Multi-hazard stress testing under extreme rainstorms in the Shenzhen metropolitan area

A severe rainstorm can trigger numerous natural hazards of multiple types in cities with hilly terrain, such as landslides, debris flows, and flooding. Such compound hazards may pose much greater risks than a single hazard to the safety of residents and infrastructures. This study focuses on a metropolitan area of Shenzhen, China and develops a multi-hazard stress testing method for hazard risk management against extreme rainstorms. The probable maximum precipitation in Shenzhen is first estimated using the storm transposition method considering orographic intensification and moisture maximisation, with the centre peak amount being 1441 mm in 24 hours at Tanglang Mountain. An integrated physically-based multi-hazard simulation model, EDDA, is then utilised to simulate the processes of initiation, propagation and transformation of landslides, debris flows and flooding in a unified manner. Under the designated extreme rainstorms of a 1000-year return period and 85% of probable maximum precipitation, deposit depths of landslides and debris flows can be large in some urban areas near the foot of Tanglang Mountain; flooding can be very extensive in the flat built-up regions, with 21.8% and 31.7% of the study area flooded, respectively. The produced multi-hazard maps can facilitate the design of risk mitigation measures against extreme rainstorms.