In Parshas Noach we find an even greater form of Hakhel. In the Teivah, there was a gathering of all of the elements of creation; mankind and animals.
Knowing that the Flood was taking place outside, would bring them to experience fear of Hashem. The Mabul went for an entire year, encapsulating all of the phases of time. This is like the purpose of Hakhel that they “fear Hashem… all of the days”.