Quantcast Chapter 7. Open Channel Meters

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Section 1. WEIRS
1.  INTRODUCTION.  Weirs are used in open channels and in conduits where the
fluid has a free surface, such as canals, streams, rivers, tunnels,
nonpressurized sewers, and partially filled pipes.
1.1 Meter Designs.  Weir designs are classed as sharp crested and not sharp
crested.  Sharp crested weirs are used for flow measurement. Not sharp
crested weirs are used for flow control and are not discussed in this manual.
1.1.1 Sharp-Crested Weirs. Sharp-crested weirs (Figure 7-1) are classified
according to the form of the notch or opening as follows:
Rectangular notch, original form.
q V-notch,
used to provide higher head readings at low flow rates.
q Trapezoidal,
hyperbolic, and parabolic, special notch types, intended
to have a constant discharge coefficient or to have a head directly
proportional to the flow rate.
1.1.2 Broad-Crested Weirs. Broad-crested weirs have a considerable thickness
of crest as measured along, and parallel to, the channel.  The crest should be
thick enough to prevent the nappe from springing free at the upstream edge
(Figure 7-2).  This requires a thickness equal to at least twice the maximum
head if the upstream edge of the crest is square.  Thickness of the crest may
be reduced one-fourth if the upstream edge is rounded.
1.1.3 Round-Crested Weirs. The curvature of the crest surface of a
round-crested weir may be radial or some other geometric curve such as a
parabola (Figure 7-3).  The upstream face of these weirs may be vertical or
sloping.  Both the degree of the crest rounding and the amount of slope to the
faces affect the rate of discharge.  Weirs of this type are seldom built with
the primary purpose of measurement.
1.2 Operating Principles. A weir is a dam-like device that obstructs liquid
flow and creates a controlled nappe and head of liquid which can be measured.
For flow measurement, the weir is considered a primary device to be used in
conjunction with a level measuring device.
The basic limitations of a weir type open channel flowmeter
are as follows:
q Conditions
must be present to provide for development of a head of
liquid .
q Weirs
are not recommended for flows that contain large amounts of
suspended solids or debris which could accumulate to Impede flow.


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