In the present study, a laboratory investigation has been conducted on the bed topography downstream the culvert structure under unsteady flow conditions. A culvert is one type of hydraulic structure whose main function is to convay flow between two unevel levels. Most natural flows are unsteady; however, most studies have been conducted in both labratory and numerical fields regarding scour phenomena around hydraulic structures, including culverts, have been carried out under steady flow conditions. Controlling the scour patterns formed around hydraulic structures is crucial to prevent harmfulfactors such as structural damege. To protect the culvert stuctures, Measures such as installing apron downstream of the culvert can be employed. for modeling unsteady flow in the laboratory, hydrographs generated by computer software have been utilized. Th generated hydrographs are designed to establish unsteady flow with four time durations, which each one are is corresponding to peak time relative to the duration time. The widths of the apron which used in the experiments were 30, 40, and 50 cm, and the effect of changes in apron’s width on the depth of the scour hole was investigated. Due to the continuous intraction of unsteady flow with the river Bed in nature, scouring will occure. To model this erosion in the labratory, the bed material height was reduced by 3cm, and the phenomenon was studied. the total number of experiments have been conducted in the laboratory to investigate scour phenomena at downstream of culvert under unsteady flow condition, was 192. peak time to duration. Among the hydraulic flow parameters, the duration of flow has a greater impact compared to the peak time in increasing the scour depht downstream of the culvert. The maximum scour depth values at zero elevation , in the precence of a pron with width (b=50cm=3.3T’) are minimized at ratio of 1.0. the greatest impact from an apron with width (b=50cm=3.3T’) results in a reduction of scour depht at