Flash Floods: Relationship Between Basins and Precipitation

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Flash Floods Relationship Between Basins and
Precipitation

• Matt Kelsch
• 10 September 2002
• kelsch_at_comet.ucar.edu

I-80 washout near Ogallala, NE 6 July 2002
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Some Generalizations on Flash Floods

• Intense rainfall rate
• High precipitation efficiency, warm rain
  processes
• Small basins
• Pre-saturation
• Impermeability
• Steep slopes

Fort Collins, CO, 28 July 1997
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More Specifically

• It is the relationship between precipitation and
  basin response that leads to rapid rises in
  stormwater
• Rainfall is intense with respect to the basins
  ability to accommodate it
• Impermeable (urban)
• Altered (fires, deforestation)
• The basin is small relative to the wet footprint
  of the precipitating system

Flash flooding in Las Vegas within an hour of a
rainburst
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Defining a flash flood (NWS)

• NWS A flood which follows within a few hours
  (usually less than 6 hours) of heavy or excessive
  rainfall, dam or levee failure, or the sudden
  release of water impounded by an ice jam.
• NWS currently claims a 52-min lead time on flash
  floods but how do we know when it starts what
  is the standard deviation?
• NATO Advanced Study Institute (1999) A flood in
  which the causative rainfall and subsequent
  runoff are occurring on the same time and space
  scales.

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Basin Size versus Intense Precip Area
Larger intense precip footprint can lead to
greater flash flood threat ?
? Minimal flash flood threat
Increased flash flood risk for small basins
completely covered by intense precip area ?
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Dallas (5 May 1995) Intense Precip Impacts Small
(12 mi2) Urban Basins
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Dallas (5 May 1995) Intense Precip Impacts Small
(12 mi2) Urban Basins
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Basin Size versus Intense Precip Area and its
Movement
? ? ? ? ?
Greater flash flood risk when the precip area is
moving downstream with time.
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Rapidan River, Virginia (27 June 1995) Intense
Rain Moving Slowly Downstream
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Virginia Topography
Radar-derived accumulation 27 June 1995
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Basin Size versus Intense Precip Area Movement
and Propagation
Rapid regeneration or training can result in a
greater wet footprint in a short time and
enhance flash flood risk over somewhat larger
basins
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Aurora, IL (18 July 1996) Rapid and Vigorous
Regeneration and Training
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Aurora, IL (18 July 1996) Large Wet Footprint,
Even Some Larger Basins Impacted
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Aurora, IL (18 July 1996) Large Wet Footprint,
Even Some Larger Basins Impacted
gt16
Illinois State Water Survey
Isohyet Analysis as of 1200 UTC 18 July 1996
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(No Transcript)
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Fire and Flash Flood

• Burn area considerations in sloped pine forests
• High-intensity fires increase the sand fraction
  of the surface soil layer
• Oils from the trees can infused into the soil
  beneath the sandy layer
• Top (sandy) layer erodes during heavy rainfall
  large sediment transport

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Buffalo Creek, Colorado (12 July 1996) 45-50 min
of Intense Rain on Fire Scar
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1-h Accumulation, 12 July 1996
?Fire Scar
Buffalo Creek, CO, flash flood
Purples50-75mm
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Just before severe weather in Fort Worth
42 kft?

Cross section ?

?Cross section
Dallas
23Z 5 May 1995 DFW Radar Reflectivity and Cross
Section
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Deadly rainstorm in Dallas
42 kft?
Dallas
02Z 5 May 1995 Reflectivity and Cross Section
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Moist ambient environment
Intense Storm?
12 August 2000, 14-inch totals 8 inch/hr rates
Enhanced low-level flowintersecting a boundary
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Sparta, NJ (12 August 2000) Low-centroid
(Warm-topped) Storm High Rainfall Rates