The flow rate (also volume flow) indicates the volume of a fluid flowing through a specific cross-sectional area during a certain time.
In the environmental engineering sector, the fluid is usually water.
There are different measuring approaches for the determination of the flow rate / discharge:
- Determination of the flow velocity: If the cross-section is known, the volume flow can be calculated from the flow velocity. In open channels, not completely filled pipes and unsaturated soils, this cross-section depends on the water level, or on the degree of saturation of the pores and thus on the water-filled porosity.
The determination of the flow velocity to determine the discharge based on the water-filled cross-sectional area is often used at larger level measurement sites of at large rivers.
- Determination of the water level: For measuring weirs or measuring channels, as well as measured flow cross sections, a volume flow can be assigned to each water level.
In the case of constant cross-sections, this method can be used permanently without redefining the water level-flow relation. This measuring method is used in the H-Flume.
- Dilution experiments: By initiating a tracer of a known concentration and determining the concentration after a sufficient mixing distance downstream, the current flow rate can be determined.
- Determination of the outflowing water quantity: Tipping counters directly determine the quantity of the effluent water and thus offer a time-resolved, automated alternative to manual volumetric measurements. Tipping counters are particularly suitable for smaller flow rates and are therefore also used in lysimeter systems for the detection of leachate water rates. Tipping counters are also used in the Run Off system of UGT GmbH, which allows to determine the surface run-off of inclined planes, which plays a decisive role in hill slope hydrology of small catchment areas. A combination with the H-Flume allows a high measurement accuracy of the runoff at low water and higher drains in open channels.