You may actually not have a clog. If your horizontal and vertical pipes are the same size you will never get it to flow full. NRCS has a table with the correct ratios (USDA Pond Design, Page 38, Tables 12/13). The other piece is the actual elbow. I cannot find it at the moment but I recall reading that extending the vertical pipe at least 1 foot below the horizontal pipe will also increase the ability of the water to move through the horizontal pipe.
If you have seen the pipe flow full prior to the clog then ignore everything I said b/c I don't actually know what I am talking about!
Definitely second what Cecil said, yesterday a guy a few miles from me was found floating in his pond. Thirty minutes prior he told his wife he was going to unclog the outlet. Scary stuff.
Man found in pond
