The scientific explanation of the occurrence of floods is simple; an excessive presence of water that overwhelms the absorption rate of the ground. All floods follow this principle, though many cases are due to large amounts of water that already exist before the flooding rather than small amounts accumulating to flood level. The explanation of the physical aspects behind this accumulation or exposure to the vast quantities of water will be explored in its relationship to the time in which flooding occurs.
Accumulation flooding occurs when rain or snow melt lead to small amounts of water that slowly meet with other quantities to form more water than the land can absorb or handle. This can be a slow process that results in flooding from over absorption, leading to saturation floods, or a fast process that absorbs little and accumulates quickly, otherwise known as flash flooding. Both cases are determined by the rate of rainfall and the state of the soil or ground surface.
In areas of forest or grassland conditions the top layer of the soil in which the plant roots penetrate are loose and have many air pockets between particles of organic matter and rock. These air pockets are necessary for quick absorption rates of water, whether from rain or melting snow. Beneath the top layer of soil are other layers that can contain sand and gravel, which will aid absorption, while areas of clay or bedrock will prevent the absorption of water. These layers determine how much water the land can hold, and the depth of the water table.
The water table is where the permanently water saturated soil is. The water table slopes with the land and in low areas meets with the surface in streams, rivers, ponds, and lakes. The higher the water table, the more likely the height of surface waters in the corresponding bodies of water is higher. Although the infiltration of the water from the top layer of soil down to the water table is slow, this rate of absorption and depth of the water table determine how much water the land can hold.
In times of steady rain, over a long period of time, the water saturated area of top soil prevents more water from being absorbed further, and the excess water runs freely over the surface. This free flowing water is the first stage of flooding through accumulation. As the free surface waters meet with streams and rivers, the water height rises. When the smaller streams can no longer drain into the rivers, they back up and run over their banks. This is the second stage of flooding, as the small streams and creeks continue to back up. If the river itself encounters an inability to move its water along its course, it will over run its own banks and create the final stage of localized flooding. Eventually the water level doesn’t receive any more water to maintain its height and recedes.
In the case of flash flooding, which occurs in arid land or urban environments, the ground is unable to absorb rainfall at all and the rate is very quick. Quick downpours from summer thunderstorms or hurricanes produce water that quickly becomes free surface water flows. These flows soon overwhelm natural and artificial means of containment and form large flows that can be very quick and damaging. Unlike saturation flooding, flash floods do not linger, and quickly dissipate once the rain stops.
In exposure flooding, and area is exposed to an amount of water that is already large and impossible to absorb. This can occur from events such as tsunamis, storm surge, dam breaks, and sea level rise. These events result in destructive flooding with few means to avoid their damaging effects.
In the case of Tsunamis, the March 11th, 2011 earthquake pushed large pulses of water into the shore of Japan. The Sendai area experiences the brunt of the flooding, and images from the event proved that the only thing that could be done is to get out of the way. This form of exposure creates walls of water that result in nothing but flooding. Once the tsunami ends, the water pushed inland is able to drain back into the ocean.
Storm surges are similar to tsunamis but on a smaller scale. They require large storm systems such as hurricanes to push the ocean onto the land and up rivers at a continuous rate. This effect results in flooding from the sea water and fresh water rivers and stream that cannot drain into the ocean. After the storm is past, the water flow returns to normal.
Dam breaks can be both natural and manmade. In these cases, large quantities of water are kept at bay by a wall of anything from dirt, to concrete, to wood, to ice. When these walls fail, the areas downstream from the dams are quickly exposed to a quantity of water they cannot overcome. Flooding persists until the water behind the dam is out and able to pass downstream beyond the afflicted area.
The last case, dealing with sea level rise, is the only flood that does not have an end at this time. Low lying islands and coastal areas experience poor drainage of rains and surface waters as the sea level rises. Eventually the land is overwhelmed and consumed by the rising waters. This kind of flooding, as stated before, does not have an ability to be resolved through natural passage of time, but can be put off through use of levees and pumps. Area such as Holland and some cities use these methods to keep the land dry, but continue to sink beneath sea level. It is only able to work for so long before it is unable to prevent the floods.
In conclusion, floods are mainly a natural phenomenon that is simply too much water in one place. Through different uses of the land and events that occur within them, the type of flooding can vary, but will always occur without the means to overcome or prevent them. Floods don’t have to happen, but knowing about why they occur helps to prevent the damage that doesn’t have to.