During periods of rapid water level change, the level in a stilling well may lag behind those in the channel that it is measuring.  This phenomenon is known as stilling well lag and is a result of head loss in the intake system. Stilling well lag may occur when the water level in the channel is rising or falling.

Uncorrected, stiling well lag can result in both totalized and instantenous flow measurement errors.

Stilling well lag can be minimized by:

• Sitting the stilling well as close as is practical to the measured channel
  • For flumes or weirs, this is generally right next to the primary device
• Increasing the number of intake ports
  • Generally not an option for short-throated flumes as there is a single, specific, primary point of measurement
• Increase the intake line size
  • This may have the secondary benefit of minimizing the change of clogging in the intake line, although it can increase sediments in the stilling well itself.

An analyis of stiling well lag can be found in Reginald Herschy's Streamflow Measurement, Third Edition.

 

Image:  U.S. Geological Survey