(c) A square parking lot is shown in Figure 2. The surface of the lot is sloping from upper edge to lower edge. Along th

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C A Square Parking Lot Is Shown In Figure 2 The Surface Of The Lot Is Sloping From Upper Edge To Lower Edge Along Th 1 (38.25 KiB) Viewed 59 times

C A Square Parking Lot Is Shown In Figure 2 The Surface Of The Lot Is Sloping From Upper Edge To Lower Edge Along Th 2 (39.5 KiB) Viewed 59 times

C A Square Parking Lot Is Shown In Figure 2 The Surface Of The Lot Is Sloping From Upper Edge To Lower Edge Along Th 3 (48.39 KiB) Viewed 59 times

(c) A square parking lot is shown in Figure 2. The surface of the lot is sloping from upper edge to lower edge. Along the lower edge is a gutter that drains the entire lot and is sloped from right lower corner towards a drainage pit A at left lower corner. The time of concentration is estimated to be 18 minutes for overland flow (from upper edge to lower edge) and 5 minutes for travel time down the gutter (from right lower corner to pit A). Assuming the Rational formula is applicable and that the runoff coefficient is 0.8, which of the following storms will cause the highest peak flow rate into pit A? a) 134 mm/hr for 10 minutes b) 116 mm/hr for 15 minutes c) 77 mm/hr for 40 minutes. (6 marks) Pit A 18 mins 300m 18 mins 5minutes Figure 2 Parking lot 18 mins Gutter 300m
QUESTION ONE (Total of 25 marks) (a) What are the effects of urbanisation on stream hydrology? (List at least three effects) (3 marks) (b) For the section of the subdivision shown below (Figure 1), find the flow into pits A and B from a 20-years ARI storm, and size pipe AB and BC. The Intensity- Frequency-Duration table for Hong Kong is attached. (16 marks) The slope in the parkland in the direction of the arrow leading to the pit A is 1%, the retardation coefficient, n* for the grass surface in this park is 0.03, the coefficient of runoff for the parkland is 0.2. The coefficient of runoff (C) and time of concentration (t.) for the paved area are: Paved area C = 0.7 and t = 5 mins For ARIs other than 10 years, the runoff coefficients can be reasonably increased by 10% (for 20-year storms), 20% (for 50-year storms) and 25% (for 100-year storms). Pipe sizes available from 225mm internal diameter up to 1200 mm, increasing of steps of 75 mm. The surface slope in the direction of ABC is 2%, distance BC is 50 m. 100 m Parkland area Pit A 130 m Figure 1 Paved area 100 m Pit B Pit C
The Rational formula is given by the following equation: Q=0.278 CIA, where, Q is in m³/sec, A in km² and 1 in mm/hr. Assume all pipes are concrete with a friction factor f-0.02. The friction loss through the pipe is calculated using the following equation. LV² D 2g where, hy-friction loss in the pipe, L-length of the pipe (m), D-diameter of the pipe (m), V-Velocity through the pipe (From Stromwater Drainage Manual) I= a (t+b) where: 1 1 b C extreme intensity (mm hr") duration of the storm (minutes) constants depending on return period (see Table 3 of Stormwater Drainage Manual) Table 3 of Stromwater Drainage Manual (2018) ARI 2 5. 10 20 50 100 8- a 499.8 480.2 471.9 463.6 451.3 440.8 IFD values for Hong Kong ARI (Yr) 2 5 10 20 50 100 1 220 255 272 284 297 303 2 202 234 249 261 273 279 b 4.26 3.36 3.02 2.76 2.46 2.26 5 166 193 206 218 229 236 C 0.494 0.429 0.397 0.369 0.337 0.316 Duration (min) 15 116 138 10 134 158 170 181 193 200 150 160 172 179 588-898 30 87 107 118 128 140 147 60 7629928 64 81 91 101 112 119 2452235 120 46 61 70 79 89 97 240 33 45 53 61 71 78