Emergencies & Your Health

Flood Watch, Flood Warning, Flood Advisory - what do they mean?

What is the difference between a Flood Watch and a Flood Warning issued by the National Weather Service?

• Flash Flood Warning: Take Action! A Flash Flood Warning is issued when a flash flood is imminent or occurring. If you are in a flood prone area move immediately to high ground. A flash flood is a sudden violent flood that can take from minutes to hours to develop. It is even possible to experience a flash flood in areas not immediately receiving rain.
  Flash Flood Warnings are changing to an Impact-Based format to improve public response. Read the factsheet.

• Flood Warning: Take Action! A Flood Warning is issued when the hazardous weather event is imminent or already happening. A Flood Warning is issued when flooding is imminent or occurring.

• Flood Watch: Be Prepared: A Flood Watch is issued when conditions are favorable for a specific hazardous weather event to occur. A Flood Watch is issued when conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.

• Flood Advisory: Be Aware: An Flood Advisory is issued when a specific weather event that is forecast to occur may become a nuisance. A Flood Advisory is issued when flooding is not expected to be bad enough to issue a warning. However, it may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.

Floods

Preparing for a flood is much like preparing for any other emergency, except:

• To avoid additional problems when water hits power, before waters reach you, turn off electricity and gas at the main switches / valves, and disconnect electrical appliances.
• Do not touch electrical equipment if you are wet or there is standing water, however!
• Do not drive into flooded areas. If floodwaters rise around your car, abandon the car immediately and move to higher ground. You and the vehicle can be quickly swept away. A foot of water will float many vehicles; two feet of rushing water can carry away most vehicles, including SUVs and pick-ups. Remember: turn around, don't drown.
• Do not try to walk or swim through floodwaters.

Rain in Santa Clara County or on the higher coastal mountains around us, ocean storm surges, dam overflows, and water system backup have all caused floods in this valley in the past - and could do so again.  And even a small-scale flood at home can cause a surprising amount of damage.

Check to see if your home is in a flood plain. If it is you may wish to take steps to prepare, including sandbagging when there are storm warnings.

After a Flood

If evacuated, do not return home until local emergency response teams declare that it is safe.

In addition:

• Listen for news reports on whether the community’s water supply is safe to drink.
• Avoid floodwaters; water may be contaminated by oil, gasoline or raw sewage. Water may also be electrically charged from underground or downed power lines.
• Stay away from downed power lines, and report them to your power company.

Cleaning up after a flood or mudslide can require a lot of hard work, and can bring its own hazards. Professional remediation work may be needed.

Mudslides or Landslides

In California, mudslides can follow after drenching storms, especially in areas that recently burned in a wildfire. Earthquakes can also trigger a mudslide or landslide. Listen to your local media for flood watches, warnings and other travel advisories to find out if there is danger of mudslides. And as always, evacuate if ordered!

Watch for warning signs of debris flows or fast moving landslides:

• Listen and watch for rushing water, mud, unusual sounds.
• Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
• A faint rumbling sound that increases in volume is noticeable as the landslide nears.
• Fences, retaining walls, utility poles, k-rails, boulders, or trees move.

Slower, more subtle signs of earth shifting may be visible in patterns of storm-water drainage on slopes (especially the places where runoff water converges) land movement, small slides, flows, or progressively leaning trees. Other signs are:

• Doors or windows stick or jam for the first time.
• New cracks appear in plaster, tile, brick, or foundations.
• Outside walls, walks, or stairs begin pulling away from the building.
• Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
• Underground utility lines break.
• Bulging ground appears at the base of a slope.
• Water breaks through the ground surface in new locations.
• Fences, retaining walls, utility poles, or trees tilt or move.
• The ground slopes downward in one direction and may begin shifting in that direction under your feet.

The USGS advises to:

• Stay alert and awake. Many debris-flow fatalities occur when people are sleeping. Listen to a NOAA Weather Radio or portable, battery-powered radio or television for warnings of intense rainfall. Be aware that intense, short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.
• If you are in areas susceptible to landslides and debris flows, consider leaving if it is safe to do so. Remember that driving during an intense storm can be hazardous. If you remain at home, move to a second story if possible. Staying out of the path of a landslide or debris flow saves lives.
• Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing or falling mud or debris may precede larger landslides. Moving debris can flow quickly and sometimes without warning.
• If you are near a stream or channel, be alert for any sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate landslide activity upstream, so be prepared to move quickly. Don't delay! Save yourself, not your belongings.
• Be especially alert when driving. Bridges may be washed out, and culverts overtopped. Do not cross flooding streams! Turn Around, Don't Drown®!. Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of possible debris flows.

After a Mudslide or Landslide

• Stay away from the slide area. There may be danger of additional slides.
• Listen to local radio or television stations for the latest emergency information, and/or check emergency alerts online.
• Watch for flooding. Floods sometimes follow landslides and debris flows because they may both be started by the same conditions.
• Check for injured and trapped persons near the slide, without entering the direct slide area. Direct rescuers to their locations.
• Report broken utility lines, damaged roadways, and railways to appropriate authorities. Reporting potential hazards will get the utilities turned off as quickly as possible, preventing further hazard and injury.
• Allow trained professionals to check roads; building foundations, sewer lines, and chimneys;  and surrounding land for damage.

Coping After a Disaster

Floods or landslides are traumatic events! Take care of your mental and emotional health as well as your physical health - and your families and loved ones.