Temporary Flow Measurement in Sewers and Drains

A measuring device, suitable for temporary measurements of flow in circular channels (or other irregularly shaped channels), is described, and the results of tests on two pairs of geometrically similar devices are reported. The device is based on the same principle as that used in Venturi flumes, that is, the formation of a narrow throat in the channel, in which a critical depth is developed. This gives a unique relationship between the discharge and the depth at a measuring point upstream of the constriction. The derivation of the relationship is based on the energy equation, subject to the usual assumptions, and the equations developed are solved numerically. A coefficient of discharge is included to allow for neglected factors. Over a certain range of depths, the relationship may be described by a simple exponential equation. The values of the coefficient of discharge are primarily in the range of 0.90 to 1.00. The throat of the new measuring device is formed by introducing a pier-shaped element, with its sharp end pointing upstream, into the channel.