Back to the original concern about the gutters going straight into the ground. I have found more times than not that the runoff of roof mix never makes it to the outlet anyhow. Limiting the runoff is a big reason why,...but,...also debris in the gutters probably helps as well,...along with the long underground slow pitch that is usually used in these underground "downspouts". Then I think it degrades,....and when it rains it's so diluted and broken down that it's a non-issue.
Ran into a situation today at a golf course. All the downspouts were rivetted together and were running down into some type of underground system.
The only thing that I can think to do because I cannot bag is to someway plug the downspouts. Then use a gutter cane in each section and flood the gutter so that it dilutes the runoff before it overflows. As it overflows flood the ground below and then some.
As I unplug the downspouts continue to run water in each section for a least 5 minutes.
Looking for suggestions before I would bid the job. Have to figure in the extra time not to kill the golf course.
Along with the dilution, I have added extra surfactant for a thick mix and taken it slow with smaller orifice tips. Takes longer but I have had hardly any run off. For downspout bagging I like the mini red velcro straps from the hardware isle at HD, they come in a 5 pack.