Steady waves in flows over periodic bottoms

We study the formation of steady waves in two-dimensional fluids under a current with mean velocity $c$ flowing over a periodic bottom. Using a formulation based on the Dirichlet-Neumann operator, we establish the unique continuation of a steady solution from the trivial solution when a flat bottom is perturbed, except for a sequence of velocities $c_{k}$. The main contribution is the proof that at least two steady solutions exist close to a non-degenerate $S^{1}$-orbit of non-constant steady waves when a flat bottom is perturbed. Consequently, we obtain persistence of at least two steady waves close to a non-degenerate $S^{1}$-orbit of Stokes waves bifurcating from the velocities $c_{k}$.