7. Scour Around Marine Structures
215
cour hole tended to be of the same size and pattern as that due to the
urrent alone.
\3 Sédiment Dynamics of Scour
rhe movement of bottom sédiment involves, transport and érosion deposiion. For the pur pose of defining major physical différences in the dynamics
>fthe sédiments founded in marine environment, bottom sédiments can be
considered under two major groups viz.,
(i ) Cohesionless sédiments in which the main stabilizing force is acting and interlocking between the individual particles that act as discrète mass.
ii) Cohesive sédiments in which the primary stabilizing force is the
physicochemical force between the grains and is stabilized in their
position on the seabed by electrostatic forces between adjacent grain
surfaces. The net attractive inter-particle surface forces which are electrochemical in nature and to some extent these forces impart the résistance to érosion.
Scour occurs when the fluid forces overcome the stabilizing forces. For
cohesionless sédiments, the magnitude of fluid shear stress needed to initiée grain movement can be determined from shields parameter [Keulegan
ind Carpenter, 1958]. But in the case of cohesive sédiments, a generally
iccepted relationship has not yet been found. The bottom current vélocités to develop the considérable fluid shear stress for the cohesionless soil is
in the order of 0.20 m/s to 0.50 m/s and for cohesive soil (clay) it is 0.1 m/s
;o2.2m/s [Niedoroda and Swift, 1981]. In cohesionless soils, after the parties are dislodged, the particles move on or close to the sea floor (bed load
transport) and cohesive sédiments tend to get suspended and transported
ivithin the turbulence. Vanoni [1975] gave a chart for estimating the critical
/elocity to cause incipient motion of quartz sédiment as a function of mean
grain size (D50). For mean grain size in the range of 0.2 mm to 100 mm, the
critical velocity increases with increase in grain size.
74 Types of Scour
74.1 General
When there is an obstruction to the flow in the océan, like an offshore structure, the bed in the immédiate vicinity will be eroded, thereby dépréssions
215
cour hole tended to be of the same size and pattern as that due to the
urrent alone.
\3 Sédiment Dynamics of Scour
rhe movement of bottom sédiment involves, transport and érosion deposiion. For the pur pose of defining major physical différences in the dynamics
>fthe sédiments founded in marine environment, bottom sédiments can be
considered under two major groups viz.,
(i ) Cohesionless sédiments in which the main stabilizing force is acting and interlocking between the individual particles that act as discrète mass.
ii) Cohesive sédiments in which the primary stabilizing force is the
physicochemical force between the grains and is stabilized in their
position on the seabed by electrostatic forces between adjacent grain
surfaces. The net attractive inter-particle surface forces which are electrochemical in nature and to some extent these forces impart the résistance to érosion.
Scour occurs when the fluid forces overcome the stabilizing forces. For
cohesionless sédiments, the magnitude of fluid shear stress needed to initiée grain movement can be determined from shields parameter [Keulegan
ind Carpenter, 1958]. But in the case of cohesive sédiments, a generally
iccepted relationship has not yet been found. The bottom current vélocités to develop the considérable fluid shear stress for the cohesionless soil is
in the order of 0.20 m/s to 0.50 m/s and for cohesive soil (clay) it is 0.1 m/s
;o2.2m/s [Niedoroda and Swift, 1981]. In cohesionless soils, after the parties are dislodged, the particles move on or close to the sea floor (bed load
transport) and cohesive sédiments tend to get suspended and transported
ivithin the turbulence. Vanoni [1975] gave a chart for estimating the critical
/elocity to cause incipient motion of quartz sédiment as a function of mean
grain size (D50). For mean grain size in the range of 0.2 mm to 100 mm, the
critical velocity increases with increase in grain size.
74 Types of Scour
74.1 General
When there is an obstruction to the flow in the océan, like an offshore structure, the bed in the immédiate vicinity will be eroded, thereby dépréssions