Flood warning

 

Flood warning is closely linked to the task of flood forecasting. The distinction between the two is that the outcome of flood forecasting is a set of forecast time-profiles of channel flows or river levels at various locations, while "flood warning" is the task of making use of these forecasts to make decisions about whether warnings of floods should be issued to the general public or whether previous warnings should be rescinded or retracted.

Description

The task of flood warning can often be divided into two parts:

• decisions to escalate or change the state of alertness internal to the flood warning service provider, where this may sometimes include partner organisations involved in emergency response;
• decisions to issue flood warnings to the general public.

The decisions made by someone responsible for initiating flood warnings must be influenced by a number of factors, which include:

• The reliability of the available forecasts and how this changes with lead-time.
• The amount of time that the public would need to respond effectively to a warning.
• The delay between a warning being initiated and it being received by the public.
• The need to avoid issuing warnings unnecessarily, because of the wasted efforts of those who respond and because a record of false alarms means that fewer would respond to future warnings.
• The need to avoid situations where a warning condition is rescinded only for the warning to be re-issued within a short time, again because of the wasted efforts of the general public and because such occurrences would bring the flood warning service into disrepute.

A computer system for flood warning will usually contain sub-systems for:

• flood forecasting;
• automatic alerting of internal staff;
• tracking of alert messages and acknowledgements received;
• diversion of messages to alternates where no acknowledgement received.

National Flood Warning Services

The type of flood warning service available varies greatly from country to country, and a location may receive warnings from more than one service.

United Kingdom

For example, in the UK, the UK Met Office may issue warnings of flooding likely to occur because their meteorological forecasts of rainfall indicate that heavy rainfall will occur - such forecasts might be available 6 to 24 hours or more before the predicted rainfall, but are not fully reliable as to whether or precisely where such rainfall will occur. Thus such warnings would need to appropriately cautious and would have to apply to wider regions than will eventually be affected by the heavy rainfall. In conjunction with this service, more location-specific flood warnings are the role of the Scottish Environment Protection Agency (covering Scotland) and the Environment Agency (covering England and Wales) who undertake flood forecasting tasks over shorter lead-times, on the basis of:

• observed rainfall over the recent past;
• forecasts of rainfall over lead-times of up to about 6 hours;
• current and past river conditions.

In the UK, the dissemination of flood warnings is moving or has moved towards a service whereby those potentially at risk of river flooding can pre-register to receive warnings by phone from an automatic system. Both warnings and updates about current conditions are also carried by local radio stations. In addition, internet sites have pages showing what locations have flood warnings in place and the severity of these warnings.

There is currently no flood warning system in Northern Ireland but the Met Office does issue weather warning. Flood risk management is the responsibility of Rivers Agency in Northern Ireland. Consideration will be given to the introduction of a warning system as part of the implementation of the EU Floods directive.

United States

In the US, the National Weather Service issues Flood Watches and Warnings for large-scale, gradual river flooding. Watches are issued when flooding is possible or expected within 12–48 hours, and warnings are issued when flooding over a large area or river flooding is imminent or occurring. Both can be issued on a county-by-county basis or for specific rivers or points along a river. When rapid flooding from heavy rain or a dam failure is expected, Flash Flood Watches and Warnings are issued.

In the USA and Canada dissemination of flood warnings is covered by Specific Area Message Encoding (SAME) code FLW, which is used in the US Emergency Alert System and NOAA Weatheradio network and in Canada's Weatheradio Canada network.

"Flood Statements" are issued by the National Weather Service to inform the public of flooding along major streams in which there is not a serious threat to life or property. They may also follow a flood warning to give later information.

Example of a flood warning

608 
WGUS44 KMRX 210433 CCA
FLWMRX

BULLETIN - IMMEDIATE BROADCAST REQUESTED
FLOOD WARNING
NATIONAL WEATHER SERVICE MORRISTOWN, TN
1233 AM EDT MON SEP 21 2009

...THE NATIONAL WEATHER SERVICE IN MORRISTOWN, TN HAS ISSUED A FLOOD 
WARNING FOR THE FOLLOWING RIVERS SOUTH CHICKAMAUGA IN GEORGIA...
TENNESSEE...

  SOUTH CHICKAMAUGA CREEK NEAR CHATTANOOGA TN AFFECTING CATOOSA AND 
HAMILTON COUNTIES

HEAVY RAINFALL SUNDAY EVENING OF AROUND 3 INCHES IN THE CHATTANOOGA 
AREA HAS CAUSED THE SOUTH CHICKAMAUGA CREEK TO RISE RAPIDLY.

GAC047-TNC065-211821-
/O.NEW.KMRX.FL.W.0013.090921T0433Z-090922T2200Z/
/CHKT1.1.ER.090921T0421Z.090922T0600Z.090922T1600Z.NO/
1233 AM EDT MON SEP 21 2009

THE NATIONAL WEATHER SERVICE IN MORRISTOWN, TN HAS ISSUED A 

* FLOOD WARNING FOR 
  THE SOUTH CHICKAMAUGA CREEK AT CHICKAMAUGA TN
* FROM THIS MORNING TO TUESDAY EVENING.
* AT 11:15 PM SUNDAY EVENING THE STAGE WAS 13.0 FEET.
* MINOR FLOODING IS FORECAST.
* FORECAST...THE RIVER WILL RISE ABOVE FLOOD STAGE AROUND 7 AM MONDAY
  AND CREST NEAR 19.0 FEET AROUND 2 AM TUESDAY. THE RIVER WILL FALL 
  BELOW FLOOD STAGE TUESDAY AFTERNOON.
* AT 19.0 FEET...WATER ACROSS MACK SMITH ROAD BEGINS TO IMPEDE 
  TRAFFIC.  WEST CHICKAMAUGA CREEK OVERFLOWS ITS BANKS NEAR THE 
  GEORGIA STATE LINE AND INUNDATES SEVERAL ROADS AND PROPERTIES IN 
  THE AREA.

$$

PRECAUTIONARY/PREPAREDNESS ACTIONS...

MOST DEATHS IN FLOODS OCCUR IN CARS! IF YOU COME TO A CLOSED OR
FLOODED ROAD, TURN AROUND! DON'T DROWN! FOR MORE DETAILS, STAY TUNED 
TO NOAA WEATHER RADIO OR COMMERCIAL TELEVISION OR RADIO THAT CARRY 
WEATHER INFORMATION.

&&

$$

TD

Source:

Example of a flood statement

000
WGUS84 KMRX 221008
FLSMRX
FLOOD STATEMENT
NATIONAL WEATHER SERVICE MORRISTOWN, TN 
608 AM EDT TUE SEP 22 2009

...THE FLOOD WARNING CONTINUES FOR THE FOLLOWING RIVERS IN GEORGIA...
TENNESSEE...
  SOUTH CHICKAMAUGA CREEK @ CHICKAMAUGA TN AFFECTING CATOOSA AND
HAMILTON COUNTIES 
 
IN HAMILTON COUNTY...THERE ARE TOO MANY STREETS CLOSED TO MENTION.
IN SHORT...BE PREPARED FOR LONG DELAYS. ALSO...FAR TOO MANY PEOPLE
ARE TRYING TO DRIVE THROUGH FLOOD WATERS. THIS IS A GOOD WAY TO DIE.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

MOST DEATHS IN FLOODS OCCUR IN CARS! IF YOU COME TO A CLOSED OR
FLOODED ROAD, TURN AROUND! DON'T DROWN! DRIVING INTO FLOODED ROADS
IS A GOOD WAY TO DIE. BETTER TO BE LATE THAN END UP ON THE NEWS.

&&

GAC047-TNC065-221608-
/O.CON.KMRX.FL.W.0013.000000T0000Z-090923T1942Z/
/CHKT1.2.ER.090921T0923Z.090922T1800Z.090923T1342Z.NO/
608 AM EDT TUE SEP 22 2009

THE FLOOD WARNING CONTINUES FOR
  THE SOUTH CHICKAMAUGA CREEK @ CHICKAMAUGA TN
* UNTIL WEDNESDAY AFTERNOON.
* AT  5:15 AM TUESDAY THE STAGE WAS 25.8 FEET.
* MODERATE FLOODING IS OCCURRING. THE FORECAST IS FOR MODERATE 
  FLOODING TO CONTINUE.
* FORECAST...THE RIVER WILL CONTINUE TO RISE AND CREST NEAR 26.5 FEET
  TUESDAY AFTERNOON. THE RIVER WILL FALL BELOW FLOOD STAGE LATE
  WEDNESDAY MORNING.
* AT 27.0 FEET...HOMES ON ARLENA CIRCLE (OFF SHALLOWFORD ROAD) ARE 
  EVACUATED.  EVACUATIONS ALSO TAKE PLACE AT THE FOUNTAINBLEAU 
  APARTMENTS ON SPRING CREEK ROAD IN EAST RIDGE.

$$

$$

BOYD

Source:

References

External links

• Environment Agency (EA) Flood Watch (live warnings) - England/Wales at environment-agency.gov.uk
• EA scheme description at environment-agency.gov.uk
• Scottish Environment Protection Agency (SEPA) Live Flood Warning Information at sepa.org.uk
• Flood warning for Ireland (Eire) at flooding.ie
• Flood Risk Management in Northern Ireland at riversagencyni.gov.uk