SEN725 Urban Stormwater Management – Assignment 1 1. A storm drain receives runoff from a street 320 m long and 13 m wide and an adjacent paved area 0.5 ha in size. Estimate the lead (Pb) concentration in the runoff for a design storm of intensity 24.3 mm/hr and duration of 30 min if the Pb loading is 0.18 kg/km/day and for an antecedent dry period of 3 days. Assume that 90% of Pb is removed during the storm event. State other assumptions made in your calculation. (10 marks) 2. A watershed contains 16.2 ha and 24.3 ha of agricultural and residential land use, respectively. The last rainfall removed all P accumulations and was followed by a 2 day dry period before a 12.7mm rain event with duration = 60 min., P loading rate for agricultural land use = 1,656 g/day, P loading rate for residential land use = 1.8 kg/ha/yr, determine: (i) the initial P load in grams. (ii) the average P concentration in the runoff if the overall (agricultural and residential areas) runoff coefficient = 0.4, 40% of P is removed from the agricultural area by rainfall, and 90% of P is removed from the residential area by rainfall. (iii) the concentration of P (mg/L) after the first 10 minutes of runoff assuming that the amount washed off from the residential and agricultural areas follows a first order reaction: Po – Pt = Po (1 – e -kRt) where Po is the initial loading (kg), Pt is the mass remaining after time t, k = 0.181 mm-1 and R (mm/hr) is the rainfall excess. Assume that the volume runoff after 10 min is 0.4 x 106 L. (20 marks) 3. (i) Explain what is meant by event mean concentration (EMC). List three precautions that should be taken to ensure that the EMC derived for a given event is accurate. (ii) Give 2 reasons why the EMC measured for different storm events at a certain catchment would be different. (iii) Give 2 reasons why the EMC measured for the same storm event occurring over different catchments would be different. (10 marks) 4. A portion of the runoff volume time series, provided at 10 min intervals, for a storm event is shown in Table 1. The catchment area is 260 ha, the initial mass of solids on the surface is 35,000 kg and the surface is impervious. Assuming that the entire mass of solids is available for washoff, determine: (i) The total solids, P (kg) remaining at each time interval. (ii) The total solids concentration C (mg/L) in the runoff, at each time interval. (20 marks) Table 1. t (min) 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 Vol (m3 ) 425 1,981 4,528 5,306 4,443 3,594 2,462 1,981 1,613 1,330
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