= Solution
The dimensionless equation is $H^3H'''=1-H^3$. At a crest, $H\gg1$, so
$$
H'''\sim-1.
$$
The imposed far-field flux is negligible there; the dominant physical balance is between local gravity-driven drainage and the capillary-pressure gradient.
At a trough, $H\ll1$, so
$$
H^3H'''\sim1.
$$
Local gravity-driven flux is negligible; the capillary-pressure gradient drives the fixed flux through the trough against viscous resistance. These alternating balances generate the strongly nonlinear <capillary wave>[capillary waves].
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