During a Flood:

Let ${\displaystyle V\left(t\right)}$ be the volume of water in the planet at any time. Let ${\displaystyle k\left(t\right)}$ be the rate of water increase in the planet at any time.

We can model the rate of water flow like this:

${\displaystyle V'\left(t\right)=k\left(t\right)}$

In other notation, it looks like this:

${\displaystyle \frac{dV}{dt}=k\left(t\right)}$

As any mathematician would tell you, the volume of a sphere of a given radius is:

${\displaystyle V=\frac{4}{3}\pi r^{3}}$

Taking the derivative of both sides:

${\displaystyle \frac{dV}{dt}=4\pi r^{2}\cdot\frac{dr}{dt}}$

Hence:

${\displaystyle \frac{dr}{dt}=\frac{k\left(t\right)}{4\pi r^{2}}}$.

If the flow is independent of time, over time, the radius will increase slower and slower, but not approach zero.