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A rectangular channel 1.5m wide has a discharge of 0.2m³/s, which is measured in a right-angled V notch. Find the position of the apex of the notch from the bed of the channel. Maximum depth of water is not to exceed 1m. Assume Cd=0.62.

Width of rectangular channel, L=1.5m
Discharge=Q=0.2m³/s
Depth of water in channel=1m
Coefficient of discharge=0.62
Angle of notch=90°.

Answer :

To find the position of the apex of the V notch from the bed of the channel, we need to apply the concept of flow through a V-notch weir.

The formula for discharge [tex]Q[/tex] through a V-notch weir is given by:

[tex]Q = \frac{8}{15} C_d \sqrt{2g} \tan\left(\frac{\theta}{2}\right) H^{5/2}[/tex]

where:

  • [tex]Q[/tex] is the discharge (0.2 \ \text{m}^3/\text{s}).
  • [tex]C_d[/tex] is the coefficient of discharge (0.62).
  • [tex]g[/tex] is the acceleration due to gravity, approximately [tex]9.81 \ \text{m/s}^2[/tex].
  • [tex]\theta[/tex] is the angle of the notch which is 90°.
  • [tex]H[/tex] is the head of the water over the notch we are trying to find.

Substitute the known values into the formula:

[tex]0.2 = \frac{8}{15} \times 0.62 \times \sqrt{2 \times 9.81} \cdot \tan\left(\frac{90°}{2}\right) \cdot H^{5/2}[/tex]

Simplify the expression:

[tex]\tan\left(\frac{90°}{2}\right) = \tan(45°) = 1[/tex]

The equation becomes:

[tex]0.2 = \frac{8}{15} \times 0.62 \times \sqrt{19.62} \cdot H^{5/2}[/tex]

Calculating further:

[tex]0.2 = 0.331 \cdot H^{5/2}[/tex]

Solve for [tex]H^{5/2}[/tex]:

[tex]H^{5/2} = \frac{0.2}{0.331} \approx 0.604[/tex]

To find [tex]H[/tex], raise both sides to the power of [tex]\frac{2}{5}[/tex]:

[tex]H = (0.604)^{\frac{2}{5}}[/tex]

Using a calculator, you'll find:

[tex]H \approx 0.78 \text{ m}[/tex]

This [tex]H[/tex] is the head of water over the V-notch, meaning the position of the apex of the notch from the bed of the channel is given by [tex]1 \text{ m} - 0.78 \text{ m}[/tex]. Therefore, the apex position:

[tex]\text{Position from bed} = 0.22 \text{ m}[/tex]

Thus, the apex of the notch should be 0.22 meters above the bed of the channel to not exceed the maximum water depth of 1 meter.

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