Abstract
Highways frequently run through the flow and accumulation areas of debris flow gullies and thus are susceptible to debris flow hazards. Assessing debris flows along highways can provide references for highway planners and debris flow control, emergency management. However, the existing assessment methods mostly neglect the essential information of the flow paths and spreading areas of debris flows at the regional scale. Taking the Gaizi Village-Bulunkou Township Section (hereinafter referred to as “the Gaizi-Bulunkou Section”) of the Karakoram highway as the study area, this research introduces a simple empirical model (the Flow-R model) and establishes a method for assessing the debris flow hazard level. The main processes include data collection, inventory of former events, calculating source areas and spreading probability, verification of the model, extraction of hazard assessment factors, and calculation of debris flow hazard levels. The results show that: 1) the accuracy, sensitivity, and positive predictive power of the Flow-R model in simulating the debris flow spreading probability of the study area were 81.87, 70.80 and 72.70%, respectively. The errors mainly occurred in the debris flow fans. 2) The calculation results make it possible to divide debris flow hazard levels into four levels. N5, N19, and N28 gullies had the highest hazard level during the study period. 3) In the Gaizi-Bulunkou Section of the Karakoram highway, during the study period, the highways with very high, high, medium, and low hazards were 4.33, 0.62, 1.41, and 1.68 km in length, respectively.
Highlights
A highway debris flow hazard assessment generally consists of two steps: 1) debris flow hazard zonation, and 2) choose a zonation method and overlap the highway elements with the zonation results to assign the spatially corresponding hazard levels
Calculated debris flow source areas occupied an area of 0.30 km2, and the density of debris flow source areas on the sunny south slope (0.0027 km2/km2) was higher than the density on the shady north slope (0.0017 km2/km2)
The study’s main conclusions are as follows: The existing debris flow source areas were obtained from the aerial photographs
Summary
A highway debris flow hazard assessment generally consists of two steps: 1) debris flow hazard zonation, and 2) choose a zonation method and overlap the highway elements with the zonation results to assign the spatially corresponding hazard levels. Eight factors (debris flow scale, loose solid matter recharge degree, drainage basin area, etc.) are extracted from the model results, and the hazard level of each debris flow gully is calculated (see section Determination and quantification of assessment factors). The energy loss to friction can be calculated from the simplified friction-limited model (SFLM), which assesses the maximum possible runout distance using the minimum travel angle (fahrböschung angle) (Corominas, 1996) It is the angle of the line connecting the source area to the most distant point reached by the debris flow (Eq 5): Eif gΔx tan φ (5). It is difficult to obtain this information in a sparsely-populated alpine area, so available secondary factors that can characterize the scale and frequency of debris flows can be adopted They can be used in combination with primary factors to constitute a multi-factor assessment model (Ji et al, 2020). After calculating the maximum deposition length (L) of the debris flow in the direction perpendicular to the highway and the distance (L) from the debris flow gully mouth to the river bank, the ratio of the difference between (L) and (L) to channel width (B) can be used for factor quantification (Zou, et al, 2019)
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