Presented in this paper are the findings of experimental investigations carried out for the determination of scour reach behind three vents regulator due to a drowned and free hydraulic jumps over a partially rigid apron extended to an erodible sand basin. The study is performed in two categories; the first is to find out the sum of the lengths of rigid apron behind the gates in addition to the length of scour hole formed downstream, while the second is to find the minimum length of rigid apron behind the gates to prevent erosion downstream it. Both categories are carried out under the conditions of symmetrical and asymmetrical under-gated regulations. A trapezoidal channel with 1:1 side slopes and 0.0001 longitudinal bed slope was used. A model of regulator with three vents and two piers was selected to study the influence of some relevant parameters on the scour reach of rigid apron behind sluice gates. The test parameters were the head difference between the upstream and downstream water levels relevant to the uniform water depth (ranges from 0.004 to 1.2), the flow rate and the gate opening height. The mean diameter of the soil particles in the sand basin was kept constant at 0.502 mm. From the comparison between the results of both the studied categories, it is found that the minimum length of rigid apron to prevent scour (Ls) is greater than the sum of the lengths of rigid apron and that of scour hole formed behind it (L+Xs ). Analysis of the results revealed the dependence of the evaluated minimum scour length on the studied parameters. Some empirical relations in dimensionless forms concerning the scour reach against some reliable parameters were obtained and compared favorably with other relations and the existing ones in the field.