Blackwater Silting Up

Major_Clanger

Major_Clanger

Well-Known Member
Joined
15 Oct 2016
Messages
1,709
Location
Home is Suffolk, boat either Suffolk or Cornwall
Visit site
Left Osea this morning and sailed the last bit to my mooring in Colliers Reach. I knew my window to get back on would be narrow due to poor tides mostly, aided and abetted by water extraction and high pressure. It was far worse than I thought though; my reference point on the lock gates at Heybridge was just covered so I should have been okay but I ploughed in to the mud a boat-length from the mooring. I now have to spend the day at 45 deg. with a hyper nine year old, no FB pies and hope that tonight's marginally better tide will be good enough to winch her in on the lines (fore-and-aft mooring).

Anyway, the reason for all this waffle is that I'm convinced the river margins are silting-up at an accelerated rate. Increased water extraction is definitely one reason, but I was told by someone that the construction of the new berths at Felixstowe has had, and continues to have, a knock-on effect for the Blackwater.

Does this sound plausible?
 
Major_Clanger said:
Increased water extraction is definitely one reason, but I was told by someone that the construction of the new berths at Felixstowe has had, and continues to have, a knock-on effect for the Blackwater.

Does this sound plausible?
Click to expand...

Not really.

How does water extraction (from where, btw?) affect silting in tidal waters? Reduced river flow means less silt, not more - surely? Not that the Blackwater even has a silt-carrying river. Equally how could constructon in Felixstowe affect silting anywhere, even significantly in Felixstowe, let alone the very top of another estuary forty miles away?
 
Old Bumbulum said:
Not really.

Reduced river flow means less silt, not more - surely?
Click to expand...

No; it can mean that silt is being dropped sooner than it would be otherwise. Any change in water velocity will result in a changed pattern of deposition; lower velocity will move deposition inland; increased will move it out.

The steady state flow isn't the main means by which silt is moved; the sediment carrying power of a river increases as a high power of the runoff, so the majority of silt is moved into the estuary during flood conditions; perhaps during a few days inthe year.
 
I don't know about the water extraction either but it would have to be massive to have any noticeable effect. Reduced flow might mean less scouring and increased sedimentation. In the Colne this has been caused by lack of shipping and the presence of the barrier but I don't think the Blackwater would be affected,

I wouldn't completely rule out the possibility of Felixstowe affecting places up the coast because these things are so complicated. Another possibility is not that the river is silting up but just that the contour is remoulding itself, as is happening with the disappearance of the sandy beach at the Rocks.
 
Old Bumbulum said:
Not really.

How does water extraction (from where, btw?) affect silting in tidal waters? Reduced river flow means less silt, not more - surely? Not that the Blackwater even has a silt-carrying river. Equally how could constructon in Felixstowe affect silting anywhere, even significantly in Felixstowe, let alone the very top of another estuary forty miles away?
Click to expand...

E&S Water have been extracting from both the Chelmer and Blackwater for many years. It's necessary due to their fairly small catchment area. Over the last two decades it's become more of an issue though, and has been discussed at various meetings with river users and ESW. Extraction has definitely had an effect on the upper navigable reaches, and since the river sees almost no commercial traffic the urgency to dredge is lacking.

Look at a Thames estuary chart. The Blackwater is esturine in nature and has a marked effect on tidal flow, the flood especially so. Removing a huge amount of material from one area and dumping most of it, sans aggregates, is bound to have a knock-on affect. I don't fully understand how it all works, but I know with certainty that the Blackwater is silting heavily above Osea; I'm currently sitting on some of it!
 
The Blackwater has been scouring in recent years, witness the necessity to place a cardinal mark for Tollesbury Pier, a structure that hasn't been seen for many years but now pokes up at low water. What's happening up river from Osea I don't know. Could it just be a movement of mud into the channel you were expecting rather than the whole river silting? Seems more likely.
 
Last edited:
It just changes. My mooring was off the Heybay for 20 years and I wintered for 37 years: the mud comes and goes. Last year which is the last I have wintered, the mud was coming - coming strongly. Probably it will change. Mind you now the lock gates hold water..... :D
 
Fresh water abstraction will have negligible or no impact on water depth in a tidal reach. And AntarticPilot is right - the greater the velocity the greater the energy to keep particles in suspension and shift them further - basically Stokes Law. Tillergirl has the plainest and sanest answer, though.
 
Fresh water would have a small effect on salinity. Would that affect the ability of the water to carry silt?
At St Lawrence Bay the sea bed changes all the time. Sinkers that are dropped on the sea bed one year can be extremely difficult to shift another year as they are buried in the silt. Another time it will be easy as it is sitting on the surface. We also find that often the chains come up fairly rusty & covered in silt & weed. At other times a mooring from the same spot will come up shiny where it has been been abraded by sand in the upper levels of the water.
My own mooring can be in sand one year & silt another.
 
There has been a change at Marconi too. It used to be that you could walk out to most of the drying moorings as there were substantial patches of stones which made it quite hard underfoot. Someone tried it earlier this year and got stuck in the mud. When you look at it now, at low tide of course, there are no signs of the stoney patches and it is all just mud.
 
I guess it's down to tides, channel morphology (and a dozen other variables) and the good old butterfly effect. Dealing with fluid dynamics with lots of variables is tricky, and any model is only a model.
Fresh water will dilute salt water, but the two don't mix very quickly - the fresh water floats on the top and generally there's a saline wedge that progresses up the channels on the flood. For the same movement / volume, salt water will carry more sediment (or of a different particle size), but engineers better than I struggle with this the world over. Its a fascinating topic. We'll never crack it.
 
No one's mentioned whether, and the heavy preponderence of Easterlies this year. Where I was born in Cornwall, there was a beach South of Land's End (Nanjizal) that would be full of sand one year and only rocks the next... Depending on what the gales had done in the winter...
 
Last edited:
There has been little rain this summer. The Blackwater system has not been dumping huge amounts of water under the Fullbridge and carrying silt further down the estuary. Hence silt accumilating further up river. If we get a wet winter I would expect scour in the upper reaches and siltation to accumulate further down the estuary.
 
Silting Up

The river just below Chatham town centre and up around the bridge has been silting up with the decline in commercial traffic and the resulting loss of funds for dredging.
Be careful if navigating at LW especially springs, some of the stuff is hard.
 
You must log in or register to reply here.

Share this page

Top