Peter Beckman

Wastewater collection systems are , in general terms, a network of pipes. To understand the performance of these pipes it is common practice to measure the flow at discrete points in the system to provide a flow versus time relationship – a flow hydrograph. The common measurement points in regional towns and cities are the pumping stations. However, these sites provide limited data, essentially the average flow over a relatively long period of time. Dedicated flow monitors may be installed in the gravity sewer and these provide an accurate record of the instantaneous flow hence the true maximum and minimum flow during the time period. The monitors also provide a powerful insight into the hydraulic behaviour of the site and that reach of pipeline by reviewing the depth versus velocity and depth versus quantity scattergraphs.

The review of the scattergraph provides information about the hydraulic character of the site. With an understanding of basic hydraulics and some experience in the review of these scattergraphs it is possible to identify from the record a range of hydraulic regimes including backwater, hydraulic bottlenecks, blockages and overflows. A changing hydraulic regime is evident from a close review of a pair of these scattergraphs.

The power of the scattergraph is in three areas:

1. Quality control by verification of the repeatability of the flow data
2. Interpreting the hydraulic behaviour of the site
3. Forecasting of service problems including siltation, increasing blockages or impending overflows

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