Using Hydraulic Models under Covid Conditions

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August 26, 2020

In part 2 of our Covid Impact series, Duncan Allen, Clean Water Hydraulic Modelling Team Leader at Scottish Water discusses how water usage patterns are changing, the steps they are taking to identify early warnings to ensure network resilience and some key findings for other modellers to consider.

It was confirmed that there has been an unprecedented use of the word unprecedented in recent times but what could we be doing as hydraulic modellers in these strange and unusual times?

As expected, domestic use has increased, figures suggest by around 15% to 20% with the data suggesting higher peak flows occurring later in the day. Based on typical Per Household Consumption Studies an example of these changes in demand profiles is shown here with the calculated average flow profile for the first 2 weeks in March 2020 compared to the calculated profile for the last week in March following lockdown.

Non-domestic use is undoubtedly reduced (rough estimates suggest perhaps 25%) but as a lot of the non-domestic demands are calculated based on periodic meter reads (be it monthly, quarterly, annually etc.) it will likely be some time before the actual reductions can be fully understood if at all.

Adding to this, the hot weather through May and June has also added an increase in demands. Based on the hot weather in 2018 we saw an increase in domestic demands of 15% to 20% as well, which now when combined with the increase during lockdown has meant increases in daily domestic volumes sometimes in excess of 30% in some areas.

The split of domestic to non-domestic properties within District Metered Areas (DMAs) is clearly known and understood but the typical daily volumetric split tends to be less defined, fortunately as hydraulic modellers this split (at least pre-Covid) is generally quite easy to extract from the models allowing us to assess where domestic volumes are much higher and therefore where a significant increase in the domestic use (even with a drop in non-domestic) is most likely to have an impact.

As modellers we can run a number of scenarios looking at increases in domestic use to identify where the networks are operating differently and therefore may start to struggle to maintain level of service. Our own assessments have shown in some cases DMAs where the volumetric usage is >90% domestic we have noted if and where potential low pressure issues would likely start occurring and at what level of increase (modelled at 15%, 25% and 30% increase in domestic demands). This has allowed us to start identifying areas where additional monitoring may help give us some early warnings.

Whilst it has not been an issue and in many areas the reduction in non-domestic use has largely offset the increases in domestic use, we have also noted some things which as modellers may be worth considering:

  • Time modulated PRVs stepping pressure back down after the pre lockdown peak flows but before (or during) the post lockdown peaks.
  • Flow modulated PRVs that with the decrease in non-domestic use meant the flow threshold where the pressure increases was not occurring.
  • Lower pressure arriving at the inlets to PRVs due to increased domestic flows upstream, reducing the differential across the PRVs.
  • Lower pressure arriving at the inlets to Pumps due to increased domestic flows upstream increasing the work they have to do.
  • Service Reservoir inlet controls not having optimum settings for the increased through flows occurring.

One of the non-modelling questions for me is what impact will this have on water company revenue – with non-domestic use being lower the financial revenue will be reduced but what about the domestic side? If a water company has full domestic water metering, then presumably revenue will be increased but if they don’t typically have domestic metering ….


This article is reproduced with the kind permission of Duncan Allen and Scottish Water having been first published in the Clean water Modelling Advisory Group (CwMAG) Newsletter.

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