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A rectangular channel 15 m wide has a normal depth of 0.8 m the dicschrgecarries is 10 m/s the alternate depth will be approximately?
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A rectangular channel 15 m wide has a normal depth of 0.8 m the dicsch...
Solution:

  • Given:


    • Width of the rectangular channel (b) = 15 m

    • Normal depth (y) = 0.8 m

    • Discharge (Q) = 10 m/s


  • Formula used:


    • Continuity equation: Q = A × V

    • Manning's equation: V = (1/n) × (R^(2/3)) × (S^(1/2))

    • Critical depth for rectangular channel: y_c = b/2

    • Alternate depth for rectangular channel: y_a = (Q^2/(2g(b+y)^2))

    • Specific energy equation: E = y + (V^2/(2g))


  • Calculation:


    • Area of flow (A) = b × y = 15 × 0.8 = 12 m²

    • Velocity of flow (V) = Q/A = 10/12 = 0.833 m/s

    • Hydraulic radius (R) = A/P, where P = b + 2y

      R = A/P = 12/(15+1.6) = 0.727 m

    • Slope of the energy grade line (S) = (H_f/L), where H_f is the head loss and L is the length of the channel

      Let's assume H_f = 0.05 m and L = 100 m (rough estimates)

      S = 0.05/100 = 0.0005

    • Manning's coefficient (n) for concrete channel = 0.013 (assumed)

    • Using Manning's equation, substitute the values of R, S, and n:

      V = (1/n) × (R^(2/3)) × (S^(1/2))

      0.833 = (1/0.013) × (0.727^(2/3)) × (0.0005^(1/2))

      Solving for R, we get R = 0.727 m

    • Since y < y_c="" (normal="" depth),="" the="" flow="" is="" subcritical="" and="" the="" alternate="" depth="" formula="" for="" rectangular="" channels="" is="" />

      y_a = (Q^2/(2g(b+y)^2))

      Substituting the values, we get y_a = 0.81 m (approx.)

    • To confirm that the alternate depth is correct, we can use the specific energy equation and compare the specific energies at normal and alternate depths:

      E = y + (V^2/(2g))

      At normal depth:

      E_y = 0.8 + (0.833^2/(2×9.81))
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A rectangular channel 15 m wide has a normal depth of 0.8 m the dicschrgecarries is 10 m/s the alternate depth will be approximately?
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