Thanos Papanicolaou
IIHR - Hydroscience & Engineering, The University of Iowa

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Application Note: Sediment Transport
 
Noncohesive Sediment: Understanding the mechanisms involved in the transport and fate of noncohesive soils in natural channel systems remains an open case in water-related engineering disciplines.  The main challenge in studying noncohesive sediments is the complex character of the bed geometry which governs the velocity, as well as the turbulence structure of the flow which in turn, control the sediment-carrying capacity of the flow.  Bed geometry is controlled by stochastic processes and subject to drastic changes due to changes in the flow condition.  Bedforms, clusters, step and pools are few examples of such complicated bed geometry.  In addition, sediment may consist of a wide range of grain sizes.
   
Step-pool configurations: High gradient streams are typically characterized by longitudinal slopes of the order of 3% and higher.  There are several geomorphologic features that are present in natural high gradient streams, namely, pool-riffle sequences, step-pool configurations, and microform clustering.  A study was conducted to improve our understanding of the near-bed flow processes affecting the formation and evolution of step-pool configurations, in particular, their spacing and frequency of occurrence in a natural stream, stability and frictional characteristics, and pool height. For this purpose, flow and sediment measurements were taken at Valley Creek, WA, a high gradient stream.  Valley Creek has an average longitudinal slope of 9%, and its substrate material has a median diameter of 350 mm.  Monitoring was performed at various locations along the creek and during different flow seasons.  An Acoustic Doppler Velocimeter (ADV) was used to measure the turbulent velocities, the Reynolds stress component, and the vortex size structures.  Measuring locations were defined atop of the step structures and within the pool. The flow measurements were coupled with bedload measurements.  Using cross-correlations techniques the effects of flow on sediment movement were examined.  Analysis of the Reynolds stress measurements provided the stress distribution atop of a step-pool configuration.  The latter information was used to determine the stability conditions for step pool configurations and examine the dependence of bed shear stress on relative submergence and the Froude number.   Finally, the spacing of the pools and the pool depth were compared against generic laboratories measurements performed at Washington State University.  The study improved the design approaches of step-pool configurations and provided a better insight of the processes affecting their evolution and stability.