East Bay Hills Fire (Oakland-Berkely, CA - October 1991) » Analysis - Fire Characteristics

In this case it is evident that the wind played a major part in every aspect of the fire’s growth and spread, when the wind died down, the fire’s progress was stopped.

The East Bay Hills fire has been described as a conflagration and as a fire storm in media accounts. The synergistic effects of the wind and the thermal energy released by the fire created unusual fire phenomena that exhibited some of the characteristics of a fire storm on a localized basis, but the term conflagration is more appropriate for the overall situation.

A fire storm is defined as a situation in which the fire’s thermal energy creates its own weather phenomena, overpowering natural meteorological forces. There were many observations of flaming whirlwinds, crowning fire in the treetops, and rolling clouds of fire moving through the air or along the ground. Firefighters described balls of fire floating through the air around them. These descriptions are evidence of localized fire storm characteristics. One witness described the fire in Hiller Highlands as a single swirling mass of flame, involving buildings, trees, and vehicles, consuming everything as if it was in a gas oven. This area was totally consumed by the fire, which spread so rapidly that residents died in the streets trying to outrun it.

Natural convective forces cause a fire’s plume of smoke and superheated gases to rise. A fire of this magnitude releases tremendous quantities of thermal energy into the plume every second. It is estimated that a wind between 15 and 30 miles per hour is sufficient to bend the thermal column and cause it to travel parallel to the ground, projecting out ahead of the fire. The superheated gases preheat exposed fuels, hundreds or possibly thousands of feet ahead of the fire, causing them to burst into flames with almost explosive force when they are ignited. The ignition may come from a flying brand or a glowing ember, or the preheating may continue until materials reach their autoignition temperatures. The ignition process is similar to the effect of a thermal layer of superheated gases within a room that radiates heat down onto the exposed contents prior to flashover.

One phenomenon that was observed at this fire was the ignition of the tops of wooden power poles ahead of the fire. The tops of the poles were high enough to project into the thermal layer and were ignited by convective heat transfer over the heads of firefighters working below. This suggests that the firefighters were working in an area that was being preheated by radiant heat transfer from the superheated gases above, as well as from the approaching flame front.

The actual spread of the fire, in most cases, was observed to be flaming brands and embers, carried by the wind and dropping onto ignitable fuels ahead of the fire front. The ignitable fuels included trees, brush, grass, and other natural fuels, as well as wood roofs, debris in raingutters, and other combustibles around structures. The preheating process dried any remaining moisture from the fuels and may have elevated their temperatures close to their autoignition point before the brands or embers landed. When an open flame came in contact with these preconditioned fuels, they would become involved in a fraction of the normal time.

In many cases the embers, blown by the strong winds, were seen to work their way in under the eaves of houses, readily igniting even tileroofed structures. Where the walls and roofs resisted ignition, the exposure caused by burning trees and brush was often sufficient to cause interior ignitions by radiant exposure through windows. The worst cases were the hillside homes, where natural fuels carried the flames directly under overhanging structures.

Very few structures in the area escaped total destruction. Most of the houses burned to the ground, as the fire totally consumed all available fuels. The combustion process was very rapid, as it took place in a superheated environment with a constant supply of fresh air, so houses became fully involved very rapidly and were totally consumed in less than an hour. The same observation was made of the more than 2,000 vehicles that were burned in the fire area; in most cases only a hulk of corroded steel and melted glass was left behind.

The most rapid fire spread occurred during the first hour of the fire. Temescal Canyon provided a natural draw for the fire with the wind blowing in over the ridge and down into the canyon, then sweeping out toward the open end. This action split the fire into two fronts and then carried the fire along the north slope, spreading more than a mile in the first hour. This initial spread carried the fire all the way to Hiller Highlands, before it jumped 2,000 feet of freeway interchange and open space to begin its run on the other side of Lake Temescal. In comparison the head of the fire advanced only three quarters of a mile in the next seven hours in the densely built-up Rockridge and Forest Park neighborhoods.

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