CHAPTER 7 - WEIRS

11. Fully Contracted Standard 90-Degree V-Notch Weir

The triangular or V-notch, thin-plate weir is an accurate flow measuring device particularly suited for small flows.

**(a) Traditional Equation for Standard 90-Degree Contracted V-Notch
Weirs**

The Cone equation is commonly used for 90degree V-notch weirs. This equation is reliable for small, fully contracted weirs generally encountered in measuring water for irrigation.

The Cone equation is:

Q=2.49h_{1}^{2.48} (7-6)

where:

*Q* = discharge over weir in ft^{3}/s

*h*_{1} = head on the weir in ft

**(b) Discharge of 90-Degree Contracted V-Notch Weirs**

Table A7-4 contains discharges in
cubic feet per second for the standard 90-degree, fully contracted V-notch
weir (figure 7-1) from the Cone equation
for a range of heads ordinarily used in measuring small flows. To be fully
contracted, all the overflow plate edges and the point of the notch must
be located at least a distance of 2*h*_{1max} from
the approach flow boundaries.

**(c) Limits of 90-Degree Contracted V-Notch Weirs**

The crest of the weir consists of a thin plate beveled 45 degrees or
greater from the vertical to produce an edge no thicker than 0.08 in. If
heads will be frequently near the 0.2-ft lower limit, then the bevel-ing
should be 60 degrees. This weir operates as a fully contracted weir, and
all conditions for accuracy stated for the standard contracted rectangular
weir apply. To be fully contracted, all the overflow plate edges and the
point of the notch must be located at least a distance of two measuring
heads from the approach flow boundaries. The head measuring station is
located a distance of at least four measuring heads upstream from the weir
crest. This 90degree V-notch weir should only be used for discharges
between 0.05 and 4.25 ft^{3}/s and should not be used consistently
near the high end of this range because a 2-ft fully contracted rectangular
weir will deliver the same flow at 40 percent less head for the same approach
channel width. All the requirements of section 5 apply. All the approach
flow conditions in chapter 2 apply.

The use of the Kindsvater-Shen method for rating V-notched weirs can considerably extend the limitations described above.