I am in a duck club that uses the dual wall HDPE pipe in several locations under levees to hold and drain water. We just replaced a 24" one which developed a split after 17 years and eroded a "suck hole" in the top of the levee which I found when it collapsed under my Ranger. Personally I would not use it for a pond spillway in preference for schedule 40 PVC or smooth steel. 18" PVC may cost close to ductile steel anyway. Certainly in your case of a 36' high dam follow the engineering recommendations.
When I built my 3 acre pond, for whatever reason, the NRCS agent did not recommend the HDPE double wall. He called for sched 40 PVC.
That could be because my pipe slants down at a decent angle and has increased flow rates when covered with water and goes into siphon mode. Maybe the suction has something to do with it, but I am just speculating. A pipe that is mostly horizontal would not have that requirement.
