Name: ______________________ S.S. No. ________________ Grade: _______

1. (25%) Rain falls on a 150-ha composite catchment which drains two subareas, as follows: (1) subarea A, draining 30%, with time of concentration 20 min; and (2) subarea B, draining 70%, with time of concentration 60 min. The phi-index is 25 mm/hr. Calculate the peak flow corresponding to the 10-yr frequency. Use the following IDF function:
   I = 650 T^0.22 / (t_r + 18)^0.75
   in which I = rainfall intensity in mm/hr, T = return period in yr, and t_r = rainfall duration in min. Assume linear flow concentration at the catchment outlet.

2. (25%) A certain basin has the following 2-hr unit hydrograph (1 cm of rainfall), defined at hourly ordinates:

   Flow (m³/s):   0 5 15 30 60 75 65 55 45 35 25 15 5 0

   Calculate the flood hydrograph for the following effective storm pattern, defined at two 3-hr increments (6-hr total event duration).

   Effective rainfall (cm/hr):    1.0   2.0
   Total rainfall (cm):              3.0   6.0

3. (25%) Given the following statistics of annual maxima for the Clearwater River: number of years n = 45; mean = 2700 m³/s; standard deviation = 1300 m³/s; mean of the logarithms = 3.1; standard deviation of the logarithms = 0.4; skew coefficient of the logarithms = -0.35. Calculate the 100-yr flood discharge using the following probability distributions: (a) normal; (b) Gumbel; and (c) log Pearson III.

4. (25%) Please answer briefly:

   (a) Why is the rational method rational?
   (b) Why does the runoff curve number method work?
   (c) Why does the Snyder synthetic unit hydrograph apply for larger basins than the SCS synthetic unit hydrograph?
   (d) How does the TR-55 method account for runoff diffusion?
   (e) For what value of skew coefficient does the Log Pearson III method plot as a straight line on log-probability paper?