Adverse Impact on Ponds of Organic Debris Deposited by Tornadoes

Article excerpt


On May 3, 1999, 58 tornadoes were reported in Oklahoma in one of the largest outbreaks of severe weather in Oklahoma history. The tornadoes killed 44 people and destroyed over 3,100 homes. In addition to the obvious damage inflicted on residential areas, the tornadoes caused extensive damage to natural resources by destroying trees and other vegetation. Several ponds in the path of the largest tornado also were affected. Analysis of water quality revealed that the ponds had become incapable of supporting aquatic life because the decay of wind-deposited organic material had created a reducing atmosphere, which caused levels of dissolved oxygen to sink. Using National Weather Service data from the period 1950-1995, researchers calculated the mean annual area swept by tornadoes in Oklahoma to be approximately 19,000 acres. Combining this information with the estimated number of ponds in Oklahoma suggests that approximately 100 ponds would be in the path of a tornado each year.


Tornadoes consist of rapidly rotating winds that circulate around a small area of intense low pressure and almost always begin as a funnel-shaped cloud attached to a large cumulonimbus cloud. The funnel cloud is labeled and recorded as a tornado only after it touches the ground and has been visually identified. Physically, all tornadoes are the same phenomenon, but they differ in size, intensity, and potential for damage. To classify the strength of tornadoes according to wind speed and observed damage, Theodore Fujita developed the F-scale in the late 1960s (Table 1) (1).

Not all tornadoes result in property damage or human injuries or fatalities. Oklahoma, for example, had 2,378 tornadoes between 1950 and 1995, and only 66 of them caused fatalities. Tornadoes typically occur in the spring; however, at least one tornado has occurred in each month in the United States between 1950 and 1999.

One of the crucial aspects of planning a response to tornado outbreaks is knowing the likelihood of occurrence. Rutch et al. formulated a model that predicted the annual probability of a tornado of a certain wind speed hitting a given area (2). This model consisted of two different probabilities--the probability that a tornado would reach a given wind speed and the probability that a tornado would hit a particular area. Tornado probability was found to be greatest in the central portion of the United States, with central Oklahoma and north central Alabama showing the highest probabilities of being struck. Rutch et al. compared the relative frequency of different F-scale tornadoes by dividing the United States into four quadrants (northeast, northwest, southeast, and southwest--latitude and longitude not specified). The wind speeds in each of the four quadrants were found to fit similar Weibull distributions, indicating that the proportion of tornadoes that fall within a given F-scale is fairly uniform among quadrants.

A number of studies have looked at the potential effect that factors such as population density, topography, and global climate patterns may have on tornado formation and frequency. Changnon conducted a five-year study of severe storms in the St. Louis, Missouri, area and concluded that "urban mechanisms tend to intensify rather than initiate convective activity" (3). Elsom and Meaden studied the suppression and dissipation of tornadoes in Greater London (4). They estimated that about 80 tornadoes occur each year in the United Kingdom. Their findings further indicated that, in the case of weak tornadoes, urban factors such as surface roughness and the urban heat island effect appear to suppress the formation of tornadoes and may have dissipated some tornadoes that were formed outside the urban area. Aguirre et al. studied the impact of urbanization and development on tornado occurrence from 1950 to 1990 in the United States and found that in metropolitan areas, the odds of tornado occurrence were higher than in rural counties (5). …