Name: ________________________ S.S. No. __________________ Grade: _______

1. (30%) 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 φ-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. (30%) 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. (40%) Please answer briefly:

   (a) Why is the rational method referred to as "rational"?
   (b) What are the four variables (or parameters) in the runoff curve number method?
   (c) Why does the Snyder synthetic unit hydrograph apply for larger basins than the SCS synthetic unit hydrograph?
   (d) What is the standard peak rate factor (PRF) in the SCS unit hydrograph methodology? How low can it be when there is a substantial amount of diffusion?
   (e) Which SCS Type storm is the most intense?
   (f) According to the Gumbel method, what is the return period of the mean annual flood?
   (g) Give an example of a bivariate uniform probability distribution.
   (h) What is the correlation coefficient ρ when the covariance σ _x,y = 0?