Here is the contraption: a five-inch diameter PVC pipe inserted about eight feet into the old CMP, with an elbow and a riser. The top of the riser is cut at a 45-degree angle for vortex prevention. If, when the spillway is in operation, a vortex or whirlpool forms on the water surface, the pipe flow will be decreased by the volume of air sucked down. We're all about efficiency around here – at least the two-thirds of us that are engineers.
The cage is intended to protect the spillway against impacts from floating debris and ice damage. We salvaged the chunk of hog panel from elsewhere on the property and fastened it to the small-diameter pipes, which in turn rest on steel posts driven into the pond bottom. The riser tips backward a bit because the CMP runs through the embankment at a slant and we used a 90-degree elbow joint.
As we inserted the long end of the PVC pipe into the CMP, we applied an expansive adhesive/sealant, the final bead of which you can see in the next image as the black-grey donut at the joint.
Early results were promising. Water rose up to the joint then over it and, putting my ear to the riser opening, I could hear nothing to indicate leakage. Water began to creep into the Isthmus, which had been dry since early last summer.
With some good help by Frank and Debbie, we took the next step in dam maintenance, moving the trees and brush we had felled on the downstream embankment into a pile and burning what we could not harvest as poles and logs. This image is looking north; the trees remaining on the slope are in the vicinity of the spillway outlet, so we're taking more care in felling the big ones, particularly so the valve stem of the drain won't be damaged.
We still have a fair amount to do, but considering the entire slope used to be covered with woody vegetation, as shown here looking the other direction, we made a good beginning.
Through the tangle of cut brush – the smaller stuff – is the drain valve stem adjacent to the principal spillway outlet. There's no shortage of work to do here!