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 FlexibleManoeuvrable Self-propelled WID

Maintenance dredging Location 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 terminals Evacuation 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 purposeEcological purpose

Evaluation of the impact of WID on the ambient turbidity Maintenance dredging, measuring campaigns Dredging campaign by WID from 30 th March to 10 th 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 NTU and peak is at 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

Without WID, high turbidityNo obvious relationship between turbidity and cycle of neap and spring tides Other processes: Waves, advection and navigation Observation:

Aerial photography No qualitative difference between with and without WID Without WID With 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