So this drainage ditch is on the north side of the big house, where most of my downspouts are (by design). It drains to the northeast corner of the property onto the street, over a wall that I'll have to protect with a trough of some kind. And it's not deep enough; it's probably around 8" deep, but this page recommends digging your trench 12" to 14" deep for 4" drainage lines.
I've also violated the recommendation of that page by including one right angle - but logistically there's just no way to avoid it. I'm going to put in a cleanout there just to be safe.
Anyway, given my recent success in getting all the wet dirt out of the basement, it was immensely satisfying to know just how much water would not be seeping in through the walls. (Not that seepage is a major problem - but seriously, if the downspouts discharge against the foundation, you're just asking for trouble.)
I'll slap in a picture later. ... I've said that before, haven't I?