How do local flood proponents explain God's promises to never do it again? Some Christians believe that Noah's Flood was a local event, and that the author of the Flood narrative never intended it to be understood as global in scope.
My question is, how then would we explain the following promises from God immediately following the Flood?

The Lord smelled the soothing aroma; and the Lord said to Himself, “I will never again curse the ground on account of man, for the intent of man’s heart is evil from his youth; and I will never again destroy every living thing, as I have done. --Genesis 8:21
I establish My covenant with you; and all flesh shall never again be cut off by the water of the flood, neither shall there again be a flood to destroy the earth.” --Genesis 9:11
and I will remember My covenant, which is between Me and you and every living creature of all flesh; and never again shall the water become a flood to destroy all flesh. --Genesis 9:15

I'm sure ancient readers were aware that local floods still happened after Noah's Flood, so what could God (and the author of Genesis) mean by this if it were just a local flood?
Local-flood proponents generally fall into one of two camps:


That the narrative is told from the point of view of the author, and from the author's point of view, the flood covered the entire (known) earth. (One example).

In this view, I believe the only logical explanation for your question would be that God's promise was taken to mean that this would not happen again.  Specifically, God promised not to destroy the entire known world by water again.

So your objection about the author knowing about local floods isn't really relevant, since the flood of Noah was a distinct event from his perspective.
The Bible's language is not unambiguous, and a strong textual case can be made for the possibility of a local flood. This article offers a good explanation of this view.

In this view, if "all the earth" doesn't mean all the physical planet, then when God says "all the earth" He also doesn't mean the entire physical planet, which would allow for local floods of a lesser degree (what degree, I'm sure is subject to debate).