Wind uplift for a Polyethylene lagoon liner

I'm really not sure why you would have to design this for uplift. Isn't it covered with, um, sludge?

At any rate, when in doubt, make it stout.

Perhaps the ballast is required for those times it is being de-sludged.
These are my initial thoughts:
For normal wind events stopping the wind from getting under the liner is the key, so that there is no positive pressure under the liner. The suction at the edge would be the negative pressure in the lee of the embankment. This would reduce to say a coefficient of -0.2 towards the centre.
I probably wouldn't design for extreme wind events as the probability of those occuring during de-sludging is very low.

If you haven't done so, check with the liner supplier and any AWWA-type standards relating to the construction to make sure this isn't somewhat codified somewhere already.

Thanks for the replies:

Structuralaggie - Yes, the uplift design is for those times when the lagoon is cleared out or washed down.

apsix - The liner is fully welded and sealed at the joints, and anchor all round the perimeter. I've been using -0.2 as my co-eff. I've got 225mm pipes filled with concrete at 3m centres laid parallel to falls, with 500mm gaps between lengths to allow drainage and washdown. It still seems a little excessive on the dwgs but this is how the numbers work out.