Embed Size (px)
Citation preview
Matthew KelschNWA Annual Meeting,18 October 2006
Hydrometeorological Training: What Makes a Flood a Flash Flood?
Matt KelschUCAR/COMET®
[email protected] Annual Meeting
18 October 2006
Matthew KelschNWA Annual Meeting,18 October 2006
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 KelschNWA Annual Meeting,18 October 2006
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 KelschNWA Annual Meeting,18 October 2006
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 KelschNWA Annual Meeting,18 October 2006
What Makes a Flood a Flash Flood?Soil Properties That Reduce Infiltration
• Small particle size (clay)
• Compaction• Intense Fire
Matthew KelschNWA Annual Meeting,18 October 2006
What Makes a Flood a Flash Flood?Faster Drainage, Less Infiltration Time
Steep drainages• “straightening” channels
increases steepnessReduce surface roughness• Vegetation removal,
pavement, culverts,… reduce “roughness”
High Stream Density• More tributaries• Psuedo-tributaries…
road and sewer networks
Matthew KelschNWA Annual Meeting,18 October 2006
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 KelschNWA Annual Meeting,18 October 2006
What Makes a Flood a Flash Flood?Urbanization: More and Faster Runoff
Impervious Cover Surface Roughness Stream Density Channel Length Slope Vegetation
All of the above lead to:1) Greater runoff volume2) Faster drainage to
stream channels and low areas
Blue: stream channelsGray: Impervious SurfacesRed: major storm drains
Matthew KelschNWA Annual Meeting,18 October 2006
Radecke Avenue
Sinclair Lane
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
Urbanization: More and Faster Runoff
Matthew KelschNWA Annual Meeting,18 October 2006
Urbanization: More and Faster RunoffLas Vegas, 19 Aug 2003
Matthew KelschNWA Annual Meeting,18 October 2006
video
Matthew KelschNWA Annual Meeting,18 October 2006
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
Rapid City, SD, June 1972
Fort Collins, CO, 28July 1997
Kansas Turnpike, 30 Aug 2003
Matthew KelschNWA Annual Meeting,18 October 2006
Rapid Release of Impounded Water9 October 2005
Matthew KelschNWA Annual Meeting,18 October 2006
near Alstead, NH
Matthew KelschNWA Annual Meeting,18 October 2006
Hinsdale, NH
Matthew KelschNWA Annual Meeting,18 October 2006
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 KelschNWA Annual Meeting,18 October 2006
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
