Unfortunately that side of the mountain becomes a major waterfall essentially. I had it free of water most of last winter with both GJ and Upper cut running free of water for the most part. However SFU gets considerable windfall yearly which causes all kinds of dam effects and water flow issues in the trees on that side of the hill.
If you rode it yesterday that would be the worst you will see it. That was a big rain storm, similar to what took out most of the upper part of GJ last winter. Very little can be done about the massive runoff cascading down that side of SFU without considerable manpower to clear trees and excavate drainage. Something that was never done properly by the city originally, and I'd imagine they have no interest in taking that on going forward. Their focus seems to be the climbing routes adjacent to the hwy over the next couple years, which is great BtW. Unfortunately, as I don't live anywhere near SFU anymore, I cannot take on the repair to GJ and Upper Cut this winter. Sorry to all Strava users of SFU lol!
Edit: I will add it's incredibly dangerous doing drainage work up there, If clearing drains, please do it on the trail. Last winter in that rainstorm the water was literally flowing like a flash flood waste high in some areas, and powerful enough for large deadfall slide downslope at a pretty good rate.