactive mechanism often self-creates step-pool or cascade bed configurations as forms of energy dissipation to control the transport efficiency of sediment. Here we aim to characterize bed structures and sediment transport in gravel-bed rivers and bedrock streams by using the concept of dimensionless stream power. We analyzed existing bed load data collected from field and experimental settings in an attempt to reach a unified outlook for both stream types and various channel bed features. We found that the mechanisms responsible for the features perceived to distinguish surface fining and surface coarsening are interrelated and triggered by different values of dimensionless stream power. The surface fining case has been attributed to fluidization of the entire bed material as demonstrated here in detail. The results also suggest that in bedrock rivers with large bedforms, such as stabled step-pool and immobile rock cascade, relatively medium-large values of stream power (i.e., floods of less than 30-year return period) do not equate with large bed load transport rates due to a portion of flow energy dissipating through local hydraulic jumps, leaving less energy to transport the bed load. Plot of transport efficiency values for each bed type and flood magnitude in bedrock rivers also helps us estimate how much fraction of flow energy is delivered to do bedrock erosive work by saltating bed load; hence, the implication for studies of landscape evolution.