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=2.49h12.48   (7-6)

where:

Q = discharge over weir in ft3/s
h1 = 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 2h1max 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 ft3/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.