7. Scour Around Marine Structures
225
3 Tunnel érosion, which occurs under the pipe and is a direct conséquence
of the increased velocity underneath the pipe, compared with the undisturbed velocity.
Mao [1986] reported the formation of three types of vortices around a
submarine pipeline resting on a plane bed. As illustrated in Fig. 7.9, one
of the vortices A, formed at the nose of the pipe, the other two vortices,
B and C, formed downstream of the pipe. It was also cited pressure coefficient distribution near a pipe in unidirectional flow measured by Bearman
and Zdravkovich [1978] and deduced that the pressure différence between
the upstream and downstream sides of the pipe may cause seepage underneath the pipe. The onset of scour according to Mao [1986], due to the
combined action of the vortices and underfiow, which leads to the formation of a small opening under the pipe as more and more sand particles
are carried away. Figure 7.10 clearly illustrâtes the process of the onset
of scour.
Fig. 7.9 Three-vortex System and onset of scour.