Turbidity modelling in InfoWorks WS

Discolouration can arise when unusual changes in velocity are experienced by water distribution pipes. This is caused by the lifting of deposits from the pipe walls. The risk of this effect and the steps needed to control it with minimum disruption to customers can now be investigated using InfoWorks WS.

Using public domain information on the PODDS project a new turbidity model has been added to InfoWorks WS. The model runs as part of Water Quality simulations and can simulate turbidity potential and pipe conditioning.
Each pipe under investigation in the network has parameters applied to simulate the behaviour of its turbidity potential. These include Initial Shear strength, Maximum Shear strength, Erosion Coefficient, Erosion Exponent and Release coefficient.

The object of the analysis is to determine the effect of increasing flow velocity on the stored turbidity volume in the pipes under investigation. The theory is that pipes can be conditioned to accept increased flow rates without causing discolouration problems. A typical course of investigation would be to modify the simulation; increasing flow rate induced by additional demand. Therefore, the simulation should include an unusual increase in flow rate induced by additional demand. This could be induced by applying exceptional demand or opening a hydrant. Once the parameters are set the simulation can be run. For each pipe under investigation, the shear stress and Stored turbidity volume ratio can be plotted from the results.

Stepped Flow Increases

Fig. 1

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In this particular example the flow rate needs to be above 1 l/s to initiate a drop in stored turbidity volume ratio. Once the pipe has been conditioned for flow rates up to 1.6 l/s there is no further drop in stored turbidity volume ratio until the flow rate exceeds 1.6l/s. These phenomena are illustrated in the graphs below

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Fig. 2

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Fig. 3

The two graphs above show the result of increasing flow rates. The Shear stress graph shows the release of material taking place. As the flow rate increases so the pipe becomes “conditioned”. The Stored Turbidity Volume Ratio graph shows that once a level has been achieved the pipe can withstand a higher level of Shear Stress with no release of material. This means that after each increase in velocity the stored turbidity volume ratio is decreased and additional velocity is required to reduce it further.

In this way the results of the simulation can be used to find the required conditioning flow rates for pipes in the network. This is particularly useful where it is necessary to divert additional flow through a pipe route which normally causes no problems but could release turbidity at the higher rate of flow.

 

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