The Tidal Effects After Dredging.

[blueboatdriver]

Just used Google to find out a bit about the influence the dredging will have on the tidal flow in Poole harbour. Quite interesting if you like that sort of thing

 

 

 

5.4 Predicted effect of the proposed scheme on water levels and tidal currents

5.4.1 It is predicted that spring tide low water levels would be lowered by up to 20mm as a result

of the effect of the channel deepening on tidal propagation, resulting in the increased

exposure of between 2.9 and 11.7ha (measured at CD) of intertidal habitat at low water on

spring tides. These estimates represent the range of intertidal slopes in Poole Harbour

from which the change in areas was calculated (i.e. 1:50 to 1:200 slopes).

5.4.2 As regards the impact of the works on tidal currents within Poole Harbour, the following

conclusions were drawn:

 

[Paul J]

There was a suggestion that some of the side channels will get silted up and there would be a loss of some moorings.

 

Time will tell

 

PJ

[Afishionado]

OK here's a prediction, all channels inside the harbour will silt up except for the middle (main) channel which will be further dredged and straightend and become known as The Poole Ship Canal. The silted up areas will be grassed over and turned into a huge travelers encampment and the sand that was pumped onto Bournemouth and Poole beaches will be ripped away in a gigantic storm and form a small island just outside UK territorial waters. This island will be taken over by Tesco who will open a huge tax free shopping center accessed by a 6 lane motorway bridge funded by the EEC and the Lottery Commission.

 

OR

 

No one will actualy be able to notice a bloody thing but go round kidding themselves that yes indeed the tide is .004873% faster than last year!

 

Mad Mike

[Maverick Martin]

I knew it...Tescos had to get in there somewhere its all their fault

 

Martin

[Fugazi]

Tesco's share price rose dramatically at the close of trading today based on unconfirmed rumours that they are moving into offshore trading. Fantastic web site this to pick up insider investment and fishing tips, buy, buy, bye

 

Gordon H

[great white]

An interesting discussion at work a few weeks ago, or was it in the bar at the last meeting?

 

 

 

That although 1 million tons of mud have been removed, none of the area was drying ground, and the rise and fall will be the same as before [all above the new depths.

 

So would the volume of water moving through the entrance not be the same as before and therfore also the speed of flow?

 

Can someone answer before Mike, but I look forward to reading his views

 

Charlie

[blueboatdriver]

That's exactly what I was thinking Charlie. My earlier search did answer a few of my questions though.

[Newboy]

Not sure if I have it correct, but if you look at the attached drawing. One on the left is the harbour as it is now, and the one on the right is after it'd been dredged. The blue bit is the new depth created by dredging. The red line is low water mark. The green line is high water mark.

 

The dredged area has no affect on the tidal flow thru the harbour entrance, as it is 'dead' water, it always sits there regardless of high or low water. The only water moving in and out is the top layer, from high to low water, this amount of water is always the same regardless.

 

There might be an effect if they dredge poole entrance as it will dictate the amount of water flowing thru, i.e., from x million gallons per minute to x+y millions gallons per minute.

[Afishionado]

Charlie says.....

That although 1 million tons of mud have been removed, none of the area was drying ground, and the rise and fall will be the same as before [all above the new depths..................................

 

So would the volume of water moving through the entrance not be the same as before and therfore also the speed of flow?

 

The removed sand/silt would have been static at all states of tide. Its volume having being removed will be replaced by water which is not static. Therefore more water will have to flow into the harbour to replace it BUT ONLY ONCE. Once the water has replaced the volume of sand/silt removed it becomes part of the total water volume at any given state of the tide. Ergo..... There will be a greater volume of water in the harbour than previously.

 

Will it affect the tidal FLOW in terms of velocity? Basicly no although a purist will insist that there will be an increase in flow speed in the choke point of the Haven it will be minute.

 

Will it affect the time taken to fully ebb or flood? Yes. As there is more water (notionaly) and the venturi (The Haven) remains a fixed width and depth then the increased flow will take longer.

 

Will one be able to notice the extra time? No due to all the other variations like air pressure, wind direction that affect the times of low or high water one could not say that a late low was a result of the dredging.

 

Conclusion..........

 

It's a done deal.......Get over it!

 

Mad (but not always) Mike