Thursday, 27 May 2010
The latest NDMA figures suggest that the freeboard is down to about 4.3 feet (1.31 metres) as of 3 pm today. That is a reduction of about 70 cm on the day, meaning that we are probably less than two days from the point of overtopping:
I am reasonably confident of these values as the Focus geologists took this picture yesterday of the spillway:
The circled boulder is the crest of the spillway, so water flow will start when this point is reached. Of course the one potential problem is a slope failure on the spillway walls, which could block the channel. This is a real possibility, but we won’t know until tomorrow.
Close up the channel at this point looks like this:
Unfortunately the large boulder will resist lateral cutting, initially at least, which is likely to increase the rate of down-cutting. This is unfortunate. Once the water reaches the top it will initially flow down this channel, crossing the summit of the dam:
And then will flow down here:
The disturbance here is from collapse of the banks induced by the seepage. Finally, it will flow down the main river channel:
The terrace in the centre left of the image is at 60 metres above the river level. Previous (comparatively recent) floods have not reached this level, which is half the height of the dam. This is the level that we believe is comparatively safe from the flood.
Source: David Petley Wilson Professor, Department of Geography, Durham University