The only homes that might be affected in Johnston would be the ones right by the soccer fields. Those are relatively new builds so I can't imagine they would have built them if they were in any flood danger?
Other than that there's not really much else in Johnston except for those industries south of Saylorville and north of I-80. I've seen that area underwater a couple of times in my life including the little league fields there just off I-80.
If the water ever gets high enough to flood up to those businesses on NW Beaver then RIP to a large portion of the Des Moines metro.