Matthew Kelsch NWA Annual Meeting,18 October 2006 Hydrometeorological Training: What Makes a Flood a Flash Flood? Matt Kelsch UCAR/COMET ® NWA Annual Meeting 18 October 2006

Matthew Kelsch NWA Annual Meeting,18 October 2006 Hydrometeorological Training: What Makes a Flood a Flash Flood? Basic Hydrologic Sciences Course: 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 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 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 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 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 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 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 volume 2)Faster drainage to stream channels and low areas Blue: stream channels Gray: Impervious Surfaces Red: major storm drains

Matthew Kelsch NWA Annual Meeting,18 October 2006 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 Kelsch NWA Annual Meeting,18 October 2006 Urbanization: More and Faster Runoff Las Vegas, 19 Aug 2003

Matthew Kelsch NWA Annual Meeting,18 October 2006 video

Matthew Kelsch NWA 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 Kelsch NWA Annual Meeting,18 October 2006 Rapid Release of Impounded Water 9 October 2005

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

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

Matthew Kelsch NWA 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 Kelsch NWA Annual Meeting,18 October 2006 Hydrometeorological Training: What Makes a Flood a Flash Flood? Basic Hydrologic Sciences Course: 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