CHAPTER 7  WEIRS
The KindsvaterShen relationship can be used for fully contracted notches of any angle between 25 degrees and 100 degrees (Kulin and Compton, 1975). The equation which includes the angle as a variable is written as:
(73)
where:
Q = discharge over weir in ft^{3}/s
C_{e} = effective discharge coefficient
h_{1} = head on the weir in ft
h_{1e} = h_{1} + k_{h}
= angle of Vnotch
The head correction factor, k_{h}, is a function of (figure 76a). However, for fully contracted traditional 90degree Vnotch weirs, equation 76 and the rating table discussed later produce comparable accuracy.


For fully contracted Vnotch weirs, the value of k_{h} is related to as given on figure 76a, and values of C_{e} are read from figure 76b. Partially contracted 90degree Vnotches only can be rated using figure 77 to obtain C_{e} values. The calibration relationships were developed with rectangular approach flow and head measurement sections for these weirs. For applications with other flow section shapes, the average width of the flow section for each h_{1} is used as B to determine coefficients.

Bos (1989) and International Organization for Standardization (1983) explain and define limits basic to the use of these figures. Precautions and restrictions concerning the use of Vnotch weirs are as follows:
(a) Vnotch weirs should not be designed beyond the range of the parameters plotted on figures 76 and 77. Only the 90degree Vnotch weir can be made partially contracted through the use of figure 77.
(b) The water surface downstream from the weir should always remain at least 0.2 ft below the notch. Lower discharge readings should be rejected if the contraction is not springing underneath for the entire nappe length.
(c) The measuring head should be greater than 0.2 ft because precision of head measurement error is large relative to smaller head depths, and the nappe may cling to the weir plate.
(d) For the fully contracted Vnotch, the maximum measuring head should be less than 1.25 ft.
(e) For the partially contracted Vnotch, the maximum head should be less than 2 ft.
(f) For fully contracted Vnotches, the h_{1}/B ratio should be equal to or less than 0.2.
(g) For the partially contracted 90degree notch, h_{1}/B should be equal to or less than 0.4.
(h) The average width of the approach channel, B, should always be greater than 3 ft for the fully contracted Vnotch.
(i) For the partially contracted 90degree Vnotch, the approach channel width should be greater than 2 ft.
(j) The Vnotch of the weir should always be located at least 1.5 ft above the invert of the weir pool for fully contracted weirs.
(k) Only the 90degree Vnotch can be partially contracted, but the point of the notch must be located at least 4 in from the channel invert.
(l) All the requirements in section 5 apply.
(m)All the approach flow conditions in chapter 2 apply.