Tree Farmer, I saw this design in a state (possibly USDA) pond design hand book, which state escapes me right now. I would have gone deeper with the outer draw pipe, but since my pond is mainly excavated, I was limited by the height of the ground level on the outside of my dam, which is more of a berm. From what I have read, they usually draw from as deep as possible, just keep it high enough off the bottom that any silt build up will not seal the end into the bottom of the pond. The deeper the better since your lower water is usually lower in dissolved oxygen than the water higher up in the column. I went with 3", so that if anything gets plugged or hung up in the bottom it will bypass the low draw design and spill over the top instead of backing up so much to go over the emergency spill way. 3" was a seat of the pants decision, not an engineered one. If it seems to be too much or too little, I can go for a swim and adjust or rebuild it since it is only 24" long.
What sizes of pipe will you be using? I will have to see how much scrap plate brass I have.
