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Floods are rare events in which a body of water temporarily covers land that is normally dry. Following from module 3, we will mostly restrict our discussion to floods in rivers, but it is important to note that floods also occur around lakes, wetlands, and the sea coast. Indeed, coastal storm surge is among the most dangerous natural disasters expected to result from global warming and sea level rise. River floods occur naturally and in many cases are beneficial for ecosystem functioning because they allow the river to exchange water, sediment, and nutrients with the floodplain and cause scour and deposition that provides habitat for a wide range of aquatic and riparian organisms. However, floods often threaten human infrastructure and livelihoods and can cause severe economic damages.
River floods are typically caused by excessive rainfall and/or sudden melting of snow and ice. Most rivers overflow their banks with small floods about once every two years. Such are the floods that tend to determine the width and depth of a river channel, as discussed in module 3. Moderate floods might occur once every five to ten years and very large floods might only occur once in fifty or a hundred years. The average time period over which a flood of a particular magnitude occurs is called that flood's recurrence interval, or return period. For example, the very large flood that only occurs, on average, once in a hundred years has a 100 year recurrence interval and is therefore called the 100 year flood. Relating this notion of recurrence interval to the section on probability, above, the recurrence interval is simply the reciprocal of the probability associated with an event (i.e., T = 1/p, where T is the recurrence interval and p is the probability that such an event will occur (or be exceeded), as computed by integrating under the dashed line shown in Figure 4, above the event magnitude of interest). The probability of a 100 year event occurring in any given year is 0.01, or 1%. We should pay careful attention to our terms here. Note that we are talking about the average time period that should be expected between events. Just because a 100 year event happened last year, there is nothing that says it can't happen again this year. In fact, the probability of two 100 year floods occurring in back-to-back years is 0.01 times 0.01, or 0.0001. This suggests that, if everything stays the same, the 100 year event should happen in back-to-back years about once every 10,000 years. Of course, over 10,000 year time periods most things don't stay the same. We'll discuss this issue, termed non-stationarity, towards the end of the module.
Flash floods are typically caused by heavy rains falling on soils that are already wet or frozen (and therefore have limited capacity to absorb more water), or on land that is covered by snow (in which case the soil has limited capacity to absorb water and the situation is compounded by the fact that melting snow adds to the runoff). Flash floods allow very little time for people downstream to be warned and are therefore especially dangerous. For example, flash floods caused by excessive rainfall from Tropical Storm Washi in the Philippines in December 2011 killed over 1200 people and caused tens of millions of dollars in damages.
Human activities such as development in the watershed and modifications of the river channel can change the frequency and severity of floods. In particular, expansion of urban areas can increase the frequency, magnitude, and flashiness of floods. This happens because impervious surfaces (roads, parking lots, and buildings) route precipitation directly to stream channels and prevent draining of water slowly through soils to groundwater (Figure 5). The term flashiness refers to the rate at which the water levels rise and fall with faster rising and falling water levels considered flashier.
Source: Image from the Federal Interagency Stream Restoration Working Group, 1998.