Depends how you're measuring and where in the cross section. Assuming no tailwater downstream, you'll see an increase in velocity in the main channel/floodway. As water levels increase upstream, your hydraulic gradient increases, and per Mannings equation, increases your velocity.

In the wider floodplain, water will typically be mostly stagnant as more trees, vegetation, and buildings increase the Mannings roughness.

Therefore, average velocity of the entire cross section could be significantly slower if the floodplain is significantly wide.

Tailwater effects also play a significant role. If your downstream water is high, say due to flooding downstream or a reservoir, the main channel may not actually increase much at all until the downstream levels recede. You see this often near deltas where storm surge or tides provide resistance to flow coming off fairly flat topography.