The water is not frozen in the pipes when the breaks begin. Unlike residential plumbing, the major water mains do not break because they are frozen. They break because the pipes are buried underground and are relatively warm. When the cold water flows through them, the difference in temperature between the main and the cold water often causes the main to rupture. The more rapidly the water temperature drops, the more likely the main breaks will be triggered.
Breaks are more common in the older, cast-iron pipes.
Although cast-iron pipes are no longer installed, a large number of these pipes remain in the system. Breaks are more likely to occur in the central and western sections of Fort Worth, which are served by the Holly Water Treatment Plant. The Holly plant draws water from Lake Worth, which is shallow and can change temperatures rapidly. The Rolling Hills and Eagle Mountain water treatment plants draw water from larger lakes, which tend to stay warmer and change temperature more slowly than Lake Worth. Because of this, the department tries to use water from the Rolling Hills and Eagle Mountain plants in as much of the city as possible during main break episodes.