Magnitude amplification of flash floods caused by large woody in Keze gully in Jiuzhaigou National Park, China

Abstract

Wood is an important component of flash floods and debris flows in forested mountainous areas. To better understand the characteristics of large wood (LW) movement and its destructive effects on check dams, bridges, and buildings, we conducted field investigations and theoretical calculations on the flash flood process in Keze gully in Jiuzhaigou National Park (China). Based on the gully characteristics and the calculation method, the magnitude amplification ratio of flash flood peak discharge caused by the clogging and breakage processes associated with LW transport reached 2.83 to 3.03 times the mean peak discharge upstream of check dam No. 1. Moreover, the limit analysis and limiting equilibrium state methods were adopted to analyze the additional impact force of LW on the sidewalls of check dams, which resulted in sidewall breaking and sediment block function loss of the check dam. Furthermore, we analyzed the LW transport pattern and formulated a conceptual model of LW transport, including the LW length, channel width, and standing tree spacing. Through this case study, we put forward that the magnitude amplification ratio and the additional impact force of LW should be considered in the design and construction of mitigation engineering projects in forested mountainous areas.

Keywords

• check dam
• clogging and breakage
• flash flood
• large wood
• magnitude amplification ratio