Discussion Of Mega-dune Characteristics And Its Formation On The East Chongming Shoal, The Yangtze Estuary

Many previous studies indicate that the great efforts have been made with respect to the dynamic geomorphology and sedimentology in the Yangtze River estuary and abundant research results have been gained. However, it is known that relevant studies on the Chongming Shoal morphological evolution are less seen. In the paper, physical observations have applied to on-site measurement of dynamic morphology of the study area in November, 2004 and September, 2005 respectively. As a result, substantial observation database was established, which helps discuss the Chongming Shoal topographical change. On the basis of data processing and simulation in the lab, mega-dune characteristics, distribution and its formation of the Chongming Shoal was discussed in-depth, in order to reveal the topographical changes controlled by estuarine tidal and runoff current flows. The results also help discuss the erosion and accumulation of the shoal face, as represented by mega-dunes movement strongly influenced by typhoon storms and tidal-runoff currents. The results achieved will provide insight healthily into land-use, environmental resource management, and especially, the post-study of 3-Gorges dam closure, by using proposed baseline of erosion and accumulation of the Chongming Shoal face. Therefore, all above would be beneficial in promoting sustainable development of the integrated local management through modified decision-making in the near future.Through observation and analysis, the following opinions can be reached: Firstly, there are gigantic clusters of mega-dunes that were found in the Eastern Chongming Shoal (proximately, 10 × 8 km). The mega-dunes are embraced in the eastern Chongming Shoal, with the arc-shaped topography parallel to the shoal. The mega-dune cluster is the natural seaward extension of the Chongming tidal flat, connected by numerous of tidal creek system developed on the middle and lower tidal flat. According to the data of on-site measurement, wave height and length ranges approximately from 0.20—0.60m and 100—530m. Data have revealed that lee-side of mega-dunes on the topographic transect profiles of 1.5 m above the mean sea level are mostly asymmetrical with seaward direction. By contrast, lee-face of mega dunes above 1.5 m altitude show semi-symmetric and/or symmetric. As far as the distribution characteristics are concerned, dunes in the northern part of the Chongming Shoal are relatively small (see observation lines I—I' and II—II' in 2004), with the trend of increase in wave length and height to the southern counterpart (see observation lines III — III' and IV — IV, 2004 database). On the mostobservational transect profiles, mega-dunes are overlaid by small dunes, with wave height at about 10cm and wave length ranging from 20 to 50m.Secondly, mage-dunes of 04 database were somewhat modified as revealed by 2005 observation. In general, wave height became longer, ranging from 0.20 to 0.75m, and wave height was higher, ranging from 100 to 630m. Dunes in the northern shoal was also relatively small, while the shape in the southern shoal became larger (see observation lines I—I' and VII—VII', 2005 database). It is the same as before, dunes on the almost all observation transect profiles at 1.5m above the mean sea level are asymmetric, but semi-asymmetric and/or symmetric above this altitude. Small-scale dunes are also overlaid on the mega-dunes.Thirdly, by tracing superimposed observation transect topographic profiles of 2004-05, this is known that the altitude of 1.5 m of the Chongming Shoal can be used as an referential baseline to gauge the accumulation and erosion of the shoal face. This meant that an obvious siltation (to 0.40 m) occurs on the shoal face above 1.5 m and erosion (to 0.10-0.40 m) of the shoal face below that altitude. In another words, sediment transport on the upper part of shoal face is characterized by accumulative movement of mega-dunes, in vice versa, an erosive movement of mega-dunes on the lower part of the shoal face. However, a reversal phenomenon occurs in the northern profile I - I' and II - II' (Figure 3.4), where erosion (to 0.25 m) occurs above 1.5 m and accumulation (0.6 m) appears below that altitude. This probably relates to the siltation of the Northern Yangtze Distributary, in which the entire channel has been largely sited due to weakening freshwater discharge over the last 100 years.Fourthly, our synchronous measurement revealed that the exchange of water and sand between distributary and shoal face is quite frequent. Sediment transport in the flood season was shoal face directed in general (referring site S3 and S4 of 2005 observation, located outside the North Branch). Also, site S2 of 2005 observation, located in the mouth of the North Channel showed southeasterly oriented in the flood season. Site S1 of 2004 observation (outside the North Branch) showed a northwest flow direction.In overall, it can be summarized that the mega-dune located in the eastern Chongming Shoal is the extension of dynamic morphology from the upper tidal flat seaward. With such an extension linked by tidal creeks from tidal zone to the shoal, the strong superimposed runoffs and tides of the river mouth area is the major sediment dynamics controlling the mega-dune sedimentation. Meanwhile, theannual powerful typhoon-triggered storms play an importation role in shaping the mega-dunes on the shoal, which is marked by the landward migration of mega-dunes below 1.5 m altitude of the study area. Of note, small-scale dunes superimposed on the mega-dunes indicate day-day tidal-wave processes. Although there were varied flow directions measured in flood and non-flood seasons, in the Yangtze estuary, the main sediment flux should be derived from the upper Yangtze river drainage basin. Shortly, the observed morphological characteristics of mega-dunes are integration resulted from the various sediment dynamics abovementioned.Comparison between 2004-05 observations reveals that the elevation of about 1.5m serves as a referential point for balancing accumulation and erosion on the upper and lower Chongming shoal face in the Yangtze estuary. Above this benchmark, most of profiles remain as accumulative and below this, erosive. This will provide an evidence to monitor coastal erosion, largely due to decrease in sediment discharging into the sea after the closing of Three Gorges dam.