CIV E 445 - APPLIED HYDROLOGY

SPRING 2009 - MIDTERM 2 - SOLUTION

PROBLEM 3: ESSAY QUESTIONS

  • 1 sq mi.

  • The rational method is rational because it responds to the process of runoff concentration, rendering it dimensionless.

    Q = CIA

    The coefficient C is dimensionless.

    The units of Q are the units of IA.

  • The four variables (or parameters) of the runoff curve number method are:

    1. Hydrologic soil group: A, B, C, D.

    2. Land use and treatment class: agricultural, urban, range, forest.

    3. Ground surface condition: poor, fair, good

    4. Antecedent Moisture Condition: I, II, III.

  • The PRF is 484. It comes from the value of 640 (ac/mi) × 1.008 (conversion factor) = 645.

    3/4 of 645 = 484.

    The value 3/4 comes from 2 / (8/3) = 3/4.

    The value 8/3 is the ratio between time base and time to peak of the NRCS triangular unit hydrograph.

    2 comes from the area of a triangle.

  • The NRCS Type IA storm, which covers Northern California and the Pacific Northwest.

  • According to the Gumbel method, the return period of the mean annual flood is 2.33 years.

  • Heavy snowstorms in May followed by an intense and early snowmelt event in June.

  • A phenomenon called "cavitation," produced when high-velocity flows are thrown upward by some small obstruction. This causes a partial vacuum which produces vapor cavities in the water. These unstable cavities then collapse, sending shock waves against the concrete lining. USBR installed air slots inside the tunnel spillway to control cavitation. The installation was successful.

  • Levees help protect rivers from overflowing their banks more frequently. However, disadvantages are: (1) they provide a false sense of security as floods increase in magnitude and frequency with upstream development, (2) they starve the floodplain of nutrients, and (3) produce the loss of "the sight of the river."

  • Because the more money we spend in flood control, the more the magnitude and frequency of floods, due to more development concentrating more flow. The emphasis now is on flood management, rather than on flood control.

 
090407