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.