SEDIMENT TRAPPING AND BYPASSING CHARACTERISTICS OF A STABLE TIDAL INLET AT KAOHSIUNG HARBOR, TAIWAN

Around the artificially stabilized tidal inlet that connects Kaohsiung Harbor to Taiwan Straight, 203 surficial samples of the sea floor wer e taken from the nearshore, in the outer harbor, and portions of the i nner harbor. The bathymetry of the same area was also surveyed. The sa nd fraction in each sample was analyzed for the grain-size composition with a custom-built rapid sediment analyzer. A total of twenty-one si ze-classes were used in the analysis. Three hypothetical sediment sour ces were assumed to have influenced the spatial grain-size patterns in the study area: the northward and southward littoral drifts, and the sediments exported from the harbor. After reducing the influence of ea ch hypothetical sediment source separately, the data were analyzed usi ng empirical orthogonal (eigen) function (EOF) analysis. The results i ndicate that the northward long-term littoral drift is the dominant di rection of sediment transport in the nearshore of the study area. A co nceptual model for four different sediment trapping and bypassing patt erns are proposed. (1) Excess bypass (or net outflux): this bypass pat tern is characterized by the export of fine-grained sediments (mud and size classes in the very fine sand fraction) from the interior of the harbor. (2) Partial bypass (or partial trapping): this bypass pattern is represented by the size-classes in fine sand fraction. The amount of these grain sizes entering the outer harbor through the inlet is mo re than the amount exiting at the inlet mouth, resulting in the retent ion of a portion of these grain sizes. (3) Total bypass (or zero trapp ing): grain sizes that exhibit this bypass pattern do not come near th e mouth of the inlet. These grain sizes include medium and coarse sand fractions. (4) Lag deposits: this group includes the size classes in the very coarse sand fraction, which are largely concentrated in the s cour pit immediately seaward of the inlet mouth. In general, surficial sediment grain-size patterns represent a time-averaged response of th e substrate to the transport processes over the time scale of at least two seasons. The differential associations of grain-size groups with various topographic features in the study area suggest morpho-textural relationships exist between the sea floor topography and grain size d istribution patterns. (C) 1997 Elsevier Science B.V.