Answers aren’t always obvious. Take the Port of Emden, in Germany which was probably the first big study on what is called ‘fluid mud’.
Mud deposits from the river started up in the water layer, eventually consolidating and dropping to the bottom.
The problem was, no matter how much they dredged (and this was no mean 2.5 million cubic metres per year) the sediment came back as rapidly as it went. It was found to be due to the relatively high density of particles in suspension just outside the harbour in the River Ems, which meant the mud crept back within days of all the hard work.
A very novel approach was used – forget about moving the mud, because it simply isn’t going to stay out.
When looked at closely, it was found that despite the density, the top layers of mud didn’t stop the ships. In fact, the space between the single particles was found to be filled with microbial slime which, unappealing as it sounds, helps shipping pass through. Further, the slime causes the mud to stay in suspension over a long period of time and in fact, the consolidation period comes about from the fluid mud passing from an aerobic (biologically lively) state to an anaerobic one.
So the approach has been to give the mud a gentle lift to oxygenate it, which helps it grow the needed bacteria. Keep the slime, keep the navigation.
Performing this operation with a converted hopper dredger every three months has virtually eliminated the port’s need for dredging, explains Rob Kirby, who has put a lifetime into getting these alternative methods accepted.
And despite the complication around the definition of ‘navigable depth’, it’s a system, that combined with the others, may be used in other places like the port of Venice which needs to keep the sedimentary ‘balance sheet’ of the fragile Venice Lagoon without substantial deficits while still allowing for transit of the large ships.
Mr Kirby adds that there are a number of ports, like Vostock and Luebeck and others in the shallow waters of the Baltic which are caught in the pinch of legislation which minimises possibilities of disposing of dredge either at sea or on land. These obviously could make use of methods to minimise harbour siltation including the fluid mud approach.
However the final point, once again, is that biology can be on your side if you know how to treat it right.
