The lot was graded in 70's or 80's, which was late enough to have modern drainage as part of the design. In their neighborhood, most houses are graded at several feet higher than the sidewalk gutter, so the trampoline just needed a regular drain pipe to bring the storm runoff to the front of the property.
They did later add a sump pump, because most of the years rain happens over a small number of very wet storms, and it can take a while to gravity drain out several inches of rain we'll often get in a single storm. (Our record from before they had the trampoline was 12 inches of rain in just over 24 hours, but that was an especially strong El Niño year, but we'll probably be getting another one soon.)
The lot was graded in 70's or 80's, which was late enough to have modern drainage as part of the design. In their neighborhood, most houses are graded at several feet higher than the sidewalk gutter, so the trampoline just needed a regular drain pipe to bring the storm runoff to the front of the property.
They did later add a sump pump, because most of the years rain happens over a small number of very wet storms, and it can take a while to gravity drain out several inches of rain we'll often get in a single storm. (Our record from before they had the trampoline was 12 inches of rain in just over 24 hours, but that was an especially strong El Niño year, but we'll probably be getting another one soon.)
Maybe they live in a place with little rain?