Understanding the Importance of Turbulent Regimes for Fish Habitat: Beyond Turbulence Intensity

Previous investigations into the effects of turbulence on fish habitat utilization have used statistical metrics which measure the amplitude and frequency of velocity fluctuations in a flow. The results of these studies have been mixed; some show increased habitat utilization with increased turbulence while others show the opposite, no, or dome shaped trends. The result for most stream restoration and fish passage designs has been either to ignore or to simply "minimize" turbulence levels. To clarify the results, a set of novel experiments has been performed, with the specific goal of investigating the importance of the physical structure of the flow and its relationship to fish habitat. Seven separate turbulent flow conditions were created and characterized in a 60 cm by 60 cm flume. Data on the size, orientation, and circulation of eddies using Particle Image Velocimetry (PIV) were collected. Creek chub, semotilus Atromaculatus, were swum in each of the seven turbulent flow conditions under an increasing velocity test. For creek chub, orientation, size, and circulation are strongly correlated to the ability to station hold in a flume. The size, circulation, and orientation of eddies are governed by channel and obstruction geometry, bulk flow conditions, and distance from the turbulence source making stream restoration and fish passage . designs possible based on a few simple relations. These relations describe the eddy composition of a flow in terms of size, vorticity, and frequency as a function of the Reynolds number and location in the flow.