Reservoirs overflow during flood season because of sedimentation cycles, which severely affects their effectiveness. Siltation is a major problem in dams constructed in waterways in arid and semi-arid areas. Therefore, the reservoirs in wadis lose their capacity due to sedimentation. This study determines an optimal design of the trapping basin on steep slope areas for Wadi Bishah in the Asir region of southwestern Saudi Arabia. The empirical design criteria of the sediment-trapping basin is used to mitigate the effects of sedimentation in the King Fahd Dam. The empirical design of the trapping basin constructed upstream of the dam located in the wadi is presented. Moreover, the annual suspended and bed sediment load (Qs and Qb) techniques for estimating the volume of sediments are used, and the relationship between the sediment trapping efficiency and size is determined. The sediment trapping in Wadi Bishah upstream sediment-trapping basins is selected to reduce the amount of sediment. One of the important results of this study tries to create a new concept to trap sediment in wadis, which are located in arid and semi-arid areas. The results obtained were evaluated using theoretical and empirical equations to determine the appropriate size of the basin. The results demonstrate that the optimal dimensions for the sediment confinement basin are Lb × Wb × hs = 3500 × 500 × 1.5 m. Also, for these dimensions, the basin efficiency was assumed to be in the range of 60−70%. The trap basin should be constructed at open check dams upstream (U/S) of the proposed basin to enhance its efficiency. Further investigation is required to understand the transport and deposition of sediments, particularly fine sediments in the basin. Additionally, the effects of sediment traps in Wadi Bishah should be assessed during the construction of these structures to aid water resource management and mitigate flood disasters.