hydrometeorlogical disasters

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Hydrometeorlogical Disasters. Matt Kelsch, Hydrometeorologist & Instructor University Corporation for Atmospheric Research (UCAR) Cooperative Program for Operational Meteorology, Education and Training (COMET) Boulder , Colorado Wednesday, 5 September 2001 kelsch@ucar.edu. - PowerPoint PPT Presentation


  • Hydrometeorlogical Disasters

    Matt Kelsch, Hydrometeorologist & InstructorUniversity Corporation for Atmospheric Research (UCAR)Cooperative Program for Operational Meteorology, Education and Training (COMET)Boulder , Colorado

    Wednesday, 5 September 2001kelsch@ucar.edu

  • Hydrometeorlogical DisastersHow are flash floods different from other weather disasters?Tropical Cyclones/HurricanesWinter StormsSlow rise floods Severe Weather/Tornadoes

    What exactly IS a flash flood?Why is the public so vulnerable to flash floods?Why is flash flood forecasting so difficult?

  • Tropical Cyclones: what specific problems do tropical cyclones (hurricanes) present?Like Flash Floods:Increasing population in vulnerable areasEven in relatively easy forecast events, uncertainty exists about the precise location likely to be hit hardestPublic does not understand uncertainty wellUnlike Flash Floods:Objective definitions about the phenomenon (> 65 kt wind)General understanding of location of greatest impact of wind and storm surge (along and near coast) Longer life spans allow tracking and detail monitoring

  • Hurricane Floyd 16 Sep 1999

  • What specific problems do tropical cyclones present?Impact larger areas than severe weather or flash floodsMost intense winds and storm surge are likely in a relatively small coastal area near eyewall:Uncertainty can be significant for forecast periods > 24hCoastal communities may need more than 24h to prepareConsequently, warnings take uncertainty into account and typically encompass larger areas than necessaryPost-landfall inland rainfall can be extreme

  • In the last 30 years, inland flooding has been the most disastrous part of tropical cyclones Hurricane Camille 1969 (Virginia)Category 5 storm at Mississippi landfall, but does more damage as a historic rainstorm in Virginia after the winds decreaseHurricane Agnes 1972 (Pennsylvania/New York)Hurricane Tico 1983 (Texas/Oklahoma)Came in from Pacific and set rainfall records as an ex-hurricaneHurricane Fran 1996 (NorthCarolina/Virginia)Hurricane Mitch 1998 (Central America)Hurricane Floyd 1999 (South Carolina to Massachusetts)Tropical Storm Allison (Texas to East Coast)Weak storm, but potent rainfall producer

  • Tar River near Tarboro, NC in the aftermath of Floyd

  • Tarboro after Hurricane Floyd, Sep 1999

  • Winter Storms: an evolving and complex relationship between nature and societyLike flash floods:Problem is from precipitation, not wind damageSocietal infrastructure is heavily impactedRisk taking puts people in wrong place at wrong timeUnlike flash floods:Winter Storms have objective definitionsThey generally occur over larger space and times scales (with the exception storms near mountain ranges and in the Great Lakes)

  • Front Range Snowstorm: 8-9 March 1992

  • Winter Storms: an evolving and complex relationship between nature and societyPrior to the 1880s, snow was often welcome for commerce because it ensured good sleighing & efficient transportationThen came:Power & communications linesNational security threatened by downed lines during 1888 Blizzard from Washington DC to New York CityInflexible work hours1888 Blizzard killed > 200 people in NYC commuting to work The automobile, and snow became transportations enemyTransportation SNAFUs like Colorados October blizzard of 97Longer commutes, expensive & difficult road maintenance

  • NOOA/LDAD Display: Eastern Colorado Blizzard, 24-26 October 1997

  • Main Stem River FloodsLike flash floods:Too much water for natural or human-made channelsSocietal infrastructure heavily impacted, can enhance the problem (dam and levee failures, blockages at bridges)Risk taking puts people in wrong place at wrong timeToxic materials, firesUnlike flash floods:Occur over larger space and times scalesallows preparation timeVictims drown in river floods, but may suffer fatal traumatic injury in flash floods

  • April 1997 Red River Flood, Grand Forks, ND

  • Severe Weather: rapid evolution of dangerous weather phenomenaLike Flash Floods:Rapid storm-scale evolution makes for very difficult forecast problemSmall scale nature difficult to convey to the publicUnlike Flash FloodsObjective definitions for threshold and verification50 kt winds and/or 0.75 diameter hailTornadoTrained spotters help with field reportsReasonable correlation between meteorological data (radar/satellite) and severe weather

  • Kansas City Brush Creek Flash Flood and Severe Winds 4 October 19984 Oct 1998 0022 UTC4 Oct 1998 0107 UTC

  • When severe weather turns into a hydrologic disasterSevere weather can evolve into a flash flood situation, especially in urban environments where it does not take as much rainfallCheyenne 1985Dallas 1995Kansas City 1998

    Cheyenne, WY, 2 August 1985

  • Flash Flood: what is it?A flood where the rainfall and the subsequent runoff are occurring on the same time and space scales.The problem is the rapid change in floodwater momentum, not the final depth, velocity, or extent.Fort Collins, CO, 28 July 1997

  • Flash Flood: what is it?Wall of Water?These typically only occur when a structural failure is involvedCan Define by VelocityIts really the rapid change in velocity rather than the actual value thats importantCheyenne,WY, 1 August 1985

  • Flash Flood: why are they so difficult to forecast and warn for?Unlike severe weather, hurricanes, and winter storms:Involves important and complex non-meteorological processesNo reliable relationship between meteorological data (rainfall) and flash flood potentialHydrologic response may dominate, causing major flash floods with relatively unimpressive rainfall Especially small basins, steep basins, urban basins, and altered basins (fire scars deforestation)

  • Flash Flood: why are they so difficult to forecast and warn for?Unlike hurricanes, winter storms, and slow-rise floods:Occurs on very small time and space scalesIn fast-response basins, the rainfall may not be the most important factorCheyenne, WY, August 1985

  • Avg ~5 in/hrAvg ~8 in/hrAccum.: 2-20+ inchesBasin Size: 5-50 miles2, avg=18 miles2Rainfall Rates: 3-12 inches/hr

  • Rapidan, VABig Thompson, COLas Vegas, NV

  • Las Vegas, NV

  • Flash Flood: Public VulnerabilityRainfall is necessary for life water just doesnt seem that threateningDifficult to know when a flood goes from a nuisance to a threatUnrealistic feeling of safety in our vehicles, especially SUVs!

  • What are the typical scales of the flash flood process?Almost always < 75 km2 (30 mi2), and sometimes less than 25 km2 (10 mi2).Less than 6 hours, and typically the real intense precipitation burst and subsequent flash flood are occurring in the 1-2 hour time frame.Rainfall rates >150 mm/h (6 in/h), perhaps only ~ 100 mm/h (4 in/h) dry climates.

  • 1-h Accumulation, 12 July 1996Purples=50-75mmFire ScarBuffalo Creek, CO

  • Omaha, Nebraska0500 UTC 7 Aug 19990700 UTC 7 Aug 1999

  • Fort Collins, CO Flash Flood 28 July 1997

  • Timetable of flash flood event5:308:008:309:009:4010:3011:001:20Rain beginsEOC ActivatedPonds Overflowing, rapid water rescues beginMost intense rain commencesNWS WarningStorm begins to dissipate and move northeastTrailer Park Flooding, Fires, Train DerailmentDeclared City Disaster

  • October 1999 Hydromet class at Spring Creek

  • Hydrometeorlogical DisastersIt Doesnt Have to Rotate to Kill Ya!Flash Floods take more lives in a typical year than Severe Weather, Hurricane Winds & Storm Surge, or Winter Storms

    Flash Floods involve complex and rapidly evolving meteorological and hydrologic processes difficult to define with objective quantitative criteria

    Floods are not viewed by many as being as violent as tornadoes or hurricanes