The dimensions of many water streams, which satisfy the proper hydraulic condition, may not be compatible with the designed dimensions of an irrigation work needed to be constructed in some locations. The design requirements of such irrigation works may need to make a contraction in the channel width in the location of constructions. This contraction, of course, affects the different flow properties and the scour hole formed downstream these structures. So, the present experimental study aims to investigate the effect of the transition angle and the contraction ratio on the flow properties and on the scour phenomenon downstream water structures. Through 454 experimental runs, carried out on 20 experimental models, the study proved that, for an efficient hydraulic performance and economic design, the best transition angle (θ) for the approaches of water structures is 30° with a contraction ratio (r) not less than 0.6.