a
Energy of crest \((\mathrm{P.E})=(2 \pi \mathrm{r} . \mathrm{dr} \times \mathrm{h} \times \rho) \times \mathrm{g} \times \mathrm{h}\)

Now, as crest spread, this energy \(E\) remains constant. So,

\(2 \pi r d r h^{2} \rho g=E\)

\(\Rightarrow \mathrm{h}=\sqrt{\frac{\mathrm{E}}{2 \pi \mathrm{rdr} \rho \mathrm{g}}}\) or \(\mathrm{h} \propto \mathrm{r}^{-1 / 2}\)