I found this question on stackexchange.com
"I have a large, steep roof section (~400sqft) connected by a long ~20' valley. During heavy rainstorms, the majority of the draining water completely overshoots the gutter, landing near the foundation. How can I fix this? I've heard of manufactured or home-brew L-shaped attachments, as alluded to in this similar post, but they didn't go in to any detail about how they work or where to get them. There is a post that attempts to describe them, but I don't really understand how to build one, and I read about potential problems with snow.
I'm open to just about anything, bigger gutters, some kind of ground drainage to deal with the overshot water, etc."
As you can see, this poor homeowner is willing to go to great lengths to solve this simple problem. Hopefully he wont replace his gutter with a larger size when there is such a simple solution. It's called a Gusher Guard and it's put out by our friends at Amerimax.
All you have to do is take what is essentially a little sheet of aluminum, bend it down the middle and screw it onto the top of the gutter. You can see in the little painting how it fits on. This guard will now catch that overshooting water and keep it in the gutter. Problem solved, thank you Amerimax!