Lecture notes of Environmental Engineering-II as per Solapur university syllabus of TE CIVIL. Prepared by Prof S S Jahagirdar, Associate Professor, N K Orchid college of Engg and Technology, Solapur

1. E N V I R O N M E N T A L E N G I N E E R I N G - I I
L-8
Stormwater sewer design

2. Storm water
drain overload

3. Storm water
drain provided

4. Introduction
Storm water flow is also known as Wet
Weather Flow (WWF)
When rainfall takes place, a part of it
infiltrates or percolates into theinfiltrates or percolates into the
ground surface while the remaining
flows over the land depending upon
permeability of the ground, its surface
slope and many other factors.

5. The amount of water flowing over the
ground surface, pavements, house
roofs etc. is commonly known as
'runoff or the storm water.'runoff or the storm water.
This storm water is ultimately drained
through the sewers, otherwise the
streets, roads etc. would be flooded.

6. The runoff or the storm water flow depends
upon the following factors
(i) Catchment area
(ii) Ground slope
(iii) Permeability of groundPermeability of ground
(iv) Extent of impervious area such as
buildings, paved yards, non-absorbent
road surface etc.

7. v) Extent of vegetation growth
vi) Rain fall intensity
vii)Rainfall duration
viii) Condition of ground prior to the rainfallviii) Condition of ground prior to the rainfall
ix) Concentration or compactness of
catchment area.
x) Climatic conditions such as wind,
humidity, temperature etc.

9. RATIONAL METHOD
The rational formula is most commonly used for
design of storm drains. In takes into account the
following three factors:
(i) Catchment area (A)(i) Catchment area (A)
(ii)Impermeability factor (I) of the catchment
area.
(iii)Intensity of rainfall (R)
This formula can be used only when catchment
area is smaller than 400 hec.

10. The rational formula
Q =K.A . I. Ri
Q = run off or storm water flow
K = constant which permits the expression of
the factors A, I and R in convenient units.
Let, Q = runoff in cubic metres per secondLet, Q = runoff in cubic metres per second
(cumec)
A = catchment area in hectares
Ai = impervious area = A x I
Ri = Intensity of rainfall in mm per hour.
I = impermeability coefficient

12. Impermeability factor
The storm water flow depends upon the
imperviousness of the surface over which
rainfall takes place.
If the ground is relatively impervious,If the ground is relatively impervious,
more runoff takes place .
The percentage of rain water that is
available in the form of runoff is known as
impermeability factor or runoff
coefficient.

15. Average Impermeability Factor
Let, A1,A2,………..An = areas of the different
surface of the catchment area.
I1,I2,……..In= Corresponding impermeability
factors for different surfaces.factors for different surfaces.
Impermeable area =
A1I1+A2I2+…………..+AnIn=∑A.I
Hence , average impermeability factor
Iavg = (∑A.I)/(∑A)

16. Intensity of rainfall (Ri)
Intensity depends upon frequency and
duration of storm.
The value of rainfall intensity (Ri) can
be determined from any given time of
i
be determined from any given time of
concentration (tc) using rainfall
intensity curves.

20. Empirical formulae
If catchment area is more than 400 hec. Then
following empirical formulae can be used.
i. Dicken’s formula
ii. Ryve’s formula
iii. Inglis’s formula
iv. Talbot’s formula
v. Fanning’s formula
vi. Metcalf Eddy’s formula
vii. McMath formula
viii. Burkli-Zeiglar formula

21. Objective Questions
1.Kuichling’s impermeability factor for tight roof surface
is _________. (0.7 to 0.95/0.85 to 0.90/0.9 to 1.0/1
to 2)
2. As per US ministry of health formula value of a
constant ‘a’ for 5 to 20 min storm duration is ____.constant ‘a’ for 5 to 20 min storm duration is ____.
(10/20/30/40).
3. As per US ministry of health formula value of a
constant ‘b’ for 5 to 20 min storm duration is ____.
(10/20/30/40).
4. 1.Kuichling’s impermeability factor for macadamized
road ways is _________. (0.25 to 0.60/0.85 to
0.90/0.9 to 1.0/1 to 2)

22. Theory Questions
Q1. Explain the term impermeability factor.
Q2. Enlist factors affecting storm water
runoff.