RAPID PREDICTION OF CHEMICAL CLOUD DISPERSION FROM CHEMICAL SPILL ACCIDENTS

Responders to spill accidents need to have at their fingertips a metho d of rapid prediction of chemical cloud dispersion behavior and other relative information to protect the public from death and injury durin g evacuation of the affected area. A survey of 123 chemical spill acci dents in the United Stares where evacuations occurred revealed that av ailable mathematical models were not used to predict cloud dispersion behavior while the event was under way, although modeling was sometime s done after the fact The varied reasons for why modeling was not done include the fact that models require complex information on meteorolo gy, the accident took place when modelers were not available, and some times, responders did not know what chemicals were released. The evacu ation distances based on a gir;en downwind concentration (level of con cern) vary considerably, depending upon the chemical released and upon atmospheric turbulence resulting from solar insolation and wind blowi ng across surface features. Examples are presented. The problem is com plicated because available mathematical models and look-up tables ofte n do not give the same answer under the same conditions, nor do author ities agree on the concentration levels representing a level of concer n on which to base evacuation. The responders to chemical spills need to be aware of these limitations, recognizing that modeling is a tool and does not represent ultimate truth. Yet modeling is useful to respo nders and is required by law in the United Stales in developing possib le accident scenarios under risk mangement plans as part of the Clean Air Act. Western Research Institute, in surveying the problems, has de termined a need to communicate the benefits of modeling in a format de signed for responders and others in the decision-making processes. A s ystem for estimating evacuation distances has been developed for a han d-held computer for field use. The major features are that the user in puts the chemical spilled, spill location, time of day, cloud cover, a nd source spill information; evacuation distances based on a level of concern are calculated in seconds. The system is designed for planners and emergency response personnel without special modeling or computer skills. The results compare favorably with real field data and with d ense gets and neutrally buoyant models in the public domain.