K-man,
Welcome to the forum; glad you joined us.
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How do we know that your spillway pipe is dropping? What empirical measurement do you have? Do you have an existing, stable benchmark anywhere on or around the pond that can be used as a reference to the full pool mark? (like a dock, nail in a tree, or whatever) Use it to shoot an elevation to the bottom of the spillway pipe to see if it's elevation has changed.
You are saying that the dam is about 210 feet across, and there is a section of about 75 feet within this 210 feet that is falling in elevation...right? And, the spillway pipe is about in the middle of this 75 ft section...right?
Once again, how much of a difference in elevation in the top of the dam between the unchanged and depressed area? Also, can we shoot the elevation of the top of the dam all the way across and compare it to the benchmark to see if there has been any settling in the dam soils?
How tall is the dam? What compacting practice was employed during it's construction?
How was the spillway pipe installed and at what point during dam construction?
There is alot of input that can change the analysis of your situation. Rainman and TomG make reasonable assessments, but something as simple as settling might be in play. Maybe not any of this and somebody like Mike Otto (our resident dirt guru) will massage the entire picture and offer alternate advice.
We installed our spillway pipe after the entire dam was constructed. The track hoe was used to cut a ditch directly into the dam. I held the laser rod and called out the elevation as my dirt-guy carefully trimmed down to the exact depth to put the spillway pipe. We then backfilled as I power tamped the soil around the pipe, filling the ditch back in. Within 18 months, the entire ditch area had settled about 4" - 6" lower than the surrounding dam. It's the difference between soil compacted by a jumping jack, compared to soil compacted by a loaded rubber tired earthmover. We fill in the sunken area this past spring and all is well.