Heavy rains and floods
Heavier rains in recent years are one of the hallmarks of climate change. As the atmosphere warms, more water evaporates from soils, plants, lakes, and oceans. So when this additional water vapor condenses into air, it leads to heavier rain, or when cold enough – to heavier snowfall. More downpours increase the risk of flooding. This expands the risk for areas that were previously safe from floods. Also, heavier rain increases erosion and runoff, removing agricultural topsoils and increasing the flow of pollutants into water pools.
Flooding is an overflowing of water onto land that is normally dry. This is the most common and widespread weather-related natural disaster. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts quickly, or when dams or dikes break. Floods can occur within minutes or over a long period, and may last days, weeks, or longer.
Flash floods are the most dangerous kind of floods. They occur when heavy rainfall exceeds the ability of the ground to absorb it. They can also occur in ravines where the incoming water exceeds the shores for a short period of time. Flash floods can happen within minutes, limiting the time available to warn and protect the public.
Densely populated areas are at a high risk for flash floods. The construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. This runoff increases the flash flood potential. Usually the rainwater flowing in the cities is directed to the city sewage system. Sewage system can become clogged during heavy rain leading to flooding of roads and buildings. Low spots, such as underpasses, underground parking garages and basements can become death traps.
Areas near rivers are at risk from floods. Embankments, known as levees, are often built along rivers and are used to prevent high water from flooding bordering land.
Dam failures can send a sudden destructive surge of water downstream.
Camping or recreating along streams or rivers in a mountainous area can be a risk if thunderstorms occur in the area. Mountain slopes and steep hills cause rapid runoff, which causes a rapid rise in the level of running water. Rocks and shallow, clayey soils do not allow much water to infiltrate into the ground. Occasionally, storms that cause heavy rainfall can occur far and wide of the campsite, making it harder to recognize a dangerous situation.
Dry canyons are often not easily identified as a source of danger. Heavy rainfall can occur upstream of the canyon and tourists can be trapped by the rapidly rising water, carrying debris that poses significant risks to life.
Melting snow cover can also contribute to flooding caused by ice jams. In winter, thick layers of ice often form on streams and rivers. Melting snow or falling warm rain can lift and break this ice, allowing large chunks of ice to get stuck under bridges or other elements of river infrastructure. This causes the water to rise quickly, causing severe flooding.