The main aim from this studies to limit runoff for valleys of Al-tharthar basins and routing the flood wave in reservoir suggested ,and limit the amount of outflow change from reservoir and the time of delay flood wave (lag time ) and the relation between the volume of in and out flow from the reservoir in addition for temporary storage , which help to limit the dimension of spillway; where the out flood wave function to height of water above of crest of spillway and the surface area of reservoir. the amount of temporary storage for reservoir represent the difference between the in and outflow, while the increase of inflow with the time increase the raising of level of reservoir and increase the surface area for it. And the outflow from the spillway let to decrease of peak flow out from inflow and delay flood wave time to take enough time to avoid the flood side affections.
In through use tow programs in computer, show the delay time between (300 – 900) min. for studying reservoirs and reducing ratio of peak flow out from (71% - 91%) for reservoir (8) and (1) respectively, which use to reducing the dimension of spillway and limit the height of the dams and the level of top crest of spillways.