Water Tanks, page-36

Since a below ground system is always full of water, unless it's been drained, I would assume it would work the same way. Both of mine have a long drain, just like that but about 15m and 20m long and running down. Since they are also connected at the lowest point and falling over 1m over that length, surely they would also act the same way and trap any sediment?

Just realised there is a difference because mine wouldn't have that fall and rise to trap the sediment.