I had to rebuild the dam on the old mill pond I live on. The pond was built about 200 years ago by damning up a river the pond is about 42 acres which is about 1,100,000 gallons per inch of surface water. I also have between 8,000,000 and 10,000,000 gallons of water flowing in and out of the pond per day depending on the season and rainfall amounts. I managed to lower the pond 10 feet using about 35 sections of 4" pipe I had to add more pipes as the pond went down because the drop decreased from around 18' to 8' as the pond went lower with the decrease in drop or head height the suction power of each pipe went way down. I used 4 inch schedule 40 electrical pipe (used for running underground cable) with bell ends that slide into each other and were glued together. The electrical pipe is a lot cheaper than plumber's grade P V C. I put a 4 inch clean out
at the top of each section and to start a section going I just had to cap the lower end fill the pipe
with water through the clean out once full close the clean out take of the lower cap and let the suctioning began. The only thing that would stop the process is if I sucked something up the pipe
and it became blocked. Just as a heads up I also tried using cheap drain pipe I think it was schedule
20 it was real cheap to buy and very easy to work with but after a couple week use the suction would collapse the pipe like a straw that is sucked on to hard.