CIV E 445 - APPLIED HYDROLOGY SPRING 2005 - MIDTERM 2 - SOLUTION

PROBLEM 3: ESSAY QUESTIONS

• A catchment is small in a hydrologic sense if:

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

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

  3. storm duration usually exceeds time of concentration

  4. runoff is primarily by overland flow

  5. channel storage processes are negligible.

• The bed slope. The rule-of-thumb for kinematic flow is bed slope greater 1 percent (0.01).

• 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 rainfall-runoff problem is well behaved because the runoff hydrograph is pinned at the origin, and the volume under it is determined by the total rainfall. Therefore, the variability in response is limited to differences in runoff diffusion only, which are usually not very marked (the time-base-to-time-to-peak-ratio varies between 8/3 and 15/3 only).

• The lowest value of peak-rate factor (PRF) of the NRCS synthetic unit hydrograph is 200 (South Carolina thesis). Compare with the standard value (484).

• The SCS Type IA storm, which covers the Pacific Northern coast (Washington, Oregon, and Northern California).

• The SCS TR-55 method is consistent with diffusion wave theory because it has built within it the capability to describe runoff diffusion (in the graphs for unit peak discharge, or peak discharge per unit of catchment area per unit of runoff depth).

• According to the Gumbel method, the return period of the mean annual flood is 2.33 years. This is because for K = 0 and n = ∞, the mean of the Gumbel variate is the Euler constant (0.5572).

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