Capital dredging for navigation purposes is the
excavation of sediments to increase depths in an
area, usually but not always for the first time,
to accommodate the draft of vessels (to a depth
that also allows for a siltation buffer zone). The
name of this type of dredging derives from the implications
that the work requires the payment of a single capital
sum. Excavation generally takes place into ‘virgin’
material that is relatively stable and has become
consolidated under the existing hydraulic regime.
However capital dredging also includes the removal
of material from previously dredged areas where
sedimentation has since occurred and has not been
disturbed by further dredging over a period of time.
In such cases consolidation of the deposited material
occurs and the physical properties of the bed will
revert to similar characteristics to the virgin
material and is therefore treated as capital dredged
As identified above, MAFF generally consider the
time period required for this process to occur to
be at least five years. With capital dredging the
full range of materials may be encountered and soft
materials, such as clays, sands and silts, can be
mixed with stiffer clays, boulders and rocks. The
additional assessment made by the consenting authorities
on capital material may demonstrate that despite
a period of consolidation, the material continues
to exhibit the characteristics of maintenance dredgings
and accordingly will be regarded as such.
Definitions are based on those provided by the
following dredging specialists and organisations:
- Bray, Bates & Land 1997.
- Institute of Civil Engineers 1995.
- ABP Research, R462 1994.
- IADC & CEDA 1996.
- Bowles, MAFF personal communication 1999.