Try a rain chain so the water won't hit the gravel so hard.

Have you talked to the installer? The gutter guards may be reducing the amount of water that can enter the gutter. You could remove a one-foot piece on either side of the valley's end and see if that fixes the problem in the next downpour.

You can also add a concrete gutter splash where the flume water hits the ground. it will prevent hole-digging by making the water splash into a larger area. Not ideal for diverting the water to the street, but for that you can add the standard black corrugated 4 inch pipe, buried.

deco, I've considered a rain chain but the water is too heavy I think. Maybe a very large one!

Applepie, It's not an installation problem, we've tried it without the guards. A typical splash guard on the ground would look really strange out in the yard when it wasn't raining. The house is on a mountain ridge so the run off isn't a problem - hence the totally permeable gravel and underground corrugated buried pipe.

We have wondered if a custom fabricated scupper at the corner of the two gutters would work. The problem would seem to be how to support it. Conventionally a scupper would feed into a downspout and in this situation a downspout at that corner would be in the way of a door and would mess up the site line of a major window. The architect really blew it design-wise.

Maybe you need extra downspouts:

Rain chains are decorative. Not functional. I would consult a roofer or the gutter guard company.

Agreed, architect/builder/designer really blew it!

Does it function better without the gutter guards in place?

It doesn't function better without the gutter guards. We put them on because we are in the woods and have pine needles, leaves and tons of pollen. The roofer says the problem IS caused by the metal roof. Applepie is correct about asphalt slowing down water flow rate. My question is what do people with metal roofs do??

Most metal roofs I've seen don't have gutters. The water just hits the ground. Of course, those were in the tropical jungles. One that wasn't was a university building where the architect and the institution didn't care what happened when it rained a downpour and the building flooded at the doorways when the water cascaded down. The manager left sandbags in place all winter which was a safety hazard, but she was also the safety manager. However, I digress.

You could retrofit some metal bars across the metal roof, using caulk to avoid nail holes in the roof. Think goldmining flumes. I take it that cultivating a zen outlook is not a viable option?

Apple- It's hard to be zen when you've just built your dreamhouse and see all the "warts". I like the idea of the metal bars glued in place. Hopefully a roofing expert on Houzz will chime in on my problem. In hindsight I would have gone gutterless and put french drains where the rain naturally hits. Ah well.....

I'm not sure this will work, but how about installing snow guards? It might slow down the flow of water enough to stop it from over shooting the gutters. They come in all different colours and styles. Google "metal roof snow guards" for examples.

Have you tried the Metal Roofing experts forum at http://www.metalroofing.com/v2/forums/ ?

A quick google search on "metal roof water overshoots gutter" turns up a lot of pages where the solution is basically the same as what you have installed already. Homeowners also say it doesn't always work in heavy rain.

For a dream house, cultivating a zen attitude would probably be the last thing I'd want to do. If the overshoot problem happens more often than you want to tolerate (say twice a year), consider alternatives. Landscaping around the problem could turn out to be a good thing.

As an extravagant response to fast water drainage off a metal roof, the lead roofs of cathedrals had stone gargoyles installed (see Washington Cathedral photos, for example). I'm not suggesting you install gargoyles because your aesthetic is probably different, but it does show that ingenuity has been applied to this problem for hundreds of years with a wide range of solutions.