Technical
Considerations in Comparing Costs of Dredging Projects
Dredging projects are often
superficially compared by some cost-per-cubic-yard of mud to be processed.
These comparisons are often invalid, either because they do not include all the
ancillary costs of the process or, compare unlike conditions. For example, many
costs cited for specific disposal options do not include dredging and its
associated costs, like disposal of debris and decanting of dredged water. Other
costs cited for disposal options are for dredged material delivered to the
site, i.e., omitting the costs of de-watering, debris removal and
disposal, and the cost of dredging and sizing. Not to mention that dredged
material in a scow has become “fluffed,” and will occupy up to one-third more
space than the material in the channel before dredging, with the resulting
additional cost. Such a project cost is per-scow-yard, not per-survey-cubic
yard.
Here follow some of the
basic considerations in comparing actual costs of dredging projects.
·
The main unit of measure as laid out in contracts by the
Army Corps and Port Authority is the Survey
Cubic Yard. (For
all practical purposes, one yard of wet mud is about one ton in weight).
·
In short, it is a cubic yard of mud, sand etc. lying on the
channel bottom.
·
Specifically, it is the
difference between the pre-dredging and post-dredging survey, ie, the true
amount of volume that needs to be
moved.
This is a yard of dredged material
as it sits in a scow after being
removed from the bottom. It is less compact (“fluff factor”) and has had some
water removed from it. It can be from one-fifth to one-third less than a Survey
Cubic Yard. For instance, 100 survey cubic yards that must be removed from the
bottom might amount to 130 scow cubic yards by the time it is processed.
The fluff factor has to be accounted
for in many disposal options. A sub aqueous confined disposal facility (CDF)
may hold significantly less volume of dredged and dropped mud than the volume
that was excavated from it. It may take nearly a year for the material to even
approximate its original consistency, affecting capitalization costs of these
projects.
Testing
and sampling is usually done prior to contracts by the contracting agency, but
additional sampling costs may pertain (like testing along the line for material
going to Pennsylvania)
The
per-yard cost of actually moving the mud from the bottom into a scow. About $
8/ yard.
Dredged
material from the bottom can contain up to 20% debris that has washed in to the
channel, from cars to telephone poles. This debris must be separated from the
mud and land-filled separately at a much greater per-yard cost
Dredged
material is about 64% water. After dredging, a certain amount of water will
collect on the top of the scow, along with the water brought on board by the
clam shell. This water must be removed from the top of the scow, usually pumped
off to another scow and either treated or piped into sewage treatment.
the
term used to describe the charge for the use of a facility exclusively. This
may sometimes include a fee for the local community, etc.
The
limiting factor for any project is the amount of mud that can be processed in a
day. Depending on the level of resources to pay for dredges and scows, it is
usually the dock-side processing facility that limits how much mud can be
processed each day. The smaller the capacity, the more costly the
operation.
1.) How does your volume equate with a Survey Cubic Yard?
2.) Can you take the material as dredged?
3.)
What will you do with the water?
4.)
Is debris removal and disposal included in the price?
5.)
Is there a restriction on the type of material you will take?
6.)
Does this per-yard cost include dredging, and all the above factors?