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Hydrometeorological Training: What Makes a Flood a Flash Flood?

Hydrometeorological Training: What Makes a Flood a Flash Flood?. Matt Kelsch UCAR/COMET ® kelsch@ucar.edu NWA Annual Meeting 18 October 2006. Hydrometeorological Training: What Makes a Flood a Flash Flood?. Basic Hydrologic Sciences Course: www.meted.ucar.edu Hydrologic Cycle

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Hydrometeorological Training: What Makes a Flood a Flash Flood?

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  1. Hydrometeorological Training: What Makes a Flood a Flash Flood? Matt Kelsch UCAR/COMET® kelsch@ucar.edu NWA Annual Meeting 18 October 2006 Matthew Kelsch NWA Annual Meeting,18 October 2006

  2. Hydrometeorological Training: What Makes a Flood a Flash Flood? Basic Hydrologic Sciences Course: www.meted.ucar.edu • Hydrologic Cycle • Unit Hydrograph • Runoff Processes • Streamflow Routing • Flood Frequency • River Ice • Snowmelt • Flash Flood Processes • FF Cases (teletraining) Snowmelt River Ice Matthew Kelsch NWA Annual Meeting,18 October 2006

  3. What Makes a Flood a Flash Flood?Intense Rainfall, Small Area Short-fused, Small-scale • Rainfall and runoff occurring on the same time and space scale • Warms season convection • Small basin or sub-basin Matthew Kelsch NWA Annual Meeting,18 October 2006

  4. What Makes a Flood a Flash Flood?Rapid Runoff, Overland Flow Overland flow: most rapid type of runoff • Infiltration excess overland flow (even when soil is dry!) • Saturation excess overland flow Matthew Kelsch NWA Annual Meeting,18 October 2006

  5. What Makes a Flood a Flash Flood?Soil Properties That Reduce Infiltration • Small particle size (clay) • Compaction • Intense Fire Matthew Kelsch NWA Annual Meeting,18 October 2006

  6. What Makes a Flood a Flash Flood?Faster Drainage, Less Infiltration Time Steep drainages • “straightening” channels increases steepness Reduce surface roughness • Vegetation removal, pavement, culverts,… reduce “roughness” High Stream Density • More tributaries • Psuedo-tributaries… road and sewer networks Matthew Kelsch NWA Annual Meeting,18 October 2006

  7. What Makes a Flood a Flash Flood?Altered Soil, Increase Sediment Transport High-intensity Fire • Soil layer becomes hydrophobic • Most notable in coniferous burn areas Hydrophobic layer  Soil layers slide downhill video Matthew Kelsch NWA Annual Meeting,18 October 2006

  8. What Makes a Flood a Flash Flood?Urbanization: More and Faster Runoff Blue: stream channels Gray: Impervious Surfaces Red: major storm drains Impervious Cover  Surface Roughness  Stream Density  Channel Length  Slope  Vegetation  All of the above lead to: • Greater runoff volume • Faster drainage to stream channels and low areas Matthew Kelsch NWA Annual Meeting,18 October 2006

  9. Urbanization: More and Faster Runoff • Baltimore’s Moore’s Run • 9.1 sq miles • Upper part, no surface channel  Flood inundation in lower Moores Run on 13 June 2003 Material from Jim Smith, Princeton Matthew Kelsch NWA Annual Meeting,18 October 2006

  10. Urbanization: More and Faster RunoffLas Vegas, 19 Aug 2003 Matthew Kelsch NWA Annual Meeting,18 October 2006

  11. video Matthew Kelsch NWA Annual Meeting,18 October 2006

  12. What Makes a Flood a Flash Flood?Rapid Release of Impounded Water Structural failure can turn a general flood into a flash flood • Dam/Levee • Road embankment • Blocked bridge or culvert Problematic for forecasters who do not have access to information about structures and impounded water Kansas Turnpike, 30 Aug 2003 Rapid City, SD, June 1972 Fort Collins, CO, 28July 1997 Matthew Kelsch NWA Annual Meeting,18 October 2006

  13. Rapid Release of Impounded Water9 October 2005 Matthew Kelsch NWA Annual Meeting,18 October 2006

  14. near Alstead, NH Matthew Kelsch NWA Annual Meeting,18 October 2006

  15. Hinsdale, NH Matthew Kelsch NWA Annual Meeting,18 October 2006

  16. What Makes a Flood a Flash Flood?Sudden Rise Associated With River Ice Rapid backwater effect upstream of ice jam Possible surge downstream of failed ice jam * Upstream flash flooding from backwater is more common and can occur within 60 min of ice jam formation Matthew Kelsch NWA Annual Meeting,18 October 2006

  17. Hydrometeorological Training: What Makes a Flood a Flash Flood? Basic Hydrologic Sciences Course: www.meted.ucar.edu • Intense rain • Small area • Rapid Surface Runoff • Steep Slope • High Stream Density • Low Surface Roughness • Low Infiltration • Soil properties • Surface Cover • Rapid runoff • *Urbanization* • Structural failure • Ice jam formation Matt Kelsch UCAR/COMET Hydrometeorologist kelsch@ucar.edu Matthew Kelsch NWA Annual Meeting,18 October 2006

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