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Coastal Engineering: Theory and Practice
to its corresponding flows and sédiment loads the flow carries. When the
sédiment budget of the entry of sédiment load is not compensated with the
actual transport capacity, then dégradation starts from upstream and proceeds to the downstream. In case the downstream posses a greater sédiment
transport capacity, dégradation starts from the downstream end. From this
description of the processes involved, it can be seen that, to obtain early
warning of channel dégradation problems at a structure, monitoring the
changes over both upstream and downstream end for a significant distance
is mandatory.
7.4.5 Boat scour
Scour may also increase due to the effects of navigation, in terms of the
“blockage” or displacement effect of the vessel, causing increased local velocities around it, the currents caused by the vessel’s propeller the surface
waves (wake) caused by the vessel (which usually hâve most effect on the
banks).
The effects of navigation on the amounts of scour may be considered by adding the associated flow velocities to those which occur in the
absence of boats. In some cases the effects are likely to be minimal, due
to the short durations and small velocities in relation to the natural velocities. In canals, the passage of boats is likely to be the principal cause of
any scour.
7.4.6 High-head scour
Erosion of bedrock occurs in high-head situations, affecting massive hard
igneous rocks as well as weaker sedimentary rocks. The primary mechanism
for rock detachment in such situations is generally attributed to pressurisation of the joints.
7.4.7 Global or dishpan scour
Apart from the scour in the vicinity of the structure, changes in the sea
bed due to the flow pattern can be witnessed over a wider région from the
local scour. The combined local-global scour development is illustrated in
Fig. 7.6. [Angus and Moore, 1982]. This type of scour is called global scour
or dishpan scour resembles that of sand waves.
Coastal Engineering: Theory and Practice
to its corresponding flows and sédiment loads the flow carries. When the
sédiment budget of the entry of sédiment load is not compensated with the
actual transport capacity, then dégradation starts from upstream and proceeds to the downstream. In case the downstream posses a greater sédiment
transport capacity, dégradation starts from the downstream end. From this
description of the processes involved, it can be seen that, to obtain early
warning of channel dégradation problems at a structure, monitoring the
changes over both upstream and downstream end for a significant distance
is mandatory.
7.4.5 Boat scour
Scour may also increase due to the effects of navigation, in terms of the
“blockage” or displacement effect of the vessel, causing increased local velocities around it, the currents caused by the vessel’s propeller the surface
waves (wake) caused by the vessel (which usually hâve most effect on the
banks).
The effects of navigation on the amounts of scour may be considered by adding the associated flow velocities to those which occur in the
absence of boats. In some cases the effects are likely to be minimal, due
to the short durations and small velocities in relation to the natural velocities. In canals, the passage of boats is likely to be the principal cause of
any scour.
7.4.6 High-head scour
Erosion of bedrock occurs in high-head situations, affecting massive hard
igneous rocks as well as weaker sedimentary rocks. The primary mechanism
for rock detachment in such situations is generally attributed to pressurisation of the joints.
7.4.7 Global or dishpan scour
Apart from the scour in the vicinity of the structure, changes in the sea
bed due to the flow pattern can be witnessed over a wider région from the
local scour. The combined local-global scour development is illustrated in
Fig. 7.6. [Angus and Moore, 1982]. This type of scour is called global scour
or dishpan scour resembles that of sand waves.