5.4 — Flood, and Days of Continuous Rain

Flooding kills more people than most other natural hazards, and the majority of flood deaths involve one of two decisions: driving into water, or walking into it.

Both are made by people who badly underestimate what moving water does.

The numbers that change behaviour

Moving water is far more powerful than anybody's intuition.

About 15 centimetres — ankle depth — of fast-moving water can knock an adult off their feet.

About 30 centimetres — knee depth — is enough to float many cars.

About 60 centimetres will carry most vehicles away, including large ones.

And water weighs about a tonne per cubic metre, moving at the speed of the current.

The phrase used by emergency services everywhere is the correct summary: turn around, do not drown.

The four rules

Never drive into floodwater

The single most common way people die in floods.

You cannot see the road surface, you do not know the depth, and you do not know whether the road is still there — floodwater scours roads out from underneath and the surface can be missing entirely.

Vehicles float and are swept sideways, and once floating you have no control at all.

And a specific and deadly case: an underpass or dip. Water pools deeply in them, the far side is hidden, and drivers who have used the road daily drive in confidently.

Never walk into moving floodwater

Same reasoning. Add: open drains and manholes with the covers lifted off by the water, which are invisible and are a documented cause of death.

If you absolutely must move through shallow standing water, use a stick to probe ahead of each step, and do not do it alone.

Go up, not out, once water is around you

Wind goes down, water goes up (5.1).

If floodwater is already at the property, moving upstairs is far safer than trying to leave through it.

And the one qualification: do not go into a sealed roof space with no way out. **People have drowned in attics with no route onto the roof. If you go up, take something to make an opening with, and go to a roof rather than into a loft where possible.

Assume floodwater is contaminated and electrically live

It contains sewage, chemicals, fuel and debris.

And it may be in contact with live electrics. Never enter water in a building until the power is off.

The flood before the flood

Because "days after days of rain" — as the brief put it — is its own situation.

Prolonged rain produces two things people miss.

Saturated ground stops absorbing. After several days, rain that would previously have soaked away runs straight off, which is why day five of a wet period floods when days one to four did not.

And it produces landslides (5.8). Slope failures overwhelmingly happen during or shortly after prolonged rainfall, and this is a much larger risk in hilly areas than the flooding itself.

What to watch during a long wet spell:

River and reservoir levels, from official sources.

Whether water is standing where it normally drains.

On slopes: new cracks, doors and windows jamming, tilting trees or poles, and any new spring or muddy water appearing from a hillside (5.8).

And urban drains backing up, which is the earliest visible sign in a city.

Flash floods

Different from river floods and much more dangerous, because the warning is minutes.

They occur in dry watercourses, canyons, hilly terrain and urban areas, and — critically — from rain that fell somewhere else.

Which means a flash flood can arrive in bright sunshine, from rain upstream.

Rules: never camp in a dry riverbed or a narrow valley floor. **Get to high ground at the first sign of rising or discoloured water, or a roaring sound upstream. Do not attempt to outrun it along the channel — move perpendicular, up the side.

Before

Six things.

Know whether you are in a flood-prone area. Maps exist; ask a long-term neighbour (5.1).

Set the trigger. If water reaches X, we go.

Know two routes out, and which one floods first.

Keep documents, medication and the kit upstairs or high, and in something waterproof.

Know where your electricity shut-off is, and turn it off before water reaches it if you have time.

And if flooding is forecast: move the vehicle to higher ground early, fill containers with clean water, charge everything, and move valuables and anything hazardous upstairs.

During

If told to evacuate, go immediately (5.1). Roads out of a flood area close fast.

If you are staying: upstairs, with water, food, medication, torch, phone, charger, and something to make an opening onto the roof.

Turn off gas and electricity at the mains.

And tell somebody outside the area where you are.

If a vehicle enters water

And this is worth memorising because the timeline is short.

Undo the seatbelt.

Open the window immediately. **Electric windows generally work for a period after entering water, and this is your best exit. Do it now rather than after assessing.

Get out through the window.

If the window will not open, break it — side window, not windscreen, and at a corner rather than the centre. A dedicated escape tool kept within reach is the difference here, and a headrest post can sometimes work.

Get children out first, oldest to youngest.

And do not wait for pressure to equalise so the door opens. That advice is a last resort and by then the vehicle is likely to be submerged.

After

Six things, because the aftermath causes many injuries.

Do not return until told it is safe.

Do not turn on electricity until it has been checked.

Assume everything the water touched is contaminated. Discard food that has been in contact, and boil or treat water until told otherwise.

Wear boots and gloves.

Watch for snakes and animals displaced by the water (3.7), which move into buildings and are a real hazard in Indian floods.

And photograph everything before cleaning, for insurance.

What to do with this page

Fifteen centimetres of moving water knocks you over; thirty floats a car. Turn around, do not drown.

Once water is around the building, go up rather than out — and take something to make an opening onto the roof.

Days of rain saturate the ground, and day five floods when days one to four did not.

Never camp in a dry riverbed.

And in a vehicle entering water: seatbelt off, window open immediately, out.

Next: 5.5 — cyclones and severe storms, where the warning is days and the danger is water rather than wind.