Floods and Flash Floods

…Today’s Severe Weather Awareness Week Topic is on Flood & Flash Flood Safety...



The flash flooding in September damaged a number of streams…and mitigation is ongoing to remove debris and shore up heavily damaged areas along the streams in and near the foothills from Larimer County to El Paso County. The snowpack in the South Platte Basin is above normal so there is concern when the melt occurs that streams may not be able to handle a fast melt. Snowpack is also well above normal in the Yampa, Upper Colorado and northern Gunnison River Basins.  The rate at which runoff occurs is dependent on a number of factors including the depth of the snow…how fast hot temperatures develop…and whether thunderstorms drop heavy rain on the snowpack with runoff combining with falling rain. In 2011 we had a similar high snowpack with minimal flood issues as the warm days and cool nights moderated the rate of snow melt.  The peak snowmelt season usually extends from late May through early June. 
In September over 8 inches of rain fell over a large area of the foothills from northern Jefferson to northern Larimer counties. All this rain filled area creeks and rivers…causing considerable flash flooding. Runoff from area streams combined to cause major flooding along the South Platte River. 
Flash flooding refers to a dangerous sudden rise in water along a creek...wash...river...or over a normally dry land area.   Flash floods result from heavy rainfall...sudden breaks in river ice jams...and dam or levee failures. Flash floods can occur within a few minutes or hours...and can move at surprisingly high speeds...striking with little warning.   Flash floods are quite destructive because of the force of the moving water...and the debris that accumulates in flood waters...such as trees and boulders...which can destroy roadways...bridges and buildings.
Other complications in Colorado are recent fires which raise the flood threat when locally heavy rain falls on recent burn scars.   Residents in and near burned areas near Mancos, De Beque or in the Front Range Foothills from Larimer to El Paso Counties should plan ahead on response actions for flooding. 

The National Weather Service will discuss flood and flash flood potential in daily Hazardous Weather Outlooks and in the “Weather Story” on National Weather Service websites.

On days with a high threat for flooding you may hear...
…a flash flood or flood watch, which means that flash flooding or flooding is possible within the watch area. 
…a flood warning, which means that flooding is imminent or has been reported along a river.
…a flash flood warning, which means that flash flooding has been reported or is imminent. When a flash flood warning is issued for your area, act quickly. If advised to evacuate, do so immediately. Go to higher ground or climb to safety before access is cut off by flood waters.

An Urban Flood Advisory will be issued for impact flooding that is not in itself life threatening. In an urban area if you were commuting during rush hour during a Flood Advisory you could expect some intersections to be underwater and a much longer commute.   A Small Stream Flood Advisory might be issued when flows are at bankfull with minor lowland flooding along the stream.  

Nearly half of all flash flood fatalities are vehicle related. Do not enter a flooded roadway. Instead, “Turn Around, Don’t Drown”. In rapidly rising waters, backing up away from water may be safer. One to two feet of water will carry away most vehicles. Additionally, roads inundated by water may be damaged or washed out with no way for you to see that the road is impassable.

Colorado’s Severe Weather Awareness Week continues through Saturday. 

...on Sunday............Introduction 
...on Monday……...Watches and Warnings
...on Tuesday……..Tornadoes and Tornado Safety
...on Wednesday...Flood and Flash Flood Safety
...on Thursday…....Downburst Wind and Hail  
...on Friday………...Lightning and Lightning Safety
...on Saturday…......Severe Weather Awareness Week in Review


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