The gaugings carried out during a flood are part of a series of gaugings, ideally covering the full range of levels measured at the site.
A flow gauging consists of measuring:
- the cross sectional area of the water in m2. This is achieved by measuring the distance across the surface and between 20 to 25 depths across the river.
- the velocity at several points at each of these depth positions, from which the mean velocity m/s is calculated.
These measurements are multiplied to provide m3/s or cumecs Q (m3/s) = A(m2) x mV(m/s).
These instantaneous flow measurements are used to create a mathematical relationship between level and flow (called a flow rating). Continuous flow data is then derived from the level data.