Monday, July 7, 2008 - 1:15PM 
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Documenting flash flood waves
(I've been intending to do a full write-up on this for the site, so this blog post may eventually graduate to a full article sometime in the future.)
One of the many natural weather-related phenomena that is fascinating to me is the development and progression of a flash flood. I've been 'chasing' these events for a few years now and have found that locating and documenting the process is a lot of fun, nearly as much as a lightning or tornado chase. On the flip side, aside from icy roads in the winter, no other weather hazard makes me as nervous and demands as much caution from me as a flash flood - they are (second to icy roads) the most dangerous things I encounter when covering weather stories. Consequently I would NOT recommend that a beginner take part in anything I'm about to describe. The main safety issue of a flash flood is having a quick escape route for both you and your vehicle to significantly higher ground (as in halfway up a mountainside). Getting yourself out of harm's way is simple (just walk uphill perpendicular to the creek), but it takes some forethought to make sure you also get your car (and all of its contents) out of the way. A major flash flood can fill a valley from mountainside to mountainside with more than 15 feet of fast-moving water, inundating all roads! It would be quite easy to lose your car in a flash flood if you don't plan ahead. Floodwater is also extremely unhealthy to come in contact with, often being contaminated with all sorts of bacteria and raw sewage that it has picked up along the way.
That said, my favorite flash flood effect is the flood wave, which is a rapid rise of water in a creek that occurs when a head of storm runoff develops and moves downstream. Most flash floods have several waves happening in succession, each causing additional water level rises, as runoff from each individual upstream tributary arrives. These waves can arrive suddenly and without warning, and during a strong event, the first wave is usually very dramatic to witness. I have seen flash flood waves cause creek rises of 8 to 15 inches in less than 30 seconds, usually the most impressive of which happens when the creek is initially at normal flow.
The best example of this that I managed to capture on video was at Mill Creek in Green Sulphur Springs, West Virginia (Summers County) on August 6, 2005. This flood wave caused a rapid rise in the water level, filling the creek bank-to-bank in a few seconds! The source of the flood was a strong thunderstorm parked on the ridges about 5 miles upstream, which I estimated dropped 2 inches of rain in about 40 minutes. I had the cameras set up in the creek bed, not expecting such a rapid rise. I had to quickly get them out before they were swept away! Here is the video from the two cameras:
Camera #1
Click for video
Camera #2
Click for video
The cause of these and all flash floods are significant heavy rainfall in a short period of time, typically associated with thunderstorms that either remain stationary or 'train' over the same area again and again. While flash floods can happen anywhere, mountainous terrain is the most prone to the phenomenon, as the steep mountainsides funnel runnoff into stream channels very rapidly. In most parts of West Virginia, a flash flood can occur with as little as an inch or two of rain in 30 minutes.
Predicting and locating a flash flood
Predicting a flash flood is fairly simple - one just has to observe radar, looking for strong storms that either are not moving or are passing over the same area repeatedly. The worst flooding will be inside and just downstream of the area where the heavy rain has fallen. A topo map is helpful in identifying watersheds and predicting where the runoff will flow. Once a suspect location is identified and traveled to, I then start looking for visual 'excessive runoff indicators' that show signs of a developing flash flood. A flash flood doesn't start in a creek, it starts all over the landscape - and therefore the clues of an imminent flood will be everywhere you look. There will be numerous 'new' waterfalls (normally not present) cascading off of rocky cliffs, water ponding on roads and in yards, and swiftly running water filling and overflowing ditches and culverts. These are all signs that the runoff volume is greater than the landscape's normal drainage pattern can handle. One of the best indicators of excessive runoff is a ditch overflowing onto a roadway, with current strong enough to sweep gravel and other debris onto the road. All of this water will soon make its way into the creek beds, where the main stage of the flash flood event begins.
A flash flood in progress will usually result in a strong, distinctive 'silty' smell permeating the air. The presence of this odor alone can be an indicator that a flood is happening nearby.
The flood wave
As the flash flood runoff arrives in a creek basin, it tends to develop a 'wavefront' or leading edge that travels down the creek. The wavefront causes a rapid rise in the water level and an increase in flow rate in a short period of time, from a few inches to more than two feet, sometimes in mere seconds! The first wave can be the most dangerous, catching fishermen and swimmers by surprise. After the initial surge of the wavefront arrives, the water will continue to rise at a slower rate until the crest of the wave is reached, which can take anywhere from minutes to hours. However, there will be a wavefront for each upstream tributary that empties into the creek, resulting in a multiple-stage water rise downstream that an observer must be wary of. This means that before the crest of the first wave arrives, additional wavefronts can arrive, causing additional rapid surges in the water level at any time. While the water rises very rapidly once the wavefront arrives, the wave itself usually travels somewhat slowly down the creek (from about a walking to running pace). You can normally easily drive ahead of a flash flood wavefront and watch it pass again and again. The wavefront and the flow behind it will often contain tree branches, leaves, trash and other small floating debris, and the water will be opaquely colored brown from upstream silt and mud.
Since creeks and streams get wider as they travel downstream (as tributaries empty in to them), a flood wave will gradually spread out and weaken as it moves downstream farther away from the storm producing it. Weaker flood waves may be barely noticable more than 10 miles away from their source, only discernable by a slight silty coloring, a few leaves and floating debris, and a water rise of an inch or two every 10 minutes.
July 6, 2008 weak flood waves
With stationary and slow-moving storms firing in the mountains to the east of Charelston, I set out Sunday afternoon to chase flood waves. For the most part, the storms did not last long enough to create any significant waves that I saw. There were likely more significant waves down on the Virginia border, where flash flood warnings were posted.
The following pairs of images are before-after shots of two weak flood waves that I caught, the first in Paint Creek near Pax, WV and the second in Pond Fork near Wharton, WV. The waves took about 5 minutes each to reach their peak rises of about 7 inches, not very impressive but still interesting to watch. The creeks were continuing to rise at a rate of about an inch every 10 minutes after the initial wave, though after seeing that no significant waves were coming from upstream, I didn't stay around after the inital rises. (Click photos to enlarge)
Paint Creek, Pax, WV:
 
 
Pond Fork, Wharton, WV:
 
 
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