Heavier rain on heavy clay: what north-west England gardens face

North-west England is already England's wettest region, and the national picture is of a wetter cool half-year: the Met Office's State of the UK Climate reporting puts the October–March half-year for 2016–2025 3% wetter than 1991–2020 and 13% wetter than 1961–1990, with very wet months roughly twice as frequent as in 1961–1990.

Intensity is the sharper change. Met Office convection-permitting modelling published in 2023 found hourly rainfall above 20 mm — the flash-flood threshold — could be four times as frequent by the 2070s under a high-emissions pathway, a larger increase than coarser models had suggested.

On compacted clay behind a sealed patio, that arrives as runoff rather than infiltration. The standard garden-scale response is to break the impermeable path the water takes: replace part of the sealed surface with permeable gravel or block paving, and give the remaining flow somewhere to pond and soak.

Monitoring of the Renfrew Close rain gardens by the University of East London recorded basins infiltrating all incoming water in many events, with an average 13.2-hour lag between peak rainfall and peak water level — the delay being as valuable as the volume when local drains are at capacity.

CIRIA's susdrain evidence library and the SuDS Manual (C753) hold the design detail, including the pervious-pavement chapter. We do not quote a specific infiltration rate in mm/hr here because those figures belong to the C753 tables and depend on construction build-up; check them there rather than assuming a number.

For a typical clay garden, the sequence is: watch where water actually goes during rain, test the drainage rate, then de-pave, shape a shallow basin at a safe distance from the building, and plant it with species that tolerate both saturation and drying out.