Matthew Kelsch

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Hydrometeorological Training What Makes a Flood
a Flash Flood?

• Matt Kelsch
• UCAR/COMET
• kelsch_at_ucar.edu
• NWA Annual Meeting
• 18 October 2006

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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
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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

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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

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What Makes a Flood a Flash Flood?Soil Properties
That Reduce Infiltration

• Small particle size (clay)
• Compaction
• Intense Fire

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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

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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
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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

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Urbanization More and Faster Runoff

• Baltimores Moores 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
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Urbanization More and Faster RunoffLas Vegas,
19 Aug 2003
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video
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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
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Rapid Release of Impounded Water9 October 2005
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near Alstead, NH
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Hinsdale, NH
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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

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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_at_ucar.edu