Armoring Process in Degrading Streams

Whenever the amount of sediment coming into a given reach of an alluvial stream is less than its sediment transport capacity, there is a general lowering of the stream bed-a process known as degradation. The most common occurrence of degradation is downstream of a large capacity reservoir on an alluvial stream. Earlier investigations on degradation have shown that composition of the bed material plays an important role in controlling degradation. If initial shear stress on the bed is such that the coarser fraction of the bed does not move, the material in the surface layer becomes progressively coarser, ultimately forming an armor coat that stops further lowering of the bed. In such a case the shear stress remains essentially constant. Under such condition, it is interesting to know the variation of characteristic size of surface layer with time at the given section.