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2. a) (2 marks) Explain the following terms. 1) Phreatic surface: 2) Seepage velocity b) (3 marks) A wooded forest is to be cleared for an industrial development in which all the trees on the site will be removed. The IDF curve for a 100-year rainfall is given by the relation: 1 = 2819/(t+16) where: i is the rainfall intensity in mmhr, and tis the duration in minutes. The storage capacity of the trees in the forest is estimated as 6 mm, the leaf area index is 7, and the evaporation rate during the storm is estimated as 0.2mm/hr. If the interception storage is given by 1 = S(1-e(-Pls)) + KEt determine the increase in rainfall reaching the ground during a 20-min storm with a return period of 50-year that will result from clearing the site. (Use linear interpolation to calculate the 20-min rainfall storm with a return period of 50-year) (10 marks) c) A 75-ha catchment has the following time-area relation: Time (min) 0 5 10 15 20 25 30 Contributing area (ha) 0 27 19 38 68 75 Estimate the runoff hydrograph using the area method for a 30-min rainfall-excess distribution given by Time (min) 0-5 5-10 10-15 15-20 20-25 25-30 Rainfall excess (mm/hr) 120 70 50 30 20 10 d) (5 marks) A fully-penetrating well with an outside diameter of 0.75 m, discharges a constant 0.1 m/s from an aquifer whose coefficient of transmissibility is 25*10 m/s. The aquifer is contact with a lake 2 km away and has no other source of supply. Estimate the drawdown at the welface. You are given the following relationship: 4. x inp>C ZATEN