Floods and Flooding

A flood occurs when a stream channel can no longer contain the water moving through it. Floods usually are local, short-lived events, others can be catastrophic, happening with little or no warning. Floods are most often caused by prolonged rainfall that saturates the ground causing surface runoff into nearby streams increasing their discharge. Flooding occurs when the water spills out of the channel and on to the adjacent terrain. Though viewed as a "natural hazard" to humans, flooding is a natural, rejuvenating process.

Causes and Conditions

Generally speaking there are two types of floods, 1) where water slowly rises and spills over the banks of a stream or river and 2) flash floods. Floods can occur at anytime of year, but particular seasonal weather patterns are more conducive to the creation of floods than others in different geographic regions. In the United States, cyclonic storms roaring off the ocean and into the Pacific coast states during the winter and early spring can cause flooding. In the southwest, summer and fall thunderstorms release torrents of water that rush down dry stream beds or arroyos as flash floods. Flooding can occur in the north central states during the winter as rain fall or snow melt runs off the frozen ground surface, or ice jams rivers causing them to flood. Flooding in the mid portion of the United States tends to occur in spring and summer as polar front cyclones march across the North American continent. Hurricanes and large convective complexes create flooding in the late summer and fall along the Gulf coast of the United States.

Figure 18.20 Flood Season in the United States Courtesy USGS (Source )

In tropical regions like Bangla Desh, monsoon rains saturate the ground causing severe flooding.  Europe floods from the sea may occur as a result of Atlantic storms pushing water to the coast and can be particularly damaging when occurring at high tide. Deforestation greatly increases the risk for flooding.

Figure 18.21 Flooding in Rochester, MN (Source: :cjohnson7 on Flickr)

Some of the worst flooding in decades also occurred during the summer of 2007. Flooding in Britain was caused by a persistent jet stream sitting further south than usual at this time of the year. System after system pounded the British Isles. During August of 2007, widespread flooding occurred in the midwest United States causing damage exceeding $115 million. A warm front advanced northward into Iowa and Illinois during mid-August where it stalled and became a stationary front. Warm, moist air overrunning the front provided the ingredients for showers and thunderstorms. Rainfall over the weekend of August 18-19, 17 inches of precipitation fell in Witoka, MN. Rochester, MN measured 6.9 inches of rainfall. The heavy rain sent streams in the region over their banks. Moisture from Tropical Storm Erin to the south enhanced the stormy conditions.

Figure 18.22 Weather Conditions during the historic rainfall and flooding event of August 18-20, 2007
Courtesy NOAA (Source)

Flood Frequency

Figure 18.23 100- Year Flood Plain. Courtesy USGS (Source )

Using frequency analysis, one can estimate the probability of the occurrence of a given flood event  The recurrence interval, also known as the return period, is based on the probability that the given event will be equaled or exceeded in any given year. For instance, a one hundred year flood has a 1% chance of occurring in any given year. One hundred year floods are rare but can be devastating. The 100-year flood plain is used for flood plain management and insurance purposes. Those living within this zone are often required to have flood insurance in addition to their regular home owner's insurance.