Instantaneous flow measurements at night in the sewer network
The flow rate of a pipe leading to a manhole can be measured with the flow rates raised by sections technique at night.
By repeating the measurements from one manhole to the other, it is possible to calculate the additional contribution of a section of pipes to the infiltration of parasitic water.
These measurements must be taken at a time when there are few users on the network, between midnight and 6 a.m, for instance.
The instantaneous night flow measurement should be considered when a high flow is observed in the network in the spring or when little difference is observed between the day and night flows.
A preliminary evaluation of the flow rates can be made from the operating times of the pumps at the pumping stations or by a campaign of continuous flow measurements in the network.
The most commonly used technique involves the use of portable weirs. The flow can be read directly on the graduated scale of the weir but only after the flow has stabilized.
Alternatively, volumetric tests will be carried out for pipes with a low flow, less than 1.67 l/s, falling.
Due to its lower accuracy, a portable H/V flowmeter will be used as a last resort, when the other techniques are not adequate.