We had 2 downspouts where rainwater didn't go through. The rainwater simply went out from the vents where the downspouts meet the roof gutters. It was ugly. Rainwater splashed everywhere - to our windows, to walls and to the soil (thus eroding it). And so we called our ever reliable plumber, Gerry Tubero. I don't know his surname but the guy's worked for me many times.
Gerry's one guy who knows his stuff. He's so good that our architect entrusts all plumbing works of his projects to Gerry. That's him working on the catch basin of the porch. It turned out that so many leaves clogged the downspout elbows. And these are leaves coming from neighbors' trees.
That "pleye" or "plehe" (drain auger) on the foreground is probly the single most useful tool that solved horrendous drainage and sewer problems. Gerry has used it for the apartment and the new house. For the 2-inch pvc downspout, Gerry just hammered the downspout a bit to shake loose the clogged leaves inside while flushing it with a pailful of water.
After his work, I asked Gerry to put Guttershield downspout strainers (shown below) in the gutter holes. This is for the 3-inch pvc downspout. Somehow Ace hardwares hereabouts don't have these. I was only able to buy some from Handyman.
Cleaning the gutters of leaves will need to be done maybe twice a year.