HEC-RAS PRACTICUM - 2 (DUE NOVEMBER 27, 2007)

— David Wagner 2007/11/20 18:47

A rectangular channel 10 ft wide carries 375 ft3/s at an initial depth of 2.46 ft. This channel has a smooth downward step of 0.49 ft. If both upstream and downstream of the step flow occurs at normal depth(2.46 ft) and Manning’s n is 0.002,

A. What is the change in depth due to the step assuming frictionless flow, zero longitudinal slope, and using hand calculations?

y2-y1 = 2.46-2.46 = 0 ft

B. Develop a HEC-RAS data file to find the change in depth due to the step assuming the step has a length of 10 ft. (Note, use the given information to estimate the slope of the channel for use in HEC-RAS.)

Using the minimum Manning's n for FlowMaster (0.009), the channel slope is 0.004377

A=by=10*2.46=24.6 ft²; P=b+2y=10+2*2.46=14.92 ft; R=A/P=24.6/14.92=1.65 ft; S=({nQ}/{phi A R^{2/3}})^2 =({0.002*375}/{1.49*24.6*1.65^{2/3}})^2 =(0.75/51.2)^2 =0.01465^2=0.000215

Zipped HEC-RAS Data Files

HEC-RAS Profile Plot

Reach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl
(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft)
2 2000 PF 1 375.00 0.22 2.68 3.73 6.28 0.000215 15.22 24.64 10.00 1.71
2 1000 PF 1 375.00 0.00 2.46 3.51 6.06 0.000215 15.22 24.63 10.00 1.71
2 999.* PF 1 375.00 -0.05 2.39 3.46 6.06 0.000221 15.37 24.40 10.00 1.73
2 998.* PF 1 375.00 -0.10 2.32 3.41 6.05 0.000227 15.51 24.18 10.00 1.76
2 997.* PF 1 375.00 -0.15 2.25 3.36 6.05 0.000233 15.64 23.97 10.00 1.78
2 996.* PF 1 375.00 -0.20 2.18 3.31 6.04 0.000239 15.78 23.77 10.00 1.80
2 995.* PF 1 375.00 -0.25 2.11 3.26 6.04 0.000245 15.91 23.57 10.00 1.83
2 994.* PF 1 375.00 -0.29 2.05 3.22 6.03 0.000249 16.00 23.44 10.00 1.84
2 993.* PF 1 375.00 -0.34 1.99 3.17 6.02 0.000255 16.13 23.25 10.00 1.86
2 992.* PF 1 375.00 -0.39 1.92 3.12 6.02 0.000260 16.25 23.07 10.00 1.89
2 991.* PF 1 375.00 -0.44 1.85 3.07 6.01 0.000266 16.38 22.90 10.00 1.91
2 990 PF 1 375.00 -0.49 1.78 3.02 6.01 0.000272 16.50 22.73 10.00 1.93
2 0 PF 1 375.00 -0.71 1.60 2.80 5.71 0.000261 16.26 23.06 10.00 1.89

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