Most of us are familiar with the concept of “triage” in medical emergencies. During large-scale crises, medical staff can only treat a certain number of patients. To prioritize which patients receive treatment first, they apply the principle of “the greatest good for the greatest number,” dividing patients into three groups.
- Those who will recover without immediate attention and can receive care later.
- Those who will not survive regardless of how much attention they receive.
- Those who will only survive with immediate attention.
These categories are commonly applied on battlefields and other emergency situations. Fortunately, in the water, wastewater and stormwater industry, we aren’t faced with such life-and-death decisions. However, these principles also can be applied when responding to widespread pipe breaks or flooding.
Widespread Pipe Breaks
Let’s explore how using triage can help prioritize pipe break response. On most days, there are sufficient utility crews ongoing pipe breaks. But what about those days when there are too many pipes breaks to deal with all at once? The obvious example is earthquakes which can break many pipes simultaneously. During extreme weather events, such as hurricanes or extreme cold, like the Texas freeze a few years ago, can create such an emergency. Should you address the biggest leaks, the ones causing the most damage, or the breaks near the mayor’s house? While size and damage are critical factors, prioritizing pipe breaks should depend on the overall impact on the system.
Note: In this blog, “break” refers to the actual break in the pipe, while “leak” refers to the water being lost. I probably should do a blog in the future about the use of those words.
Value of Zones
It is easier to respond to a large number of widespread leaks when a distribution system is divided into zones. These can be pressure zones, district metered areas or simply areas that can be isolated easily from the rest of the system.
Isolate vs. Repair
There are two main steps when dealing with a zone with very high inflow and low pressure: 1. isolating the leak and 2. repairing it. Stopping the leaking is the first priority while the long-term solution is repair. If you know where the leak is, you can isolate the affected segment in the distribution systems with some valve closing. If you don’t know the exact location, your best bet may be to isolate the zone in which it is located. This can lead to a number of customers not getting water, but that’s the nature of triage. You can’t fix everything at once. Isolating 5% of the customers so you can restore service to the other 95% may be worthwhile. After the other zones are getting adequate flow, you can now pinpoint and repair the leak in the problematic zone. Meanwhile, alternative water sources, like tanker trucks or bottled water, can be provided to the affected area.
If a zone shows no signs of widespread or very large leaks that have caused customers to lose service, it may be best to defer action in those areas and go back to them after the emergency subsides. Isolation can often be managed with a small crew armed with valve keys, whereas repair requires larger resources like backhoes, trucks, repair parts, road closures, and larger crews. Using the ideas from triage, I have put together the following table that outlines possible actions during leaks.
The decision to repair or simply isolate a break depends on whether that pipe is hydraulically important, which depends on whether the pipe serves a lot of customers and whether there are alternative paths around the isolated section. If a pipe serves few customers or has a good way to be bypassed, the segment needing the repair can be isolated and the repair can be deferred temporarily to focus on higher priority pipes.
Earthquake Response
Consider the example system below that has just been hit by a major earthquake. Assume it can be divided into four zones. The table below illustrates potential responses:
Isolating and repairing breaks in Zone 1 first benefits the entire systems, while repairing breaks in zones 2, 3 or 4 doesn’t help Zone 1 and the other zones. All other things being equal, it is best to start near the source because everyone in the system benefits from fixing those breaks.
These aren’t necessarily rigid rules, but they provide an example of how a utility can prioritize pipe break repairs. Some of this may need to be modified if there are critical customers. (Hospitals are the usual example.) Road blockages (e.g., collapsed bridges) may also limit access to important breaks. Even when equipment and crews are available, asphalt may not be available to repave roads once breaks are repaired. Don’t dig a hole unless you have the resources to fill it (and preferably pave over it) or barricade it to keep people away.
Value of Sensors
Systems with a large number of SCADA/IoT sensors have the best chance to locate the largest leaks. Areas with excessively low hydraulic grades can indicate a large leak. Unless the area is very flat, pressure values should be converted into hydraulic grade values to identify the location with the worst leaks. Hydraulic grade values from the system hydraulic model can be compared with sensor values to show what the hydraulic grade should be at any point in the system. Sensor data and models can indicate which portions of the system will require boil water advisories due to low or zero pressure.
Isolating a zone that is hemorrhaging water may seem unfair, but those water losses are preventing the rest of the system from recovering. It is better to initially isolate breaks or zones and repair them only as crews become available from the higher priority zones.
Using the hydraulic model to simulate what is happening can be useful in identifying which responses should be prioritized. Expect to get a lot of user notifications such as: “Junction xx has demands but is disconnected from reservoir or tank.” You can check on these using Network navigator query, “Find disconnected.”
Data During Emergency
During the widespread emergency, reports need to be maintained to record customers who reported:
- No water due to leaks
- No water due to the main in the street being isolated
- Low pressure
Such data from sensors in the field need to be continuously monitored.
Reopen Valves
A major risk in isolating part of the system during an emergency by closing valves is failing to open all of the valves once repairs have been made. This oversight can lead to a loss of hydraulic capacity if, for example, four valves are closed to effect an isolation and only three are opened afterward. Unless the water utility runs a rigorous valve exercising program, these closed vales may not be noticed until they result is poor fire flows at the worst possible time.
Planning for Resilience
The U.S. EPA has published an Earthquake Resilience Guide but it has more to do with planning for earthquakes rather than real-time response. It has a nice table ranking pipes by their vulnerability. Old, small cast iron and asbestos cement are poorest, while ductile iron (especially with seismic joints) and polyethylene are the least vulnerable.
Summary
Widespread pipe breaks are rare. But when they do occur, it is good to have done some planning beforehand. This blog provides guidelines and practical tips similar to triage for prioritizing responses during such emergencies.
Note: I’d like to hear from readers who have been through this kind of widespread failure to find any lessons learned or have planned for such emergencies. Contact me at tom.wlski@bentley.com.
References
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https://doi.org/10.3390/su12030914.
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Read more of Tom’s blogs here, and you can contact him at tom.walski@bentley.com.
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