CIV E 445 - APPLIED HYDROLOGY

SPRING 2008 - MIDTERM 2 - SOLUTION

PROBLEM 3: ESSAY QUESTIONS

  • A catchment is small in a hydrologic sense if:

    1. rainfall can be assumed to be uniformly distributed in time

    2. rainfall can be assumed to be uniformly distributed in space

    3. storm duration usually exceeds time of concentration (concentrated catchment flow)

    4. runoff is primarily by overland flow

    5. channel storage processes are negligible.

    In midsize catchments rainfall cannot be assumed to be uniformly distributed in time.

    In midsize catchments, runoff is subconcentrated.

    In midsize catchment, runoff is alos by channel flow.

    In midsize catchments, storage processes may not be negligible.

  • 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 II storm, which covers the mainland U.S. (excluding all coasts, Atlantic, Pacific, and Gulf).

  • The NRCS TR-55 method emulates Creager's formula because both account for the increased diffusion of runoff as the basin grows in size. Both have peak discharge per unit of area in the ordinates. The TR-55 method has time of concentration in the abscissa, while Creager has the size of the basin. These two are related, the greater the basin, the longer the time of concentration.

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

  • The correlation coefficient varies between -1 and +1.

    The value ρ = 0 means that there is no linear dependence between the variables.

  • In The U.S., return periods vary from 5-10 to 100. Beyond 100, fractions of the PMP-P100 are used.

    In places where the PMP has not been established, return periods vary from 5-10 to 10,000.

  • Global warming could place the records at risk of obsolescence.

    In areas where moisture decreases (desertification), an existing or planned design will become less conservative.

    In areas where moisture increases (humidification), an existing or planned design will become more conservative.

 
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