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Aflood warning orflood alert is the task of making use offlood forecasts to make decisions about whether warnings should be issued to the general public or if previous warnings should be rescinded or retracted.The task of providing warning for floods is divided into two parts:
The decisions made by someone responsible for initiating flood warnings must be influenced by a number of factors, which include:
Acomputer system for flood warning will usually contain sub-systems for:
The type of flood warning service available varies greatly from country to country, and a location may receive warnings from more than one service.
In thePhilippines, thePhilippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) branch of theDepartment of Science and Technology (DOST) issues general flood advisory (for non-telemetered river basins whenever there is a significant amount of rainfall recorded).[1]
| Warning name | Description | Forecast | Action/Response |
|---|---|---|---|
| Flood Monitoring | Telemetered: Slow rise in water level but still below alarm level Non-Telemetered: Monitor for possible flooding area | Telemeterd: Flood is possible Non-Telemeterd: Light to Moderate Rainfall | Non-Telemetrd: Advised to take Precautionary Measures |
| Flood Alert | Telemetered: Water level is continuously rising but still below critical level Non-Telemetered: Alert for possible flash floods and landslides | Telemeterd: Flood is threatening Non-Telemeterd: Moderate to Heavy Rainfall | Non-Telemeterd: Advised to be alert for possible flood, flash flood and landslides |
| Flood Warning | Telemetered: Water level is above critical level Non-Telemetered: Flood is occurring immediate action is recommended | Telemeterd: Flood is occurring Non-Telemeterd: Heavy to Intense Rainfall | Non-Telemeterd: Advised to take appropriate action |
| Severe Flood Warning | Telemetered: Water level is continuously rising above critical level Non-Telemetered: Flood is persisting force evacuation is recommended | Telemeterd: Flood is persisting Non-Telemeterd: Intense to Torrential Rainfall | Non-Telemeterd: Advised to force evacuation |
| Final | Telemetered: Slow recession of water level Non-Telemetered: Light rains | Flood is no longer possible |
Arrangements for flood warnings vary across the United Kingdom with several agencies leading on warnings for emergency responders and the public. TheEnvironment Agency,Natural Resources Wales andScottish 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:
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.[3] Both warnings and updates about current conditions are also carried by localradio stations. In addition, live updates are carried by the Environment Agency's website,[4] 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 theMet Office does issue weather warnings.Flood risk management is the responsibility ofRivers Agency in Northern Ireland. Consideration will be given to the introduction of a warning system as part of the implementation of the EUFloods directive.
In theUnited States, aflash flood watch is issued by theNational Weather Service (NWS) when weather conditions are favorable for very heavyrain that could cause flooding orflash flooding.[5] A watch does not mean that flooding is occurring, only thatweather conditions have created or will create a significant risk for it. If flooding occurs, a flood warning orflash flood warning would be issued, and immediate action should be taken. A flood warning or flash flood warning is issued when flooding is imminent or already occurring. When flood warnings are issued, it means that areawaterways will likely soon be in flood.[6] Not all flood watches suggest that large-scale flooding, such as duringlandfallingtropical cyclones, is possible.
In theUnited States, theNational 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.[7] 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 andwarnings are issued.
In the U.S. andCanada, dissemination of flood warnings is covered bySpecific Area Message Encoding (SAME) code FLW, which is used by the U.S.Emergency Alert System andNOAA Weather Radio network and in Canada'sWeatheradio Canada network.
"Flood statements" are issued by theNational 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.[8]
The following is an example of a "Flood Warning." TheSabine River is used as an example:
394WGUS44 KSHV 070219 RRAFLWSHVBULLETIN - IMMEDIATE BROADCAST REQUESTEDFlood WarningNational Weather Service Shreveport LA819 PM CST Wed Jan 6 2021...The National Weather Service in Shreveport LA has issued a FloodWarning for the following rivers in Texas... Sabine River Near Hawkins affecting Wood, Smith and Upshur Counties.PRECAUTIONARY/PREPAREDNESS ACTIONS...Do not drive cars through flooded areas.Caution is urged when walking near riverbanks.A Flood Warning means that flooding is imminent or occurring. Allinterested parties should take necessary precautions immediately.Turn around, don't drown when encountering flooded roads. Most flooddeaths occur in vehicles.For more hydrologic information, copy and paste the following websiteaddress into your favorite web browser URL bar:water.weather.gov/ahps2/index.php?wfo=shvTXC423-459-499-080230-/O.NEW.KSHV.FL.W.0009.210109T0430Z-210111T0224Z//HAKT2.1.ER.210109T0430Z.210110T0000Z.210110T2024Z.NO/819 PM CST Wed Jan 6 2021The National Weather Service in Shreveport has issued a* Flood Warning for the Sabine River Near Hawkins.* From Friday evening to Sunday evening.* At 7:45 PM CST Wednesday the stage was 19.1 feet.* Flood stage is 23.0 feet.* Minor flooding is forecast.* Forecast...The river is expected to rise above flood stage late Friday evening to a crest of 23.7 feet Saturday evening. It will then fall below flood stage early Sunday afternoon.* Impact...At 23.0 feet, minor lowland flooding. Move livestock and equipment to higher ground.&&LAT...LON 3263 9535 3257 9509 3253 9508 3254 9521 3257 9535$$19
Source:[9]
000WGUS84 KMRX 221008FLSMRXFLOOD STATEMENTNATIONAL WEATHER SERVICE MORRISTOWN, TN 608 AM EDT TUE SEP 22 2009...THE FLOOD WARNING CONTINUES FOR THE FOLLOWING RIVERS IN GEORGIA...TENNESSEE... SOUTH CHICKAMAUGA CREEK @ CHICKAMAUGA TN AFFECTING CATOOSA ANDHAMILTON COUNTIES IN HAMILTON COUNTY...THERE ARE TOO MANY STREETS CLOSED TO MENTION.IN SHORT...BE PREPARED FOR LONG DELAYS. ALSO...FAR TOO MANY PEOPLEARE TRYING TO DRIVE THROUGH FLOOD WATERS. THIS IS A GOOD WAY TO DIE.PRECAUTIONARY/PREPAREDNESS ACTIONS...MOST DEATHS IN FLOODS OCCUR IN CARS! IF YOU COME TO A CLOSED ORFLOODED ROAD, TURN AROUND! DON'T DROWN! DRIVING INTO FLOODED ROADSIS A GOOD WAY TO DIE. BETTER TO BE LATE THAN END UP ON THE NEWS.&&GAC047-TNC065-221608-/O.CON.KMRX.FL.W.0013.000000T0000Z-090923T1942Z//CHKT1.2.ER.090921T0923Z.090922T1800Z.090923T1342Z.NO/608 AM EDT TUE SEP 22 2009THE FLOOD WARNING CONTINUES FOR THE SOUTH CHICKAMAUGA CREEK @ CHICKAMAUGA TN* UNTIL WEDNESDAY AFTERNOON.* AT 5:15 AM TUESDAY THE STAGE WAS 25.8 FEET.* MODERATE FLOODING IS OCCURRING. THE FORECAST IS FOR MODERATE FLOODING TO CONTINUE.* FORECAST...THE RIVER WILL CONTINUE TO RISE AND CREST NEAR 26.5 FEET TUESDAY AFTERNOON. THE RIVER WILL FALL BELOW FLOOD STAGE LATE WEDNESDAY MORNING.* AT 27.0 FEET...HOMES ON ARLENA CIRCLE (OFF SHALLOWFORD ROAD) ARE EVACUATED. EVACUATIONS ALSO TAKE PLACE AT THE FOUNTAINBLEAU APARTMENTS ON SPRING CREEK ROAD IN EAST RIDGE.$$$$BOYD
Source:[10]
TheIowa Flood Center at theUniversity of Iowa operates the largest real-time flood monitoring system of its kind in the world. It includes more than 200 real-time stream stage sensors that feed data into the Iowa Flood Information System where data can be viewed, online, by disaster management staff and the general public. The stream stage sensors, mounted on bridges and culverts, use ultrasonic sensors to monitor stream and river levels.[11] The Iowa Flood Information System is considered "one of the best in the country", according to theWashington Post.[12]
InAustralia, flood warnings are handled by theBureau Of Meteorology as well as local councils. The method of distributing the warning can vary but is usually transmitted by radio, social media and dedicated apps.[13]
InCanada, aheavy rainfall warning, which indicates rainfall amounts thatcould produce flooding are expected, has basically the same meaning as a flood watch.[citation needed]
In Europe, there is theEuropean Flood Awareness System and the commercial product including app and data servicesFloodAlert.[14]
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