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Flash Floods: Relationship Between Basins and Precipitation

Matt Kelsch

10 September 2002

kelsch@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 basin’s 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.

Basin Size versus Intense Precip Area

Minimal flash flood threat

Increased flash flood risk for small basins completely covered by intense precip area

Larger intense precip footprint can lead to greater flash flood threat

Dallas (5 May 1995): Intense Precip Impacts Small

(12 mi2) Urban Basins

Dallas (5 May 1995): Intense Precip Impacts Small

(12 mi2) Urban Basins

Basin Size versus Intense Precip Area and its Movement

Greater flash flood risk when the precip area is moving downstream with time.

Rapidan River, Virginia (27 June 1995): Intense Rain Moving Slowly Downstream

Virginia Topography

Radar-derived accumulation 27 June 1995

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

Aurora, IL (18 July 1996): Rapid and Vigorous Regeneration and Training

Aurora, IL (18 July 1996): Large Wet Footprint, Even Some Larger Basins Impacted

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Isohyet Analysis as of 1200 UTC 18 July 1996

Illinois State Water Survey

>16”

Aurora, IL (18 July 1996): Large Wet Footprint, Even Some Larger Basins Impacted

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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

Buffalo Creek, Colorado (12 July 1996): 45-50 min of Intense Rain on Fire Scar

1-h Accumulation, 12 July 1996

Purples=50-75mm

Fire Scar

Buffalo Creek, CO, flash flood

23Z 5 May 1995 DFW Radar: Reflectivity and Cross Section

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Cross section

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Cross section

Dallas

42 kftJust before severe weather in Fort Worth

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02Z 5 May 1995 Reflectivity and Cross Section

Dallas

42 kft

Deadly rainstorm in Dallas

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12 August 2000, 14-inch totals

8 inch/hr rates

Intense Storm

Moist ambient environment

Enhanced low-level flow

intersecting a boundary

Sparta, NJ (12 August 2000): Low-centroid (Warm-topped) Storm; High Rainfall Rates