Question
QUESTION3 (150 marks) 1) A rainwater tank has a capacity of 3 and is connected to a 100 m2 roof. The tank is closed at the top to prevent evaporation. The tank is used for water supply and the estimated daily water use is 500 L day. Undertake a water balance analysis to determine whether the tank will be empty at the end of the following scenario: the tank is half full at the start of Day 1 there is no rainfall for six consecutive days on Day 7, a storm occurs with a rainfall depth of 15 mm. Assume that 90% of the rainfall will run off from the roof and flow into the tank Also provide an estimate of the overflow volume from the tank (if any) and the total volume of water in kL supplied from the tank during the 7 day period (100 marks) 2) Temporary flood control works are to be installed at a construction site. A cost analysis indicates that an acceptable risk of flooding during the 5 year construction period is 5%. Estimate the Design Flood ARI for the temporary works. (50 marks) QUESTION4 (100 marks) 1) Describe the Thiessen Polygonal method to determine areal rainfall from point rainfalls. Use sketches to support your answer (50 marks) 2) The Muskingum equation is used for channel routing of discharge hydrographs. Describe the channel storage concepts that have been applied to formulize this equation. Use sketches to support your answer (50 marks)
Explanation / Answer
Question 3 -
1)
a) Tank half-full at the start of day -1 -
At the end of day 1 the tank will not be empty. Water in tank at end of day 1 = 1500 - 500 = 1000 L.
b) There is no rainfall for six consecutive days -
Water available in tank at end of each day -
End of day 1 = 1000 L
End of day 2 = 1000 - 500 = 500 L
End of day 3 = 500 -500 = 0
Therefore there will be no water in tank from the end of day 3 or start of day 4.
c) On day 7 storm of 15mm observed and 90% runoff flowed in tank .
Total water recieved in tank = 100 x .015 x 0.9 = 1.35 m3 = 1350 L
So tank will not be empty at the end of day 7 and it will have water = 1350 - 500 = 850 L
There is no overflow volume of water in tank.
Total volume of water supplied in 7 days period = 500 x 4 = 2000 L = 2 kL.
