Within commuting distance (40 to 60 miles) of metropolitan areas, development most often takes the form of relatively expensive residences on medium-to-large lots. Unless prohibited by well designed and effectively enforced ordinances, developers tend to leave as much native vegetation as possible to maintain and promote the rural effect that the buyers seek. Similarly, developers usually provide wood shingle or shake roofs, picture windows, and sun decks. Water and other utilities are usually provided only in amounts adequate for normal use unless fire codes are strictly enforced. Because of high land values, road rights-of-way tend to be narrow and, if in mountainous areas, steep. In such circumstances, residents often have only one route of ingress and egress (Orange County Bd. Sup. 1976; Los Angeles County 1973).
In addition to suburban subdivisions, hundreds of thousands of residences and their outbuildings have been built in the past few years beyond practical commuting range from cities. Such buildings can be found in any of several arrangements. One is the rural subdivision composed of second (weekend or vacation) and/or retirement homes. Except for location and the inclusion of some recreation and light commercial development, these subdivisions appear similar and present fire problems similar to those posed by suburban subdivisions.
Although fewer than those in subdivisions, the structures that individually cause the greatest fire protection problems are ones built by or for their occupants on individual parcels of land, each usually with its own water system. The most dangerous of these problems result from so-called lot splitting. Lot splits tend to create densities approaching those of subdivisions without any of the inherent advantages of subdivisions (e.g., access to a public thoroughfare, water system, refuse disposal, sewage system). All individually developed residences, whether on large farms or small parcels of land, are difficult to protect from fire. Their water supply is seldom adequate. Access is usually long, narrow, and slow. Electric power, if available, is subject to interruption, often by the fire itself. A relatively new structural fire problem is the mobile home, which presents all the fire problems of the conventional home and several more. Because the structure itself is more susceptible to flash fire, occupants find it more difficult to escape. If the structure is not fitted with adequate skirting, wildfire will run quickly under the floor. A mobile home is easier than a conventional house to set up with little or no clearing of native vegetation. And when it is placed in a mobile home park, the spacing between structures normally is insufficient to prevent a fire from spreading by radiation or convection (Los Angeles County Fire Dep. 1970; Org. St. Dep. For. 1978a).
Figure 2 shows how fuels increase with time. In timber stands and heavy brush fields, this available fuel may reach 50 tons per acre. In areas of 1- to 2-year-old logging slash, fuel loading may run up to 200 tons per acre. When hundreds or thousands of acres of such volumes of fuel are burned in short periods of time-as often happens under conflagration conditions-the amount of heat and energy released approaches that of an atomic bomb. Flame lengths can exceed 100 feet. Radiated heat can ignite exposed flammable materials at distances of 100 or more feet. Convection columns carrying flaming leaves and other fire brands often extend many thousands of feet into the atmosphere and have been known to drop such firebrands several miles downwind. Thousands of homes can be exposed to these conditions (Task Force on California's Wildland Fire Probl. 1972, Helm-and others 1973, Nerd and Countryman 1972).
Of particular concern is the wildland fuel known as
chaparral, because so much of it is in or near metropolitan areas. Chaparral also represents an unusually
dangerous fire hazard because of its inherent qualities.
As a "fire climax plant community, ' chaparral has for
thousands of years not only survived repeated fires but
has adapted itself to depend on fire for regeneration and
survival. Thus wherever chaparral exists large-scale fire
can be predicted with great certainty to occur sooner or
later, and the longer between fires the larger they are
In southern California, the average time between fires in any given area (cycle time) is about 30 years (Hanes 1974, Nerd and Countryman 1972, Philpot 1974, Wright 1972).
Structures built in wildlands can also be hazardous fuel. They are mostly of wood frame construction, often with wood siding. Wood shingle or shake roofs are common, particularly in suburban and rural subdivisions. Attic and floor vents are often left unscreened. Picture windows and stilt or cantilever balconies facing directly into or over heavy wildland fuels are common. Many roofs and rain gutters hold large quantities of dry leaves or needles. Dooryards often are not kept clear of flammable vegetation. Any or all of these qualities contribute to make these structures one of the most hazardous fuels in the urban, suburban, rural or wildland setting (Alger 1971, Task Force on California's Wildland Fire Probl. 1972).
Weather, or more specifically "fire weather," can properly be termed a fire hazard because it aids ignition and accelerates combustion. Almost all conflagrations have occurred during periods of extreme fire weather. We can do little about the weather except to understand it and its interactions with the other hazards. Yet weather is at least as important as fuels in the urban/ wildland interface fire problem.
The weather elements responsible for very high or extreme fire danger are strong winds, high temperatures, low humidities, and low fuel moisture contents. This combination can happen, and has, at almost any time of the year, including the middle of winter. In areas with Mediterranean climates, such as most of California, extreme fire weather is actually quite common during late summer and fall, and should be expected to be present one or more times at any point in the State during the period from mid-June to mid-November (Alger 1971, Task Force on California's Wildland Fire Probl. 1972, Deeming and others 1977, Phillips 1971).
By far the most critical factor in fire weather is wind. A study of fires 5000 acres or larger during the period 1961-70 revealed that at the time of start of 66 fires for which weather records were available, the average wind speed was about 30 mph; in some it went as high as 75 mph (Los Angeles County Fire Dep. 1970, Task Force on California's Wildland Fire Probl. 1972).
The most dangerous wind is the foehn (subsidence)
wind, the most notorious being the Santa Ana of southern California
In other parts of the world this wind is known by other local names, such as mono, north wind, mistral, chinook, tramontana, and williwa. The foehn wind produces the usual effects of winds: fanning and supplying oxygen, preheating fuels by bending flames from the vertical, and carrying burning firebrands ahead of the fire front. But it also brings dry air from continental high pressure areas, then heats and dries it further by compression as it flows to lower elevations at a velocity of up to 100 mph (Alger 1971, Orange County Bd. Sup. 1976, Task Force on California's Wildland Fire Probl. 1972, Phillips 1971).
High temperatures bring the fuels, both vegetative and structural, closer to their ignition temperatures. Low humidities dry the moisture from the fuels. Low fuel moistures, by reducing the amount of heat used in vaporizing moisture, reduce the total amount of heat needed to raise the fuel to ignition temperature. Even green fuels, particularly chaparral, can have remarkably low moisture contents after long dry summers or under adverse fire weather conditions (Deeming and others 1977, Hanes 1974, Nerd and Countryman 1972, Philpot 1974, Wright 1972).
The third major hazard contributing to wildland and structural fire danger is mountainous topography. Although it increases the costs of construction and development, such topography attracts thousands of homeowners with its feeling of openness, an attractive view, and the possibility of getting above or away from the smog. Sidehill construction often makes structures more ignitable (e. g., stilts, cantilevers). Topography affects fires in some of the same ways that wind does as well as modifying, and often intensifying, the effects of the wind. Generally fires run faster uphill than down. Higher elevation fuels (e.g., houses on ridgetops) are preheated by flames and convection columns even in the absence of wind. Canyons act as chimneys trapping heat and intensifying combustion. Canyons, saddles and ridgelines deflect, and often intensify, winds. Thus the mouths of canyons on the lee side of main ridges often become raging infernos during a Santa Ana. Roadbuilding is difficult and expensive in mountainous terrain, often making ingress for firefighting manpower and equipment and egress for residents slow and difficult. 1
The fire risks involved in the structural/wildland fire problem are almost all man-caused. In the populated portions of California, 90 percent or more of the fires involving vegetation are caused either by people directly or by their developments (e.g., arson, various types of machines, power lines); only 4.2 percent of the conflagration fires are caused by lightning. The mere acts of developing, constructing and occupying structures in formerly wildland areas, therefore, expose both the wildlands and the structures to increased risk of destruction by fire (Alger 1971, Task Force on California's Wildland Fire Probl. 1972, Moore 1977).
The risk of structural ignition during wildland fires comes from any of three sources: direct exposure to flame, radiated heat, or firebrands carried by winds or convection columns or both. The first two of these sources can be fairly easily guarded against by proper clearance of native vegetation, landscaping, and maintenance. The last (most common) is much more complex and requires a combination of defenses, including all those described in the chapters on Land-Use Planning and Zoning; Property Development; Structural Design and Construction; Maintenance; and Undeveloped Areas. The most usual structural fuel bed for ignition by these flying firebrands, which are often carried from one-quarter mile to 2 miles ahead of the fire front, is the roof. Therefore, roofing materials and cleanliness are of prime importance as protective measures (Alger 1971, Los Angeles County Fire Dep. 1970, Orange County Bd. Sup. 1976, Task Force on California's Wildland Fire Probl. 1972, Helm and others 1973).
Firefighting is at best one of the most hazardous activities of man. Under conflagration conditions, the risks of entrapment, asphyxiation, heat exhaustion, falling, and other injuries become so great that chief officers and other leaders will usually not allow their men to be at the head of the fire. Also under these conditions smoke and air turbulence often preclude the use of air tankers or helicopters. The only effective perimeter control, therefore, is on the flanks of the fire until it reaches some barrier or the weather changes (Los Angeles County Fire Dep. 1970, Los Angeles CityCounty Fire Bd. Inquiry 1971, Howard and other 1973).
The strategy of wildland fire control requires perimeter control, containment of spread, and eventual extinguishment. Due to the scarcity of water, the tactics of wildland fire control usually consist of constructing fire lines, either by hand or with machines, burning out prepared or preexisting control lines, cooling limited areas by air drops of fire retardants, and similar measures. Such work cannot be carried out safely or effectively on steep hillsides or in close proximity to a high intensity fire in heavy fuels. It must usually be done at some natural or artificial barrier, e.g., a ridgetop or interstate freeway. If the wind is very strong, even these barriers often will not hold because of spot fires caused by flying firebrands (Alger 1971, Los Angeles County Fire Dep. 1970, Los Angeles City-County Bd. Inquiry 1971, Howard and others 1973). Consequently wildland fire control is basically incompatible with the aim of protecting structures.
Most wildland fire protection agencies consider their primary statutory mission to be resource protection, and have budgeted accordingly. In spite of this limitation, wildland firefighters normally react to human moral values when faced with a choice of either saving life or improved property, or saving several hundred acres of brush or timber. Therefore, even though their equipment and training are often poorly designed for structural firefighting and adequate water is usually lacking, firefighters tend to let the fire spread while they do the best they can to save lives and structures (Alger 1971, Los Angeles County Fire Dep. 1970, Oreg. St. Dep. For. 1978b).
Mountainous terrain complicates fire suppression in several ways. The combined effects often make it impossible to save houses from burning during major wildland conflagrations, and, as discussed earlier, steep slopes, canyons and ridges complicate and intensify the effects of wind. Even in the absence of wind, fires race much faster up a steep slope than they do on the level. It is difficult, slow and dangerous to fight fire on steep, rocky side hills. The combined effects of terrain and wind often create smoke and air turbulence conditions which preclude the use of aircraft. Mountain roads make access and response time slow and difficult for firefighters and for heavy fire trucks and other equipment. Sometimes access becomes impossible when roads are blocked by debris, other vehicles, or the fire itself or if construction of the roads themselves has made them too steep or with curves too tight to be traversed by fire engines or bulldozer transports (Alger 1971, Los Angeles County Fire Dep. 1970, Green 1977, Howard and others 1973).
The control of major wildland fires, usually involving structures, is further complicated by the communication and coordination problems inherent when the fire is being fought by multiple agencies, which is usual for two reasons. First, wildland fires do not respect political, jurisdictional, administrative, or ownership boundaries. A fire of any appreciable size will usually spread from the protection jurisdiction of one agency to those of several other agencies. Secondly, no single agency, even the largest, has the resources to handle a major conflagration alone. Aid from other agencies, often from considerable distance, must be called in. As a result, differences in training, equipment, and radio channels regularly exist. The effects of these differences are usually minimized before a fire through joint planning and coordination between adjoining agencies. But if agencies are responding from a distance in emergency situations, coordination of the efforts can be a serious problem (Alger 1971, Los Angeles County Fire Dep. 1970, Los Angeles City-County Fire Bd. Inquiry 1971, Oreg. St. Dep. For. 1978b, Lowden and Degenkolb 1972).
The fire protection available to structures in mountainous areas in or near rural or wildland fuels is often 1imited in certain other ways not usual for similar structures located in cities. Federal and State wildland fire protection agencies are funded only during the wildland fire season. At other times of the year, manpower is sharply reduced, outlying stations are closed, and 24-hour service is terminated. In some areas, a local agency (county or fire district) can make up this deficiency by entering into a contract with the State for firefighting services. Other areas receive structural protection from low-budget volunteer fire departments funded by counties, fire districts, or informal donations from private citizens. Their equipment may be old, training inadequate, and response time excessive. Even when professionally manned and well equipped, county or district fire departments generally have longer response times than do city departments and water is not as easily available. Thus the fire protection for structures in rural and wildland areas is at a somewhat lower standard (Alger 1971, Oreg. St. Dep. For. 1978 b).
That hillside soil bared to the elements by fire is susceptible to accelerated erosion is rather self-evident and has been noted by many researchers and fire officials. What is not so widely known is that many soil types are made hydrophobic (water-repellent) by fire. The overland flow of water during rainstorms is increased by 10 to 40 times the normal (unburned) amount because of this phenomenon. This tremendously accelerated runoff not only creates high flood waters, but provides the force needed to move soil, gravel, and boulders downslope and downstream and to transport deposits accumulated largely by dry erosion between fires (Houts 1974, Rice 1974, Zivnuska 1974.)
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