7. Scour Around Marine Structures
211
7,2
Mechanism of Scour
7.2.1 General
When structures are founded in potentially mobile sédiments, it is important to quantify the degree of mobility and hence scouring of those sédiments. Because a wide variety of installation types are placed in the marine
environment, a general methodology for assessing the scour potential is
required for each type. Considering the flow near an obstruction like pile,
the dominant feature is the development of System of vortices around the
pile. These vortices act as a vacuum cleaner and aid the érosion process at
the cylinder. Some of the basic mechanisms responsible for the scour can
be explained for a two-dimensional flow in the following manner. The flow
boundary layer approaching a vertical cylinder generates a pressure gradient on the upstream face of the cylinder and the secondary flow results due
to the interception of the incoming velocity by the obstruction as shown
in Fig. 7.1. This has the effects of (i) formation of vortex (ii) downward
reflection of the flow streamlines around the cylinder as shown in Fig. 7.2.
The dimensions of the vortex System primarily dépend on the shape and
Fig. 7.1 Pressure gradient around the cylinder.
211
7,2
Mechanism of Scour
7.2.1 General
When structures are founded in potentially mobile sédiments, it is important to quantify the degree of mobility and hence scouring of those sédiments. Because a wide variety of installation types are placed in the marine
environment, a general methodology for assessing the scour potential is
required for each type. Considering the flow near an obstruction like pile,
the dominant feature is the development of System of vortices around the
pile. These vortices act as a vacuum cleaner and aid the érosion process at
the cylinder. Some of the basic mechanisms responsible for the scour can
be explained for a two-dimensional flow in the following manner. The flow
boundary layer approaching a vertical cylinder generates a pressure gradient on the upstream face of the cylinder and the secondary flow results due
to the interception of the incoming velocity by the obstruction as shown
in Fig. 7.1. This has the effects of (i) formation of vortex (ii) downward
reflection of the flow streamlines around the cylinder as shown in Fig. 7.2.
The dimensions of the vortex System primarily dépend on the shape and
Fig. 7.1 Pressure gradient around the cylinder.