This presentation includes the estimation of storm sewage generated as a result of storm/rainfall events. It includes the detailed usage of rational formula for quantity estimation with solved examples.

1. Estimation of Storm
Sewage
Resource Person
Engr. Sidra Rashid
sidradar9@gmail.com

2. Textbook
WATER SUPPLY AND SEWERAGE
By
E.W. Steel and T.J. McGhee
6th Edition
2

3. Factors Affecting Storm Sewage
The surface run-off resulting after precipitation contributes to the storm
water. The quantity of storm water reaching to the sewers or drains is very
large as compared with sanitary sewage.
The factors affecting the quantity of storm water flow are as below:
i. Area of the catchment
ii. Slope and shape of the catchment area
iii. Porosity of the soil
iv. Obstruction in the flow of water as trees, fields, gardens, etc.
v. Initial state of catchment area with respect to wetness.
vi. Intensity and duration of rainfall
vii. Atmospheric temperature and humidity
viii. Number and size of ditches present in the area 3

4. Estimation of Quantity of Storm Sewage
Rainfall is the primary source of storm flow. Estimation
of flow is the first step to design the Storm sewer.
 Rational Method is widely used
 Most widely used formula for urban areas
 Based on rainfall data
 For very large area empirical formula methods are used i.e.
Burkli – Zeiglar formula, Fuller’s formula etc.
• In both the above methods, the quantity of storm water is considered as
function of intensity of rainfall and coefficient of runoff. 4

5. Cont.
Rational Method
The total volume which fall upon an area “A” per unit
time under a rainfall of intensity “i” is
Q = iA
A portion is also lost by evaporation, percolation and
ponding
The actual amount which appears as run off may then be
calculated from
Q = CiA
Where,
C = Run off Coefficient 5

6. Cont.
“C” for an area is not invariant but tends to increase as
the rainfall continues.
For impervious surfaces
C = 1.75 t1/3
or
C = t / (8 + t)
These depends on duration of rainfall in minutes.
For improved pervious surfaces
C = 0.3 t / (20 + t)
Where “t” is the duration of the storm in minutes.
6

7. Cont.
Average values of C commonly used for various surfaces
7
Sr. # Type of Surface Value of C
1. Watertight roof 0.70 – 0.95
2. Asphalt cement streets 0.85 – 0.90
3. Portland Cement Street 0.80 – 0.95
4. Paved Driveways and Walks 0.75 – 0.85
5. Gravel Driveways and Walks 0.15 – 0.30
6. Lawn
Sandy soil with slope
2% 0.05 – 0.10
2-7% 0.10 – 0.15
>7% 0.15 – 0.20
Heavy soil with slope
2% 0.13 – 0.17
2-7% 0.18 – 0.22
>7% 0.25 – 0.35

8. Cont.
Some engineers use values of “C”
8
Sr. # Type of Surface Value of C
1. Densely built areas 0.70 – 0.9
2. Well-built areas 0.5
3. Residential areas 0.25 – 0.5
4. Suburban section 0.15 – 0.25

9. Sample Problem
Determine the runoff coefficient for an area of 0.2 Km2.
Out of this 3000 m2 is covered by buildings, 5000 m2 by
paved driveways and walks and 2000 m2 by Portland
cement streets. The remaining area is flat, heavy soil,
covered by grass lawns.
9

10. Solution
Surface Type C Avg C % of Area Runoff C of Area
Roof 0.7 – 0.95 =(0.7+0.95/2)
=0.825
=3000/(0.2*10002)
=0.015
=0.825*0.015
= 0.012
Driveways & walks 0.75-0.85 0.80 0.025 0.02
Portland Cement
Streets
0.8 – 0.95 0.875 0.01 0.00875
Flat, heavy soil grass
lawns
0.13- 0.17 0.15 0.95 0.1425
Overall Runoff Coeff 0.1836
10
Select values of C for each type of area/surface form given values
and calculate percentage of land area for each type
Total area 0.2km2

11. Cont.
Time of Concentration
The time required for the maximum runoff rate to develop is
known as the time of concentration
or
It is equal to time taken by a drop of water to run from the
most remote point of the drainage area to the point for which
the runoff is being estimated.
The time of concentration has two parts
Time of concentration = Inlet time + time of travel
11

12. Cont.
12

13. Cont.
Consider two areas “A” and “B” as shown in Figure.
 I1 is entrance for area “A” and
 I2 is entrance for area “B” and I1  I2
 The water flows from A enters the sewer at I1 and that from B
at I2.
 Time of flow is a function of the velocity in the line I1 – I2
and its length.
 The inlet time is time of concentration at I1.
13

14. Cont.
• The time of concentration at I2 is
either the time of concentration
for area B or the inlet time plus
the time of flow from I1 to I2 ,
whichever is greater.
• The time of concentration for
each sewer line is calculated in a
similar fashion.
• The time of Concentration will
largely depend upon the slope of
the ground surface and slope of
the sewer. 14

15. Cont.
• Nomogram is also used to
calculate the time of
concentration.
• In nomogram flow distance, type
of surface and slope are used to
calculate time of concentration.
• This procedure neglects effect of
rainfall intensity but is adequate
for most urban drainage project.
15

16. Rainfall intensity
16
Where, a , b and n are constants

17. Cont.
17

18. Problem 1
Determine the runoff coefficient for an area of 0.2 Km2. Out of this 3000
m2 is covered by buildings, 5000 m2 by paved driveways and walks and
2000 m2 by Portland cement streets. The remaining area is flat, heavy soil,
covered by grass lawns. If the maximum intensity of rainfall is 40
mm/hour, calculate the quantity of storm water which will reach sewer
lines.
18

19. Solution
Surface Type C Avg C % of Area Runoff C of Area
Roof 0.7 – 0.95 =(0.7+0.95/2)
=0.825
=3000/(0.2*10002)
=0.015
=0.825*0.015
= 0.012
Driveways & walks 0.75-0.85 0.80 0.025 0.02
Portland Cement
Streets
0.8 – 0.95 0.875 0.01 0.00875
Flat, heavy soil grass
lawns
0.13- 0.17 0.15 0.95 0.1425
Overall Runoff Coeff 0.1836
19
Select values of C for each type of area/surface form given values
and calculate percentage of land area for each type
Total area 0.2km2

20. Solution
20
i = 40 mm/hr.
A = 0.2 km2
C = 0.1836
Plug the values in the following equation
Q = C (i )(A)
Q =0.1836 (40 )(0.2)
= 0.40 m3/s

21. Problem 2
Drainage Area: 80 acres
30% - Rooftops (24 acres)
10% - Streets & Driveways (8 acres)
20% - Lawns @ 5% slope (16 acres) on sandy soil
40% - Woodland (32 acres)
Time of Concentration (TC) = 15 min.
Location: Tallahassee, Florida (Leon County), Zone 2
Calculate Peak runoff rate from 10-year frequency storm.
21

22. Cont.
22

23. Cont.
23

24. Solution
Surface Type C Area Area * C
Rooftops
0.9 24 21.6
Streets
0.9 8 7.2
Lawns
0.15 16 2.4
Woodland
0.1 32 3.2
Overall Runoff Coeff
24
Total area 80 Hectares
C

25. Solution
25
i = 6.2 in/hr.
A = 80 Acres
C = 0.43
Plug the values in the following equation
Q =C (i )(A)
Q =0.43 (6.2 )(80)
= 213.28 cfs.