How are you going to get the crap that will collect at the bottom out? If left there without oxygen flow through it (the bottom layer) it will be anerobic and take oxygen from the water (plus be stinky rotten eggs).
I think you need to rethink the design.
The elevated sides are a good idea as it prevents the soil being eroded and then filling the pond, clogging the pump and filling the barrel with soil.
Do you know what plants you will put in?
Water lilys
Water hawthorn fine on 60cm shelf
Nymphoides peltata - very pretty and natural looking, 60cm shelf
Amphibious bistort - homes for dragonflies - 90cm shelf
white zantedeschia (arum lily) mine overwinter in the pond and come up every spring You would need to raise up the pot though it does not want to be in the 30cm deep zone
iris, iris iris!
Barred horsetail 30cm shelf
Bog bean 60cm shelf
Aponogeton desertorum 30 cm shelf
Marsilea mutica 30cm or 60cm shelf
Also how will you tackle a deluge of rain?
Would a rainfall in England really raise a pond like mine 5 inches? Couldn't I just open the outlet valve and drain the pond to the desired level?
Just thinking - better to have a automatic level/overflow. Given the surface area, the pond will collect rain fast, if there's large amount of rain then it will likely do that faster than you think. Just create a drainaway (long tube with rocks in) and put a grill in front of it to stop the tube from getting blocked.
I suppose I could have the outlet pipe right right at the bottom instead to flush the crap out?
