The Reston North Park & Ride commuter lot is covered with flood water following remnants from tropical storm Lee in 2011. Many of the cars are submerged and are not visible in this photograph. Photo by Olga Bogatyrenko

Compared to flash floods, tornadoes and hurricanes are more sexy and get more coverage on TV.   There are no flash flood chase teams.  However, flooding and flash flooding combined are a leading cause of weather-related fatalities in the the United States.

According to a 20-year National Weather Service study ending in 2006, an average of 99 people per year perished due to flooding and flash flooding. While the number of fatalities for tornadoes and hurricanes has decreased, the number of fatalities due to flooding and flash flooding has more or less remained constant (the current 30-year annual average through 2012 is 95). Most people who die in flash floods make one fatal mistake – driving into flood waters.

Flash floods have impacted every state in the union. And they are a major hazard internationally.

On Wednesday alone, news broke of:

* Multiple water rescues in Roanoke from flash flooding. The city had its rainiest July day on record and areas just to its north were reportedly deluged by up to 4 inches of rain in one hour.

* Major flooding in the Pittsburgh area

* Dozens of fatalities in the Sichuan province in China from flash flooding.

* Cars being swept off the road along Highway 24 in Colorado in the Waldo Canyon area after flood waters poured over burn scars from recent wildfires (the soil and vegetation that would absorb the water was charred).

Definition of flash flooding

The National Weather Service (NWS) defines a flash flood as a rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a pre-determined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). They caution that the length of time between the heavy rain and flooding may vary from minutes to hours.

The NWS also notes that flash flooding can occur even if no rain has fallen; for instance, after a levee or dam has failed, or after a sudden release of water by a debris or ice jam.

Twenty notable flash floods

Twenty notable fatal flash floods since 1960 are listed in the table below.  Some of the top killers were associated with a combination of heavy rains and dam failures (usually earthen dams).

A list of some notable flash floods that led to multiple fatalities. The floods are color-coded for the season in which they occurred. Red were during June-August, blue occurred in December-February, while magenta were during September-November and the lone green was March-May.

Our country has approximately 76,000 dams of which 80 percent are earthen dams according to the NWS.  Such dams are susceptible to damage and failure when intense and/or prolonged heavy rainfall occurs.   People living downstream of an earthen dam should pay close attention to local officials when flash flood watches have been issued and should evacuate and move to higher ground if a dam or levee is compromised.

Dam failures are one of the leading causes of the proverbial wall of water that people envision with flash flooding.  The 1972 flash flood in Rapid City, South Dakota and the 1972 flood disaster in West Virginia are two striking examples of how deadly dam failures can be.

A large percentage of the killer flash floods noted in the table were associated with inland flooding from the remnants of or decaying tropical systems, especially when interacting with fronts.

The frontal interaction allows the heavy rainfall to develop well to the north of the storm center. This can lead to an extended period of rain as a series of storm cells line up like a train, repeatedly dousing or “training over” the same area.

Flash flooding associated with the remains of Hurricane Camille in 1969 killed 153 people in and around Nelson County, Virginia as the storm remains interacted with the mountains.  56 people lost their lives from inland flooding in North Carolina when the moisture associated with Hurricane Floyd interacted with a front in eastern North Carolina.  Around half of those people died in cars.

According to a NOAA study, “More than 60 percent of U.S. hurricane deaths from 1970-1999 occurred in inland counties, with more than half of tropical hurricane deaths related to freshwater flooding.”

The deadly August 1971 flash flood that impacted the Baltimore area and the 2011 flooding from the remnants of tropical storm Lee are red flags that serious, deadly flash floods can happen in the Washington-Baltimore region.

Factors that contribute to flash flooding

In addition to the amount of water, other critical factors help determine whether a serious flash flood might develop.  The shape and characteristics of the stream basin help determine how quickly and easily flooding might develop along the stream.   A stream basin with steep, rocky terrain with few trees will typically flood easier than a stream with a basin that is less sloped and has more trees and permeable soil.

Areas where forest fires denuded the terrain, especially in mountainous regions, are particularly susceptible to flash flooding and mud slides.

The antecedent conditions of the basin are also critical.  When a series of days have been plagued by numerous thunderstorms leaving the soil extremely moist, it takes much less rainfall to produce flooding than when soil moisture and stream flow is low because of an extended drought.

Washington, D.C.’s streak of rainy days during late June and the very high soil moisture were the primary reason that the NWS issued such a broad Flash Flood Watch on July 1 and 2.  As little as an inch of rain in an hour had the potential to produce flash flooding in some areas.

Occasionally in winter in our area, rapid snow melt can produce flash flooding. In January 1996, warm and moist air, strong winds from the south and rain led to rapid snow melt and a significant flood and flash flood event across the mid-Atlantic and Northeast (link: more information on 1996 flood).

Across the northern tier and in the mountainous terrain out west, snow melt can sometimes be the primary player in causing a flash flood.  The Red River Valley, which covers parts of Minnesota and North Dakota, routinely floods each spring because the river drains to the north where the snow and ice are slower to melt leading to ice jams and dams.

Stay tuned for more on flash flooding tendencies in the Washington and Baltimore area in a follow-up to this post.