How does the energy of tornadic winds compare to floodwaters?
The force exerted by a moving fluid (gas or liquid) per unit area is
proportional to its density $D$ and to the square of its velocity $V^2$.
The power delivered by a moving fluid (gas or liquid) per unit area
is proportional to its density $D$ and to the cube of its velocity $V^3$.


Water is about $800$ times as dense as air.
So water moving at $(\frac{1}{800})^{1/2}$ or about $\frac{1}{28}$ as fast as air exerts the same force per unit area,
and water moving at $(\frac{1}{800})^{1/3}$ or about $\frac{1}{9}$ as fast as air delivers the same power per unit area.

Or in other words air must move $800^{1/2}$ or about $28$ times as fast as water to exert the same force per unit area, and $800^{1/3}$ or about $9$ times as fast to deliver the same power per unit area.

Thus water moving at $6$ mph exerts the same force per unit area as air moving at about $170$ mph (a $170$ mph wind), and delivers the same power per unit area as air moving at about $55$ mph (a $55$ mph wind).

So it is not the power of 6 mph water that equals that of an EF4 (not EF5) tornado, but the force.
$6$ mph water has the same power as a 55 mph whole gale.