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about whether warnings of floods should be issued to the general public or whether previous warnings should be rescinded or retracted.
The task of providing warning for floods is 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- The need to avoid situations where a warning condition is rescinded only for the warning to be re-issued within a short time, again because of the wasted efforts of the general public and because such occurrences would bring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.
Arrangements for flood warnings vary across the United Kingdom with several agencies leading on warnings for emergency responders and the public. The Environment Agency, Natural Resources Wales and Scottish Environment Protection Agency all undertake location specific flood warning activities for communities at risk depending upon the scale of flood risk, technical challenges and investment needed to deliver a reliable service.
Prior to issuing a flood warning consideration is given to:
- the needs of communities to activate emergency response plans
- the nature of the catchment or coastline and the lead time that may be provided
- meteorological observations and forecast information on rainfall and coastal water levels
- hydrological observations and flood forecasts
- reference to thresholds of historic or forecast flood levels
Dissemination of flood warnings has moved towards a service whereby those at risk can pre-register to receive warnings by phone, email or text message from an automatic system, Floodline. Both warnings and updates about current conditions are also carried by local radio stations. In addition, live updates are carried by the Environment Agency's website, showing which 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 warnings. 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.
In the United States, 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 U.S. and Canada, dissemination of flood warnings is covered by Specific Area Message Encoding (SAME) code FLW, which is used by the U.S. Emergency Alert System and NOAA Weather Radio 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
The following is an example of a "Flood Warning." The South Chickamauga Creek is used as an example:
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
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
- Flood Risk Management in Northern Ireland at riversagencyni.gov.uk