Wow......I had the same problem this year.....took me a few days to get the pool from green to clear enough to see the bottom. Saw a good amount of dirt, etc down there, so I started the automatic vacuum. It was sucking up fine, but when I went to backwash it the water seemed not that dirty. After rinsing I'd start the filter again to vacuum and dirt was coming out of the return. Never had that happen before.
My sand wasn't "channeled" I don't think, just clogged up with dead algae....and backwashing didn't help.
I took the head off the sand filter and did the "hose overflow" thing for about 15 minutes. A lot of junk overflowed. Put it all together, and now everything is working fine.
I have not been very active at all here on the forum, but I used to be very active back in the "old days" on Ben Powell's poolforum. I have a Taylor test kit and am a many year devotee of the BBB method. I've had my 30' round pool for almost 10 years and this is the very first time I've ever seen green in my pool.....but that was mostly my fault....(my pump died).... All is well now, and just getting it going. Thanks again for the great website! I'm just trying to get the pool looking great because I'm selling the house. The kids are gone now so we're downsizing. No more pool soon......
Frank