Leakage Allocator

Required information

To make use of the Leakage Allocator tool, you'll need the following:

• An identified area ("scope") where total outflows combine actual demands with leakage flows. The Leakage Allocator recognizes this area as a set of pipes, which can be:
  • All the pipes in the network
  • The pipes in the Domain
  • The pipes in a Selection Set
• An estimate of the percentage of the combined outflow in the scope that corresponds to leakage.

  For reference, the 2021 Report Card for America's Infrastructure estimates that at least 15% of extracted water for drinking supply is lost to leakage in the United States.

• A "reference" model (scenario) that roughly represents:
  • The total outflow that combines actual demands and leakage flows (and its dynamic behavior)
  • The typical pressure conditions to which the pipes in the scope are subjected
• A rough estimate of the average degree of rigidity of leaks in the pipes within the scope. This is recognized as a percentage, with 0% corresponding to fully flexible leaks and 100% corresponding to fully rigid leaks (fixed-orifice-area behavior).

Allocating leakage

Once you've gathered the required information, follow these steps to allocate leakage:

1. Activate the scenario that represents the reference network conditions.
2. Define the scope.
3. Run the reference standard extended period simulation.
4. Open the Leakage Allocator tool from the Allocator group on the ribbon.
5. Fill in the input parameters in the dialog:
   
   1. Demand Dis-Aggregation Scope: The set containing the pipes to assign leakage to. Pipes with no connections to junctions are ignored. The scope can be the entire network, the pipes in the domain, or the pipes in a selection set. Junctions in the scope are automatically selected as those connected to pipes in the scope.
   2. Leakage Percentage (%): The percentage of the total outflow from the junctions in the scope, during the reference simulation, to be converted to leakage.
   3. Global Pipe Rigidity (%): Determines the ratio of fixed and variable leakage area (see the FAVAD leakage paradigm) to be assigned to all the pipes in the scope. The ratio is evaluated at the average pressure for each pipe. With 0%, the entire leakage area is set as variable; with 100% the entire leakage area is set as fixed.

   4. Store Leakage Parameters In: The leakage set in which to store the resulting leakage parameters. The active set is selected as default. Click on the More button (…) to create a new leakage set.

      Warning: The current contents of the selected leakage set will be erased.

   5. Store Modified Base Demands In: The demand set in which to store the reduced base demand values. The base demand values are copied from the active demand set and, for each junction, these values are reduced proportionally to make room for leakage. The active set is selected as default. Click on the More button (…) to create a new demand set.

      Warning: If the default (active) demand set is selected, the original base demand values will be modified.
6. Run the leakage allocation by clicking on the Allocate button.
7. If the allocation is successful, click OK to return. If an error occurs, address the specific problem displayed.
8. Create and activate a scenario with leakage:
   • If both the active leakage set and demand set were selected for the allocation, skip this step.
   • If a scenario was created before running the allocation, simply activate that scenario.
   • Otherwise, create a new scenario based on the reference one, and assign to it the leakage set and the demand set that were used for the allocation.
9. Run the simulation with leakage and inspect the results.
10. (Optional) You can manually modify the demand patterns assigned to the scope junctions to redistribute the actual demand. The redistribution should move demand away from the periods with high pressure and high leakage (e.g., early morning), and towards the actual demand peak periods (with low pressure and therefore low leakage). This refinement is intended to better match the overall behavior of the model to the calibrated outflow totals in the reference simulation.
11. (Optional) You can further refine the leakage (and base demand reductions) by, for example:
    • Adjusting based on pipe age and number of connections
    • Adjusting based on individual leak rigidity percentages
    • Adjusting based on separate estimates for long-term pipe operating pressures
    • Transferring leakage to skeletonized sections of the model

Leakage allocation algorithm

The goal of the leakage allocation algorithm is to convert a portion of the total outflow into leakage without significant changes to the model's behavior. The total outflow volume is aggregated from junctions at the ends of pipes within the scope and throughout the duration of the reference simulation. The algorithm assigns leakage values to the pipes in the scope so that, assuming the pressure time series portrayed in the reference simulation, the total leakage volume is equal to the target percentage of the total outflow volume. The algorithm also shrinks the base demand values in the scope junctions, with the reductions corresponding to the newly assigned leakage. The remaining demand values therefore represent actual demand.

In general, the leakage parameters per unit length assigned to pipes are not the same. Aside from length, leakage is distributed according to both the hydraulic and the deteriorating effects of pressure:

• Hydraulic effect of pressure: Leakage volumes are proportional to the square root of pressure, by Torricelli's law. The allocator tracks the changes in pressure throughout the reference simulation to account for the leakage flow at each pipe.
• Deteriorating effect of pressure: The AWWA's M36 Manual posits that, statistically, the amount of leakage in a system is directly proportional to the water pressure. Following this recommendation, the allocator first assumes that the average pressure throughout the reference simulation corresponds to the long-term conditions the pipe has operated under. Then, it assigns leakage flow to pipes in proportion to this assumed average operating pressure.

Similarly, base demand reductions are not uniform for all junctions. The reductions are distributed based on the amount of leakage in adjacent pipes. The reductions are proportional in all the demand categories for a single junction, except for negative demands, which are left unmodified. When target reductions exceed the total outflow at junctions, the remaining reductions are similarly distributed among junctions with remaining capacity. Given that there are multiple aspects to be considered simultaneously, it is normal that the resulting leakage percentage does not exactly match the requested value, and that the reduced base demands do not exactly compensate for the total leakage volume.

The modified model would ideally behave as similarly as possible to the reference model, in terms of both mass and energy balance. While the algorithm is designed to try to achieve this goal, in its current form, it does not modify the demand patterns to redistribute actual demand according to the time. This usually means that the total outflow will be overestimated at night, when leakage is highest, and underestimated during peak demand periods. Conversely, pressure will be underestimated at night and overestimated during peak demand. This limitation thus necessitates the manual step #10 above, in which the demand patterns are modified so that they represent the temporal dynamics of actual demand and not those of combined demand and leakage.