posted 11 years ago
Welcome to Permies.
Where are you located?
(You can add this info to your profile, if you are willing - in any format you like. It helps others who may know your region to give more pertinent advice.)
If you're in the Western USA, with the typical arid summer weather, you may be looking at watering that garden through 2 or 3 months of every summer, no matter what annual rainfall you get. Think about making big storage capacity for that runoff water in a swale, pond, or hugel bed (dig down a foot or two, put logs in, then bury them with the dirt you removed, add more topsoil if needed, and mulch. If you have enough topsoil available e.g. upslope, you don't need to dig the logs in, you can just drop them on level and surround them in dirt). I like how my hugel beds are performing in our eastern-Washington arid climate, and they'd also give your plants a raised-bed boost above the flooding when it does happen. But a hugel bed that will hold enough moisture for 2 or 3 months would be huge.
If you're in the south-east, or Midwest, or New England, (which seems more likely from the glimpse of hardwood trees around your house), then your summers may be more muggy, with thunder-and-rainstorms every few weeks or even most days. In that case, you may not need to hoard moisture for as long, although 'climate weirdening' means it may still be a good idea to build in some extra storage capacity. A simpler solution like a log barrier to slow the water runoff might be fine. Or raised beds of any type. Hugels in this context, as described above, might eliminate the need to irrigate at all. A hugel 2 or 3 feet tall seems to hold water for a week or two; larger ones longer.
Regardless of your location, slowing the water and collecting any silt or value from it is likely to be a better idea than simple drainage. it's easy to move water downhill, but hard to get it back. If you do use a gravel French drain, consider having it dump into a buried tank, cistern, barrel, etc. so you can pump it back up for irrigation. You could even put that drain on the slope above, so that the water storage becomes a feature conveniently accessible from your garden level.
Heck, you could build in a mini-waterfall.
-Erica