Assessment of secondary mountain hazards along a section of the Dujiangyan-Wenchuan highway

Conducting a hazard assessment for secondary mountain hazards is the technical basis for reconstructing destroyed highways and for disaster prevention. It is necessary to consider the role and influence of structural engineering measures as an important assessment factor. In this study, based on six substantial field investigations conducted between July 2008 and July 2012, a 2 km wide zone along both sides of the Dujiangyan-Wenchuan (Du-Wen) Highway was selected as the study area. Microgeomorphic units and small watersheds in the study area were extracted with GIS software and used as basic assessment units. Through field investigations, remote sensing surveys and experimental analysis, a structural engineering effectiveness assessment was conducted using the technique of principal component analysis. The results showed the following: 1) A total of 491 collapses, 12 landslides, 32 slope debris flows and 17 gully debris flows were scatted across the study area. The total overall areal density of all mountain hazards was 25.7%. The distribution of secondary hazards was influenced mainly by seismic intensity, active fault zones, lithology, slope and altitude. More than 70% of secondary hazards occurred in zones with a seismic intensity of XI, a distance to the fault zone of between 0 and 25 km, a slope between 25° and 50°, and an altitude of between 1,000 m and 1,800 m. 2) Different structural engineering measures play different roles and effects in controlling different types and scales of secondary mountain hazards. 3) With a secondary mountain hazard area of 128.1 km² and an areal density of 34.9%, medium, high and very high hazard zones accounted for 74% of the study area and were located on the high, steep slopes along both sides of the highway. The low hazard zone was located mainly in the valley floor, on gentle slope platforms and at locations 1.5 km away from the highway ? the hazard area was 45 km² and the areal density was 3.3%. 4) The methodology for hazard assessment of secondary mountain hazards, which is based on five factors, solves such key technical problems as the selection of assessment units, multi-source data fusion, and the weight calculation for each assessment index. This study provides a new and more effective method for assessing secondary mountain hazards along highways, and the proposed models fit well with validation data and field observations. The findings were applied to reconstruction and disaster mitigation in the case of the Du-Wen Highway and proved to be feasible.