What is dredging?

Why do we dredge?
We don’t want any ships to run aground!
What is dredging?
• Dredging is a mechanical means of removing sediment from
• The sediment is scraped, scooped, or pumped up to the
dredging barge to be transported away.
Dredging at Port Saint John
Why do we dredge?
The movement of sediments into the Saint John Harbour is a natural phenomenon.
The Saint John River and the Bay of Fundy move silt back and forth all day long
due to the river flow and the tide movement.
This silt is pushed around and then dropped off at the Port where it collects. If the
harbour floor has too much silt built up it could create a problem and potentially
lead to ships running aground. This would be considered a hazard.
In Saint John we have a naturally deep harbour but because of the movement of
sediment at the mouth of the river (where it meets the Bay of Fundy) dredging
must occur every year for several months.
The forces of the river and tide are very strong. In these photos you can you see
two forces pushing against each other.
How does dredging make our Port safe?
• By dredging, we can increase the depth of our harbours to allow ships to
navigate through.
• The main shipping channel must be 8.4 m deep when tidal waters are at their
lowest to allow large cargo and cruise ships to enter. (That is the height of a 2 ½
storey building).
• Harbour Pilots use navigational charts to determine the base depth.
An example of a
Navigational Chart. The
numbers represent depth
in metres. Notice how
shallow the water is
around the island.
• Port Saint John dredges every year so that the numbers represented on the
chart are true. For example, if the ship needs 8 m of water and the chart says
there is 10 m at that location,
it will be safe to enter.
Chart of
Saint John
How do we know when to dredge?
• First we discover where
dredging is required. To do
this we use a technique called
hydrographic surveying.
• This uses a computerized
sonar sounding system.
• Sound waves are sent out to
determine the depth of the
area below.
• If the sonar results show that
there is not enough depth for
the ships which come to Saint
John, dredging is needed in
that location.
What happens before we dredge?
• We consider the environment.
• After the hydrographic survey is completed,
samples of the silt in the areas noted for removal
are then taken and studied in a lab.
• Contaminated silt must be treated and deposited
on shore under special controlled circumstances.
• Silt that is not contaminated is deposited at a “dumping site” identified by
government agencies that protect the environment and wildlife
(Environment Canada and the Department of Fisheries and Oceans). Our
dumping site is at Black Point.
Did you know that lobsters use the silt beds
at Black Point for breeding grounds? The
area has the perfect:
• Water temperatures
• Texture
• Light
• Nutrients
How do we dredge at Port Saint John?
• A dredging company is hired to remove the silt.
• The dredging machine is a large crane with a
scoop on a specialized barge which has its own
• The scoop is first lowered into the water, then
raised bringing up the silt where it deposits it on
the barge.
• Once the barge is full of silt,
it moves to the spot assigned
for dumping and releases it
When do we dredge?
• Dredging occurs between July and November.
• During this time, the lobster and fishing season are
over so this industry and activity will not be
• Port Saint John has
been dredging the
harbour for many
years. This photo is
from 1950.
What is the cost?
Dredging is the single largest business expense for Port
Saint John.
About 1/3 of all the revenue made from business at the
Port is spent on dredging each year.
In 2012, the cost of dredging was $4.2 million.
The cost always depends on how much silt must be
removed and how much the dredging company charges
per unit of material.
Due to the high expense of dredging the Port is
partnering in a 4-year study with scientists at UNB
to look at ways we can predict the sediment each
year, and, perhaps, alter the water flow to reduce
the amount of dredging.
Other locations, such as the Fraser River in British
Columbia, spend up to $10 million on dredging
every year. They can lessen the cost by selling the
gravel they dredge from the river. We cannot do
that here because our dredge material (silt) is too
wet and the particles of sediment are much finer;
therefore it is not useful for construction.
Who benefits from dredging?
• We all do!
• Dredging gives harbour pilots
confidence navigating all types of
ships into the harbour
• Dredging means larger ships can
visit Saint John with cargo and
cruise passengers
• Dredging means more people
come to visit, more cargo comes
through the Port, and as a result…
o Jobs are created in the region by the activity generated from
both cargo and cruise.
o Businesses are expanding and new ones starting up.
o All this expands our world.
For more information about
Port Saint John…
Visit Port Saint John’s website at www.sjport.com .
You will find:
– Daily vessel reports (ships in port for the day and what
cargo they are carrying)
– Cruise ship schedules
– A link to tidal information
– News about the Port
Follow us on Twitter @PortofSaintJohn

similar documents