Water Supply Runtime Calculator
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The Water Supply Redundancy Runtime Calculator calculates how many days a water supply redundancy can support its critical loads during a water outage. This information is an input for characterizing redundant systems in the TRN Lite risk assessment methodology. The calculator can account for three types of

systems: All Systems: The site must provide the ability to achieve the appropriate water quality that is required for the site and critical loads, such as potable water or ultra-pure water. Necessary treatment methods should be conducted at the load in case the redundant supply does not meet those standards.

  • Onsite storage tank(s) connected only to the primary water supply,
  • Onsite or offsite secondary water source that supplies onsite storage tank(s), and
  • Onsite or offsite secondary water source that directly supplies critical load(s)

After providing answers to the questions below, you will obtain an output estimating the number of days current supply redundancies will be able to meet critical loads. This analysis provides a high-level estimate. For users who have done or are planning to do more refined assessments of the runtime capabilities for their supply redundancies, those estimates may be more appropriate to use as inputs for the TRN Lite analysis. Note that if your system is more complex than one of the system types listed above, it may require a more involved analysis to estimate system runtime. Review the system definitions to check to see if your system meets the criteria of analysis for this tool.

To see the calculations behind the results, click the “Show Calculations” checkbox. Click “Example” to see an example analysis including entered data and results.

Calculator related questions, suggests, and issues can be submitted here:

Type of water supply redundancy
Select the description that best describes your redundant water system from the dropdown list. An onsite secondary water source could refer to a well or surface water for example. An offsite redundant source, e.g., a regional water tower, must be able to supply the site in the case that the primary source fails. The 'System Definitions and Requirements' section in the Introduction tab contains important descriptions of the supply redundancy types for the purposes of this calculator.
Average daily water load for the system during an outage
gallons/day
This may be larger than the critical load(s) if the supply redundancy is connected to a larger system including non-critical loads. Account for any loads that will be curtailed during an outage, such as irrigation. If there is no storage tank, check that the supply can meet the peak load, which may be greater than the daily average load.
This may be larger than the critical load(s) if the supply redundancy is connected to a larger system including non-critical loads. Account for any loads that will be curtailed during an outage, such as irrigation.
Average daily water load for the system during an outage
gallons per minute
ERROR: Average daily water load must be greater than 0 gallons/day.
WARNING: The average daily water load you entered is larger than is typical, please confirm before proceeding.
Onsite Storage Tank

Enter the data below to characterize your

onsite storage tank. Onsite storage tanks:

An onsite storage tank could be connected to only a primary water supply (e.g., municipal or utility) or connected to a secondary, onsite or offsite water source. Whether or not it is additionally connected to the primary supply, if the tank is connected to a secondary supply it falls into the category, “Onsite or offsite secondary water source that supplies onsite storage tank(s)”. Rainwater storage tanks are often considered secondary water sources, but for the purpose of this tool, rainwater does not qualify as a secondary source as it is not reliable enough to model as a consistent secondary source. The storage tank itself still counts as a supply redundancy as the water that is available in the tank can be used during an outage, so the user should select Onsite storage tank(s) connected only to the primary water supply for rainwater systems with no additional secondary source.

The amount of residual disinfectant (e.g., chlorine) in the water, both in the storage tank and throughout the distribution network, should meet the criteria described in American Water Works Association (AWWA) Manual 20 or equivalent standard, both during regular use and during an outage. The amount of residual disinfectant will be dependent on the tank management, pressure, and network characteristics. Residual disinfectant must be maintained for potable water or non-potable water that comes into contact with humans. Some strategies to maintain proper levels of residual disinfectant include periodic cleaning, cycling water through the tank, aeration, or chemical dosing.
Refer to the
schematic water tank Water Tank Definitions:

Useable volume: The storage capacity of the tank(s) that can be used during an outage to supply the critical load(s) assuming the tank were full. It is equal to the total storage volume minus the dead volume and reserved volume.

Dead volume: The volume of water at the bottom of the tank that, when the water level reaches this level, there is not enough head to supply the system. This level depends on the elevation of the tank(s) and the system pressure requirement. This volume may be nominal, but it is important not to include it in the usable tank volume to receive an accurate runtime estimate.

Reserved volume: Tank volume that must be reserved for emergency fire suppression requirements or similar uses and therefore is not included in the useable tank volume.

Empty volume: The amount of empty volume at the top of the storage tank before it would be refilled during normal operations. Storage systems could be operated with automatic setpoints, using an automatic control rule to fill from the supply source when the water drops below a certain volume, or with a manual process by the operator. It's not possible to know exactly what the water level will be at the moment of an unexpected outage, so using the greatest typical empty volume that the storage tank could have at any given time will yield a conservative runtime estimate.
and the tool tips next to each question for more information.

Tank Storage Diagram
Useable tank volume
gallons
This value is the storage capacity of the tank(s) that can be used during an outage. If there are multiple tanks, treat them as one aggregated system for this section. Be sure to include Dead Volume or Reserved Volume.
ERROR: Usable tank volume must be greater than 0 gallons.
WARNING: The useable tank volume you entered is larger than is typical, please confirm before proceeding.
Empty tank volume at any given point in time pre-outage
gallons
The amount of empty volume at the top of the storage tank before it would be refilled during normal operations.
ERROR: Empty tank volume must be greater than 0 gallons and less than the useable tank volume.
Water loss factor for the piping between the storage tank and the load(s)
Estimate what the loss factor is based on your knowledge of the piping system. The age of pipes will be the biggest factor (older pipes will leak more, PVC tends to be newer), followed by distance to load (longer distance means more opportunities to leak). If you are unable to estimate this value, a resilience gap strategy could be to conduct a water audit to obtain a precise value and possibly identify the location of leaks to reduce water loss.
Secondary Water Sources

Enter the data below to characterize the water available from

your secondary water source. Secondary supply systems:

Onsite: When fully operational, the source is not expected to be exhausted within a reasonable timeframe (e.g., months) and it must be able to continuously provide the design flow rate over the entire duration of the outage. An onsite secondary water source, such as a well, aquifer, or lake, should have enough capacity such that it can be considered infinite for the purpose of this tool.

Offsite: An offsite secondary water source, such as a regional water tower or industrial-grade storage facility that can supply the site in the case of an outage to the primary municipal water supply, must not be exhausted by the typical demand of the site and other sites that may be expected to draw upon the source during an outage event. If these systems have limited capacity, such as if the supply is dependent on other customers' usage, a more involved analysis should be pursued instead of this tool. An offsite secondary supply redundancy must not have any known co-locations or shared vulnerabilities with the primary water system. This could include shared distribution pipe infrastructure, shared reliance on a substation, or shared water source supply. These types of shared systems increase the likelihood that the supply redundancy is vulnerable to the same outage events as the primary system.
Refer to the tool tips next to each question for more information.

Flow rate that can be provided by the onsite or offsite secondary water source supply
gallons/minute
This information could be provided by the system/pump specifications. Enter the maximum flow rate that the offsite or onsite secondary water system is designed to provide the site.
ERROR: Flow rate must be greater than 0 gallons.
WARNING: The flow rate you entered is larger than is typical, please confirm before proceeding.
Water loss factor for the piping between the onsite or offsite secondary water source and the onsite storage tank (if present) or the load(s) (if no storage tank)
Estimate what the loss factor is based on your knowledge of the piping system. The age of pipes will be the biggest factor (older pipes will leak more, PVC tends to be newer), followed by distance to load (longer distance means more opportunities to leak). If you have no idea, a resilience gap strategy would be to conduct a water audit to obtain a precise value and possibly identify the location of leaks to reduce water loss.
Usable secondary flow rate
gallons per minute
Set peak water load estimation type
Peak water load for the system during an outage
gallons/minute
ERROR: Peak water load must be greater than 0 gallons/minute.
WARNING: The peak water load you entered is larger than is typical, please confirm before proceeding.
Peak water load for the system during an outage
gallons per minute
The daily load will likely not be evenly distributed throughout the day, and if there is no storage tank onsite, then the secondary source must meet the peak load to achieve full functionality of the system. If the peak load is known, this cell can be overwritten with that data, but many sites will not have this information. If the peak load cannot be met, in reality there would be partial outages, but the TRN only gives credit for meeting the entire critical load(s). In line with a conservative approach, the estimate provided here assumes a peak load that is 1.5 times as big as the average load.
Results

The results below show the calculated runtime for the supply redundancy described by the inputs to this calculator. The runtime is shown in days and in hours. Use the runtime in hours as an input to characterize your water supply redundancy in the TRN Lite Detailed Information – Supply Redundancies subsection.

Complete the form and address any errors above to view calculated water supply redundancy runtime.
Longest duration of outage this supply redundancy could support:
days The expected runtime is longer than the greatest outage modeled in the TRN. The expected runtime is zero days because the secondary supply is not sufficient to meet the peak load.
Input for 'Estimated Run Time (hrs)' in the TRN Lite, Detailed Information – Supply Redundancies:
4,500 0 hours
Use this value as an input for 'Estimated Run Time (hrs)' in the TRN, Baseline Development Action 3. The maximum modeled runtime relevant for the TRN analysis is 4500 hours