Last weekend you heard the weather services issuing a flood watch, a flood warning and a flash flood warning. All of these advisories are based on a number of factors that can be different with every storm. Consequently, much needs to be taken into consideration before these advisories are issue.The primary factor, of course, is the amount of rainfall. However, there are instances when a rainfall of as little of one to two inches can create flooding problems and a rainfall of twice that much may have only a modest impact. When the rainfall is in excess of five or six inches, there is almost always a good chance that some level of flooding will occur in our local watersheds.
Weather services have a great deal of information available to them today. Predicting rainfall timing and amount is much more precise than in years past. It is, however, still difficult to predict precisely how much rainfall will occur in a specific area. For example, in the storm last weekend my rain gauge near West Chester measured four inches while rainfall in areas five to 10 miles away varied by as much as two inches.
When the weather service has a fix on the amount of rainfall expected, there are several other factors to consider. One factor is the intensity of the rainfall. A storm of two or three inches that falls over a 24 to 48 hour period will have less impact than the same storm that takes place in six hours. When rain falls over a greater period of time, the land is able to absorb the rainfall easier than it can if the rain comes in a very short time period. In the storm of last weekend the rain lasted perhaps 12 hours, but had its greatest intensity over a four to five-hour time span.
Another factor to consider is the soil condition. If it has rained before a major storm and the ground is already saturated, most of the rain will run off. Likewise, if the ground is frozen or hard due to lack of rain, a very intense storm will largely run off. Ideal conditions for infiltration occur when the soil is slightly dampened. Last weekend a small amount of rain came early which softened the hard soil and allowed the more intense rain which followed to infiltrate better.
A good example of a saturated soil condition occurred in September of 2003. While we were all waiting for a storm to occur at the end of the week, an unexpected storm dropped nine to 10 inches over a six to eight hour period from Sunday night into Monday morning. Because the ground was already saturated, virtually all of the rain ran off. Because the rain occurred in a very short period of time at a very high intensity, this increased the runoff. The result was true flash flooding and damage in areas that have otherwise been free of flooding.
One other factor is important in gauging flood levels. The amount of impervious surface in a watershed plays a major role in flood levels. Where watersheds are highly developed and contain high percentages of roofs, driveways and other impervious surfaces, there is little for the rain to do but run off. In these conditions, even lesser storms can have flooding impact.
Where watersheds are largely fields and forested areas, there is more opportunity for rainfall to infiltrate, even at higher intensity and volume. In those more rural areas impervious surfaces generally cover less than ten percent of the water-shed, often nearer to five percent. Even in these rural areas an intense storm, such as the September 2003 event, can cause serious flooding. When a storm has significant rainfall in a short period of time on soils that are saturated there is very little that can be done to prevent flooding.
Weather services take all of these factors into consideration when they predict flood levels. In the Brandywine Watershed long term stream gauge measurements have also helped to predict flood levels downstream. This history provides information for streamside residents who can apply flood proofing to their homes or businesses or are able to move cars to higher ground.
Flood warning has become a much more scientific and specialized practice as more information is available on the conditions that produce flooding. Nevertheless, it is not yet a precise science that can predict exact levels in every stream system. As the science evolves predictions will get better. In the meantime, it is a good practice to pay attention to and heed the flood warnings.
o Robert G. Struble Jr. is executive director of the Brandywine and Red Clay valley associations, the oldest and second oldest watershed organizations in the country. He lives in East Bradford Township.