Sizing Stilling Well Inlet Ports

Trapezoidal flume with connection port and detached stilling well The primary purpose of a stilling well is to provide a area were level measurements can be taken free of wave action or water surges. The surges and wind waves present in the main body of flow can be dampened by restricting the opening area of the inlet port.

For many flume and weir applications, the stilling well is next to (or a part of) the flow structure. Here the connection size can be relatively small (1/1000 of the horizontal cross-sectional area of the stilling well).

However, as the stilling well is moved away from the main body of flow, the connection size between the main flow and the stilling well must increase to produce the same effective dampening.

The table below provides inlet port sizes for a variety of stilling well sizes and locations.

Stilling Well Size	Attached (or Close)	Detached (20-30' Remote)
Ø12"	Ø1/2"	Ø1/2"
Ø16"	Ø1/2"	Ø3/4"
Ø20"	Ø5/8"	Ø3/4"
Ø24"	Ø3/4"	Ø1"
Ø30"	Ø1"	Ø1-1/2"
Ø36"	Ø1-1/4"	Ø2"
3' x 3' square	Ø1-1/4"	Ø2"
3' x 4' rectangular	Ø1-1/2"	Ø3"
4' x 5' rectangular	Ø2"	Ø4"

For applications where the discharge changes rapidly (or where the stilling well is very remote), the inlet port is commonly increased in size so that it is 1/10^th the diameter of the stilling well.

Finally, keep in mind that the intake pipes on most stilling wells used in surface water applications will require occasional cleaning - especially on streams and canals carrying sediment. As the stilling well gets further away from the main flow, it may be necessary to increase the inlet port size to make the connecting pipe easier to clean and maintain.