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Analyses of tidal observations and a numerical model of the M2 and M4 tides in the Uldolmok waterway located at the southwestern tip of the Korean Peninsula are described. This waterway is well known for its strong tidal flows of up to more than 10 knots at the narrowest part of the channel. Harmonic analysis of the observed water level at five tidal stations reveals dramatic changes in the amplitude and phase of the shallow water constituents at the station near the narrowest part, while survey results show a decreasing trend in local mean sea levels toward the narrow section. It was also observed that the amplitudes of semi-diurnal constituents, M2 and S2 are diminishing toward the narrowest part of the waterway. Two-dimensional numerical modeling shows that the M2 energy flux is dominated by the component coming from the eastern boundary. The M2 energy is inward from both open boundaries and is transported toward the narrow region of the channel, where it is frictionally dissipated or transferred to other constituents due to a strong non-linear advection effect. It is also shown that the M4 generation is strong around the narrow region, and the abrupt decrease in the M4 amplitude in the region is due to a cancellation of the locally generated M4 with the component propagated from open boundaries. The superposition of both propagated and generated M4 contributions also explains the discontinuity of the M4 phase lag in the region. The tide-induced residual sea level change and the regeneration effect of the M2 tide through interaction with M4 are also examined.