A 50 foot wide river with a rectangular cross section

Question

Civil Engineering Assignment Test - Hydraulics

Solve all the problems for water at 20^oC (=68^oF) for which

	US	SI
Viscosity μ	2.09 x 10-5 lb s/ft2	1.00 x 10-3 Ns/m2
Density ρ	1.937 slug/ft3	998 kg/m3
Kinematic viscosity v	1.082 x 10-5 ft2/s	1.00 x 10-6 m2/s
Specific weight γ	62.4 lb/ft3	9790 N/m3
Gravitational acceleration g	32.2 ft/sec2	9.8 m/s2

1 ft³/s = 449 gal/min, 1 m³ = 1000 Litters, 1 horsepower = 550 ft lb/s, 1 foot =12 inches

1. A pump running at 800 rpm has the following performance data:

Q (gal/min)	80	120	160	200	240
H (ft)	270	260	240	210	170
bhp (hp)	13	14	16	18	19

(a) Calculate which of these flow rates the best efficiency point (BEP) is.

(b) If the pump capacity Q is 200 gal/min, and the pump is equipped with a variable frequency drive (VFD), find the capacity and head if it is operated at 1000 rpm.

2. A 50 foot wide river with a rectangular cross section flooded last week and you are told to estimate the peak discharge. You find a cross section where fill for a road reduces the width to 30 feet. High water marks left by debris indicate that upstream from the constriction the water depth was 8 feet. The energy loss for a change in cross sectional area is K_e(α₂v₂²/2g - α₁v₁²/2g) in which k_e = 0.1 for a contraction and 0.3 for an expansion. With only this information, what is the best estimate of peak discharge?

3. Given the compound channel shown with a water depth of 4 m in the main channel and a channel slope of 0.0001. Assume uniform flow and calculate the velocity and discharge in the main channel and two floodplains.

4. The triangular channel shown has a Manning coefficient of 0.02 and a bed slope of 0.0001. For a design discharge of 30 ft³/s , calculate the critical and normal depths.

5. Multiple choice: There may be more than one correct answer for multiple choice problems. Circle or mark all correct answers for full credit.

1. Which of the following are correct:

(a) Axial pumps provide greater head and greater flow than centrifugal pumps.

(b) Axial pumps provide less head and less flow than centrifugal pumps.

(c) Axial pumps provide less head and greater flow than centrifugal pumps.

(d) Axial pumps provide greater head and less flow than centrifugal pumps.

(e) None of the above

2. Which of the following are correct:

(a) Cavitation occurs when vapor pressure is exceeded.

(b) The distance a pump can be located above a reservoir it is pumping from is limited by the head produced by the pump.

(c) The pump manufacturer provides the available NPSH (Net positive section head)

(d) Axial pumps are not susceptible to cavitation

(e) If specific speed is less than 7900 cavitation will occur.

(f) None of the above.

3. Net positive suction head is a function of

(a) Power coefficient

(b) Pressure head on the suction side of the pump.

(c) Pressure increase caused by the pump

(d) Velocity head

(e) Vapor pressure head

(f) Dimensionless Darth Vapor suction head coefficient, an imaginary number

(g) None of the above.

4. A general equation for flow over a weir is:

(a) Q = 1.49 A R²/S^1/2/n

(b) Q = vA

(c) Q = bCH^1.5

(d) Q = bDV

(e) None of the above

5. Water surface profiles are controlled:

(a) From downstream in subcritical flow

(b) From downstream in supercritical flow

(c) From upstream when the Froude number is less than one

(d) From upstream when the Froude number is greater than one

(e) None of the above

6. For steep slopes:

(a) The small angles assumption does not apply

(b) Normal depth is greater than critical depth

(c) Uniform flow is always supercritical

(d) Non-uniform flow is always supercritical

(e) None of the above

7. A water surface for non-uniform flow is calculated

(a) With the manning equation for normal depth

(b) Proceeding downstream for supercritical flow

(c) Iteratively solving the energy equation

(d) Assuming depth is critical

(e) None of the above

8. For subcritical flow in the lab flume, a small bump on the bottom would

(a) Reduce the water surface elevation over the bump

(b) Increase the water surface elevation over the bump

(c) Cause critical flow if the bump were large enough

(d) Flood the lab if the bump were taller than the sides of the flume

(e) None of the above

9. Hydraulic jumps

(a) Are transitions from subcritical to supercritical flow.

(b) Reduce the Froude number of the flow

(c) Can be assumed to have negligible energy loss

(d) Are features of uniform flow

(e) None of the above

10. Which water surface profiles are possible (D_s = normal depth, D_c = critical depth)?

(a)

(b)

(c)

(d)

(e)

(f)

(g) none are possible