Permeable paving stops being permeable

Semi-permeable paving is the cheapest drainage available. No gully, no pipe, nothing anybody has to come and clear. That is a good argument and it is also the argument that hides the maintenance.

The voids fill. Silt and leaf litter work into them, and unless somebody brushes the surface out every few years it becomes ordinary paving with a drainage strategy attached to it. Nothing announces this. There is no flood on the day it stops working — the run-off simply starts going somewhere else, slowly, and by the time anyone notices, the reason is several years behind them.

At the Sensory Garden in Bernburg the paths were laid semi-permeable against an explicit cost test. The city is losing inhabitants, so the municipality has less to spend each year, and anything that cannot be built cheaply and kept cheaply is not a proposal. Permeable paving passes that test on the day it is built. It only stays passing if somebody turns up with a brush.

Which is the general point. Low maintenance and no maintenance are different specifications, and the second one does not exist outside a brochure. The honest way to specify a permeable surface is to write the cleaning interval into the handover documents beside it, so the thing keeping it working is a line somebody owns rather than an assumption nobody made out loud.

Cheap to build still needs somebody to turn up.