Turbulence Observations in Cobble-Bed Rivers

Improving descriptions of natural stream flow fields is a critical step in restoring aquatic ecosystems. Current methods used for evaluating aquatic habitat rely on simplified representations of the flow field. These methods can be improved by incorporating important spatial and temporal flow field variations and more advanced habitat metrics. However, knowledge of velocity and turbulence distributions in natural streams is limited to laboratory derived empirical equations. Further, only limited experiments have been conducted in natural streams. The objectives of this research were to evaluate the adequacy of existing empirical relationships for describing natural stream flow fields and to investigate spatial distributions of flow variables. In this research, acoustic Doppler velocimeter (ADV) measurements were conducted at the reach scale (approximately 5 stream widths) in two cobble-bed streams. The measurements encompassed riffle, pool, and run stream units. The results showed that velocity distributions were adequately predicted with the log-law for all stream units and transverse locations. However, empirical turbulence intensity and turbulent kinetic energy equations inadequately described measured values. This was likely due to turbulence generation from stream banks, bedforms, obstructions, and other stream features.