Debris flow formation conditions and optimal characteristics of drainage canal following Wenchuan earthquake

The debris flow formation conditions changed after the Wenchuan earthquake. For studying the optimal characteristics of the drainage canal under the changed formation conditions, the Zhangjia gully in Beichuan County, a typical debris flow gully, was selected as the study area. Two debris flow events occurred on September 24, 2008 and July 17, 2009, respectively, which threatened the safety of the inhabitants and destroyed the properties and transportation routes on the alluvial fan. Huge amounts of solid loose materials triggered by the earthquake and the effective antecedent precipitation were responsible for the debris flow occurrence. For eliminating the damage caused by debris flows, the construction of a drainage canal on the alluvial fan became necessary. A design method for the optimal cross-section characteristics of a debris flow drainage canal is discussed in this paper. Using the standard measurements for the depth (h) and the width (b), the cross-section size parameter (F) and the cross-section configuration parameter (M) under optimal hydraulic conditions are defined and deduced. Subsequently, formulas for calculating the cross-section measurements are deduced under optimal hydraulic conditions when peak discharge (Q), grain composition (D ₅₀ and D ₁₀), and longitudinal slope (I) are known. Finally, this method is applied for the cross-section design of the debris flow drainage canal in Zhangjia gully.