Construction of porous barriers to collect flood flows and gradual release of water, later, is one of the effective techniques for flood control. In this study, by carrying out a set of experiments in a flume with the width of 0.6, depth of 0.8, and length of 14 m, the effects of gradation of stones and downstream slope on the stability of such structures has been investigated. Moreover, the influence of mean diameters of stones along with their standard deviations as regards the stability of the structure have also been scrutinized. Utilizing dimensional analysis technique as well as two-thirds of the observed data, a new relationship for calculating the stable downstream slope in term of discharge, gradation of material and water depths upstream and downstream, has been developed. To evaluate the new relation, the remained one-third of the observed data was compared with those calculated from the new relation. The root of mean square errors and the mean of relative errors were found as 0.04 and 0.09, respectively, indicating a reasonable accuracy of the new relation established.