What causes flooding? FIG F-2: Image courtesy of Photodisc A flood typically occurs when a river (or other body of water) overflows its banks. As you can read in Physical Geography: The Global Environment, Canadian Edition, annual floods can even be a normal part of a floodplain's development. These floods deposit sediments that build a river's natural levees (broad ridges that run along both sides of the channel). Figure F-3 shows the relationship between floods and natural levee development. As the river spills out of its channel, the coarsest material it is carrying is depostied closest to the overflow, hence along the levees. When the river contracts after the flood, it stays within its self-generated levees.     FIG F-3: Image from Physical Geography   However, not all floods are so regular and productive. Infrequently - perhaps once in a century - a river may experience a flood of such magnitude that its floodplain is greatly modified. Water up to several metres deep may inundate the entire floodplain, destroying submerged levees, eroding bluffs, and disrupting the entire system. These sorts of floods have cost millions of lives in the densely populated floodplains of Asia's major rivers. Canada, where the damage can also be enormous. No reinforcement of natural levees or construction of artificial levees can withstand the impact of a powerful "100-year" flood.       What are the different types of floods? a. Flash Floods. Flash floods can occur in several seconds to several hours, with little warning. Flash floods can be deadly because they produce rapid rises in water levels and have devastating flow velocities.   Several factors can contribute to flash flooding. Among these are rainfall intensity, rainfall duration, surface conditions, and topography and slope of the receiving basin. Urban areas are susceptible to flash floods because a high percentage of the surface area is composed of impervious streets, roofs, and parking lots where runoff occurs very rapidly. Mountainous areas also are susceptible to flash floods, as steep topography may funnel runoff into a narrow canyon. Floodwaters accelerated by steep stream slopes can cause the floodwave to move downstream too fast to allow escape, resulting in many deaths. Canada experiences, on average, 40 flash floods a year. In terms of quantity of rain over a short period of time, the worst flood in Canadian history involved two hundred and fifty-four millimetres of rain falling in under an hour from clouds described as 'brownish, pinkish, and greenish'.   Floodwaves more than 30-feet high have occurred many miles from the rainfall area, catching people unaware. Even desert arroyos are not immune to flash floods, as distant thunderstorms can produce rapid rises in water levels in otherwise dry channels. Early-warning gages upstream save lives by providing advanced notice of potential deadly floodwaves.   FIG F-4: Image courtesy of NOAA   b. Ice-Jam Floods. Ice-jam floods occur on rivers that are totally or partially frozen. A rise in stream stage will break up a totally frozen river and create ice flows that can pile up on channel obstructions such as shallow riffles, log jams, or bridge piers. The jammed ice creates a dam across the channel over which the water and ice mixture continues to flow, allowing for more jamming to occur. Backwater upstream from the ice dam can rise rapidly and overflow the channel banks. Flooding moves downstream when the ice dam fails, and the water stored behind the dam is released. At this time the flood takes on the characteristics of a flash flood, with the added danger of ice flows that, when driven by the energy of the floodwave, can inflict serious damage on structures. An added danger of being caught in an ice-jam flood is hypothermia, which can quickly kill. At Cap-Rouge in 1874 the St. Lawrence River flooded (sinking many boats) because of an ice-jam flood.   c. Storm-Surge Floods. Storm-surge flooding is water that is pushed up onto otherwise dry land by onshore winds. Friction between the water and the moving air creates drag that, depending upon the distance of water (fetch) and the velocity of the wind, can pile water up to depths greater than 20 feet. Intense, low-pressure systems and hurricanes can create storm-surge flooding. The storm surge is unquestionably the most dangerous part of a hurricane as pounding waves create very hazardous flood currents. Nine out of 10 hurricane fatalities are caused by the storm surge. Worst-case scenarios occur when the storm surge occurs concurrently with high tide. Stream flooding is much worse inland during the storm surge because of backwater effects. Storm-surge floods are very dangerous. In September 2003 the storm surge associated with Hurricane Juan caused severe damage to property on the Halifax waterfront, as well as washed many objects into the sea. In the United States, half of the deaths from hurricanes are because of storm surges.   d. Dam- and Levee-Failure Floods. Dams and levees are built for flood protection. They usually are engineered to withstand a flood with a computed risk of occurrence. For example, a dam or levee may be designed to contain a flood at a location on a stream that has a certain probability of occurring in any one year. If a larger flood occurs, then that structure will be overtopped. If during the overtopping the dam or levee fails or is washed out, the water behind it is released to become a flash flood. Failed dams or levees can create floods that are catastrophic to life and property because of the tremendous energy of the released water.   e. Debris, Landslide, and Mudflow Floods. Debris or landslide floods are created by the accumulation of debris, mud, rocks, and (or) logs in a channel, which form a temporary dam. Flooding occurs upstream as water becomes stored behind the temporary dam and then becomes a flash flood as the dam is breached and rapidly washes away. Landslides can create large waves on lakes or embayments and can be deadly. Mudflow floods can occur when volcanic activity rapidly melts mountain snow and glaciers, and the water mixed with mud and debris moves rapidly downslope.     Where do floods occur? Floods can occur nearly anywhere on Earth. The NASA web site contains a real-time map showing where floods are currently taking place throughout the world, as well and links to NASA photographs of the flood conditions.