Scour regime effects on the time scale of wave scour below submerged pipes

• Time scale parameter was studied for both clear-water and live-bed scour regimes.
• Experiments were conducted with different grain sizes of bed materials.
• The time scale covaries with both Keulegan–Carpenter number and Shields parameter.
• New equations were suggested for clear-water and live-bed scour regimes.

In this study, a non-dimensional functional relationship was constructed to characterize the time scale of the scour process below submerged pipelines exposed to waves. The data used in the study were obtained from an experimental set-up which mainly consists of a wave channel, a wave generator, a test pipe and several measurement devices. The values for Shields parameters and non-dimensional time scale were calculated for non-cohesive bed material with different grain sizes. The results from both the present work and previous studies, despite being slightly different, indicate that both Keulegan–Carpenter number and Shields parameters are effective in the variation of time scale. In consequence of the results, the non-dimensional time scale is found proportional with both Shields parameter and Keulegan–Carpenter number. Also modified Ursell parameter was evaluated instead of Keulegan–Carpenter number to predict the non-dimensional time scale. The study reveals that the grain size of bed material and the scour regime have effects on the scour process. New empirical relations for each scour regime, clear-water and live-bed, were suggested.