Boswall Bittern – Silt agitation Catamaran

Anybody looking at the history of Granton Harbour will be aware that it used to be a busy harbour with many large ships coming and going.  In both World Wars it was a significant base for warships.  Looking at it now it is hard to believe, as it is full of mud and dries out in the East Harbour when the tide recedes.  For more about the history there are some fascinating pages available on the web:  see Granton Harbour History

Without dredging Granton harbour silts up and would in time become a salt marsh.  But there is a lack of availability of traditional dredgers, and the cost is high.  The club therefore investigated the option of silt agitation as an alternative to dredging in 2007 and, having carried out successful trials, commissioned Jet-Pumps UK Ltd to build a 10 metre silt agitation catamaran.  This allows the Club to carry out its own constant maintenance dredging with a view to achieving a depth in berthing areas of 1.5 metres below chart datum. The vessel was delivered in November 2008 and subsequently named by our Patron HRH The Princess Royal.

The club’s silt agitator catamaran is Boswall Bittern.

Boswall Bittern at work

Boswall Bittern at work

The principal of silt agitation is that it mimics the natural process whereby silt is carried from the head of the estuary into the sea.  Some of this is swept into Granton Harbour and is deposited there.  With silt agitation a large nozzle is lowered into the mud and a jet of water injected into it to stir up the mud.  If this is done as the tide ebbs, the silt stirred into the water is carried back out into the estuary where it is naturally dispersed.

Boswall dredging 2

Any dredging has to be licenced.  We have had two 3 year maintenance dredging licences from Scottish Ministers.  The first was restricted to summer month operation. This was then extended to all year round operation, for a further 3 years.  And wBoswall dredging 1e are now applying for our next 3 year period.  The Club is grateful for support in this from RYA Scotland, The Edinburgh Marina Ltd and the Royal Society for the Protection of Birds.

The initial period of operations highlighted some design issues which have required some fine tuning.  But Boswell Bittern has proved itself with marked improvement in the depth of the water, particularly around the pontoon and the approach club crane and dinghy slipway (see the photos).

In 2014 the clubs commissioned a full bucket dredge from Shearwater around the EML pontoons, but this needed to be supplemented in 2015 by the use of Boswall Bittern on the inside of the pontoons where Shearwater was not so effective due to the ground chains.  For 2016 onwards Boswall Bittern will be used annually to maintain the depth inside the EML pontoons.

2016 will see further maintenance dredging around the approach to the dinghy slip and the approach to the zone earmarked for club marina pontoons. For some of the work involved in gathering the information to renew the dredging licence, see the post “Mud, Mud, Glorious Mud!”

 

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