LITERATURE REVIEW
2. Introduction
2.2. Grain size studies
width of the braid bars was always about 16 km, the mean increased with time from ca. 9 to 11 km, and the minimum width changed from ca. 3 km (until 1980) to a constant of ca.
7 to 8 km in the early 1990s. They also observed frequent occurrence of channels more than 2 km wide between 1967 and 1992, but rarely after 1994. Coleman (1969) also described wide channels in the mid-1960s.
only small amounts of sediments can be extracted from river water and most of the analyses require several grams of sediments. In such cases, grain size analysis may be used as a tracer for identification of sediment provenance as modern laser particle size analyzers are capable of analyzing low concentration samples (Kurasighe and Fusejima, 1997).
Stummeyer et al. (2002) used content of quartz as an indicator for the grain size distribution of the sample material, because the limited amount of suspended matter samples did not allow grain size analysis. Many researchers have analyzed grain size distribution to characterize sediments (Buller and McManus, 1972, 1973; Duck, 1994;
Machida et al., 1975).
Singh et al. (2007) reported that cumulative size distribution curves might give an idea of the transportation dynamics of sediments in a river. They found that nearly 20%
of the bed load of the Ganga River moved as bed load and the rest 80% moved in suspension. This phenomenon was reflected by overlapping of the coarsest parts of the cumulative curves for bed and suspended sediments. They observed that all the cumulative grain size curves for the bed load sediments showed distinct “break” in slope, which represented the maximum grain size transportable in suspension at maximum shear stress.
Frequent and costly dredging operations become necessary to address the problem of silting up of harbours and channels. Knowledge of transport directions of sediments is useful in planning the location and layout of new harbours, construction of groynes or other structures to control sedimentation at existing facilities or possible dispersal patterns of contaminants adsorbed to sediments. Various studies have related grain size
trends (spatial variation of grain size parameters within the same environment) to net sediment transport (Miller et al., 1977; Komar and Reimers, 1978; Mclaren, 1981;
McLaren and Bowles, 1985, 1991; Gao and Collins, 1992). So, sediment transport patterns within different environment may be inferred using the grain size trends. Trend vectors to a central sampling station are defined based on mean grain size, sorting coefficient (standard deviation) and skewness. These vectors are summed up to produce a resultant. This is done for each sampling station and the resultant vectors are compared to decide transport direction. There are computer programs to do the trend analysis for defining net sediment transport pathways (Gao, 1996; Chang et al., 2001). Roux et al.
(2002) have cautioned that observed transport directions from computer programs should be compared with additional information on the dispersal patterns and if no such information is available, the trend type producing the highest vector magnitude should be employed.
Ex-situ analyses may not represent field-conditions. Aggregates of sediments are often broken up in sample collection and handling. Again, allowing the particles to settle may create more aggregates than existed in the original sample (Wren et al., 2000).
Researchers emphasize the importance of measuring particle sizes for fine sediments without disturbing aggregates (Walling and Moorhead, 1989; Droppo and Ongley, 1992, 1994; Woodward and Walling, 1992; Walling and Woodward, 1993). Phillips and Walling (1995) recommended in-situ measurements of fines in suspended sediment as they observed increases in volume mean particle size up to 24% in an experiment of settling for 1 hour followed by resuspension.
2.2.2. Some examples of grain size studies
Siddiquie (1967) studied the sediments of the Bay of Bengal for various particle size parameters. He observed that the median diameters of the sediments ranged from 0.74 µm to 700 µm. Coefficient of sorting varied from 1.75 to 10.69. Most of the sediments were normally sorted (coefficient of sorting varying from 2.5 o 4.5), few samples were well sorted and about a third was poorly sorted. Skewness ranged from 0.027 to 8.81. Most of the sediment samples exhibited positive skewness, suggesting predominance of coarser admixture. Kurtosis ranged from 0.02 to 0.34.
Kanjilal et al. (1970) studied the sedimentary parameters of Ganga sands. The sands were medium to very fine size, moderately to very well-sorted with negative to very positive skewness and platykurtic to very leptokurtic. Majority of the sediments were bimodal and according to them, this was due to the presence of considerable amount of silt.
In the particle size analysis of the Nile River suspended sediments, Shalash (1982) observed that, as a result of increase in suspended sediment concentration, the percentage of fine sand (0.2-0.02 mm) decreased, the percentage of clay (<0.002 mm) remained constant and was not affected, but the percentage of silt (0.02-0.002 mm) increased. He reported that presence of coarse sand (>0.2 mm) in suspended sediment was negligible.
Singh and France-Lanord (2002) studied the grain size of the Brahmaputra sediments (suspended) collected during post-monsoon (October, 1999) and the pre- monsoon season (July, 2000) at Dibrugarh, Tezpur, Guwahati and Dhubri. They observed
that the sediments consisted mostly of silts with clay fractions (< 2 µm) representing 0.5- 3% of the total sediments.
Based on the fact that silt is a major component of most continental margin sediments, Gorsline (1985) reported that silt was the dominant class among the fine- grained textural classes.