A prototype experiment of debris flow control with energy dissipation structures

Large-volume debris flow events are defined when the volume of solid materials exceeds 1 million m³. Traditional engineering measures, such as check dams, diversion channels, and flumes, are effective for normal debris flow control but are not sufficient to control large-volume debris flows. Experiments were conducted with an artificial step-pool system on the new Wenjiagou Gully to mitigate large-volume debris flows. The old Wenjiagou Gully was buried by 81.6 million m³ of loose solid material created by a landslide that was triggered by the Wenchuan earthquake on May 12, 2008. The new gully was formed during the scouring process caused by debris flows in 2008. Large-volume debris flows were initiated by rainstorm flood with high kinetic energy. The artificial step-pool system was constructed with huge and big boulders on the new Wenjiagou Gully in 2009. The step-pool system dissipated flow energy in steps and hydraulic jumps. Analysis proved that the step-pool system dissipated two-third of the kinetic energy of flow; thus, the critical discharge for triggering debris flow increased threefold. Due to the step-pool system maximized the flow resistance and protected the bed sediment and banks from erosion, the rainstorm floods in 2009 did not trigger debris flows. In 2010, the step-pool system was replaced with 20 check dams. Huge boulders were broken into small pieces of diameter less than 0.5 m and were used as building materials for the 20 dams. Without the protection of the step-pool system, a rainstorm flood scoured the base of the dams and caused failures for all of the 20 check dams in August 2010. The flow incised the gully bed by 50 m. The loose bank materials slid into the flow mixed with water and formed a large-volume debris flow with a volume of 4.5 million m³. Many houses were buried by the debris flow, and 12 people were killed. Comparison of the two strategies proved that energy dissipation structures are necessary for controlling large-volume debris flows. Check dams, if they are stable, may reduce the potential of bank failures and control debris flows. The step-pool system dissipates flow energy and control gully bed incision and bank failure. A combination of check dams and step-pool systems may be the most effective for mitigating debris flows.