Observations of boundary layer parameters and suspended sediment transport over the intertidal flats of northern Jiangsu, China

A current-turbidity monitoring system (CTMS) was deployed on the intertidal flat at Wanggang, northern Jiangsu during October 16-17, 2000, to measure the tidal current speeds and seawater turbidities at 5 levels above the seabed. Based upon the logarithmic-profile equation, the boundary layer parameters, i.e., u, z0 and C60, were obtained for 247 tidal flow velocity profiles. Around 90% of the profiles were logarithmic according to the critical correlation coefficient. Internal consistency analysis shows that these parameters derived by different methods are consistent with each other. In addition, the height of the bedforms observed is close to the seabed roughness lengths calculated from the velocity profiles, indicating that the boundary layer parameters obtained can reveal the conditions at the sediment-water interface on the intertidal flats. Suspended sediment concentrations were obtained from the 5 CTMS turbidity meters using laboratory and in-situ calibrations. The results show that the in-situ calibrated SSCs have a much higher accuracy than the laboratory calibrated ones. Calculation of suspended sediment fluxes on the intertidal flats, with a magnitude of 104 kg/m per spring tidal cycle, indicates that suspended sediment moves towards the northwest, which is reversal to the transport pattern controlled by the southward Northern Jiangsu Coastal Current in the sub-tidal zone and adjacent shallow waters.

SEDIMENT TRANSPORT IN YALU RIVER ESTUARY

Tidal cycle measurements of tidal currents, salinity and water tempe rature, and suspended sediment concentrations were measured at four stations, to gether with surveys along two profiles short core collection within the Yalu Riv er estuary. Grain size analysis of the three core sediment showed that: 1) the s ediment from B1 to B3 became finer, worse sorting and positively skewed; 2) the diversification of matter origin became more and more evident from east to west; 3) the sediments over the region were of the same origin, as indicated by their similar colors and grain sizes. The data indicated that stratification occurred in the flood season, from upstream to downstream, and a salt wedge was formed. The water column was well mixed, but the longitudinal gradient of the salinity w as larger on spring tide. The results also showed that the dominating mechanism of suspended sediment transport in the Yalu River estuary was T1, T2, T3 and T5. The non-tidal steady advection transport was restricted by the net transport o f suspended sediment induced by mass Stoked drift directed to landwards, then th e net sediment transport rate were decreased and the turbidity maxima was also f avored to forming and extending.

Sedimentation rates in the Wanggang salt marshes,Jiangsu

Coastal salt-marshes represent an important coastal wetland system. The total area of coastal wetlands exceeds 5000 km2 in Jiangsu Province, China, but it is decreasing rapidly in response to the intense reclamation activities and coastal erosion along a part of the coastline. Hence, two types of plants, Spartina angelica and Spartina alterniflora, were introduced successively into the Jiangsu coastal areas, in order to protect the coastline from erosion and to increase the accumulation rate. Pb-210 and Cs-137 analyses were carried out for sediment samples from the salt-marshes of Wanggang to determine the sedimentation rate, on the basis of an evaluation of the background activity values and the factors affecting the enrichment of Pb-210. Analysis of a typical sediment column of the tidal flat shows that there is weak absorption of Pb-210 in the silt-dominated sediment. Because of the influences of factors such as storm events, bioturbation, material sources and analytical error, some abnormal data points appear in the Pb-210 record. After ignoring these data the calculated sedimentation rate was 3.3 cm yr-1 on average. Based upon analysis of the Cs-137 dating, the rate since 1963 was 3.1 cm yr-1 on average, similar to the data by Pb-210 dating and the previous studies. The dating results show that there were three stages of sedimentation, with the most rapid accretion being taking place after Spartina angelica was introduced into the area. The study also shows that at the stage of Spartina alterniflora growth, the accretion rate was higher than on the flat surface with the same elevation without the cover of this plant.

Preliminary analysis of grain-size populations with environmentally sensitive terrigenous components in marginal sea setting

Terrigenous components were separated from the bulk sediment of Core A7 from the Okinawa Trough and Core A37 from the Ryukru Trench and grain-size distribu-tions of these sub-samples were analyzed. Based upon an analysis of the grain-size data of the two sedimentary se-quences, grain-size populations are identified to be sensitive to sedimentary environmental changes. The modal values and size ranges of the two main grain-size populations in Core A7 are evidently different from those of Core A37, in-dicating the spatial variability of sediment sources and transport processes between the two places. The downcore variations in the content of the environmentally sensitive grain-size populations reveal that during the accumulation of sedimentary material the environment remained relatively stable at the site where Core A7 was collected, except for the apparent events for the formation of two turbidite layers and a volcanic ash layer. However, the sedimentary sequence of Core A37 shows six sedimentary cycles, indicating a highly variable sedimentary environment at this location.

Spatial variability and representation of seabed sediment grain sizes: An example from the Zhoushan-Jinshanwei transect, Hangzhou Bay, China

Grain size analysis was undertaken for the 2-cm interval sub-samples of eight short cores collected along the Zhoushan-Jinshanwei transect in Hangzhou Bay, using a Malvern Mastersizer 2000 laser particle size analyzer. The result indicates that there are different vertical distribution patterns of mean grain size for the short cores. In the study area, the thickness of the seabed moving layer on an annual temporal scale is much larger than that of the deposited layer, i.e. the sedimentary environment is highly dynamic. As a result, the vertical distributions of mean grain size within the short cores represent different types of sedimentary records formed in the same environment, rather than signals of long-term environmental evolution. The seabed sediment consists mainly of silts in the study area, and the verti- cally-averaged value of mean grain size has a tendency of convergence when the thickness over which the mean value is derived increases. Such patterns indicate that the grain size composition of the deposit is controlled mainly by the source of material supply; nevertheless, to some degree hydraulic sorting is effective, which has resulted in the differences in distribution patterns along the transect between the grain size values of the surficial sediment and the verti- cally-averaged values. For long-term sediment transport modeling for an environment associated with strong tidal action and silty sediment, it may be more appropriate to use the vertically-averaged grain size than the value for the surficial sediment, as the model input.