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:
   • Why is the rational method referred to as "rational"?
     
   • What are the four variables (or parameters) in the runoff curve number method?
     
   • Why does the Snyder synthetic unit hydrograph apply for larger basins than the SCS synthetic unit hydrograph?
     
   • 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?
     
   • Which SCS Type storm is the most intense?
     
   • Why does the TR-55 method apply to midsize catchments?
   • According to the Gumbel method, what is the return period of the mean annual flood?
     
   • What type of flow produces degradation in alluvial rivers? What type of flow produces aggradation in alluvial rivers? What is an artificial flood?