Turbulence statistics of flow through degraded channel bed of sand–gravel mixture

• Experimental study of turbulence characteristics of flow over the degraded bed composed of sand–gravel mixture.
• Quadrant analysis is performed which demonstrates the importance of ejection and sweep phases in sediment detachment and transport.
• The magnitude of turbulence intensities and Reynolds stresses are observed to reduce, as one moves towards the downstream along the degraded bed profile.

This paper describes the results of an experimental study on the turbulence characteristics of flow through bed profile of a laboratory channel that was degraded by detachment of its bed material consisting of sand–gravel mixture. The measurements of velocity and turbulence characteristics over the degraded bed profile were made at three locations along the degraded bed profile using an ADV. The maximum value of turbulence intensity and the Reynolds shear stress were found to occur at the level of bed surface existing before the detachment and these values decreased towards the flow surface. The magnitude of turbulence intensities and Reynolds shear stress are observed to reduce, as one moves towards the downstream along the degraded bed profile which signifies that vortex structure weakened by bed profile degradation. Quadrant analysis was performed which demonstrated the importance of ejection and sweep phases in sediment detachment and transport. Highest occurrence probabilities are found for sweep and ejection event. The other two events i.e. outward and inward interactions are found to have very small occurrence on the probabilities. Variation of occurrence probability with hole size (H) is also discussed.