I'd like to intercept and divert as much runoff coming down the yard slope as possible, about 2 or 3 feet behind the retaining wall. The wall does have toe drains, but the manufacturer is pretty stern that those are for "incidental ground water" and steps need to be taken to mitigate water BEFORE wall.
I'm pretty sure I've got sufficient slope to do this, but I'm torn with how best to implement it.
Water would follow the path shown below, roughly. Should I do a traditional french drain system (which I've been told are not the proper application for SURFACE water)? Solid 4" pipe with a collection basin every 10" or so? Try to get away with an open swale? If I can get to the corner of the house near the driveway, I can get to the front yard or curb for discharge. The 4" PVC downspout system already does this very well. I've got sufficient slope to go below grade with the downpout line (and retaining wall line), just haven't had time to bury them yet.
The "low point" of the gentle bowl shape of the yard is a couple feet behind where the wall steps from 3 courses down to 2. I've also got to manage the water between the house and retaining wall, but now that the grade is corrected and water runs away from the foundation that is an easier task. I'm leaning towards an outdoor sump for that, since there isn't much available slope for gravity drainage.
Input needed: french drain vs. collection basins vs. swales
Try first for surface drainage, namely a swale.
Underground drains are used when there is ground water that needs to be removed and/or the
water table is too high for the basement, or when you don't have enough slope on the surface.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.