When it rains heavily on a flat area, the rainwater does not drain away and accumulates on the ground. Water flows into low areas from surrounding small elevations. Drainage canals and small rivers are the first to overflow as water levels rise. Floods that occur in this way are called internal floods and are distinguished from external floods that occur when the levees of main rivers break or overflow. In general, internal floods include the overflows of relatively large drainage rivers that have their source in the plain and the overflows of small rivers on plateaus and hillsides into lowlands at the bottom of valleys. Floods caused by internal flooding are particularly problematic in cities and surrounding newly developed urbanized areas. What is called “urban flooding” is the flooding of urban areas, which is intensified by the structure of the city and creates new types of damage, such as the inundation of underground malls.
