:STORM DRAINAGE
The functions of storm drainage to collect and carry away the rain water
."through drainage system, this system called "Roof Drainage
Its designed is based upon 100% fill because there is no problem of
.pressure variation
:Storm Drainage Pipe sizing
:The pipe in storm drainage depends on three factors
Rainfall: it is amount of rainfall in a specified period, the
maximum short period rainfall not the average rainfall.
The minimum recommended design rainfall is 4 inch/ft.
Roof area: the effective area is a projected area, in flat ceiling or
attic ceiling. The amount of rain water must be collected based on
other factor rainfall angel, this angel between (15-20) degrees.
Pipe fill: the degree of pipe fill design 100% for horizontal drain
and minimum fill is 50% for vertical drain.
Estimation the number of storm drains:
For each 100 m2 must be drained by one roof drain.
Area of roof building is 215 m2 (Flat Roof) then,
Number of roof drains = Projected area/100 = 215/100= 2 Drains.
The minimum rain fall is 4 inch/hr.
Rain fall angel is 20 degree.
215 m2 = 2314.32 ft2
Volume of water collected on roof = (4/12)*1*1 =1/3 ft3
1.0 cf 7.48 gallon/hr.
1/3 cf X
X= 7.48/3 = 2.5 gallon/hr = 0.042 gpm.
Each 24.0 ft2 collected 1.0 gpm.
Pipe fills Horizontal 100%.
Vertical 50% minimum fill.
Each drain must be covered = 2314.32/2 = 1157.2 ft2
Horizontal storm drain (Design) as follows:
From table 10.10
Allowable projected roof area at various rates of rainfall 1157.2 ft2.
Slope 1/8 inch/ ft2.
Rainfall rate is 4 inch/hr.
No. of horizontal drains is 2.
Horizontal Flow rate is 78 gpm.
The Diameter of horizontal drain is 4 inch.
Pipe Diameter Slope
(inch) %
Horizontal (1) 4 1/8
Horizontal (2) 4 1/8
Vertical Storm drainage (Conductor):
From table 10.9
Allowable projected roof area at various rates of rainfall 1157.2 ft2.
Rainfall rate is 4 inch/hr.
No. of vertical drains is 2.
Diameter of conductor is 4 inch.
Pipe Diameter
(inch)
Conductor (1) 4
Conductor (2) 4
Horizontal connecting between Manholes:
Slope 1/8 inch/ ft2.
Fall = 13.335-cm.
Rainfall rate is 4 inch/hr.
Horizontal Flow rate is 41 gpm.
No. of Manholes = 2.
The Diameter of horizontal drain is 4 inch`
Horizontal connecting between Manholes:
Slope 1/8 inch/ ft2.
Rainfall rate is 4 inch/hr.
Horizontal Flow rate is 139 gpm.
The Diameter of horizontal drain is 5 inch`
Pipe Diameter Slope
(inch) %
Horizontal Drain
connecting between 4 1/8
Manholes
Horizontal Drain to 5 1/8
garden
