leaders in hydraulic network analysis
Demand Assessment & Loading Tool for Mains Design and Developer Studies

An intrinsic shortcoming in the theory and application of network analysis is the loss of realism regarding the demands of small numbers of properties. This issue arises due to the apportionment of a DMA’s inflow to all the individual customers contained within. This approach is unavoidable but does apportion appropriate ‘daily volumes’ to customers in the model, however, it unfortunately results in unrealistic ‘instantaneous’ customer demand. Whilst this does not have significant implications when using the model for less detailed analysis, it becomes increasingly problematic when analysing very small parts of networks. An example of this would be several customers at the end of a cul-de-sac where normal network analysis methodology could be underestimating demands by a large factor. This has obvious implications when using a model to help design mains which feed smaller numbers of properties or when analysing the effects of additional demands from new properties. For these reasons HydroCo have developed a tool to help network modellers apply realistic demand loading for use in such studies.

The tool draws upon research conducted by the WRc (Water Research Council), who explored the relationship between instantaneous flow rates and the number of domestic properties downstream of the flow measuring point. The tool developed by HydroCo has translated this data into formulae to be used in mains design with modelling developer enquiries. In addition, the tool also considers commercial developments using standardised loading units to estimate peak instantaneous flows.

The ‘domestic dwellings’ instantaneous formula derives an estimated instantaneous flow rate using the number of properties downstream of the proposed main and the typical average demands of an area. The formula generates a scaling factor which considers the probability of a number of domestic dwellings drawing their maximum flow rate at the same time. If only 1 customer connection is to be considered, the probability for drawing a typical peak flow is very high, whilst if there are 150 connections the probability of all these connections taking their peak flow rate at the same time is greatly reduced. The formula trends towards 1,000 properties, where the scaling factor is equal to 1 (i.e. the peak instantaneous flow rate is negligible to mains design).

The instantaneous flow calculation is just one tool that has been incorporated into the HydroCo demand analysis spreadsheet. It can also derive estimated demands for use in a modelling environment when performing developer enquiries. This spreadsheet has been developed to provide a consistent approach to estimating demands for new developments and uses a robust methodology that can be demonstrated to the water regulator. The spreadsheet uses published average daily demand figures from the water company to generate demand values.

This methodology / software tool is currently used by four UK based water companies as part of their standardised developer enquiry process. It is hoped that other companies will embrace this tool to supplement their existing methodologies. Screenshots from the latest version of the tool can be viewed overleaf.