CIV E 530 - OPEN-CHANNEL HYDRAULICS

FALL 2003 - MIDTERM 2 - SOLUTION

PROBLEM 3: ESSAY QUESTIONS

1.

    (a) n = 0.008; lucite.

    (b) n = 1 (justified based on extreme effective roughness of Florida Everglades).

2.

    At low flow, it is high due to ripples and dunes.

    At bank-full flow, it is low due to erasure of ripples and dunes (upper regime)

    At average flood stage, it is high due to roughness of trees and biuldings.

    At extreme flood stages, it is again low due to low relative roughness.

3.

    The permissible tractive force is preferred because it is founded on a more solid theoretical basis.

4.

    The typical range of angle of repose of noncohesive materials is 20o to 40o, with an average of 30o.

5.

    Because when a canal has clear water, it wants to transport more sediment ("hungry water"). Therefore, for the same slope and flow depth, a clear-water canal will have more eroding capacity than a normal-sediment-load (200 ppm) canal.

6.

    So (geomoprphology/gravity), Sc (friction), and Froude number (hydraulics).

7.

    So There are twelve (12) water-surface profile types. C1 and C3 are completely horizontal.

8.

    So, Sc, 0, +∞ and -∞

    In the upstream end, Sy is asymptotic to Sc and 0.

    In the downstream end, Sy is asymptotic to So and 0.

9.

    1. i. Direct-step method is EASY.

      ii. Standard-step method is DIFFICULT.

    2. i. Direct-step method is restricted to a PRISMATIC (usually artificial) channel.

      ii. Standard-step method is used for ANY-SHAPE (usually natural) channel.

    3. i. Direct-step method ASSUMES FLOW DEPTH and calculates distance.

      ii. Standard-step method ASSUMES DISTANCE and calculates flow depth.

    4. i. Direct-step method uses CHANNEL LENGTH as dependent variable.

      ii. Standard-step method uses FLOW DEPTH as dependent variable.

    5. i. Direct-step method advances DIRECTLY.

      ii. Standrad-step method advances BY ITERATION.

    6. i. In the direct-step method accuracy increases with A SMALLER DEPTH INCREMENT.

      ii. In the standard-step method accuracy increases with MORE CROSS SECTIONS.

    7. i. Direct-step method uses ONE TYPICAL cross section.

      ii. Standard-step method uses MANY DIFFERENT cross sections.

    8. i. Direct-step method has MINIMAL data needs.

      ii. Standard-step method has EXTENSIVE.

    9. i. Direct-step method uses SPREAD SHEET.

      ii. Standard-step method uses HEC-RAS.

    10. i. In the direct-step method uses an answer is ALWAYS POSSIBLE

      ii. In the standard-step method an answer is SOMETIMES NOT POSSIBLE.

10.

    Gabions are preferred because they provide intermediate properties, between rigid and unfriendly to the environment, as in the case of concrete, and friendly to the environment but unstable at high velocities as in the case of unlined channels.

    Too much vegetation clearing is bad for the enviroment, because riparian vegetation does serve a useful purpose. Too little clearing represents a flood risk, because roughness increases and conveyance decreases.

    A happy medium needs to be determined.

 
060403