Scour at culvert outlets as influenced by the turbulent flow structure

Scouring downstream of culvert outlets is a common problem that can lead to damage to the culvert structure and neighbouring land. This paper presents measurements of the turbulent flow structure within scour holes downstream of pipe culvert outlets at different stages of development. Velocities have been recorded in scour holes at four stages of formation to enable changes in flow structure to be observed as the scour hole develops. The analysis of mean velocities, turbulence intensities, Reynolds stresses and near-bed bursting structure has led to a foundational understanding of the flow structure in scour holes. Initial scour hole development is a result of high velocities exceeding the critical velocity for sediment transport whilst further development results in a reduction in the magnitude of the near-bed velocities and an asymptotic increase in scour depth associated with the turbulent structure of the flow. Towards the downstream end of the scour holes the jet comes into contact with the bed and flow structures similar to those observed downstream of backward facing steps are noted.