Risk Assessment and Control for Geohazards at Multiple Scales: An Insight from the West Han River of Gansu Province in China

Risk assessment provides a powerful tool for the early warning and risk mitigation of geohazards. However, few efforts have been made regarding risk assessment and dynamic control at multiple scales. With respect to this issue, the West Han River catchment in the Gansu Province of China was taken as a study area, and geohazard risk assessments at three different scales were carried out, namely regional, local and site scales. Hazard assessment was performed using the combination of the information value and hierarchical analysis models, infinite slope stability model, and FLO-2D model. Vulnerability was estimated from two viewpoints, including physical vulnerability and social vulnerability, by applying remote sensing and semi-quantitative methods. Finally, risk mapping and zonation was obtained from the products of hazard and vulnerability, and corresponding measures of risk management and control at different scales were recommended. The results indicated that the geohazard risk at the regional scale was the highest under the earthquake and rainfall conditions with a 100-year (100a) return period, respectively, and the area of very high risk level reached 5%. When the rainfall condition had a return period of 50 years, only 1% of the area was located in the very high-risk region. Additionally, the overall risk was higher in the central and northeastern parts of the region under heavy rainfall and earthquake conditions. The overall risk level in Longlin-Leiba Town (at the local scale) responded more significantly to heavy rainfall conditions, with higher risk in the southwestern, central, and northeastern parts of the region. For the site scale (Wujiagou debris flow), only 2% of the total area was identified as very high-risk even under heavy rainfall with a 100a return period, but the proportions for the low and moderate levels reached 30% and 56%, respectively. The present study can provide scientific references for geohazard risk assessment and control.