i think the nrcs may be slightly different here in oklahoma not sure why. they only recommend and will only pay share on sch. 40 smooth steel pipe. they say there records over the past 30 to 40 years proves it last longer than anything. absolutely no pvc or corrugated. they say no to pvc due to sun and fires and no to corrugated because unless its smooth inside it reduces water flow and that water sets on the bottom and rots them out even on a 12 to 1 slope the water sets in the spirals and doesnt drain dry. but i see a lot of folks on here using both which surprised me but it may just be different in different areas. at first i didnt like the steel pipe but i'm kinda changin my mind on it. once in place you can get pretty ruf with it. the nrcs doesnt require us to use a packer but to add 10% for shrinkage using dozer. so once pipe is in we put a couple inches of cover on it and drive over it to get some compaction. you have to be way more careful with plastic.anyway havent had any problems yet and going back to fish some of these ponds that are 10 to 15 yrs old the metal pipe looks almost as good as new. by no means am i sayin using a packer isnt better i think they are trying to keep it cheap and it seems to be working. these pipes may or may not have a riser but they always start at ditch behind pond and come out at desired water level in pond so they are probably 8 or 9' ft deep in places on the dam. and they usually have 2 anti seep collars welded on. hope this helps you a little although compared to some of these other replies oklahoma is a much warmer climate so it may be different here.