Soil and Water Conservation Engineering · Fluid Mechanics · 2 marks · Numerical answer
A trapezoidal weir has side slope1H:4V, crest shorter than channel width, head .70 m and discharge .85 m³/s. Cd=.62,g=9.81 m/s². Find crest length in m, to two decimals.
The1:4 side slope identifies a Cipolletti weir, whose triangular side flows compensate end contractions. Use $Q=(2/3)C_d\sqrt{2g}LH^{3/2}$. Thus $L=.85/[(2/3)(.62)\sqrt{19.62}(.7)^{3/2}]=\mathbf{.79}$ m. Adding an independent triangular discharge without the contraction compensation would use a different model.
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