Classification of yearly extreme precipitation events and associated flood risk in the Yangtze-Huaihe River Valley

Fifty cases of regional yearly extreme precipitation events (RYEPEs) were identified over the Yangtze-Huaihe River Valley (YHRV) during 1979-2016 applying the statistical percentile method. There were five types of RYEPEs, namely Yangtze Meiyu (YM-RYEPE), Huaihe Meiyu (HM-RYEPE), southwest-northeast-oriented Meiyu (SWNE-RYEPE) and typhoon I and II (TC-RYEPE) types of RYEPEs. Potential vorticity diagnosis showed that propagation trajectories of the RYEPEs along the Western Pacific Subtropical High and its steering flow were concentrated over the southern YHRV. As a result, the strongest and most frequently RYEPEs events, about 16-21 cases with average rainfall above 100 mm, occurred in the southern YHRV, particularly in the Nanjing metropolitan area. There have been 14 cases of flood-inducing RYEPEs since 1979, with the submerged area exceeding 120 km(2) as simulated by the FloodArea hydraulic model, comprising six HM-RYEPEs, five YMRYEPEs, two TC-RYEPEs, and one SWNE-RYEPE. The combination of evolving RYEPEs and rapid expansion of urban agglomeration is most likely to change the flood risk distribution over the Nanjing metropolitan area in the future. In the RCP6.0 (RCP8.5) scenario, the built-up area increases at a rate of about 10.41 km(2) (10 yr)(-1)(24.67 km(2) (10 yr)(-1)) from 2010 to 2100, and the area of high flood risk correspondingly increases from 3.86 km(2)(3.86 km(2)) to 9.00 km(2)(13.51 km(2)). Areas of high flood risk are mainly located at Chishan Lake in Jurong, Lukou International Airport in Nanjing, Dongshan in Jiangning District, Lishui District and other low-lying areas. The accurate simulation of flood scenarios can help reduce losses due to torrential flooding and improve early warnings, evacuation planning and risk analysis. More attention should be paid to the projected high flood risk because of the concentrated population, industrial zones and social wealth throughout the Nanjing metropolitan area.