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I'd go a bit deeper if you can as in the depths of winter cold, water temperatures are only stable at 60cms and more. By the time you've lined that with sand or stone proof protector fibre to protect the butyl or plastic liner you have almost no safe depth left. It's important for hibernating underwater critters in their larval stage. Otherwise the size is fine but if you intend to grow marginal plants in baskets (easier to control and prevent invasions) you'll need a shelf around some or all of the sides.
I'd go a bit deeper if you can as in the depths of winter cold, water temperatures are only stable at 60cms and more.
Quote from: Obelixx on February 06, 2018, 13:22:48I'd go a bit deeper if you can as in the depths of winter cold, water temperatures are only stable at 60cms and more.Ignore this advice - it's completely unnecessary. It's London, not a Scottish mountaintop. 30 or 40 cm depth would be fine. Maximising the surface area is more important than depth (indeed, deeper is not better when it comes to ponds). Native aquatic insects are perfectly well able to cope with winter cold.
The RHS knows a thing or two about wildlife ponds and advises shelves for marginal plants and a centre depth of 60cm to suit most plants and creatures - https://www.rhs.org.uk/advice/profile?PID=622 Have a read and see what you think but at depths of less than 60cms the water will freeze to quite a depth in a bad winter and there will be a shortage of oxygen for hibernating critters. It will also stay cooler in hot weather and be less prone to algae bloom.London is not immune form cold spells and certainly gets hot in summer. Best, in my opinion, to get it right from the start and not live with regrets and constant tweaking.