Flash Flood Awareness

There are a number of factors that contribute to the potential for a flash flood. The most obvious factor is the weather, but it is not the only one. You must also consider watershed and potential runoff.

The surface flow of all precipitation is called runoff. The amount of runoff in an area depends on several variables including the amount of precipitation, size of the catchment, surface slope, permeability of surface materials, density of vegetation cover and evaporation rate.

For short distances, runoff may occur in unconcentrated sheets, or it may subdivide into tiny rivulets. Eventually this flow covers the land with a pattern of well-defined channels that join even larger ones and ultimately form streams. Each major stream and all of its tributaries make up a stream system. Streams flow through valleys that are usually of the stream’s own making. The higher ground that separates one valley from the next is called an interfluve. On an interfluve that separates two entire stream systems, there is an imaginary line called a divide. On one side of the divide all runoff flows into one stream system, while on the other side runoff flows toward another stream system. The land surface drained by a stream system is called its watershed (also known as catchment area). The larger the catchment, the greater the risk of flash flood.

Estimating the surface area of the watershed requires that you first identify the divide lines that surround the stream system. This can be accomplished by studying a topographic map of the area using contour lines. From the watercourse, move your eyes up the side of the canyon from one contour line to the next until you come to a ridge, hilltop or saddle. Mark that high point with a pencil. Repeat this process in several places on both sides of the canyon. Next, connect your marks, following the ridges, hilltops and saddles. If you drew it correctly, it should be obvious that water on the inside of your outline will run downhill into your canyon. Water on the outside of your outline will run downhill into other drainages.

Catchment would be measured in square miles or square kilometers, but it is not necessary to calculate the exact size. It is more important to evaluate the relative size – is it significant enough to drain large volumes of water even if precipitation is light?

Next you should evaluate the surface slopes of the terrain within the Catchment by examining the contour lines of the topographic map. This will provide an indication of how rapidly precipitation will drain into the stream system. If the surface materials of the watershed are relatively impermeable (i.e. rock or clay) and there is very little vegetation to slow its movement, precipitation falling on steep slopes will drain into the stream system very rapidly. Conversely, a significant percentage of the precipitation that falls on gradual slopes with dense vegetation may move slow enough to be absorbed (especially in highly permeable surface material) or to evaporate before it reaches the stream system.

Upstream Precipitation
It is a beautiful, sunny day – not a cloud overhead. But common sense tells us that we need to be concerned, not simply with the sky directly above, but with what is happening upstream. How far upstream depends upon the size of the watershed that feeds the canyon. If the watershed is relatively small, upstream could mean only a few miles, so we can probably see the sky above it to watch for clouds. If the watershed is large, upstream could mean a hundred miles and the sky above it will be too far away to watch. Checking local weather patterns will give you an idea of the direction from which weather systems develop. If they tend to develop from a direction that happens to be downstream, you may see changes in the weather forming as it passes overhead moving upstream. If they tend to develop from a direction that happens to be upstream, or in either direction perpendicular to the canyon, changes could occur behind you without your knowledge. It will be even more critical to seek accurate and timely weather information and to analyze all the other factors described in this section.

Snowmelt
If snow is present anywhere within the watershed or along the divide lines, you must also consider snowmelt as a factor in evaluating risk. This is especially true in spring and summer months when it is common to experience rain on a warm day when the snow is melting upstream. The combined water volume can create a deluge, even in canyons with small watersheds.

Other Upstream Factors
As you study your maps and scout the watershed, you should also look for other upstream factors that could add to the risk of a flash flood. Of particular importance are dams, both natural and manmade. A beaver dam, or even a log jam, will create a reservoir of water that could be released all at once if it breaks. Even if the watershed is not wooded, don’t discount the possibility that a natural dam could have been created by a rock slide or the accumulation of other vegetation.

While there may not be much risk of a manmade dam breaking, it is common for water to be intentionally released when the reservoir becomes too full or irrigation is needed downstream. With a telephone call you can determine if a release is likely and to inform the dam operator of your plans.

Recent Precipitation
Determine how much precipitation has been received within the watershed during the past few weeks. If a significant amount has been received it is possible that the water table beneath the surface is already at or near capacity. As a result, additional amounts may not be absorbed as readily, even with highly permeable surface materials. Conversely, the lack of precipitation over an extended period may result in dry, hardened surface materials, adversely effecting absorption rates.

Quantifying the Risks
Unfortunately, there is no mathematical equation for determining flash flood risk given a certain amount of precipitation on a given size watershed, etc. There are simply too many variables involved, most of them immeasurable. The intent of this section is only to point out the factors that you need to consider in making your own subjective determination of relative risk. It is extremely important for you to invest the time to study your maps, scout the canyon and its watershed and check the weather forecast. Your decision to enter a canyon when the risks are high could have very dire consequences.

Article by Rich Carlson ACA