A method is proposed for the approximate solution of the problem of the decay of a small-amplitude discontinuity in the case of a strictly hyperbolic system of conservation laws which has linearly degenerate characteristic fields. The method is used to analyze the qualitatively different flow regimes occurring in the solution of the problem of the decay of a small-amplitude discontinuity for two-layer shallow water model with a free upper boundary. The most detailed study is made for the special case — the dam-break problem.