Free surface flows in macro-and intermediate roughness conditions have a high aeration potential causing the flow characteristics to vary with slopes and discharges.The underlying mechanism of two-phase flow characteristics in macro-and intermediate roughness conditions were analyzed in an experimental setup assembled at the Laboratory of Hydraulic Protection of the Territory (PITLAB) of the University of Pisa,Italy.Crushed angular rocks and hemispherical boulders were used to intensify the roughness of the bed.Flow rates per unit width ranging between 0.03 m 2 /s and 0.09 m 2 /s and slopes between 0.26 and 0.46 were tested over different arrangements of a rough bed.Analyses were mainly carried out in the inner flow region,which consists of both bubbly and intermediate flow regions.The findings revealed that the two-phase flow properties over the rough bed were much affected by rough bed arrangements.Turbulence features of two-phase flows over the rough bed were compared with those of the stepped chute data under similar flow conditions.Overall,the results highlight the flow features in the inner layers of the two-phase flow,showing that the maximum turbulence intensity decreases with the relative submergence,while the bubble frequency distribution is affected by the rough bed elements. Free surface flows in macro-and intermediate roughness conditions have a high aeration potential causing the flow characteristics to vary with slopes and discharges. The underlying mechanism of two-phase flow characteristics in macro-and intermediate roughness conditions were analyzed in an experimental setup assembled at the Laboratory of Hydraulic Protection of the Territory (PITLAB) of the University of Pisa, Italy. Crushed angular rocks and hemispherical boulders were used to intensify the roughness of the bed. Flow rates per unit width ranging between 0.03 m 2 / s and 0.09 m 2 / s and slopes between 0.26 and 0.46 were tested over different arrangements of a rough bed. Analyses were mainly carried out in the inner flow region, which consists of both bubbly and intermediate flow regions. The findings revealed the two-phase flow properties over the rough bed were much affected by rough bed arrangements.Turbulence features of two-phase flows over the rough bed were compared with those of the stepped chute data under similar flow conditions. Overall, the results highlight the flow features in the inner layers of the two-phase flow, showing that the maximum turbulence intensity decreases with the relative submergence, while the bubble frequency distribution is affected by the rough bed elements.