BIE 5300/6300 Assignment #3
Weir Calculations
24 Sep 04 (due 28 Sep 04)
Show your calculations in an organized and neat format. Indicate any
assumptions or relevant comments.
I. Hydraulically, what is the difference between a suppressed weir and a
contracted weir?
II. A rectangular sharp-crested weir is installed in a straight section of a rectangular
open channel with B = 10 ft, L = 7 ft, P = 3 ft, and hu = 1.37 ft. The downstream
water surface is well below the weir crest elevation.
(a) Do the stated conditions meet all the guidelines for setting and
operating weirs, as given in the lecture notes? If no, which are
violated?
(b) You are required to estimate the discharge over the weir for the stated
conditions. Assuming negligible approach velocity, estimate Qf in cfs.
(c) Estimate the discharge, Qf, without assuming a negligible approach
velocity (hint: use Hu instead of hu in the calibration equation).
(d) Now suppose hd = 0.31 ft and everything else is the same as given
above. Estimate Qs in cfs.
III. A Cipoletti weir is installed in an open channel. The approach velocity is
negligible. If the crest length is 0.90 m and the upstream depth, hu, is 0.22 m,
referenced to the crest elevation, what is the estimated free-flow discharge?
IV. An overshot gate with L = 8.0 ft and Gw = 12.0 ft is installed in a canal. At the
downstream side of the gate is a reservoir with a constant water surface
elevation which is 0.29 ft above the gate hinge. The irrigation district needs you
to develop and plot calibration curves for gate openings of: θ = 15, 20, 25, 30,
35, 40, 45, 50, 55, and 60 degrees. The plot should have hu on the abscissa
and Q on the ordinate. Both the abscissa and ordinate must start at zero. Each
curve on the plot must be labeled with its corresponding gate opening angle.
V. A new trapezoidal concrete canal with a base width of 2.0 m and inverse side
slopes of 1.5 has a total lined depth of 2.5 m. The bed slope of the canal is
0.00015 m/m and the length is 2.35 km, all straight in alignment (no curves or
bends). At the end of the section there is a sudden drop in the bed elevation of
3.5 m, then the same channel cross section continues downstream, with the
same bed slope. Design a sharp-crested weir, just upstream of the elevation
drop, for a maximum flow rate of 7.0 m3/s. Make sure the canal lining won’t be
overtopped upstream of the weir.
