Short answers, pulled from the story.
Floods occur when water overflows its usual boundaries to submerge land that remains dry under normal conditions. This process involves water or rarely other fluids moving across terrain and escaping channels to flood property.
The 1931 China floods killed between 2,500,000 and 3,700,000 people making them the worst natural disaster in history. The 1887 Yellow River flood claimed between 900,000 and 2,000,000 lives while the 1938 Yellow River flood resulted in 500,000 to 700,000 deaths during that year.
Climate change intensifies the water cycle leading to more frequent and stronger extreme weather events that cause intensified flooding. Sea level rise contributes to more intense floods and increased flood risk for coastal populations while global environmental issues influence causes of floods.
Every year flooding causes countries billions of dollars in economic destruction that threatens individual survival. Almost 40 percent of small businesses never reopen their doors following a flooding disaster according to FEMA data and yearly in the United States floods cause over $7 billion in property and infrastructure damage.
Waterways prone to floods are carefully managed through defenses such as detention basins levees bunds reservoirs and weirs. These structures prevent waterways from overflowing their banks into surrounding communities and dams can be built and maintained over time to reduce occurrence and severity of floods globally.