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

PROBLEM 3: ESSAY QUESTIONS

• The rational method is referred to as "rational" because being conceptual, it is based on the principle of runoff concentration, which renders it dimensionless. This sets it apart from older formulas which were dimensional and fully empirical.

• The five variables in the theoretical time-of-concentration formulas are:
  
  
  1. Plane length L

  2. Plane slope S

  3. Effective rainfall intensity i

  4. Friction coefficient n

  5. Exponent of the rating m

• 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 Snyder method applies for larger basins than the SCS synthetic unit hydrograph because it is a two-parameter method, unlike the SCS synthetic unit hydrograph, which is a one-parameter method.

  In the Snyder method, the ratio of triangular timebase to time-to-peak can be varied, while in the SCS method it is constant and equal to 8/3. By increasing the ratio of triangular timebase to time-to-peak beyond 8/3, up to about 15/3, the Snyder method is able to provide larger amounts of diffusion which are applicable to larger basins.

• The standard peak-rate factor (PRF) of the SCS synthetic unit hydrograph is 484, which is 3/4 of the value of 645, which arises from unit conversions in the U.S. customary system of units.

• The SCS Type II storm, which covers the entire U.S., excluding the Eastern seaboard, most of the Gulf Coast, and the California and Pacific Northwest coasts.

• The SCS TR-55 method is physically consistent 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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