Bon, I think a perc test for drainage can be done any time, with the understanding that results will vary from one time to another based on the soil's moisture level. I also think asparagus will do fine atop a hugelkultur bed.

In order to plant my asparagus on the north side of the garden so it wouldn't shade out any plants to its north, I had to put it at the low end of the garden. This made having a raised bed imperative. The raised bed is only 8-10" above grade level, so I did have a certain level of concern about what might happen to the asparagus plants in a very wet year. Since you plant asparagus plants deeply, by the time mine were planted at the proper depth in the raised bed, they were almost down to grade level. Since my garden slopes and our neighbor's property to the south sits higher than ours, a river of water seeps off of their property and into ours all the time whenever there is moisture available and it is obviousy most notable in a very wet year. Normally that river of water is seeping underground, but last year it was on the surface and easy to observe for months.

So, in an average year, we get about 39" of rainfall. In drought years we might get as little as 18-25" and the asparagus tolerates that low rainfall well too. Last year we had 78+". The water stood in pathways between raised beds for about 3 months without ever really drying up. It was horrible. It was like every raised bed was an island surrounded by water in the pathways. The raised beds' wooden boards were like dams holding back the water. I feared my asparagus might stay too wet and that the roots would drown and rot. So, guess what? None of the asparagus died. It was up extra-early this year and is extra-vigorous. Every day I am muttering and mumbling to myself about what a PITA it is to have to harvest asparagus daily (sometimes twice daily) just to keep up with the harvest. I have to preserve a lot of it or give it away because we get more than we can eat fresh. If ever there would have been a year when high rainfall and high moisture levels might have damaged our asparagus plants, last year was it, and no damage occurred. Therefore, I have concluded that asparagus will not die and you cannot kill it. Get busy planting and don't fret over the drainage. Asparagus can handle it, especially since that sort of really wet year is uncommon here.

Dawn

Thank you Dawn. I'll continue forward and not overthink it.

*OP should read "storm water run-off", not "sewer"

(For anyone else reading about hugelkulture beds in Oklahoma: I dig my hugel beds in ground where the bulk of materials is resting below grade leaving a slight 8" - 12" raised bed above grade.)