Do you always need a pump for basement groundwater drainage, or will a gravity system do?

[RaybanSam]

Hello

We're building a 3m extension to the lower ground floor of a Victorian townhouse.

Detached, Flood Zone 3 (150m from River but with flood defences and no history of flooding for 100+ years).

The lower ground finished floor level is 700mm below garden ground level. We're installing channel drainage around the perimeter, surface mounted on the concrete slab and below the insulation and flooring.

The rainwater drainage manhole pipe invert level is 150cm and I think we should be able to use a gravity to drain the channels rather than a mechanical pump. Can anyone advise?

Even if we meet the required gradients for gravity drainage, I'm also mindful we need to meet Building Regulations (I think gravity fed is acceptable for internal channels), plus will the waterproofing supplier/installer provide a 10 year guarantee if we don't install a pump, and will Home Insurance be difficult to get without a pump?

Why would you not install a pump? Personally I wouldn't risk leaving it to gravity in a basement.

What's the risk? There's no motorised pump that's operated reliably for as long as gravity has!

Gravity drainage will fail if the pipe it discharges to is blocked or flooded, but wouldn't the same issues also cause a pumped system to fail? I have no great knowledge in this area - I don't even have a basement - but I don't see what problem a pump would be solving if there's a drain at the right depth for a gravity system to discharge into....

I feel the same - a pump will never be as reliable as gravity ...