Hard to tell, but over time the gravel-only solution will silt up.Yorkshire Kris wrote:But do I need the the water to drain that quickly away?
You are right about the raised beds draining well. I have raised beds in my vegetable garden, they are only a few inches (I dug the paths out, and put the soil on the beds, so the lower paths are, in effect, providing the lower drainage level) and even though I have heavy clay the beds are much easier to work than anywhere else in my garden. Also, during perishingly cold snaps in winter when my mates are telling me their ground is frozen solid and they can't harvest any Parsnips, I've never had a problem getting root veg out - so presumably less water, in my beds, equates to less freezing too.
We have a number of French drains, all with perforated drainage pipe. When we get massive downpours, that flood the whole area, the areas with piped drains have drained away completely within 20 minutes.
My aim would be to get the excess water out of the garden in a reasonably short period of time, plus making sure that the drains will still be flowing freely in 10-20 years time.