Flash Floods – Causes and Remedies
A flash flood is a rapid inundation of geomorphically low-lying areas: washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields. Flash floods are known to occur in the highest mountain ranges of Uttarakhand and are also occasional occurrences in the arid plains of Rajasthan. In June 2013, a multi-day cloudburst centered over Uttarakhand caused devastating floods and landslides, becoming the country's worst natural disaster since the 2004 tsunami. The flash floods which were triggered by very heavy rainfall and cloudburst on 16-17 June 2013, affected 12 out of the 13 districts in Uttarakhand. The 4 districts that were worst affected were Rudraprayag, Chamoli, Uttarkashi and Pithoragarh. The deluge washed away roads, bridges and other infrastructure. About 1000 people were reported dead and many more went missing. In Kedarnath alone about 75,000 pilgrims were stranded due to flood triggered landslides. In August 2016 the army launched operations to rescue villagers stranded in flash floods in Rajasthan where heavy rains had killed 11 people, including six children and rendered hundreds homeless in four districts – Pali , Jodhpur, Bhilwara , Chittorgarh – where low lying areas were completely inundated.
Urban Areas are also prone to flooding in short time-spans and, sometimes, rainfall (from the same storm) over an urban area will cause flooding faster and more-severe than in the suburbs or countryside. The impervious surfaces in the urban areas do not allow water to infiltrate the ground, and the water runs off to the low spots very quickly. Flash Flooding occurs so quickly that people are caught off-guard. Their situation may become dangerous if they encounter high, fast-moving water while traveling. If people are at their homes or businesses, the water may rise quickly and trap them, or cause damage to the property without them having a chance to protect the property. Flash Flooding is possible everywhere. The normally tranquil streams and rivulets in your neighborhood or area can become raging torrents during a heavy downpour – or even upstream of your location. Flash Flooding can also occur on city streets and highway underpasses.
What are the main causes of floods?
Flooding occurs most commonly from heavy rainfall when natural watercourses do not have the capacity to convey excess water. However, floods are not always caused by heavy rainfall. Flash Floods can be caused by a number of things, but is most often due to extremely heavy rainfall from thunderstorms. Flash Floods can occur due to Dam or Levee Breaks, and/or Mudslides (Debris Flow).
The intensity of the rainfall, the location and distribution of the rainfall, the land use and topography, vegetation types and growth/density, soil type, and soil water-content all determine just how quickly the Flash Flooding may occur, and influence where it may occur.
Flash floods occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or sudden release of water held by an ice jam. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more.
Deforestation plays many roles in the flooding equation because trees prevent sediment runoffs and forests hold more water than farms or grasslands. The flood equation is simple. If a river cannot handle the load of water it is required to carry, it will rise above its banks. This is when floods occur.
Human factors increasing flood risk: Urbanisation, because towns and cities have more impermeable surfaces. Deforestation, because removing trees reduces the amount of water intercepted and increases run-off.
How can we avoid flash floods?
Control of Floods: Some methods of flood control have been practiced since ancient times. These methods include planting vegetation to retain extra water, terracing hillsides to slow flow downhill, and the construction of floodways (man-made channels to divert floodwater).
During heavy rains, trees reduce the risk of flooding. There are two major ways in which trees provide protection against flooding. Experts say that woodland acts as a barrier to floodwater, while trees also prevent soil erosion, reducing sediment going into rivers and increasing water absorption into the ground.
Other methods include construction of levees, dams, reservoirs and channels diverting floodwater, called floodways.
What to do during a Flash Flood:
1. Bring in outdoor furniture and move indoor furniture to a higher floor if possible.
2. Unplug all electrical appliances.
3. Turn off your gas and electricity at the main valve (if instructed) to help prevent fires and explosions.
4. Climb to the highest place you can.
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Department of Geology
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