3. INSTALLATION. To obtain reliable flow information, weir installation must
include sufficient upstream and downstream preparation. The following
paragraphs are general suggestions only. A design engineer is required to
provide specific details.
3.1 Upstream. A weir should be preceded by a straight uniform section of
channel to ensure uniform velocity distribution in the flow. The length of
this channel should be 4 to 6 times the width of the approach channel upstream
of the weir.
3.2 Downstream. On the downstream side of a weir plate, the edges of the
channel end wall, to which the plate is mounted, should be cut away so that
the nappe will fall freely without adhering to the end wall. Also, the width
of the downstream channel at the weir should be sufficiently greater than the
crest length, so that the sides of the nappe fall free.
4. MAINTENANCE. All types of head and area meters are used for open flow
measurement, and their operation depends on the absence of any kind of
interference at the discharge opening. Weirs require the following
4.1 Daily Maintenance. Check the weir edge daily to make sure it is clean
and free of algae growth and other interfering material.
4.2 Monthly Maintenance. Make sure the breather pipe is open. In cold
weather make sure frost is not blocking the opening.
4.3 Annual Maintenance. Drain the weir and make sure that water breaks
evenly over the crest. Check the weir edge for irregularities and correct if
5. ACCURACY AND RELIABILITY. The accuracy of flow measurements made with
sharp-crested weirs varies depending upon the sharpness of the weir edge, the
exactness with which the weir dimensions were determined, and the method and
accuracy of head determination. Weir accuracy is determined to vary between
0.5 and 4.0 percent. The reliability of weirs is not a quantitative
characteristic since the crest must be free of debris to function correctly.