Carport drainage, calculate water quantity

If the ground is somewhat permeable, just dig a hole, say 50cm in diameter, 70cm deep. Place a concrete ring in it and fill it with gravel. If possible, lay a drainage pipe in the upper third heading towards the sewer/street for the hundred-year rain.

207L/ha should be about 2.5l per second for 60sqm, if I'm not miscalculating. The ring with 50/70 is a bit tight. Gross volume 0.5 cubic, most of that goes to gravel. Not much is left.

I have a soakaway for about 250sqm roof area with 2m/2m/2m, directly on porous sandstone, so with a quickly percolating substrate. That is sufficient for a standard rainfall of 280l/ha.

Emergency overflow is not a bad idea.

My suggestion was to have a soakaway calculated by Graf and base the self-built one on the Graf soakaway's dimensions. That way you have all the measurements for your rainfall. Alternatively, buy the Graf part if self-building is not possible for whatever reason.