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Open Channel Flow Part 2

ERT 349 SOIL AND WATER ENGINEERING. Open Channel Flow Part 2. Open Channel Flow. ERT 349 SOIL AND WATER ENGINEERING. Siti Kamariah Md Sa’at PPK Bioproses, UniMAP. Channel Design. Open Channel function: Convey water between 2 points in safe and efficient

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Open Channel Flow Part 2

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  1. ERT 349 SOIL AND WATER ENGINEERING Open Channel FlowPart 2 Open Channel Flow ERT 349 SOIL AND WATER ENGINEERING Siti Kamariah Md Sa’at PPK Bioproses, UniMAP

  2. Channel Design • Open Channel function: • Convey water between 2 points in safe and efficient • Channel Design for uniform flow can be divide to two types: • Lined Channel • Unlined Channel

  3. Section factor, Z Conveyance Factor, K • K = AR2/3 /n • From Manning: AR2/3 = Q/So½ n • K = Q/So½ • K is called the conveyance of the channel and express the discharge capacity of the channel per unit longitudinal slope • Z = AR2/3 • From Manning: AR2/3 = Qn/So½

  4. Most Efficient Section • Also known as ‘Best Hydraulic Section’ • Characteristics: • Low cost (P min) • Higher Flowrate (Q max)

  5. From Manning Equation: With assuming A, n and So is constant, Q max when P min Best hydraulic section has P min Best Hydraulic Section

  6. Best Hydraulic Section for Rectangular Channel A = By ------------(i) P =B+2y = A/y + 2y P = A/y + 2y -----------(ii) Best hydraulic section when dP/dy = 0 -Ay-2 + 2 =0 A = 2y2 ---------(iii) y B

  7. Best Hydraulic Section for Rectangular Channel From eq (i) A = By And from eq (iii) A = 2y2 B = 2y -----------(iv) R= A/P = 2y2/(B+2y) = 2y2/(2y + 2y) = 2y2/4y = y/2

  8. Best Hydraulic Section for Trapezoidal Channel T A = (B + zy)y B = A/y – zy P = B + 2√(z2 + 1) y P = A/y – zy + 2√(z2 + 1) y -------------(v) 1 l y z B

  9. Best Hydraulic Section for Trapezoidal Channel dP/dy =-Ay-2 –z + 2√(z2+1) A = (2 √(1 +z2) – z)y2 -------------- (vi) Substitute (vi) in (v) B = 2y(√(1 +z2) – z) P = 2y(2√(1 +z2) – z) R = y/2

  10. Best Hydraulic Section for Trapezoidal Channel Side slope, z = 1/√3 And θ = 60o

  11. Example 1 • A slightly rough brick-lined trapezoidal channel carrying a discharge of 25 m3 is to have a longitudinal slope of 0.0004. Analyse the proportions of (a) an efficient trapezoidal channel section having a side 1.5 horizontal: 1 vertical (Ans: R=y/2, y=2.830m, B=1.714m) (b) most efficient section of trapezoidal shape (Ans: z =0.57735, y=3.045m,B=3.516m)

  12. Example 2 • If z = 1, n = 0.017, So = 1/1200 and Q = 142 m3/min • Determine width of the channel and water depth for most efficient section. • Ans: B = 0.89 m, y = 1.1 m

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