Why would the District use above-ground plastic pipes in a fire zone?
The CZU fire destroyed a total of 7 miles of above-ground high-density polyethylene (HDPE) pipes that transferred water from creek intakes to the Lyon water treatment and storage facility above the town of Boulder Creek. Additionally the treated water lines to the three water storage tanks burned, causing catastrophic loss of water pressure and contamination of two of the tanks. If your initial reaction on hearing that the plastic pipes burned was "What were they thinking using plastic in a fire zone?" Why didn't they use steel?, as it was mine, read on. It's not as crazy as it seems.
Advantages of HDPE pipelines: Flexibility and cost
All the District pipelines were installed more than 20 years ago. Using above-ground HDPE was and has been the standard for rural water districts like ours. HDPE has several advantages. HDPE is more flexible than ductile iron (a type of iron with carbon added, which is more flexible than steel), making it more resistant to earth movements during earthquakes or the long-term down-slope creep of landslides. This trait is important in the SLVWD because the east side of Ben Lomond Mountain where SLVWD facilities have been located has abundant landslides and is close to the active Zayante-Vergeles fault, a strand of the San Andreas fault zone. The greater flexibility of HDPE makes it easy to create bends that avoid obstacles. It is easy to install and repair, whereas joining sections of iron pipe requires more skill and a great deal more work, making costs per mile for ductile iron about twice as much as for HDPE.
Putting the HDPE pipes above ground avoided environmental issues that arise when pipes are buried in a forest. In particular, the environmental effects of trenching, tree removal, and going up and down ravines along the 5-mile line traversing the east slope of Ben Lomond Mountain would have been large and would have met significant public outcry and environmental regulatory hurdles. As built, the HDPE pipeline traced a snaking path, avoiding trees.
Disadvantages of HDPE pipelines: Susceptibility to fire and VOC contamination
Thus the only disadvantage of above-ground HDPE pipes at the time they were installed was their susceptibility to fire (and even ductile iron pipelines can be damaged in fire because the sections are joined using rubber gaskets, which can burn). It was not until the aftermath of the 2017 Tubbs and 2018 Camp fires that it was known that burning of plastic pipes could contaminate otherwise undamaged infrastructure with toxic and carcinogenic volatile organic compounds such as benzene. So, at the time the SLVWD pipes were installed, more than 20 years ago, this potential problem did not factor into the decision to use HDPE.
Current context for deciding HDPE vs. ductile iron and above-ground vs. buried
he context for current decisions on whether to use HDPE or ductile iron for pipelines and whether to bury them or leave them at the surface is different from what it was more than 20 years ago. In addition to the recognition of the potential for the contamination by volatile organic compounds if HDPE pipes burn, two other factors have changed since the time when the HDPE pipelines were installed: the fire hazard is greater now, and the environmental concerns regarding a post-fire landscape versus a forest.
Since the early 1980s, the frequency of autumn days with extreme fire weather conditions has more than doubled in California. Rainfall during the season has decreased by about 30%, while average temperatures have increased by more than 2°F (Gose et al., 2020). The most pronounced warming has occurred in the late summer and early autumn, causing tinder-dry conditions to coincide in time with the strong, dry “Diablo” and “Santa Ana” winds that occur in the autumn in northern and southern California. These conditions have fed large, fast-spreading wildfires across California in 2017, 2018, and 2020.
The environmental considerations in installing pipelines are different now that the District's watershed has burned. In the past, installing ductile iron or burying pipelines would raise objections due to the need to remove trees; now, many trees are destroyed and many damaged and dangerous trees need to be removed. Today, construction needs to occur with an eye toward minimizing conditions that could cause debris flows.
Decision will involve balancing risks, cost, and environmental issues
Given all the factors involved—cost, environmental issues, the balance between risks of damage from earth movements and wildfire--it will be a complicated decision on whether to use ductile iron or HDPE in reconstruction efforts, and whether to leave pipelines at the surface or bury them. It could be a combination: use buried, ductile iron for pipelines immediately adjacent to storage tanks (as the District Manager plans to do) to prevent future catastrophic loss of water and pressure in a future fire, while using HDPE for long runs of distribution lines. One could create defensible space adjacent to HDPE lines, whether buried or on the surface, by removing dead and dangerous trees. Making the decisions on how to proceed with reconstruction will also involve discussions with FEMA to see if additional funds could be made available to not simply rebuild the system, but to make it more resilient to natural hazards.
In other words, it’s not as simple as my initial knee-jerk reaction made it out to be. The engineering and financial decisions on how to proceed with reconstruction will engage the SLVWD staff and Board of Directors for many months to come.