Science And Environment: How Floods Work

Flooding occurs when rivers and other waterways are filled beyond capacity and excess water has to find a new pathway.

In order to understand the dynamics of flooding, it is essential to realize how relentless water can be. As soon as water makes contact with the earth, it is forced by the pull of gravity to seek the lowest level possible. This could mean soaking into the ground or even into the crevices of rocks. If the water is not absorbed into the ground, it will continue to seek out a lower level. After years of continuous flow, the water will carve out natural troughs called tributaries. These tributaries eventually meet and form larger troughs called streams or creeks. In turn these streams may form large rivers which can carry significant amounts of water. These rivers must have outlets, which may be oceans or lakes. Eventually water levels are controlled by evaporation or absorption into the ground. Every step along the way, from raindrop to stream to ocean, has developed a natural capacity and this balance is usually maintained.

But occasionally this natural order is changed by man. Many large rivers would regularly overflow their banks and create problems for settlers. Navigation of these rivers was nearly impossible and the resulting floods would destroy everything in their paths. Government engineers eventually developed plans for taming these wild rivers. Large dams were constructed at strategic points along the river's path, allowing engineers to control the flow of water through spillways located on either side of the main dam. Water which backed up behind the dam would form large man-made lakes. If a large rainstorm or melting ice added more water upstream, then the spillways would be opened in a controlled pattern. Random flooding would theoretically be a thing of the past after dams were installed.

But flooding still occurs, primarily because of water's relentless nature. In a flood, water from a rainstorm or melting snow collects at the headwaters of a river, generally located in a mountain system. The ground at the top is quickly saturated, sending the excess water rapidly downhill. Tributaries cannot handle this excess, so the water finds new paths to the lower levels. Eventually the river receives all of this water, plus whatever rain continues to fall. As the rate of flow increases, the river water puts tremendous stress on the ground near the banks. Any object located too close to the current may be pulled into the river, possibly causing logjams and other diversions. The diverted water may flow over the riverbank and into low-lying areas. If the area is already saturated by rain, the additional flood waters will flow further inland and overwhelm storm drains. Streets will soon be completely under water, and houses become swamped as the water seeps into basements through any opening in the foundation.



Meanwhile, the river will continue to rise in volume until it has reached a level called the 'flood stage'. At this point, the water will overflow the banks and add to the problems inland. The river will continue to rise until it has reached its maximum depth, called the crest. This may not occur for several days, which can mean that flood waters inland will have nowhere to go. Eventually the storm system or the snow melt should subside, but areas downstream will continue to experience flooding. Once the river has crested and the water level begins to fall below the flood stage, street flooding is reduced through storm drains. Home owners may have to assess damage and pumps are often needed to remove excess water. City engineers often examine the effects of flooding in order to prevent similar disasters in the future. Rivers may be dredged to create deeper channels or the banks may be reinforced with artificial reefs or levees. In an emergency situation, sandbags may be employed in an effort to redirect floodwater from populated areas. Homes and businesses located in areas prone to flooding may have to carry extra insurance for repairs, or owners may be encouraged to relocate on higher ground.

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