FYI - M.Otto of Otto's Dirt Service is a very experienced, highly qualified pond builder with 40+ yrs experience and he has authored a very good new book about pond building: " Just Add Water". Three of the chapters apply to your drain pipe situation: Dam Maintenance and Repair, Erosion Repair on the Front Slope, and Repairing Pipes. Book available through the Pond Boss Office.
http://www.pondboss.com/store.asp?c=8 
