ABSTRACT
The methods for measuring the discharge of a natural load in a river or a channel fall into four main categories:
1. Volumetric methods make it possible to determine the discharge directly by measuring the time needed to fill a container of a given capacity with water. Given the practical problems inherent in this method of measurement (size of the container necessary, uncertainty regarding the measure of time, possibly requiring a specific installation), this method is generally used only for very low flows, a few "/s at the maximum. 2. Hydraulic methods take account of the forces that govern flow (gravity, inertia, viscosity…). These methods follow the laws of hydraulics. 3. Velocity-area methods involve determining the flow velocity at various points in a flow cross section and measuring the flow area of the cross section. These techniques require a specific device (current meter, sounding-weight …) and personnel trained in their use. Among the many methods used to explore the velocity of water, we describe several below (gauging with current meters and with floats), as well as the principle of the operation of electromagnetic sensors. 4. Physico-chemical methods take into account the variations, during the flow, of certain physical properties of the liquid (the concentration of certain dissolved elements). These methods generally consist of injecting a solute into the river, and following the evolution of its concentration over time. These are called chemical methods or dilution methods.
