When handling contaminated sediment a number of environmental friendly techniques
exist. It is often economic restrictions connected to the individual dredging job that
restrict the use of these more environmental friendly dredging and depositing techniques.
The present report lists dredging techniques, ways of transportation and methods of
sediment disposal that are most often used by Danish Contractors.
The report also briefly describes how the different dredging, transportation and
depositing methods are used and under which circumstances. An estimate of cost and
environmental consequences related to each of the techniques is also included. The
economic estimates were acquired in the autumn of 2000 from Danish contractors, who
specify that prices vary, depending on each assignment. Permits and fees associated with
dredging are also contained in this rapport.
When choosing a method for a particular dredging job a number of considerations have to
be taken into account. Primarily, the quantity of sediment to be dredged, the type and
degree of contamination that exist, have to be considered, as well as access way, working
conditions in the area and the distance to suitable disposal areas.
As all these factors need to be taken into consideration, it is difficult to set up
general guidelines for the best choice of handling methods for a dredging operation. On
the basis of gathered information however, the following general guidelines for the
dredging of clean sediment are set forward. It shall be stressed that the capacity of the
contractor’s equipment is important when choosing equipment for a specific
assignment. The intervals stated below are therefore only meant as guidelines.
When dredging large quantities of clean harbour sediment (>
30.000 m3) a bucket-dredger is preferable. The bucket dredger however is
expensive to mobilise, mostly because it is also necessary to mobilise a barge for
transportation of the dredged sediment. Due to this, the costs will be disproportionately
high when cleaning small quantities of sediment.
When cleaning quantities between 10.000 m3 – 30.000 m3 of
clean soft sediment, the use of a trailing suction hopper dredger are advantageous.
However, if the loading capacity is exceeded and the distance to the disposal area is long
it will be a costly and slow method.
As a suction dredger requires less space and is cheaper to mobilise than the trailing
suction hopper dredger, it would from an economic point of view, and depending on the type
of assignment, be more suitable for cleaning smaller quantities of sediment (~ 10.000 m3).
Smaller quantities of sediment (< 10.000 m3) can be cleaned by a dipper
dredger, bucket dredger or closed bucket dredger. These methods are cheaper to mobilise
than for example the bucket-dredger or trailing suction hopper dredger.
The same guidelines as above apply when dredging contaminated sediment. However,
measures must be taken to ensure that contaminated matters are not spread to the
surroundings while dredging. These measures could be either to use modified dredging
methods, a silt curtain or to cause sedimentation by using air.
It is necessary to mobilise a barge if dredged sediment has to be transported over a
long distance.
If the dredged sediment has to be disposed at sea then the use of a split/capping barge
is recommended.
If the sediment is contaminated it should be transported either in a closed barge or a
pipeline, depending on the distance, quantity and the local weather-, water- and access-
conditions. Contaminated sediment dredged using either a trailing suction hopper dredger
or a suction dredger, can very well be transported in the hold of the vessel.
Sediment suitable for recycling or disposal at sea should not be deposited on land.
Contaminated sediment should only be disposed of in tight depots when it is necessary
due to environmental concerns. Slightly contaminated sediment should not be deposited in
tight depots, but instead pumped ashore to inland lakes or upland sites where the runoff
water passes through a clay embankment before reaching the recipient. In this way most of
the heavy metals contained in the runoff water would be retained.
As contaminated sediment is deposited in tight depots, it is important to be able to
control whether the depot is actually tight, and to ensure that potential runoff water can
be treated, before it is released to the recipient.
When establishing depots, the possibility of land recovery, whether for industrial or
recreational purposes should be considered. If the sediment is dried/drained before being
deposited on land, special depots can be established where the sediment is deposited
higher that the surrounding banks, following the same principle as when depositing on a
landfill.
The cost of disposal primarily depends upon demands to the tightness of the depot and
then secondarily on the location, need for surveillance and monitoring of the site.
Establishing many small tight deposit sites is very costly, so the possibility of
establishing large tight deposit sites, available for an entire region, should be
investigated.
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