Tidal characteristic adjustment due to dyke and seawall construction in the Mokpo Coastal Zone, Korea

The results of tidal analyses of long-term (1971–2001) sea-level elevation records at the Mokpo observation station on the western tip of the southwest coast of Korea showed significant changes in tidal amplitudes, together with advance in phases for the major constituents as a result of constructing a dyke and two seawalls in the Mokpo Coastal Zone (MCZ). According to the stages of construction, the amplitudes of each tidal constituent, rather than the phases showed different variability patterns. The semidiurnal (M 2 and S 2), and fourth-diurnal (M 4 and MS 4) amplitudes increased significantly post-dyke construction, whereas the diurnal (K 1 and O 1) amplitudes decreased slightly post-seawall construction. That is, the semidiurnal and fourth-diurnal amplitudes were markedly affected by the dyke construction, and less affected by the construction of the two seawalls. In contrast, the diurnal amplitudes were little influenced by the dyke construction, and slightly influenced by the construction of the two seawalls. Three numerical simulations with different stages of construction reveal that an increase/advance in tidal amplitudes/phases in Mokpo Harbour is closely related to change in the MCZ from a choked coastal system to a non-choked system. Before the construction, amplitudes/phases inside the MCZ were lower/more delayed than those that are outside due to narrow channels in comparison with large capacity of water storage inside the MCZ. In addition to results of the numerical model, a simple analytic model showed that semidiurnal tides are more affected than the diurnal tides under the choked system because of their different frequencies. Tidal amplitudes increase in the shallow Youngam and Geumho Basins due to the shoaling effect, while remaining low in the Youngsan River Estuary due to intensive tidal dissipation within the river. The presence of the Aphae Channel, together with relatively active tidal dissipation in the Muan Basin accounts for the low amplitudes compared to those of the Youngam and Geumho Basins. Reduction in capacity of water storage induced by the series of constructions in the MCZ shifts the complex hydraulic system to a simple non-choked system. Therefore, the difference in tidal amplitudes and phases between inside and outside of the Mokpogu Channel was significantly reduced after construction.