Short answers, pulled from the story.
A flood is an overflow of water that submerges land that is usually dry. Floods are caused by prolonged heavy rainfall, accelerated snowmelt, severe winds over water, unusually high tides, tsunamis, or the failure of dams, levees, and other structures that retain water. They occur when ponds, lakes, riverbeds, soil, and vegetation cannot absorb all the water.
The 1931 China floods were the deadliest on record, with an estimated death toll between 2.5 and 3.7 million people. The list of deadliest floods, counting only events with at least 100,000 deaths, is dominated by China and the Yellow River.
The main natural types of floods are river flooding, groundwater flooding, coastal flooding, and urban flooding, which is sometimes known as flash flooding. Areal flooding happens on flat ground when rain or snowmelt arrives faster than the land can absorb or shed it, and tidal flooding can combine river and coastal processes in estuaries.
Floods directly cause death mostly through drowning, and they spread waterborne diseases such as cholera, hepatitis A, hepatitis E, and diarrheal diseases when clean water is contaminated. Still water left behind raises vector-borne diseases including malaria, dengue, West Nile, and yellow fever, and chronically wet houses grow mold that worsens respiratory health.
Yes, smaller and more frequent floods recharge ground water, make soil more fertile, and add nutrients to soils. Periodic flooding was essential to ancient peoples along the Tigris-Euphrates, Nile, Indus, Ganges, and Yellow Rivers, and freshwater floods help maintain floodplain biodiversity and improve fisheries.
Floods are forecast by analyzing annual maximum flow rates statistically to estimate the 100-year flood and using physical process models such as HEC-RAS and TUFLOW to map flood depths. The Global Flood Monitoring System, GFMS, maps flood conditions worldwide using precipitation data from NASA satellites and the Global Precipitation Measurement satellite, with forecasts running 5 days ahead.