# Calculator for the calculation of flow velocity and flow rate of water

This Calculator serves the cal­cu­lation of average flow velocity and flow rate in rivers, creeks and canals when the cha­racteristic of the flow cross-section is known.

All input fields (white background) are filled out from the start, these values can be changed according to the in­di­vi­dual re­quire­ments.

The cal­cu­lated values are output in the green fields after a click on "Calculate" or pressing the enter button.

Category

kSt   Slope %

Flow cross section bo m h m bu m x m

 Cross-sectional area m² Wetted extent m Flow velocity m/s Flow rate m³/s

## Background to this Calculator

The Calculation is based on the formula of Gauckler-Manning-Strickler.

### Explanations concerning Strickler-coefficient kST

The Strickler-coefficient kST is a so-called roughness-coefficient. The larger this coefficient, the smoother is the channel bed and the lesser resistance is put against the flow. Therefore flow velocity increases with growing Strickler-coefficient.

Newly concreted channels have a very low roughness, whereas naturally or near-naturally rivers and creeks show a higher roughness, especially if they are characterised by dense bank vegetation and/or big stones.

In this calculator the Strickler-coefficient is output in the field "kST" after selecting the category. The value can be changed as required.

### Explanations concerning slope

Slope expresses the proportion of height difference and distance. It is indicated in percent usually.

Example: 10 cm height difference over a distance of 10 m gives a slope of 0.1 m / 10 m * 100 = 1 %

Slope provides the energy for the motion of the water. Therfore flow velocity increases with growing slope.

### Explanations concerning flow cross-section

Flow velocity also depends on the ratio of flow cross-section and wetted extent.

This Calculator allows the selection of simple flow cross-sections. Cross-sectional area and wetted extent are cal­cu­lated.

If you miss your desired flow cross-section in the drop-down menu, you have to choose "custom" and fill in values for cross-sectional area and wetted extent in order to calculate flow velocity and flow rate. The following figure shows what the term wetted extent means. It is plotted as a darkblue line:

### Explanations concerning the calculated average flow velocity and flow rate

The cal­cu­lated values for average flow velocity and flow rate are ap­proxi­mate values. De­via­tions from the true values are possible. In case of greater water depths the cal­cu­lated values may be to high, especially if slope is large at the same time.

## Related site

last modification: march 2018