Civil Engineering Reference
In-Depth Information
Sandbanks may change shape, move, or even disappear completely. This
can interfere with navigation and nearby moorings may start to silt up and
become unusable. All these effects must be investigated by an analytical
model to develop a solution that is acceptable to all parties.
As mentioned, sediment accretion rates in the dredged trench are of great
importance to the contractor, so an accurate assessment of these is essential.
The rate of accretion is dependent on the volume of sediment being carried
by the water, the water velocities, and the shape of the trench. The profile
of the trench is generally chosen to suit the maximum slope gradient that
will remain stable over the duration of construction. The pattern of accre-
tion is affected by the orientation of the trench toward the direction of the
water flow and the side slopes of the trench. Depending on the geotechnical
parameters of the material being excavated, these can vary from almost
vertical sides in hard material to very flat side slopes of up to 1:8 in soft
alluvium. Accretion is likely to occur at the base of the trench if it has steep
side slopes. If the trench side slopes are at shallow gradients, this accretion
may only affect the slopes and not the central part of the trench. However,
if the trench is exposed for a lengthy period of time, it can tend to migrate
in the direction of the current as material is deposited upstream and eroded
from the downstream side.
Another effect that has to be considered is the impact on the sediment
dynamics due to spillage from the dredging operations. Increases in sedi-
ment concentrations, even temporary ones, can affect the nearby river eco-
system. Often, the marine authorities will already have specified limits for
sediment spillage that the contractor will have to adhere to. Sediment trans-
port from the construction works and how, and if, any additional sediment
disperses has to be modeled. Relevant spill scenarios can be determined
based on the types of soil to be dredged, the types of dredging equipment
proposed, and the duration of the activities. The results from the sediment
transport simulation show the extent of the bed affected by the works.
These effects then form part of the environmental discussions and suitable
mitigation measures will need to be introduced. This also links to the over-
all proposals for spill monitoring during construction, which are required
to demonstrate compliance with the authorities' requirements.
As an example of the type of investigation that is required, consider
the Øresund Tunnel. This is part of the fixed link between Denmark and
Sweden and the waterway it crosses is one of the main channels that enable
water to flow in and out of the Baltic Sea. The Baltic Sea supports a very
delicate ecosystem and a restriction on the consent to build the crossing
was that construction had to have zero impact on the water flow in and
out of the Baltic Sea, so that this balance would not be upset. The crossing
was part bridge and part tunnel and required the creation of an artificial
island to form the transition between the two. Demonstrating to the satis-
faction of the authorities that the shape of the island had no overall effect
