Chapter Thirteen Tidal flats and associated muddy coast of China

Abstract

Summary Tidal flats are a major geomorphic feature along the coasts of China and cover an area of 2 M ha. Two major physiographic types may be identified, namely, (i) those fringing the shorelines of major plains which occur extensively in the north of China, and (ii) those occurring along embayed steepland coasts, which are found mainly along the south east coast. The former are associated with the reworked alluvial sediments deposited within the coastal nearshore and shelf environments by the two major rivers, while the latter reflect more localized catchment and tidal embayment depositional processes and exist mainly within elongated embayments along steepland coasts. The tidal flats developed off the North China Plain are between 3 to 18 km in width and are much more extensive than those associated with estuarine embayed areas. Typical geomorphic features of the tidal flats include a relatively steeper slope within the high tide zone, a broad near flat mid-tidal zone, and a steeper low tidal to sub-tidal zone. The average slope across the entire system is less than 1 m/km. A striking characteristic of the tidal flat deposits is the very high proportion of mud-sized sediment. Generally, muddy clay-rich sediments predominate within the high and supra-tidal zones. The broad and extensive mid-tidal flat is generally dominated by silt with muddy lenses, while the low-to-sub-tidal zone features both sand and muds. Tidal processes play an important role in the evolution of the tidal flats. For the extensive fringing tidal flats of the North China Plain, tidal range is about 3 m and the velocity of tidal currents is less than that found in the embayment tidal flat environments. The tidal flats along the East China Sea and the South China Sea are developed in narrow embayments or along other types of sheltered coasts. Fetch within such environments limits wave generation, and tidal currents likewise play important roles. Thus, major factors influencing evolution of the tidal flats as the dominant morphology of muddy coast are tidal dynamics, the gentle coastal slope, and the abundance of available fine-grained detrital sediments.