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Civil Engineering Exam Questions on Stilling Basin, Pumps, Culvert Design, and Sediment Transport

## Stilling Basin Design

Stilling Basin Design

1.    Find the dimensions for a stilling basin (Figure 1 below) assuming a free hydraulic jump providing energy dissipation for a reinforced concrete box culvert.

(a) Determine the velocity and depth at the culvert outlet, along with velocity and depth (TW) in the receiving channel.  (4 Marks)

(b) Estimate the conjugate depth for the culvert outlet conditions using Equation SB.1and the transition length from Equation SB.3. (refer to Appendix) (6 Marks)

(c)     Calculate the conjugate depth for a free hydraulic jump using Equation SB.4. (4 Marks)

(d)    Calculate at (2nd iteration) the conjugate depth for a free hydraulic jump using Equation SB.4. (refer to Appendix)  (4 Marks)
(e)     For the slope change from the outlet to the transition, determine the needed radius of curvature using equation SB.8 and the(r).  (3 Marks)
(f)    Calculate the size of the basin elements and Sketch for the free hydraulic jump stilling basin. (4 Marks)  (Total 25 Marks)

Pumps
2.    The characteristics of centrifugal pump running at constant speed

 Q (m3/s) 0 0.12 0.24 0.36 0.48 0.6 0.72 0.84 0.96 HM (m) 40 40.6 40.4 39.3 38 33.6 25.6 14.5 0 η (%) 0 41 60 74 83 83 74 51 0

The pump is used to pump water against gravity between two reservoirs whose surface leves differs by 13 m. The pipeline is 130 m long, 450 mm diameter, f=0.005, and contains two gate valves (ζ loss coefficients = 0.2) and ten 90o bends (ζ loss coefficients = 0.35)

(a)     Determine the flow rate and power absorbed. (11 Marks)

(b)    The flow rate is to be increased by adding second, identical, pump to the pipeline.

Determine the flow rate and power absorbed if the second pump is:

(i)     Connected in series with the first pump.  (7 Marks)

(ii)    Connected in parallel with the first pump.(7 Marks) (Total 25 Marks)

Culvert design
3.    A single-barrel precast concrete box culvert, shown in Figure 2 below, is proposed to span 45 m across a railway embankment. The mean bed level at the outlet is 68.65 m AD, and at the inlet is 68.2 m AD. The culvert must accommodate for a 1:100 year flood river flow of 12.5 m3/s (with a corresponding surface water level of 69.5 m AD).

(a)    Propose appropriate dimensions for a standard rectangular box culvert design with its invert set at ground level, to carry the flood flow without increasing upstream flood water level, and operating under free-surface inlet control conditions. (8 Marks)

(b)    Subsequently, calculate the flow velocity and critical depth within the designed culvert. State any assumptions made.  (8 Marks)

(c)    Assuming the only available precast section is 1.0 x 0.6 m2, how many sections are needed to accommodate the above flow in a multi-barrel design with square-edge soffit, with submerged entrance. (9 Marks)

Sediment Transport 4.
(a)    Discuss the following factors which affect the amount of sediment carried by a river:

(i)    The size, shape and density of the practices on the bed of the river (and sometimes on the banks).  (3 Marks)

(ii)    The terminal velocity of the particles. (2½ Marks)

(iii)    The speed of flow of the river (2½ Marks)

(iv)    The degree of turbulence of the flow. (2 Marks)

(b)    Calculate the critical shear stress for a particle of diameter D and density 2650kgm-3 in a river with water of density 1000 kgm-3, You may assume that the diameter size ‘’D’’ puts the particles in the gravel size range, where Tcr / (Dg (ρs – ρw)) = 0.06. From this determine the shear stress on a particle width D = 3mm (6½ Marks)

(c)    A 25 m wide channel carries a flow 4m deep with a slope of 0.0010 over a gravel bed with a D50 = 36mm. If this condition is sustained for 1 week, how much bed load will have been transported in that period? (8½ Marks) (Total 25 Marks)