Be prepared for flash floods


By Thushara Dissanayake

(Charted Civil Engineer)

A short time after the onset of rain, you may have witnessed rapidly rising river or stream water levels or quick inundation of roads in urban areas barring the traffic, and then these are indications of probable minor flash floods. Major events of flash floods are considered severe natural disasters with nearly 5,000 lives lost annually besides economic and environmental damages.

According to the World Meteorological Organization, a flash flood can be defined as any flood that develops in less than six hours. A flash flood can occur almost anywhere but more commonly in low-altitude areas with insufficient rainwater drainage systems. However, the time thresholds may vary across regions depending on characteristics such as land use, topography, soil type and many other factors relevant to the region

Although there have been no significant flash flood events in Sri Lanka, to date, there have been many of them elsewhere. In 1889, Johnstown in Pennsylvania experienced a flash flood when a dam failed after prolonged heavy rainfall, resulting in the deaths of more than 2,000 people. Big Thompson flood in Colorado in 1976, due to an extremely heavy rainfall in the steep mountain Canyon, caused more than 140 fatalities.

When the higher elevations of a river catchment receive high intensity, rainfall flash floods can occur downstream. Similarly, urban flash floods can occur when drainage systems cannot remove the fast accumulating surface water quickly. With the ongoing climate change the chances are higher of flash floods in Sri Lanka as well; the flood caused by the overflowing of the Kelani Ganga, in 2016, is considered by some experts as a flash flood.

In addition to heavy rains, dam or levee failures, coastal storms and tsunami can also cause flash floods, though other causes like snow melts are not relevant to Sri Lanka. Densely populated areas close to rivers are more prone to flash floods than others. They can experience flash floods due to the paved surfaces like roads, parking areas, concrete drainage systems and buildings, which can prevent the soil from absorbing rainwater.

When river catchments consist of steep slopes, chances are higher of flash floods and the situation becomes worse if these catchments are without a good forest cover. In that sense, Kelani, Gin, Nilwala and Kalu Ganga have similar catchment characteristics and are prone to flash floods during heavy rains. More often than not, it is very difficult to issue early warnings regarding flash floods based on traditional flood forecasting approaches.

Safety measures

Basically, damage from floods occurs either due to rising water level or rapidly flowing water or both in some cases. Rapidly flowing water has a momentum, which is strong enough to uproot trees, sweep away people and vehicles and damage infrastructure.

Following are some useful safety measures to be adopted in case of flash floods.

When signs of a flood such as extremely heavy rain emerge people in high risk areas have to evacuate immediately and move to higher ground or the upper floors of building previously identified as safe locations, in order to prevent injuries or death. In that sense, you have to have access to the media to get updates on prevailing and expected weather conditions, flood forecasts, and instructions by the relevant authorities.

It is advisable to stay away from streams, rivers, and large scale storm water drainage lines. Walking, swimming and driving through flood waters must be avoided. Generally, six inches of moving water is enough to knock down a person and one foot of moving water can sweep a vehicle away. Staying on bridges over fast-moving water is also dangerous as flash floods are strong enough to cause bridge failures.

If flood water, not flowing fast, is rising around your vehicle, leave the vehicle and quickly move to higher place. If your vehicle is dragged by the moving water, best thing is to stay inside. In a situation, where inside of the vehicle is getting filled with water, get on to the roof of the vehicle. If you are trapped in a building, then climb up to its highest floor; get on to the roof, only if necessary. Always follow relevant authorities’ advice and never return to your homes until they say they are safe.

Things to be done after floods

Before entering a flood affected building look for possible structural damages. Thoroughly examine walls, slabs, ceilings, doors, and windows as there is the risk of their collapsing. Be extremely cautious when entering the building and it is a must to wear safety shoes, as foot-cut injuries are common.

Another risk of entering a flooded building is electrocution and you should not touch any switch or electrical equipment, if it is wet, or if you are standing on a wet floor. Therefore, disconnect the electricity supply to the building before commencing cleaning works, and use a non-electric source for illuminating the building, if required.

There can be poisonous snakes, other animals and dangerous debris in the building and it is advisable to wear thick gloves and safety boots during clean up. It is noteworthy that children are more vulnerable to toxic exposures that may result from a flood disaster and should not be allowed to enter the affected areas until clean-up work is over.

Drinking water contamination is another key issue mainly due to petroleum products, chemicals, and waste in flood waters. Until the drinking water sources are confirmed safe by the public health officials it should not be consumed.

With the ongoing technological innovations, extending accurate warnings on flash floods will soon be a reality. So, we can hope that flood hazards will not lead to disasters in the future.


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