SGS Aqua
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Especially at overflow crests the measurement of the overflow discharged to the receiving water is of high interest to operators and governmental agencies. In several places it is required to record this overflow already.


The biggest proble with the measuring of such overflows is the accuracy of the measurement and therefore the reliability of the recorded data. Especially at the start and at the end of storm events large amounts of flow might not be recorded with some measurement techiques. Mostly this is because the storm water overflows over concrete spillways without clearly defined geometry. The deciding factors are often not completely level crests, over the length of the crest varying geometry of the crest and undefined surface roughnesses.

The factors listed above reflect the predominant condition of CSO spillways in operation today. Under these conditions, even the most accurate measuring systems for overflow level measurement are not able to compensate the calculated error based on the unknown weir coefficient.

The critical factor to optimize the accuracy of overflow measuring systems is the manufacturing and intallation of a defined overflow crest with known flow coeffiient.

In order to increase the accuracy of the recorded data even more, we suggest to survey further influence factos like upstream baffles, of the direction of overflow, etc. and include these factors in the calculations for the Q-h curve.

Should the condition of a drowned overflow, i.e. based on backwater levels accur, this backwater level needs to be measured with a separate downstream water level sensor and needs to be integrated to the flow calculations.


  • known weir coefficient
  • works well for retrofitting
  • low investment costs
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