How do floods occur? – English explanation

HOW FLOODS HAPPEN

Floods begin when soil and vegetation cannot absorb falling rain or melting snow, or when water runs off the land in such quantities that it cannot be carried in normal stream channels or retained in natural ponds and human-made reservoirs. Flash floods are the result of too much rain falling in too small an area, in too short a time. Flash floods frequently occur in seconds and minutes, while floods occur over hours and days. River Forecast Centers issue flood forecasts and warnings when the rain that has fallen is enough to cause waterways to overflow their banks, and when melting snow combines with rainfall to produce similar effects.

FLASH FLOODS

Typically, flash floods occur primarily at night and when there is an abundance of atmospheric moisture; in addition, there is usually little, if any, vertical wind shear present. Flash flooding can be produced by large, slow-moving storms or as a result of “train effect” storms (i.e., sequential mature storms that release precipitation over the same area). Train effect storms can be part of multicell cluster or squall line storm systems.

Flash-flood waves, moving at incredible speeds, can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Killing walls of water can reach 10–20 ft.

On small streams, especially near the headwaters of river basins, water levels may rise quickly in heavy rainstorms,and flash floods can begin before the rain stops falling. There is little time between detection and flood crest. Swift action is essential for the protection of life and property.

Flood Safety Rules

Before the flood:

Become familiar with local flood areas and dams; know if floodwaters might affect your home and property. Know your flood risk and elevation above flood stage and flood plain. Do local waterways, or rivers, and washes flood easily? If so, be prepared to move to a place of safety.
Learn flood warning signals and community evacuation routes and shelters.
Keep a stock of food that requires no cooking or refrigeration; electric and gas services may be interrupted.
Keep a portable radio, emergency cooking equipment, lights, and flashlights in working order.
Keep first-aid supplies and any medicines your family may need on hand.
Store materials like sandbags, plywood, plastic sheeting, and lumber to protect your house from flood waters and to make quick repairs.
Keep your car fueled. In an emergency, filling stations may not be operating.
Contact your insurance agent or local government to discuss flood insurance coverage.
Install check valves in building sewer traps to prevent flood water from backing up in sewer drains.
Arrange for auxiliary electrical supplies for hospitals and other operations that are critically affected by power failure.

When you receive a flood warning:

11. Store drinking water in clean bathtubs and in various closed containers as water service may be disrupted.
12. In coastal areas, board up windows or protect them with storm shutters, or tape to prevent flying, broken glass.
13. Put sandbags or other protection in place, but away from outer walls. In the case of deep flooding you may opt to flood a basement with clean water.
14. If forced to leave your home and time permits: move essential items to safe ground or to upper levels of the house; turn off utilities at main switches, but do not touch electrical equipment if you are wet or standing in water; fill tanks to keep them from moving away; grease immovable machinery; leave a note on your house to advise authorities that you have evacuated.
15. Move to a safe area before access is cut off by flood water. Watch for mud slides, downed electrical lines, and areas with high or rising water levels.

During the flood:

16. Avoid areas subject to sudden flooding.
17. Do not drive into flooded areas. Even 2 ft of water will carry away most vehicles. If flood waters do rise around your car, abandon it and move to higher ground.
18. Do not swim or dive into the water.

After the flood:

19. Do not visit disaster areas; your presence might hamper rescue and other emergency operations.
20. If you have flood insurance, contact your agent that you have a loss.
21. Tune in to local radio and television for advice on where to obtain medical care and other assistance.
22. Do not enter structures if floodwaters have covered the first floor. Seek expert advice to determine if the building is safe to enter.
23. Use battery-powered lanterns or flashlights (not oil or gas lanterns); if the building may have a gas leak, do not use any kind of light.
24. Flood waters may have swollen doors tightly shut; use windows or other openings.
25. Check with local authorities before using any water; wells should be pumped out and water tested before drinking.
26. Do not use fresh food that has come into contact with flood waters.
27. Do not handle live electrical equipment in wet areas; have an expert check all equipment before returning to service.
28. Pump water out of basements gradually (one-third of the water per day) to lessen damage to walls and foundation.
29. Report broken utility lines to appropriate authorities; have the gas company check for leaks and to turn the gas back on.
30. Watch out for poisonous snakes in previously flooded areas.