Onset of sediment motion 

The onset of sediment transport is fundamental to sediment transport predictions, river restoration design, and channel evolution calculations. However, large uncertainties remain in predicting the flows that cause the initial movement of sediment.  To better understand and predict sediment motion, we are using a combination of laboratory experiments, field measurements, mechanistic theory development, and Discrete Element Method modeling.  Some topics we are investigating include:

• the role of turbulence in sediment movement

• the temporal variations in the onset of motion that are driven by sediment supply and flow histories

• granular mechanics

Flow regulation and climate change impacts on river systems 

Flow regulation through dams and diversions can significantly impact downstream flow hydrographs, channel morphology, sediment mobility, and aquatic ecosystems.  We are using laboratory experiments, as well as hydraulic and aquatic habitat modeling to better understand and potentially mitigate for these impacts. Some questions we are investigating include:

• the influence of hydrograph shape on sediment transport rates and hysteresis

• flow regulation and climate change impacts on riparian ecosystems, in-stream habitat, and channel morphology

Vegetation and geomorphology

Vegetation can partly control many geomorphic processes and is often used in river restoration projects to increase habitat, decrease stream temperatures, and provide bank stability. However, many complex feedback mechanisms exist between vegetation and river channels, which complicates predictions of geomorphic change and river restoration success. We use models and laboratory experiments with real and simulated vegetation to investigate:

• vegetation impacts on flow turbulence, sediment transport rates, and channel morphology

• vegetation succession and cottonwood recruitment

Interactions between salmon and channel processes

Many studies have shown that salmon habitat is partly controlled by the flow and grain sizes present in river systems.  We are investigating the reverse question, which is how salmon impact their own environment as well as the long term evolution of landscapes. We use laboratory experiments and numerical models to answer the following: 

• how do salmon influence the onset of sediment motion, channel bed structure, and sediment transport rates

• the effects of salmon on nutrient exchange in rivers

• the role of salmon spawning on river profile evolution over millions of years

Post-fire hillslope erosion

Fires can significantly impact hillslope erosion rates and processes. The eroded sediment that is subsequently supplied to rivers can have detrimental effects on water quality, aquatic habitat, and channel stability.  We are using detailed field measurements of hillslope erosion and mechanistic theories to understand:

• how do aspect and fire severity influence the magnitude of post-fire erosion

• the spatial and temporal scales over which equations for hillslope erosion can accurately predict post-fire erosion

• the role of hillslope curvature on post-fire erosion processes

Steep mountain streams

Steep, boulder-bed channels comprise the majority of channel network lengths in mountainous areas and supply sediment to downstream river systems.  The hydraulics, sediment transport rates, and morphologies of these channels are very difficult to predict.  We are using a combination of detailed field measurements, laboratory flume experiments and numerical flow (e.g. Large Eddy Simulation) models to test and develop mechanistic equations for fundamental processes in steep streams. Some examples of our research topics include:

• coupling between hillslope erosion and sediment transport in steep streams

• predictive equations for flow hydraulics and sediment transport rates

• sediment supply influences on channel morphology

• formation of step-pool channels

Spatial variations in flow and grain sizes in rivers

Calculations of river stability and long-term evolution are often based on the assumption that the flow hydraulics, morphology, and grain sizes within a river reach are relatively uniform.  The actual considerable spatial variations in these parameters are often ignored, which could cause large errors in predicted channel processes.  We are using a combination of flume experiments, detailed field measurements, and numerical modeling to:

• determine how rivers create spatially sorted grain size distributions (patches) within a reach

• quantify the controls on spatial flow variations

• incorporate flow and grain size distributions into predictions of sediment transport 

Processes in bedrock rivers

Through a number of collaborative projects we are investigating processes that provide insight into long-term evolution of river systems. 

• We are using laboratory flume experiments to understand how bedrock erosion rates and particle impacts vary with bedrock roughness.

• We are using field measurements of sediment cover and bedrock exposure to better understand the role of sediment supply on bedrock channel evolution. 

Scour around bridge piers

Sediment transport and channel bed erosion around bridge piers can cause bridge failure.  Many equations exist to predict pier-induced scour in sand-bedded rivers but these equations are inaccurate in rivers dominated by gravel. We are using a combination of laboratory flume experiments, numerical flow models (LES), and field measurements to better understand and predict scour around bridge piers in gravel-bedded channels.