19 May 2011

Conversations regarding excess water on Tesco site.

From Hugh Barlow, 18th May

I have had three conversations today, with Tom Shipp of Jubbs, with Garry Mountain the site manager, and with Fran of the Express and Echo.

Fran was following up the report that EDDC gardeners had not been able to replant the flower beds between the tram station and the car park, because water levels were too high: they were full to a few inches below the surface with salty water. I raised this with Tom Shipp, and he referred me to Garry Mountain.

Garry's understanding was that it was the water level rather than its salinity which was the problem. He pointed out, with some justification, that the previous bedding plants had still been flourishing. We differed somewhat over the question of salinity, and I suspect that he has been over-reassured by Delta Simons, the consultants who have been measuring salinity, that any extra salinity represented only “a drop in the ocean”. My point was that it depended where that drop was concentrated. He knew that the piped outlet had been to a point well away from the coral reefs and sensitive protected area of Lyme Bay.

I suspect that the salt has been more concentrated than they recognise: I have not previously seen the water of the ditch so blue, and we saw the white deposits round the settlement lagoons, where the salt water had evaporated. Fran had very sensibly asked how salt water from the bay became more salty on the site, and I believe that is because it has absorbed salt from the dredged material, particularly as it warmed up out of the sea. Garry also thought the water could not be any different out of the sea rather than in it.

Garry had a mixed explanation for the levels of water oozing out from the site. In the first instance, the sand was super-saturated (more so than the intended gravel would have been), and simply oozed out as it was squeezed down. Then groundwater was being forced up, through fence-holes and other breaks in the surface, as the weight of fill (or surcharge) compacted the underlying silt: this water, though mixing with the site water, would be only brackish to a normal degree. Lastly, spring tides are currently keeping the groundwater levels raised above normal. He expects levels to go down steadily without pumping out or releasing into the estuary.

One explanation here was especially helpful. We had, many of us, been puzzled by the earlier trial embankment, to test the extent and rate of settlement. This was not to test how far the fill would sink, but how far the underlying silt would be compacted by that weight (so it did not matter what the test material was). So they have imported 4 metres of sand and gravel (as surcharge), to compact the underlying silt down to the required 4 metre level. This Garry expected to take about 28 days: I suspect this is more hope than expectation, but it is probably based on what happened with the trial embankment. It clearly explains why the mountain of fill is so much higher than we expected.

My own interpretation of events includes the fact that, according to Mr Shipp and the original plans, there is a ten- or twelve-metre wide gravel-filled berm between the raised site and the ditch, but this has been virtually covered over by the fill material: it was intended to absorb any water and solid materials that were washed out from the site. There were also supposed to be sandbags and straw bales to absorb these, but we only saw sandbags, and no distance between the fill heap and the ditch.

Jubb's site inspection is due tomorrow, and it would be interesting to know what they make of the situation.

Mr Shipp and Jubbs are working together with other consultants on the longer term water management measures. He would be concerned if the present situation persisted.

The Environment Agency and Environmental Health have been kept informed, and I understand something had been done about the mosquito infestation. Otherwise, whatever is in the ditch is not being released, but left to soak away naturally, so the environment is not likely to suffer any sudden impacts, but Garry Mountain is happy to answer questions as best he can, and Mr Shipp says that our concerns have been justified.

1 comment:

  1. Hugh, thanks for the update. So it appears that the sand was super-saturated what ever that means ( probably very wet ), so will have had more than 300kg of sea water per cubic metre of sand imported. Doing the arithmetic, that is over 90,000 tonnes of sea water that was brought in to the site. With all the warm and dry weather we have had recently, then it would be expected that the salt level of the water will rise due to evaporation of the water. This is good as less water will be running off, but bad in that it concentrates that salt in the sand where it can be leached out later ( ground water or rain will do this ), affecting future planting around the site.

    What is amazing to me is that the 'experts' are refering to the site manager. Surely this makes a mockery of the EIA. What else is occuring that was not anticipated, despite being pointed out in advance of this operation taking place.

    What was the outcome of the site inspection? What happened to the material lining the lagoons? Was it dug up or just covered over? How might that affect the hydrology under the new housing development?!?