Backgroud: Affected by global warming, extreme weather events have occurred frequently in recent years, and many places have suffered floods. Since the flood season in 2023, to September 11, there have been 97 cities at or above the regional level in the country have suffered varying degrees of flooding, heavy losses, and even paid the price of life. To this end, the professional team of water Environment, with professional sensitivity and dedication, collected and sorted out a large amount of public information, statistically analyzed the characteristics of disaster, analyzed the problem from a professional perspective, and expected to cause reflection. In general, most of our cities lack high-quality flood risk maps, and their risk carrying capacity is weak.
Statistics are collected from official notifications, media reports and wechat public accounts as of September 11, 2023. The map below shows cities affected by flooding this year. Yellow indicates both flooding and waterlogging, pink indicates flash floods, and red indicates cities or districts with casualties.
Map of flood disaster cities in China in 2023
Analysis of results:
There are 4 municipalities directly under the Central Government, 333 prefecture-level administrative regions, 2 special administrative regions, a total of 339 administrative regions at or above the prefecture level, which are called cities at or above the prefecture level for convenience of understanding. So far, 97 urban flash floods/waterlogging events have occurred. Among them, four municipalities directly under the Central Government and the Hong Kong Special Administrative Region suffered floods. Hong Kong suffered the heaviest rainstorm in nearly 139 years. Beijing and Chongqing suffered heavy casualties; Fuzhou, Quanzhou, Shenzhen, Huizhou, Dongguan, Chaozhou and other southeastern coastal cities were hit by typhoons twice, causing urban flooding. In Yangzhou, Taizhou, Wuxi and other cities in the south of the Yangtze River, heavy rainfall caused urban waterlogging twice. Xi 'an, Gannan prefectures under the jurisdiction of the county due to short-term heavy rainfall caused two flash floods, causing casualties. In cities affected by flooding:
l Waterlogging accounted for 64%, floods (flash floods) accounted for 36%, and the remaining 9% of the city suffered from both waterlogging and flooding.
l 50% of urban floods are caused by typhoons and heavy rains.
l 16 cities suffered casualties due to floods or flash floods, or resulting geological disasters, such as mudslides and landslides.
Statistics on the proportion of major types of flood disasters in 2023
Risk Maps - Important means of exposing flood risks:
A flood risk map is a professional tool that represents the risk characteristics of floods or waterlogging, and shows the scope and objects that floods may affect. The risk characteristics are usually represented by one or more combination factors such as inundation range, water depth, flow rate and inundation time. Risk map is an important basis for further assessing flood losses, deciding disaster reduction measures and emergency response. It is usually based on hydrological statistics and topographic and geomorphic parameters to build professional models, analyze hydraulic elements, and draw them with GIS professional tools.
Therefore, in the early stage, through technical means such as modeling, flood risk can be predicted, risk distribution under different terrain characteristics and drainage conditions can be understood, flood inundation range under different rainstorm conditions can be estimated and other key elements can be obtained.
This study uses risk maps to illustrate the vulnerability of these cities to floods from extreme rainfall events. These risk maps not only help to identify flood risks, but also provide an important basis for urban planning and emergency response:
l Mountain features, such as Wanzhou in Chongqing, Xiahe County in Gansu, Mentougou and Fangshan in Beijing, Pingyang in Wenzhou, and many other mountain towns, have a significant proportion of buildings clearly within the risk range. When the rainstorm occurs, the flash floods converge sharply, the water depth and velocity are large, which may cause mudslides or landslides. If the upstream reservoir dispatching flood discharge, hidden risks will be more difficult to deal with. Accurate flood risk maps can ensure urban construction to avoid high-risk areas and reduce casualties and property losses when floods occur.
Flood risk diagram of a street town in Mentougou District, Beijing
Distribution diagram of flood risk of Shunxi reservoir, Pingyang County, Wenzhou city
l Foothill cities, such as Fuzhou, Fujian Province, Fuyang district of Hangzhou. The upstream mountain catchment passes through the urban area, while the internal urbanization river channel is complicated. Often, cities protect the city by channeling floodwaters downstream through a flood interception canal. However, when the rainstorm exceeds the standard, the flash flood may break through the capacity of the flood interception system and invade the city. If it is superimposed with heavy rain and waterlogging in urban areas, the situation is more critical.