23 February 2011

More Than a Cubic Metre of Gravel

I noticed when scanning the Non Technical summary of the Environmental Impact Assessment (EIA) for the filling operation, that :

"The settlement lagoon in the north east part of the site which will control the return of water back to the sea and reduce suspended solids will include a membrane to prevent saline sea water entering the ground water and, as such, there will not be an impact to salinity. Water within the settlement lagoon will be allowed to stand during the dredger’s cycle period until enough settlement has occurred in the water to allow safe discharge back to the sea. "

This seems to be backed up by the main EIA.

Now I am not an expert, but I have shifted plenty of gravel and sand mixes by hand, and can say for certain that when wet, a given cube of the material is much heavier than when dry. I have found no mention of washing the gravel to remove the saline pumping water, so can only assume that this will be spread over the site with the gravel.

Figures I have seen show that a cubic metre of sand/gravel when dry is about 1650kg and when wet about 2000kg (this will I am sure vary, depending upon type of rock, grade range of material etc). Calling it 300kg of saline water per cubic metre of sand/gravel would probably not be miles out i.e. about 15% of the total fill weight.

If as is implied by the EIA, saline water entering the groundwater is not a good thing, and mitigation measures are in place for the settlement lagoon, then what about the balance of the site. I see that Natural England had some concerns, though I can't find any mitigation measures in place to address these.

I would like to understand what if any affect this will have on the local ecosystem? Any experts out there care to comment.

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