The Art of Dredging

Dredging and shipping

Dredging the Westerscheldt

 

Between 1988 and 1999 I worked onboard "James Ensor" and "Amerigo vespucci". These ships were mostly dredging on the Westerscheldt river, the lifeline for Antwerp.

 

                                       

 

 

        

 

Both vessels had a length of appx. 100 metres, a draught of 6+ metres loaded, and were very manoeuvrable. We often said that they were the "motorcycles of the Scheldt". 

The Westerscheldt has always been a tough school for skippers.

The river sees continuous heavy traffic. It's a tidal river with currents depending from ebb /  flood, and a tidal range up to 6 metres.

The sandbanks are eternally shifting and changing the face of the navigation channels. Hence the necessity of continuous maintenance dredging to preserve channel depths.

 

MECHANICS OF CURRENT PATTERNS 

At every buoy, on every hour, the current has a different direction and speed. There is a logic in that chaos. When sailing or dredging on the Scheldt river, one needs to know the logics behind the current patterns.

From Vlissingen to  Antwerp, the river is dotted with sandbanks. The main navigation channel meanders around these banks.

Let's explain current patterns with the example of Hansweert:

At the start of flood tide (=low water) the water moves into the delta from Vlissingen towards Antwerp. The sandbanks in the river are still uncovered, so the waterflow follows the main channels inward.  When the waterlevel rises, the sandbanks become covered and the tidal currents start to cross the shallows. Thus currents start to cross the main navigation channel. These cross currents appear from 2-3 hours after lowwater up to 3 hours after highwater, untill the sandbanks dry out again.

 

 

With ebb tide; the process is reversed: as soon as the shallow banks dry out, water flows around them.

 

 

 

 

Current velocities can be very high, depending from location, lunar phase and wind. Personally I experienced a few times current velocities up to 5 knots. These will only run during a short interval of appx. 15-30 minutes, and only at springtide, and in certain parts of the river.

Northwest storm winds tend to funnel seawater in the delta. If they coincide with springtide,  high water levels can be extreme, flooding the quaysides in Antwerp.

There are some "bottlenecks" in the river, causing high current velocity, because of the smaller area section: Borssele, Hansweert , and the part from Walsoorden to Valkenisse. 

The current may be a nuisance for through going ships, but they are a real hazard for trailer dredgers. A dredger typically  dredges with a speed of 1-2 knots. With a given cross current of 1-2 knots this makes for a driftangle of 45 degrees. These large driftangles are equally confusing for passing seaships, who cannot determine the course over the ground of a dredger.

Ships cannot fight these currents. But current can be used, and then it may become an extra engine for the ship, getting the ship where the skipper wants.

 

SHIPPING

Shipping on the Westerscheldt is dense. Vlissingen sees about 150 movements a day; that is one ship every 10 minutes. This does not take in account inland navigation craft, tugs, pleasure craft, etc... 

All shipping is tightly controlled by an elaborate VTS-system. Larger seaships are all piloted, though smaller seaships may enjoy a pilotage exemption. The latter may lead to some dangerous situations, since all VHF-communications are handled in Dutch, except for the pilot-exempted vessels.

Normally, the pilotage service is on a very high level. Young pilots on coasters may scream through VHF that they need the whole width of the fairway, but experienced pilots on the so-called "super" ships (the largest ships that can navigate the river) never make a fuzz, they just pass by, sticking to their side on the channel, being used to the acrobatics of the dredgers.

TSHD "Manzanillo" during dredging in the vicinity of Valkenisse, as seen from the wing of a passing containership.

 

 

 

 

 

 

The opening up of European borders meant that East-European inland craft are seen more often on the river, further worsening the language problem.

The Dutch part of the Westerscheldt sees a fairly constant traffic flow, but on the Belgian part shipping tends to converge, depending on the operation of the main locks to the port of Antwerp.

 

An example: "Zandvliet dredging area": the dredging area is roughly in the middle of the photo.

  • Inward bound vessles are slowing down, (coming from NW on the photo), awaiting turns in the Berendrecht- and Zandvliet locks (to the east), often already attended by tugs,
  • Large containerships swing around for berthing at the two terminals south and north of the locks,
  • Large groups of coasters may come from Boudewijn- en Van Cauwelaert locks (from the south), often on full speed
  • Variable currents
  • At night: blinding background lights from the conatinerterminals
  • An overhead powercable causes false radarecho's

Dredging on a chokepoint like this, is not a piece of cake.

The continuous presence of a dredger in high-risk area's like these, gave rise to a few collisions already, mostly "touch-and-go".

Early nineties dredger "Schelde-II" was rammed by a reefer. The ship sank, one crewmember died. The accident may be caused by overreliance on the use of VHF to avoid collision.

The "Schelde-II" was later repaired and renamed "Jade River".

                               

                  

On one occasion, "Amerigo Vespucci" to was involved in a "brush", when a large bulker (overtaking us) lost rudder control and sheared towards our stern. "Amerigo Vespucci" had some paint-damage, but everybody on the bridge was near a mental collapse.

The one near-miss that almost caused "James Ensor" to be constructive-total-loss occurred also around 1990, during my watch. When I tried too hastily to reach the dregding area and threw "James Ensor" in a melee of ships, we were almost rammed midships by a reefer.

Misuse of VHF was (again) one of the root causes of this near miss. The other root cause was perceived economical pressure I felt, rather than the urgency to play ity safe.

Marc Van de Velde

 

 

http://users.skynet.be/p.woinin/dgk/u6sche.htm

 

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