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