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Continuous flow measurements in the sewer network make it possible to know the degree of demand on a network and evaluate its residual capacity. Combined with the installation of rain gauges, this technique makes it possible to determine the impact of precipitation on the flow rates and levels of water flowing into the sewer network. These measurements provide essential data for the validation of theoretical modeling within the framework of master plans for storm and sanitary sewer networks. This data also makes it possible to determine the vulnerability of sanitary networks in infiltration and direct or indirect parasitic water capture, in addition to deducting the flow rates of its domestic origin. We also believe that flow measurements should be considered as a starting point for any parasitic water research project. This approach makes it possible to determine the infiltration and catchment flow rates of various sections, to prioritize interventions and to guide possible monitoring techniques which will make it possible to locate problematic sources of catchment and infiltration of parasitic water. Knowledge of flow rates therefore allows network managers to obtain essential information to explain wastewater backflow or overflow problems, or to plan future developments (new residential developments, addition of industrial users, etc.).
The measurement system that is generally used is the Doppler area-velocity flowmeter, which allow the continuous measurement of water height and speed, and the calculation of the flow rate based on the diameter of the pipe. For measurements in sanitary networks where flow rates are low, the addition of primary elements such as thin-walled weirs or gauge channels in parallel with the use of area-velocity flowmeters, usually makes it possible to obtain a more reliable outcome for both periods of low flow in dry weather and of higher flow in rainy weather. For measurements in large pipes (e.g. 1200 mm and more), the use of velocity profiling flow meters (pulsed Doppler) ensures better measurements covering the entire pipe. It is also possible to use non-contact radar-speed measurement systems, which are particularly ideal for heavily silted pipes.
As part of the search for parasitic infiltration water, it may also be interesting to measure the flow rates on an ad hoc basis, at various successive locations in a network. These one-off flow measurements, called Flow monitoring at different manholes along the sewer line, are generally carried out at night, between 12 a.m. and 6 a.m. when there are few users on the network during periods of high-water tables in dry weather. By repeating the measurements from one manhole to another, it is possible to calculate the additional contribution corresponding to the infiltration of parasitic water along the section of pipes. The technique of lifting in sections is usually considered when a high flow rate is observed in the network during the high spring water table, or when little difference is observed between day and night flow rates. A preliminary assessment of the flow rates can be made from the analysis of the operating times of the pumps at the pumping stations or by a continuous network flow measurement campaign. To carry out these point measurements, the most commonly used technique is that involving the use of portable weirs. Using this technique, the flow rate can be directly read on the graduated scale of the spillway, after a sufficiently long wait following its insertion into the pipe to ensure the stabilization of the flow rate. Alternatively, volumetric measurements will be carried out for falling pipes with a flow rate of less than 1.67 l/s (limit for the use of a 20 liter boiler). When portable or volumetric spillway technicians are not applicable (silting, warped pipes, etc.), a portable H/V flow meter is used, but usually as a last resort due to its higher uncertainty.
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