SEDIMENT SORTING IN AN AGGRADING STREAM

Abstract

An alluvial stream is said to be in equilibrium when its bed and banks do not undergo significant. change with the passage of time. Such a condition is obtained when the sediment load entering the reach under consideration is equal to the sediment transport capacity of the reach. A stream in equilibrium is also called a graded stream, a balanced stream or a stream in regimen. Mackin (11) 1948, has defined a graded stream as one in which, over a period of years, slope is delicately adjusted to provide, with available discharge and prevailing characteristics, just the velocity required for the transportation of the load supplied from drainage basin. Natural streams are seldom under true equilibrium condition over long periods of time because the water discharge, the sedi-ment size and sediment load in natural streams vary with time and distance, and changes in slope required-to prevent aggradation or degradation do not take place fast enough for the stream to be considered to be in true equilibrium. But from a practical point of view,, the definition of a stream in equilibrium' is extended to include even those cases where the channel 2 returns to a particular condition during a particular season each year.