When we were designing the house, John Kim at Res4 explained a rough engineering version of what effect water has as it moves downhill against a wall - I didn't really get it, but it sounded scary. And because our retaining wall opens on the uphill side of the house, it acts kind of like a giant catch basin for water coming down the hill underground. So it was really important to get the drainage right and send whatever water gets trapped by the wall through to the other side. The guys who poured the foundation took care of that with pretty extensive drainpipes around the footers that funnel the water through a pass-through in the wall (where it travels through an underground pipe to an outlet below the house down in the woods).
Because we also get a lot of water from the roof through the scupper above the retaining wall, the same process needs to happen - it all needs to get funneled through to the other side of the wall. So as they infilled the retaining wall they connected some black sewer pipe to the footer drains and ran it up the side of the wall to right below the scupper. I built the walkway with a box for a gravel catch basin:
When the stone for the driveway came a few weeks ago, I stole some of it and filled the bottom half of the box, cutting down the pipe:
Graded the stone toward the pipe and lined it with plastic and some chicken wire:
And filled it (almost) the rest of the way with stone:
Remains to be seen how it will handle a lot of falling water - how much splashing we'll get, how quickly it will drain through the gravel, etc. And I'll finish the open edges of the box with some leftover ipe.