Response Capability of Tigris River Bed to Armoring Phenomenon after Mosul Dam Operation

Existence of small percentages of sand and coarse clay in alluvial rivers bed downstream of dams constructed on these rivers with the degradation processes caused by released discharges from dams clear of sediments as a result of sedimentation processes in the upstream reservoirs will lead to washing, segregation and removal of fine materials from the bed leaving coarse sizes gradually. This will cause an increment in the bed roughness and lowering in the flow velocity which in-turn lead to the equilibrium condition. This state of bed roughness is called (armouring). The present research work focuses the light on the armouring phenomenon in rivers and trying to apply on Tigris river downstream Mosul dam identifying the capability of river bed to this phenomenon after Mosul dam operation for about sixteen year since 1985 using various number of criteria laboratory and field relations predicted by many researchers. The results proved that the armouring conditions was existed in Tigris rivers bed in the present study reach and probably had been reached before number of years as a result of Mosul dam operation . The value of geometric standard deviation of the armour layer was 1.58 and the mean diameter was 33 mm. The results proved lowering in the geometric standard deviation of the surface bed material in Mosul city since Mosul dam operation. The average ratio between the mean diameter of the original layer and the armour layer was 0.82, which confirm the conclusion concerning the arrival of the river bed to the armouring state.