The Model for Dilution Process of Landslide Triggered Debris Flow —A Case of Guanba River in Tibet Southeastern Plateau

Jun Li,Ningsheng Chen

doi:10.15446/esrj.v22n2.68177

Abstract

Understanding and modeling the downstream dilution process of a landslide triggered debris flow is the foundation for recognizing the boundary condition and dilution mechanism of this type of debris flow, and this serves as the theoretical basis for the categorized control of viscous debris flows, diluted debris flows, hyperconcentration flows and flash floods in a drainage basin. In this study, taking as an example a typical debris flow that occurred in the Guanba River on Tibet’s southeastern plateau on July 6th, 1998, empirical models are used to calculate the density, water flow discharge, debris flow discharge, average depth of loose materials and channel gradient at 11 cross-sections upstream to downstream in the debris flow. On this basis, the dilution characteristics and debris flow dilution process are analyzed in this study. According to the correlation between the debris flow density and the water-soil ratio and channel gradient, we have established the density evaluation model for the debris flow dilution process, which can predict the dilution process of a landslide triggered debris flow. The study results include the following four aspects: (1) The key factors in the dilution process of landslide triggered debris flows are the water flow discharge, average depth of loose materials and channel gradient. (2) The debris flow dilution characteristics in the Guanba River in 1998 include the occurrence of the debris flow dilution process after a significant increase in the water-soil ratio; an increase in the proportion of fine particles after dilution of the debris flow; and the size distribution of grain is “narrowed.” (3) In accordance with the density and dilution characteristics, the debris flow dilution process in the Guanba River can be divided into the upstream viscous debris flow section, midstream and downstream transitional debris flow section and downstream diluted debris flow section. (4) The density evaluation model for the debris flow dilution process is expressed by the Lorentz equation, and this model can reflect the debris flow dilution process such that the debris flow density will decrease gradually with an increase in the water-soil ratio and decrease in channel gradient. The density evaluation model for the debris flow dilution process has been verified by three debris flow cases, which include Gaoqiao Gully, Haizi Valley, and Aizi Valley

Highlights

Debris flows can be mobilized by the occurrence of landslides, which are usually viscous, high-density debris flows (Iverson et al 1997)
When the water-soil ratio increases and channel gradient decreases, the water-soil ratio is the water flow discharge divided by the average depth of loose materials, and such debris flows will be gradually diluted into a diluted debris flow, which is a flow with a high concentration of sediment or a flash flood (Pierson and Costa 1987, Smith 1991, Svendsen et al 2003); we call this the dilution process of a landslide triggered debris flow (Kang et al 2004)
While some studies have been performed on the dilution process and mechanism (Pierson 2005, Felix and Peakall 2006), there are still unsolved questions for the following 3 aspects: (1) some studies on the dilution index and its related factors have been completed (Pierson and Costa 1987, Dasgupta 2003, Pierson 2005), and the results indicate that the sediment content indexes can be used to distinguish between a debris flow and hyperconcentration flow or other type flows

Summary

Introduction

Debris flows can be mobilized by the occurrence of landslides, which are usually viscous, high-density debris flows (Iverson et al 1997). There are few studies on the dilution process and characteristics based on the water source, loose materials and channel gradient, etc.; (2) the existence of dilution is well known (Pierson 2005, Pederson et al 2015), there is still little understanding about the detailed predicted model on the dilution process due to absence of the investigation methods and typical cases; (3) The absence of a dilution process prediction model hampers quantitative study of the dilution process and mechanism This makes it difficult to implement the relevant mitigation measures for various types of debris flow.

Location The Guanba River is on the southeastern margin of the Qinghai-Tibet

Methodology

Dilution characteristics and dilution process of the Guanba River debris flow

Findings

Conclusions