City parking lots A and B slope to join at a common gutter that discharges to a
ID: 1709969 • Letter: C
Question
City parking lots A and B slope to join at a common gutter that discharges to a storm sewer inlet located at one end of the lots, as shown. Both lots are 100 ft wide, with lot A being 120 ft deep and lot B being 90 ft deep. Lot A slopes at 1 % and lot B Slopes at 1.5%. Both lots are newly paved with smooth asphalt. For a rainfall storm with an intensity of 0.80 in/hr over a 1 hr period, what is the time of concentration for flow to the gutter? How much flow will be generated for the storm? The City Engineer wants to know what will be size of pipe to catch the runoff at the inlet?Explanation / Answer
Kirpich Time Of Concentration formula
where L is in ft
t is in min
for LOT A L= 120 ft
Tc = 0.0078(1200.77/0.010.385) =1.83 min
for LOT B
L= 90 ft
tc = 0.0078*900.77 /0.0150.385 =1.26 min
total catchment area = (120+90)*100 = 21000 ft2 =0.4821 acer
pick flow q= CiA
C= Runoff Coefficient
I= INTENSITY OF RAINFALL in in/hr
A= Area of catchment in Acer
for asphalt paved road C=0.95
q= =0.95*0.8*0.4821 = 0.3664 ft3/sec
assume average velocity in pipe drain is 3 ft/sec
pipe will be design for max flow
area of pipe A= flow/velocity= 0.3664/3=0.122 ft2
dia D= (4A/3.14)0.5 =0.39 ft = 4.73 in = 5 in
thanks