Capital dredging

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 material.

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.

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