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-8Stormwater sewer design

Storm water drain overload

Storm water drain provided

Introduction

�Storm water flow is also known as Wet Weather Flow (WWF)

�When rainfall takes place, a part of it infiltrates or percolates into the infiltrates 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.

�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.

The runoff or the storm water flow depends upon the following factors

(i) Catchment area

(ii) Ground slope

(iii) Permeability of ground Permeability of ground

(iv) Extent of impervious area such as buildings, paved yards, non-absorbent road surface etc.

v) Extent of vegetation growth

vi) Rain fall intensity

vii)Rainfall duration

viii) Condition of ground prior to the rainfall viii) Condition of ground prior to the rainfall

ix) Concentration or compactness of catchment area.

x) Climatic conditions such as wind, humidity, temperature etc.

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.

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 second � Let, 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

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.

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)

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.

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

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)

Theory Questions

Q1. Explain the term impermeability factor.

Q2. Enlist factors affecting storm water runoff.