Flood:
One of the most common weather-related hazards around the world, flooding is the overflow of water onto land that is usually dry. Often, this is caused by excessive rain and/or dam failure. However, floods can also be caused by a variety of other processes that have little to do with ongoing meteorological conditions, such as earthquakes, tsunamis and glacial lake outbursts (see Chapter 6).
Flooding damages homes and crops, and it can lead to the spread of disease. The water-borne bacteria that cause hepatitis A and cholera, for example, tend to spread after flood events, as do the mosquitoes that carry malaria. Floodwaters also contain sewage, chemicals and pesticides that can poison the water supply or harm delicate ocean ecosystems such as coral reefs.
Despite their destructive power, floods are necessary for many ecosystems to function properly. In fact, many of Earth’s most fertile regions are floodplains along rivers and streams.
Within-trend flooding surfaces are lithological contacts of lesser stratigraphic significance than the transgressive systems tract boundaries that mark maximum flooding surfaces at lower hierarchical ranks (Fig. 6.61). For instance, the flooding surface at the contact between a transgressive shoreface sand and the overlying transgressive shelf shales at the site in Western Canada displays no lag deposits or other evidence of scouring. Consequently, the flooding surface is not a system contact and cannot be used to identify a tectonic discontinuity. However, this type of flooding surface can be useful in interpreting sedimentation patterns and the stratigraphic sequence of a depositional environment.
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