Flash flood safety tips: before, during and after

A flash flood leaves little time to think. In a hill valley, a quiet stream can become a torrent within minutes, sometimes with no rain falling where you stand [6]. These flash flood safety tips come from the National Disaster Management Authority's (NDMA) do's and don'ts and from national weather and emergency services, for anyone who lives, travels, treks or drives in the hills.

Why are flash floods so dangerous?

Flash floods are dangerous because they rise fast, often far from the rain that causes them, with very little warning. The World Meteorological Organization (WMO) describes them as rapidly rising flood waters from excessive rainfall or a dam break, developing within a few hours of the rain, usually in mountains [4]. The failure of a natural dam of debris or ice can also release a large volume of water suddenly [4]; see glacial lake outburst floods.

Three features catch people out:

• The rain may be somewhere else. Distant rain is channelled into gullies and ravines and can turn a quiet stream into a torrent in minutes [6].
• They wash away what you rely on. Flash floods commonly wash away houses, roads and bridges over small streams [4].
• They often strike in the dark. In the United States, the National Weather Service (NWS) reports that most flash flood deaths happen at night, and most victims are people trapped in vehicles [6].

A study cited by WMO found that about 4 % of people affected by flash floods died, against less than 1 % for other floods [4]. Why the warning time is so short is explained in flash floods vs river floods.

Before: how to prepare for a flash flood

Know whether you are at risk

NDMA asks people to be aware of flash flood areas such as canals, streams and drainage channels [1]. If your house, hotel or campsite sits beside a stream, you are in one. Learn the safe route to the nearest shelter or raised pucca house, and make a family communication plan [1].

Know the warnings

The India Meteorological Department (IMD) issues weather warnings for each district in four colours [3]:

• Green: no warning, no action needed.
• Yellow: watch; be updated.
• Orange: alert; be prepared.
• Red: warning; take action.

The colour reflects both how likely the event is and its expected impact [3]. An orange or red heavy-rain alert for your district is the time to check your plan. River gauges have their own thresholds; see warning level vs danger level.

NDMA advises keeping phones charged, using SMS in an emergency, following radio and TV, ignoring rumours, and evacuating immediately when officials tell you to [1]. If your village has a siren or loudspeaker system, learn what its signal means and treat it as that instruction. See also early warning dissemination.

Keep an emergency kit ready

NDMA's emergency kit [2] includes a battery torch, spare batteries and a battery radio; a first aid kit and essential medicines; dry food and sealed drinking water; candles and matches in a waterproof container; a knife; chlorine tablets; documents (ration card, voter ID, Aadhaar) and cash; thick ropes; and shoes.

Keep documents in waterproof bags, store ready-to-eat food and water for at least a week, and keep cattle untied for their safety [1].

How to survive a flash flood?

To survive a flash flood, move to higher ground immediately and stay out of the water. If you are outdoors, never enter floodwater on foot or in a vehicle, and if you notice even a slight rise in water level, climb [6].

Never walk or drive through moving water

The NWS "Turn Around Don't Drown" guidance gives these depths [5]:

NDMA agrees: do not walk or drive in floodwater, because two feet of moving water can wash away big cars [1]. In the United States, more than half of flood drownings happen when a vehicle is driven into floodwater [5], and the road beneath may have collapsed [5]. Turn around and find another way [6].

If you must cross shallow water, NDMA says not to enter deep or unknown water and to check the depth with a stick [1]. Keep away from drains, culverts and gutters, and from electric poles and fallen power lines [1].

How to survive a flash flood in your house?

The safest choice is to leave early for higher ground or a safe shelter [1][7]; stay inside only if the water already blocks every safe route. If you must leave, NDMA advises switching off power and gas, raising furniture and appliances, and taking your emergency kit, documents and valuables [1].

If you are trapped by the water:

• Go to the highest level of the building [7].
• Do not climb into a closed attic or loft, where rising water can trap you [7].
• Go onto the roof only if necessary, and signal for help once you are there [7].
• Stay away from electricity. Do not touch electrical equipment if it is wet or if you are standing in water [7].

Flash flood safety for trekkers and campers in the Himalaya

Trekkers, pilgrims and campers often walk and camp on valley floors and stream banks, where a flash flood arrives first. NWS advises never camping on low ground next to a stream, because a flash flood can reach you while you sleep [6]. Remember that the rain that matters may be falling on the slopes above you, out of sight [6].

NDMA's landslide guidance applies in the same terrain: avoid low-lying areas and valleys, stay together with your companions, and if you hear unusual sounds such as trees cracking or boulders knocking together, move quickly away from the slide path and the valley below it [1]. NDMA also lists muddy river water among the warning signs [1]. Check the IMD district warning before setting out and before choosing a campsite [3]. Why hill catchments respond so quickly is explained in time of concentration.

After the flood: staying safe once the water falls

The danger does not end when the water falls. Go home only when officials say it is safe [1][7].

• Roads and bridges. A road may have collapsed under the water [5]. In the hills, stay away from slide areas, because more slides may follow [1].
• Drinking water. Drink boiled or chlorinated water; do not drink tap water until the health department says it is safe, and do not drink untreated water from rivers, springs or wells [1].
• Food. Do not eat food that has been in floodwater [1].
• Electricity. Do not use damaged electrical goods until they have been checked, and watch out for broken poles and wires [1].
• Other hazards. Keep children away from floodwater, use mosquito nets, and watch for snakes, which are common after floods [1].

Key points

• Flash floods rise within minutes to a few hours, often from rain far upstream, and often at night [4][6].
• Before: know whether you live, stay or camp near a stream, know the IMD colours and your local warning signal, and keep a kit ready.
• During: go to higher ground at once. Never walk or drive into moving water; 15 cm can knock you down and 30 cm can carry away a car [5].
• At home and trapped: highest floor, never a closed attic, roof only if necessary.
• After: wait for the all-clear, avoid damaged roads, drink only safe water and keep away from wires.

Where ClimMaTech fits

At ClimMaTech we work on getting a warning to people in time to act on these steps. RAKSHAK is a multi-hazard warning siren with a built-in library of 15–20 user-selectable siren tones and voice messages; it is used for flood warning and also for other disasters, civil defence and air-raid warnings, and public voice announcements. More on how sirens fit into a complete system is on our flood warning page.

References

1. National Disaster Management Authority, Be Smart Be Prepared: Disaster Survival Guide (flood and landslide do's and don'ts), Government of India, September 2019. ndma.gov.in
2. National Disaster Management Authority, Floods (emergency kit), Government of India. ndma.gov.in
3. India Meteorological Department, Standard Operation Procedure: Weather Forecasting and Warning Services, Ministry of Earth Sciences, 2021. mausam.imd.gov.in
4. World Meteorological Organization, Manual on Flood Forecasting and Warning, WMO-No. 1072, 2011. old.wmo.int
5. US National Weather Service, Turn Around Don't Drown, NOAA. weather.gov
6. US National Weather Service, Blacksburg Forecast Office, Outdoor safety: flooding, NOAA. weather.gov
7. US Federal Emergency Management Agency, Floods, Ready.gov. ready.gov