Quantitative effects of antecedent effective rainfall on ID threshold for debris flow

Abstract

Abstract. Studies have shown that the antecedent effect precipitation (AEP) is closely related to rainfall intensity-duration (ID) threshold of debris flow. However, the quantitative relationship between the AEP and ID threshold is still undetermined. In this study, a hydrological process based numerical model (Dens-ID) that can derive the ID threshold curve is adopted to address this issue. Jiangjia Gully (JJG) in Dongchuan District of Yunnan Province was chosen as the study area, Dens-ID was used to derive a series of ID threshold curves corresponding to different AEP. Based on calculated data sets including AEP, ID curves, parameters of ID curve equation (α and β), and debris flow density, the influence of AEP on the ID threshold curve is deeply explored. We found that although solid materials and runoff are the two necessary conditions for the formation of debris flow, the specific roles played in which are different: the volume of loose solid sources provides a basal condition for debris flow and determines the scale of debris flow, while the runoff volume will have a sudden change during the rainfall process, which is a key factor promoting the formation of debris flow. In the condition of AEP ranging from 20 mm to 90 mm, AEP and α can be described by the equation α = −0.0078AEP2 + 0.68AEP + 6.43, and β shows a linear change law with AEP. The error of the two equations were evaluated using 45 historical rainfall data that triggered debris flows, which is equal to 37.85 % and 11.1 %. Due to the two functions, the ID threshold curve can regularly move in the I-D coordinate system rather than a conventional threshold curve stay the same regardless of AEP variation, it is beneficial to improve the prediction capacity of the ID threshold.