The flood was caused by heavy rain, the rapid flow phenomenon, the snow melts and the ice storm surge, the flood is caused by the natural factors such as rivers and lakes or the rapid increase of water level. When the rainstorm or snowmelt occurs in the basin, the runoff is gathered at the exit section of the river according to its distance. As the nearby runoff arrives, the flow of the river begins to rise, and the water level rises accordingly.
When most of the high intensity surface runoff reaches the exit section, the maximum discharge is called peak flow, and the corresponding maximum water level is called the peak water level. After the storm stopped, the river surface runoff and the amount of water stored in the ground, surface soil and river network have all flowed out of the outlet section, and the water flow and water level of the river have dropped back to their original state. The curve from the flood to the peak and back down is called the flood hydrograph, and the total amount of water flowing out is called the total amount of flood.