Where does silt go

Twister_Ken

Twister_Ken

Well-Known Member
Joined
31 May 2001
Messages
27,584
Location
'ang on a mo, I'll just take some bearings
Visit site
MDL newsletter just out announces winter dredging programme. Speaking about one marina it says “This winter’s dredge will take about one month to complete and we will be removing approximately 4400m³ of silt from A, B, C and X pontoons. This equates to just under two Olympic sized swimming pools. For those not into swimming, that’s 43 Double Decker buses full to the brim with silt.”

But where do they dump it - does it just end up being someone else's silt problem?

(One suggestion - Studland. It will fill in the holes left by wild and reckless anchorers) ;)
 
Dredging

They have stopped dredging the harbour here because it is now so difficult and expensive to find a place to empty the silt, instead they just drag out about 1/4 mile with a big steel plate behind a small tug,to where the water is a couple of metres deeper it must take at least a week to comeback in again.
 
They are using a new system in Portsmouth harbour. Silt is pumped into a temporary pipe laid to the main channel ready for the ebb tide and therefore let nature re-distribute it.

Silt from the Pompey gravel works which is left over after washing the gravel, is dumped south of the Nab Tower right on top of a couple of our favourite wreck dives.

Pete
 
We used to sit and watch the mudbank that is now Haslar Marina getting dredged out. The barge used to get topped up a couple of times a day and then dissapear somewhere out into the channel and dump it in the tide.

Historically, in Naval circles, the Isle of Wight was always known as the Pile of 5h1te, so mariners may have always had the ability to read the future.
 
At Chichester they tried a new system of dredging the silt, taking it to the entrance of the harbour and dumping it on the flood. Saves the expense of disposal and retains the nutrient rich silt for the environment as the tide simply distributes it around the harbour.
 
They're pumping it back into the river at Conwy (after all, that's where it came from!) - 20,000 cubic metres this winter, apparently. There's a motorised barge with a big diesel engine on it driving a water pump and a giant egg whisk. The egg whisk stirs up the silt, and the pump hoovers it up and discharges it through a floating pipeline back into the estuary on the ebb tide.

I'm sure in previous times it was dug out, loaded onto a barge and dumped near Puffin Island (ISTR there's a "spoil ground" marked on the chart). Otherwise landfill, as someone above said (I think the dredgings from Pwllheli had to go to landfill).

It all depends on what's in the silt and the whim of the Environment Agency (crosses self).

Andy
 
Thank you, Uncle Google

"Since 1990 the Nab Tower site has received all the capital dredge material for sea disposal arising from within the Solent. The most significant capital disposal operation at the site in recent years arose from the deepening of the navigation channel by ABP Southampton during 1996 and early 1997. This resulted in the production of approximately 7 million cubic metres of fine sands and silts, the majority of which was deposited at the Nab Tower site. The site also receives routine maintenance material arising from the many wharves and marinas in and around Southampton Water, the Rivers Test, Hamble and Itchen, Chichester, Portsmouth and Langstone Harbours and the Isle of Wight coast. "

Taken from:
http://www.cefas.co.uk/media/2823/wight_csac.pdf
 
MDL (and RHYC) up here on the East Coast just drag it out into the main channel of the Orwell by agreement with the Ipswich harbour Authority, it is then dealt with in their main dredging programme. They move to various agreed sites further down river where presumably it slowly makes its way back :D

This provides long term employment for the dredging companies ;)
 
I don't know about harbours and the like, but where I moor on the River Wey there is a constant problem with silt thrown up against moored boats from passing narrow boats. I was told by the Trust's foreman that under current regulations once they load silt onto a barge it becomes a landfill issue and subject to the Governments Landfill tax, so they are limited to moving it around within the scope of the dredging machines boom reach, which usually means chucking it back into the deeper channel. They would attract tax if they moved it elsewhere on the system, even on their own property.
 
I suspect Conwy is using the same system Port Dinorwic used three years ago. The egg whisk contraption is mounted on a sturdy boat and they commenced dredging on every ebb tide, even in the middle of the night, much to the consternation of the local residents.

No pipes, just a lot of 'emulsifying' of the silt/rubbish/water which just floated out on the tide.

I understand there were a lot of environmental studies made before dredging could start.
 
Twister_Ken said:
MDL newsletter just out announces winter dredging programme. Speaking about one marina it says “This winter’s dredge will take about one month to complete and we will be removing approximately 4400m³ of silt from A, B, C and X pontoons. This equates to just under two Olympic sized swimming pools. For those not into swimming, that’s 43 Double Decker buses full to the brim with silt.”

But where do they dump it - does it just end up being someone else's silt problem?

(One suggestion - Studland. It will fill in the holes left by wild and reckless anchorers) ;)
Click to expand...

In respect of Lymington & Yarmouth, its dumped between Fort Albert & Hurst Castle, on an outgoing tide.
 
OK I guess I am stating the obvious, but the facts remain.

Silt is exactly what its name says; fine particulate sediments that are carried by strong tides/ currents & precipitate out when the water speed slows. It is the reason meanders form & sandbanks. Docks and Marinas are bound to attract deposits as the water surges in at half tide carring a huge load of it, & then as the tide hits the flood & virtually stops flowing for an hour or so - and it all drops out.

Stir it up & move it on or dig it out & carry it away, it will only be carried while the current is strong & will stop wherever it gets to when the tide turns. Even dumping it at HW in a spoil ground, the lighter stuff will be swept up & away on every tide & find a new home where the currents eddy & sandbanks form, filling any pools or harbours it can find. The port of Parkgate on the Wirral now has grass growing a couple of feet below where the ships of the Royal fleet once tied up. The tide still arrives a few times a year but is only a few feet deep & threatens the houses on the prom rather than floating their boats.

ParkgateSummer.jpg
 
Last edited:
misterg said:
They're pumping it back into the river at Conwy (after all, that's where it came from!) - 20,000 cubic metres this winter, apparently. There's a motorised barge with a big diesel engine on it driving a water pump and a giant egg whisk. The egg whisk stirs up the silt, and the pump hoovers it up and discharges it through a floating pipeline back into the estuary on the ebb tide.

I'm sure in previous times it was dug out, loaded onto a barge and dumped near Puffin Island (ISTR there's a "spoil ground" marked on the chart). Otherwise landfill, as someone above said (I think the dredgings from Pwllheli had to go to landfill).

It all depends on what's in the silt and the whim of the Environment Agency (crosses self).

Andy
Click to expand...
The big bank at the mouth of the harbour in Pwllheli is the dredgings, The muddy hollow drying moorings are being dredged/put in wagons and put on the bank. Alledgedly the muddy hollow bits have weathered and are now suitable for putting on the beach to the west of pwllheli front, the dredging this year will go back to muddy hollow to weather for the next removal.
Stu
 
You must log in or register to reply here.

Share this page

Top