Selecting a Weir for Flow Measurement

This overview provides information on the advantages and disadvantages of thin-plate, sharp-crested weirs for measuring the flow of water in open channels. Additional information is also provided on unique features and application notes on various weir types.

Advantages of Weirs

Simple design
Low construction costs
Easy to install
Easy to verify
Point of measurement flexibility
Good accuracy when properly sized / installed
Range of notch styles / sizes
Compound weirs for dual flow range measurement
Widely used - operator familiarity

Disadvantages of Weirs

High head loss
Peroidic weir pool maintenance
Well maintained crest critical to accurate flow measurement
Excessive approach velocities affect accuracy
Zero reference elevation above weir pool floor
Potentially large upstream weir pool requirements
Not suitable for cold weahter flows
Little resistance to submergence
Downstream scour
0.2' minimum head - regardless of weir type

Types of Weirs

Triangular (V-Notch): Suited to measuring low flows. Provides the best weir profile for discharges of less than 1 CFS and may be used for flows up to 10 CFS. Availalbe in notch angles from 20-120º.
Rectangular (Contracted): Able to measure higher flows than v-notch weirs. Discharge equation more complex than most other weir types. Widely used to measure high flow rates in channels suited to weirs.
Rectangular (Suprpressed): Able to handle same range of flows as Contracted Rectangular weirs but is easer to construct. Simple discharge equation. Care must be taken to ensure proper aeration of the nappe. Channel sidewalls act as weir ends.
Cipolletti: Similar to Contracted Rectangular weir except the notch ends angle out (4:1). Simplified discharge equation. Less accurate than V-notch or Rectangular weirs - and less used because of it. Compound A combination of two or more weir styles or sizes to provide optimal resolution at targeted flow rates. Ambiguous discharge in the transition zone between weir sections.
Proportional: provides discharge proportional to head. Suttro style most common. Most common in man-made channels.
Circular: Designed to mount in pipes / conduits. Flow rate is determined by open weir area and NOT pipe size.