Much has been expressed about the risks associated with high-profile activities such as the storage of spent nuclear fuel in Skull Valley, the destruction of chemical weapons at the Tooele Army Depot and the disposal of low-level radioactive materials at the Clive Site west of our valley.

Recently, a risk of 1 chance in 10 million per year for an aircraft crash into the fuel storage canisters was reported. This translates to a risk to residents of Skull Valley of about one chance in 100 million per year. It is informative to put these risks in perspective with other risks that we unknowingly assume. There are many risks in a modern society, but consider the following unusual risks to residents in the greater Salt Lake Valley.

More than 360 flights (both takeoff and landing) of large commercial aircraft occur each day (or 130,000 flights per year) at the Salt Lake International Airport. These landing and takeoff operations essentially span a flight path bounded by Bangerter Highway and Redwood Road from West Jordan to about 10 miles north of the airport.

The risk of an aircraft crash during this critical phase of operation is about three chances in 10 million aircraft flights per year. So for the Salt Lake International Airport this gives about one chance in a 100 per year.

In 1965 a Boeing 727-22 crashed at the airport, killing 43 passengers. Other aircraft accidents in the valley occurred in 1963, 1969, 1970, 1972, 1987 and 1997. Along the southern flight path over the valley are hospitals, public schools, colleges, libraries, parks, the Valley Fair Mall and many residents.

Consider the crash of a Boeing 737 into Salt Lake Community College during a landing approach. Typically about 10,000 students occupy this campus in a land area of about 20 acres. The risk to a student at this college is about one chance in a million per year. Residents in the west and south of the valley face some risks.

What about to the north of our valley? In the Woods Cross and North Salt Lake area are many large petroleum tanks, each containing about a million gallons of fuel. These tanks lie under the northern flight path of these same transiting aircraft.

The equivalent energy in a few large tanks is about that of the Hiroshima atom bomb. Although the most likely consequence of an aircraft crash into these tanks would be a major firestorm, it is possible that a catastrophic explosion could level much of Woods Cross and North Salt Lake. An estimate of risk to a resident in this area is about two chances in a million per year.

What about residents in the east of the valley? The Wasatch and east-bench fault zones roughly parallel Highland Drive from the northern Avenues to Wasatch Boulevard to Draper. About 10 percent of the valley's population or nearly 100,000 people reside along this fault zone, including 25,000 students, faculty, workers and hospital patients at the University of Utah. The frequency of a major seismic event of Richter magnitude 7 or greater along this fault is about one in 3,000 per year. Assuming only 1,000 fatalities occur from such an event, the risk to the residents in this area is about three chances per million per year.

Perhaps it is safer to live in the city itself. But, alas, the same earthquake of magnitude 7 or greater could rupture either or both Mountain Dell and Little Dell reservoirs in Parleys Canyon. Release of the 3,300 acre-feet of water in Mountain Dell and 20,000 acre-feet in Little Dell would create a wall of water perhaps 20 feet high moving down the canyon at a hundred miles per hour. The impact on traffic in the canyon is obvious, but the final result would be a couple of feet of water deposited on the valley floor from I-15 to Highland Drive and North Temple to the western leg of I-215.

This is the major population zone for the valley with about 200,000 residents. The frequency of such a massive seismically induced failure of these dams is only about 1 chance in 10,000 per year, but the risk of loss of life to so many valley residents and traffic in the canyon is about 5 chances per million per year.

We may conclude from these unlikely but real risks to valley residents that we would find greater safety in Skull Valley or Tooele or even Clive, Utah. But, alas, the greatest risk we face then is traveling along our highways and freeways to these areas. The risk of death to do this is about 200 chances per million per year, undoubtedly one of the riskiest activities in the valley.


Gary M. Sandquist is a professor of mechanical engineering at the University of Utah.