Modeling of breaching parameters for debris flow dams

The debris flow dam is a common type of barrier dams,which shows significant differences from other types of barrier dam such as landslide dam,moraine dam in their formation processes,dam body shapes,and internal compositions.The basic breaching parameters such as flood peak discharge are vital indicators of risk assessment.In this study,we elucidated the failure process of the debris flow dam through the flume experiment,and built the calculation equation of the breaching parameters by selecting critical factors.The result shows that the overtopping failure process of the debris flow dam is capable of forming significantly retrogressive scarps,and the failure process experiences three stages,the formation of the retrogressive scarp,the erosion of the retrogressive scarp,and the decline of the retrogressive scarp.Five factors used for establishing the calculation equations for peak discharge(Qp),final width(Wb)of the breach,and duration(T)of the debris flow dam failure are dam height(h),reservoir capacity(V),the fine grain content(P0.075)of the soil,the nonuniformity coefficient(Cu)of the soil,and the upper limit grain size(D90)of the soil,respectively.In the three equations,the correlation coefficients between Qp,Wb,T and the five factors were 0.86,0.70,0.63,respectively.The equations still need to be modified and verified in actual cases.

The siltation of debris flow behind check dam in the midstream of Bailong River

Siltation gradient and siltation length are important parameters for designing gravity check dams for debris flows,which directly affect the accuracy of estimates of interception capacity.At present,siltation gradient calculations are based primarily on empirical values,and range from 0.4 to 0.95 times the channel slope coefficient.The middle reaches of the Bailong River are one of the four areas in China that are most severely affected by debris flow hazards.Gravity dams are widely employed in this mountainous area.However,field studies of their capacity are lacking.In this paper,the operations of check dams were investigated.Based on field investigation results and theoretical analysis,calculations for siltation gradient,siltation length,and dam storage capacity are established.The impact of debris flow density,channel slope,and particle size weight percentages are discussed.The calculations show that the theoretical values for siltation gradient are consistent with measured values with 83.6% accuracy;and theoretical values of siltation length are consistent with measured values with 91.6% accuracy.The results of this research are an important reference for optimal height and spacing of dams,estimation of dam storage capacity,and disaster prevention.

A catastrophic debris flow in the Wenchuan Earthquake area,July 2013：characteristics,formation,and risk reduction

In the Wenchuan Earthquake area,many co-seismic landslides formed blocking-dams in debris flow channels. This blocking and bursting of landslide dams amplifies the debris flow scale and results in severe catastrophes. The catastrophic debris flow that occurred in Qipan gully(Wenchuan,Southwest China) on July 11,2013 was caused by intense rainfall and upstream cascading bursting of landslide dams. To gain an understanding of the processes of dam bursting and subsequent debris flow scale amplification effect,we attempted to estimate the bursting debris flow peak discharges along the main gully and analyzed the scale amplification process. The results showed that the antecedent and triggering rainfalls for 11 July debris flow event were 88.0 mm and 21.6 mm,respectively. The event highlights the fact that lower rainfall intensity can trigger debris flows after the earthquake. Calculations of the debris flow peak discharge showed that the peak discharges after the dams-bursting were 1.17–1.69 times greater than the upstream peak discharge. The peak discharge at the gully outlet reached 2553 m^3/s which was amplified by 4.76 times in comparison with the initial peak discharge in the upstream. To mitigate debris flow disasters,a new drainage channel with a trapezoidal V-shaped cross section was proposed. The characteristic lengths(h1 and h2) under optimal hydraulic conditions were calculated as 4.50 m and 0.90 m,respectively.