wrong concept for flood zone

Most flood zones are not related to a reservoir.  The flood can come from ocean, lake, river, stream, or just rain storm.  In particular, flood zones are  not related to a dam failure.  Even if your house is right downstream of a dam, you are not necessarily in a flood zone.  If the dam satisfies proper safety requrients (most of them do), it is assumed not to fail when determining the flood zone.  In fact, a dam is considered a flood protection feature.

As long as you are in the 100-year flood zone, you have to pay flood insurance if you have a mortgage.  If you do not have mortgage, then it is your own decision.