Sediment

Sediment solid fragmental material that originates from weathering of rocks and is transported or deposited by air, or water, or that accumulates by other natural agents, such as chemical precipitation from solution or secretion by organisms, and that forms in layers on earth's surface at ordinary temperatures in a loess, alluvium. Typical sediments are, in order or increasing coarseness, clay, mud, silt, sand, gravel, pebble, cobble, and boulder. From environmental management point of view, the movements of fluvial sediments are critically important for channel management, river and irrigation canal maintenance, reservoir and harbour siltation, debris dumping and floodplain accretion. The size of particulate material transported as sediment-load by rivers ranges between fine clay and colloidal particles of less than 0.5 m in diameter to large boulders moved during peak flood stage. River sediment loads may move downstream as bed load and as suspended load. The bed loads mainly consist of inorganic materials. The suspended loads generally involve particles <0.2 mm in diameter and the particle size characteristics are largely determined by the nature of the source material.

The total amount of sediment load that passes through Padma-Meghna-Jamuna to the bay of bengal is estimated to be about 4 billion tons annually. The typical grain size characteristics of the major rivers in Bangladesh differ. For instance, bed material in the brahmaputra is mostly fine sand to silt, the clay fraction generally represents less than five percent of the total sediment. Bank materials are mostly varying amounts of fine sand and silt, with only occasional amounts of clay. In general, silt-size sediments are common in the load of the ganges and the bed material is predominantly sandy. [Mohd Shamsul Alam]