We have built 3 ponds now and found we seem to like one approach since its simple and probably the cheapest way. I so much wanted to try a siphon system just for the fact they pull towards the bottom of the pond but our latest pond has a minimum of six springs feeding from the bottom. One is so powerful it split the native soil 150' wide x 15' deep, it's actually changing the shape of the pond almost on a daily bases, fortunately it's not near the dam.
This being said, we basically have fresh water being introduced year round so the whole siphon thing lost its appeal. We also have fish and a lot of plant life that naturally establishes and there is just something about having a screened suction in a storm event unseen at the bottom of the pond just a little scary, the siphon system was also ridiculously expensive to build in our scenario because we felt 8" was't adequate. You also add a considerable amount more places for water to seep in and cause problems at a later date since proper compaction around the PVC is harder than it sounds. Keeping the siphon vent clean also poses a problem for us. One "plug" here and kiss your pond goodbye. This could be easy to fix I'm guessing by building a manifold and having multiple siphon vents. Regardless if something can go wrong, it will go wrong, at least for us
We opted for the same way we did our last pond, cheap and effective for 5 years now. We just put a large horizontal pipe in the native soil at the outer most edge of the Dam, 24" pipe this time with an earthen emergency spillway 4" below the pipe about 6' away and covered it in some road cloth we had left over then filled with 4" minus rock ( slightly smaller than rip rap)
We will use this pond to fill another pond through a siphon pipe so we have our draining covered if ever needed. I do like the siphon system for this purpose though and would do this if it weren't being siphoned purposely. A 3" siphon with a valve on the outlet would be really cheap to install just below the surface and could also be opened in an emergency situations as well.......our 2" siphon we installed really moves a lot of water in a day, we sized it for the GPM we pull for irrigation daily and actually got slightly too much flow if needed
We can also adjust the height of our water by 20" by building a box on the end of our pipe and since we used a hub bed pipe it should work extremely well. We're going to just use the pipe through the winter to make sure the compaction "settles" in properly before adding the "box" next spring and let the springs fill the remaining 20"
