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Effect of dredging sequence on the efficiency of water injection dredger and trailing suction hopper dredger,
a case study of the port of Calais
Abbass Tavallali, Kathleen De Wit
Water injection dredger (WID)
WID is mostly used for maintenance dredging in low strength fine-grained soils
WID injects high volumes of low pressure water into the sediment
Calais port in Northern France
Maintenance dredging
Most of the maintenance dredging Trailing suction hopper dredger (TSHD)
Ferry terminals
Very busy traffic
Flexible Manoeuvrable
Self-propelled WID
Maintenance dredgingLocation of the dredging sites
Maintenance dredging
“Jetsed” : The main WID in Calais;
belonging to Van Oord;
the first vessel of this type;
built in 1987, upgraded in 2003;
maximum dredging depth: 23 m.
Maintenance dredging, Jetsed
Ferry terminals
Main characteristics:
More than 70% of the sediments consist of silt and clay (< 63 µm);
Exploitation level between 1 and 2 m above the one of the adjacent basin
Yearly dredging volume (by WID) about 7000 m³
Sequence of dredging can affect the efficiency:
It is wise to
deepen the basins by a TSHD first
and only then
dredge the ferry terminals with WID
The basins TSHD
Ferry terminals WID
Maintenance dredging, bathymetry
The advantage of the sequence : TSHD first and then WID
Maintenance dredging, sequencing
TSHD dredge more consolidated sediments in the basins Increases the TSHD’s efficiency
The basins become deeper than the terminalsEvacuation of the sediments from the terminals towards the adjacent basins highly improves
WID of the ferry terminals becomes more efficient
The above procedure is done usually two times per year (spring and autumn). The
relevant data from 2002 to 2010 are analysed.
The Operational monitoring for WID is very important
Maintenance dredging, operational monitoring
Before Dredging campaigns After
Check evacuation possibilities Check if the sediments are efficiently being removed as anticipated
Evaluate if some preparatory works with some other equipment are needed
Visualise how far the sediments are re-shifted
Optimisation purpose Ecological purpose
Evaluation of the impact of WID on the ambient turbidity
Maintenance dredging, measuring campaigns
Dredging campaign by WID from 30th March to 10th April 2012
A turbidity measurements campaign
Two campaigns of aerial photography
1 RCM-9 Current, turbidity and CTD sensor
1 OBS-3A Turbidity
1 RCM-9
With WID, high turbidity With WID, high turbidity, unrelated to dredging
Turbidity 30-60 NTU and peak is at 120-180 NTU at slack water at the end of ebb deposition of finer sediments in the absence of current, disappears with peak velocity of flow
Effect of WID does not observed Effect of dredging area does not observed
Observations:
Turbidity measurements
Turbidity measurements
Without WID, high turbidity No obvious relationship between turbidity and
cycle of neap and spring tides
Other processes: Waves, advection and navigationObservation:
Aerial photography
No qualitative
difference between
with and without WID
Without WID
With WIDWith WID
Turbidity generated by the
ferries clearly visible
Conclusions
Required crew and auxiliary equipment on a WID is very limited
Low cost technique At certain boundary conditions, can be applied very efficiently
WID installation is applied on a flexible and manoeuvrable small vessel
Suitable option for a location with high vessel traffic (e.g. Calais ferry terminals)
Operational monitoring for WID is very important
Both before and after the dredging campaigns
Effect of WID and dredging area on water turbidity are not observed in Calais