How Flooding Occurs

Flooding will either occur naturally or because of how people have altered the environment. Before these two are explained, it is probably best to provide the details of the path that water takes. Everyone knows that rain falls from the sky but where does it go after that?

Basics of flooding

Before water enters rivers or larger bodies of water, it usually has quite a journey and is hindered along its way. Rainfall first either hits water bodies, the ground, or trees and other plant life. The leaves of trees and plants can intercept rainfall and the water remains on the leaf surface. The water will either be absorbed by the leaf, fall off the leaf at a later time, or evaporate back into the atmosphere. At the same time, rain fall hitting the ground can run long the surface of the ground, seep into the ground, or even start pooling in areas that are flat or concave.

Water seeping into the ground will only do so until the ground has become saturated. Only a certain amount of water can enter the ground before the voids in the soil become full and the ground can no longer hold anymore water. After that, any extra water will either pool or travel elsewhere. The water in the saturated ground slowly seeps out into local waterways until the ground returns to its original condition before the storm.

The water that does not evaporate or seep into the ground will travel either on the surface to increasingly larger bodies of water that may or may not eventually lead to the ocean. The trees, grass, and other plants on the surface will also absorb water through their roots in the ground.

With that explained, the water that does not get intercepted, evaporate, or seep into the ground is called runoff and has the potential for causing a flood. There are three methods by which floods occur naturally. The first is from rainfall and the second is from thaw (melting of snow and ice), and the third is from natural disasters.

1- Rainfall

Rainfall can cause flooding naturally if the storm produces a large amount of water in a very short time, which is referred to as a flash flood, or if the rainfall lasts for a very long time. The longer the rainfall, the more water is intercepted and at a certain point, no more water can be intercepted and all of the rainfall becomes runoff. Runoff will fill up rivers and streams until they overflow onto the flood plain or perhaps even beyond that. A flood plain is an area around a body of water that is prone to flooding if a large enough storm occurs. The exact amount of rainfall that will cause flooding can be determined for different areas based on many different factors.

Excessive rainfall in higher mountains that comes down into valleys can also cause flash floods. Water coming down a mountain can accumulate very fast and completely overflow most waterways. Once it reaches the valley, it is a torrent of water and can flood the area for a short time.

2- Melting snow

The melting of snow and ice has the potential to cause flooding. This can because of snow and ice that have accumulated during the winter months and that start thawing in the spring and summer months. Flooding such as this usually occurs in areas with higher elevations such as mountains. It is seasonal and depends on the snow accumulation and temperatures. The melted snow/ice will travel down the mountain and can flood rivers and streams.

A large amount of snow that melts extremely fast because of rising temperatures can produce enough water to flood. This depends on how fast the snow melts over a given time. Ice can also clog or dam up rivers and streams to cause flooding.

3- Natural disaster

Natural flooding can also occur because of disasters such as hurricanes and tsunami. Most of the effected area is right along the coast but these disasters can flood quite far inland depending on the waves and size of the storms. While hurricanes are tracked with ahead of time and the areas affected are plotted out, a tsunami can have very little warning at all for those living near the coast and occur very suddenly.

Flooding because of changes by people

Flooding will also occur because of the changes made by people to their environment. In a city or even suburban landscape, water acts very differently. Impervious structures are created by people all the time and unlike the ground or trees, they don’t allow any water to infiltrate or be absorbed by them. They include, the roofs of buildings and houses, streets, parking lots, and even the disturbing or compaction of the ground (possibly caused by construction) will cause a semi-impervious area that will limit the amount of water that is intercepted. These surfaces can’t absorb any water and they even help water travel faster because they are smoother surfaces with less roughness than the ground.

The problem with these surfaces is that they not only create more runoff in a given area but they also increase the speed of the runoff. So not only will the water volume be greater but it will be traveling faster to its destination, which will still be bodies of water. This is further increased by the construction of artificial conveyances like storm drains and other features that get the water out faster. These conveyances are created because so many surfaces are impervious and no one wants water to start pooling in a parking lot or on roadways. The problem is that while one area might be fine, flooding might occur just downstream because of the increase in runoff and less time it takes to travel. While those upstream are fine, those downstream experience flooding.

Water conveyance structures are only built to accommodate a given storm. Water data is collected and the sizes of storms are figured out. Based on this data, structures like storm drains, dams, ditches, culverts and other water conveyance structures are created to handle a given storm. You might be wondering why they don’t make them to handle the biggest storm possible, and that has to do with cost. It costs increasingly more to build for a very large storm and storms that large or often very rare. People don’t want to pay that kind of money if there is a very slim chance it will ever occur and engineers won’t design that big. The size of the storms and the design are based on historical data of the rainfall in the area as well as calculations. With a big enough storm or a storm that produces a lot of runoff in a short amount of time, even a good sized town can flood because the constructed conveyances can’t handle it.

Even structures like a dam or levee system have the potential for failing because of a large enough storm or continuous rain. Dams break because they fill up too fast and can’t hold all of the water back. The sudden release of the water being held back by a dam can cause extensive damage and flooding to a downstream area and it would occur so fast that no one could prepare for it. A levee system can fail because it can no longer hold back all of the water behind it or because the water had seeped underneath the structure and destroyed the levee foundation, which would cause it to collapse.

Thus, flooding occurs in many different ways. It occurs naturally from features of the land and the precipitation experienced by an area as well as occurring because of the changes that people have made to their environment. Flooding will continue to be a problem because there is no solution that will completely prevent it. When you get a big enough storm, enough snow or ice, natural disaster, or even enough rain in a very short time, there will always be the chance of a flood occurring.

Other sources:

Mays, L. W. 2005. Water Resouces Engineering. John Wiley & Sons, Inc.