Since when are there prescriptive and performance based laws?Ty J and Rick 18071, you both make good comments. The prescriptive provisions of the law exists for public safety, yet other technologies can provide alternate means of achieving a similar level of safety. I saw one of those British homebuilding shows on Netflix where a guy built his house on a barge in a river floodway.
My point is, if the OP can understand the underlying safety issues behind the original prescriptive provisions of the law, there may be an alternative performance-based solution that provides for equivalent safety.
Ty J, if impedence of floodwaters was the "most important" issue behind the original prescriptive requirement, then why does the WAC allow commercial construction in a floodway?
This is not building code that we are discussing, it is a State Administrative Code. Totally different aspect than building codes.