catchment tank overflowing the top

[Eric1600]

One of the biggest problems with the overflows here is during times where the flow rate is very high (storms) it will generate a whirl pool on the intake, if that starts to spin hard enough to pull air into the pipe it will kill the siphon ("lose prime"). Adding a second overflow helps. Also I've found that putting several small holes in the upper top loop will draw in a little air and prevent strong whirl pools from forming because it will draw in a little air and slow the flow down. I think I saw this in the Keaau Ace Hardware as a recommendation and it has worked at stopping them from losing prime.