Sewerage System

Sewerage system
Dr. Akepati S. Reddy
School of Energy and Environment
Thapar University
Patiala (PUNJAB) – 147 004

Sewerage system
Sewage – sewer – sewerage
• Sewage: municipal wastewater (domestic sewage, storm
water and infiltrated ground water)
• Sewerage: system for the collection and conveyance of
municipal wastewater to the STP or the point of disposal
• Sewer: conduit carrying the sewage
– Sanitary sewers, storm sewers and combined sewers
– Sanitary sewers carry sewage
• Residential, commercial and institutional sewage
• Industrial wastewater
• Infiltration water and some storm water

Sewerage system
Sewers
• Conduit carrying the sewage mostly by gravity
• Asbestos cement, ductile iron, reinforced concrete,
prestressed concrete, PVC, vitrified clay material are used in
sewer manufacturing
• Sewerage is converging network of sewers (building
connections, lateral sewers, main sewers, trunk sewers and
intercepting sewers)
– Building sewers/building connections – begins beyond a building
foundation conveying sewage from the building to (lateral) sewer
– Lateral/branch sewer – first element of the sewerage system –
receives sewage from buildings and conveys to main sewers
– Main sewer – receives sewage from lateral sewers and conveys to
trunk sewers or intercepting sewers
– Trunk sewers – large sewers conveying sewage from main sewers to
STP or disposal facilities or to large intercepting sewers
– Intercepting sewers – large sewers used to intercept a number of main
or trunk sewers and convey sewage to STP/disposal facilities

Sewerage system
Sewer
• Lateral sewers are sized larger than the building sewers
– Building sewers are either 100 or 150 mm size and 150 mm
is the recommended minimum size for a gravity sewer
• Flow in sewers is considered as steady and uniform
• A functioning sewer has to
– carry peak flow
– Transport suspended solids with minimum of deposition in
sewers
• Curved sewers are not usually preferred
– Can be used if compatible cleaning equipment is available
– Curved sewers do not allow use of laser type survey
equipment during construction to maintain sewer slope

Design of sewers
• Design involves finding slope and diameter of the sewer
– Slope for ensuring self-cleaning velocity for present peak flows
– Diameter to run partially full (d/D=0.8!) at the design peak flow
• Manning’s formula used in the design of sewers
• Nomographs for the use of manning’s equation are
available for the sewer design
– These relate discharge (Q) and flow velocity (V) with the sewer
diameter (D) and slope for different Manning’s n values when
circular sewer is flowing full
• Hydraulic elements curves developed from Manning’s
equation for circular sewers are used for obtaining the
following when the sewer is not flowing full for the known
flow (q)
– velocity (v), depth of flow (d), hydraulic radius (R), flow cross
sectional area (a) and even the Manning’s n value

Peak factor, and present & design peak flows
• Flow in sewers vary from hour to hour and also seasonally
• Peak factor is defined as the ratio of maximum hourly flow to
average hourly flow
• Peak factors depend on population density, topography of the
site and hours of water supply
• Peaking factor is taken as
<20000 3.00
20000-50000 2.50
50000-750000 2.25
>750000 2.00
• Peak factor for commercial, institutional and industrial areas
are taken as 1.8, 4.0 and 2.1 respectively
• Minimum flow may be 1/3rd to ½ of average flow
• Sewers are designed for the peak flows
• Slope of sewers is based on the present peak flow
• Diameter of the sewer is based on the design peak flow

Present and design peak flows
• Sanitary sewage generation can be assessed by using the
water supply information
• Population and per capita water supply (135 or 200 LPCD!)
• Return factor of the sewage (typically taken as 0.8)
For arid regions as it may be as low as 0.4, and for well developed
area it may be 0.9
Use of other than municipal water supply (industries, commercial
buildings, etc.!) can upset the return factor
• Forecasting sewage generation at the end of the design period may
require
• Sewers are designed for a minimum of 100 LPCD sewage
• Land use pattern (contained in the master plan) and zoning
regulations
• Land of a typical city may be
56% - residential area
20% - roads, 15% - gardens
5% - institutions (schools)
2% - hospitals and dispensaries, 2 % - markets
Industrial area - ?

Present and design peak flows
• Ultimate (saturation) population densities are often used for
anticipating the population
• Floor Space Index (ratio of total floor area to plot area) can be
used in finding out the ultimate population densities basis
• Per capita floor area is also needed in the assessment (9 m2 per
capita !)
• Population densities depend on the size of the town/city
<5000 75-150/ha.
5000-20000 150-250/ha.
20000-50000 250-300/ha.
50000-100000 300-350/ha.
>100000 350-1000/ha.
• Design period
– Length of time upto which the sewerage system will prove
adequate
– Depends on the life of the structures and equipment to be used,
anticipated rate of population growth and economic justification
– Recommended design period is 30 years

Infiltration of ground water
• Ground water infiltrates through sewer joints
– Depends on the workmanship in laying the sewers and the level
of ground water table
– For sewers laid above the ground water table sewage may lost
from the sewers
– Sewers require hydraulic testing after laying
• Suggested infiltration rates for sewers laid below the
groundwater table
– 5-50 m3/ha/day or
– 0.5-5 m3/km.day or
– 0.25 to 0.5 m3/manhole/day

Manning’s Equation
• ‘n’ is reported to reduce with increasing pipe diameter
and also vary with the depth flow
• Manning’s n of 0.013 is used for new and existing well
constructed sewers, and for older sewers it is taken as 0.015
• Typically applied for open-channel flow conditions
• Design of sewers involves finding slope and diameter of
the sewer with peak design flow capacity
• Flow velocity should be 0.6 to 3.0 m/sec. during (present and
design) peak flow
n
SR
V
2/13/2

n
SAR
Q
2/13/2

2
3
2
1
.
4 






S
vn
D
8
3
2
1
3
5
.
..4









S
Qn
D

V = velocity (m/sec)
Q = flow rate (m3/sec.)
R = hydraulic radius (m)
S = slope of the energy grade line
n = Manning’s roughness coefficient
D = Diameter of the pipe

Properties of circular sewer section
Flow through sewer is open channel flow
Parameters of interest are
• Breadth of flow (b)
• Depth of flow (d)
• Diameter of the sewer (D)
Breadth of flow is needed for the calculation
of the risk of H2S generation
Derived parameters
• Angle of flow () in radians
• Area of flow (a)
• Wetted perimeter (P)
Escritt’s definition of hydraulic radius (a/p)
 2
360
N
flowofAngle 






 
D
d
21cos2 1








2
sin

Db





 

8
sin2 
Da
2
DP 

A few important terms

d
D
p
b
a
‘a’ is area of flow
‘b’ is breadth of flow
‘d’ is depth of flow
‘D’ is sewer diameter
‘p’ is wetted perimeter
‘’ is angle of flow in radians
(2 radians = 360)
‘a/p’ is hydraulic radius ‘r’
‘d/D’ is proportional depth of flow
Here,
8
)sin(22  
 DDka a
2
Dp   2
sin Db 
    
sin1
4
 DDkr r
  D
d21cos
2


 


2
sin

A
a

sin
1
R
r
Tables are available in sewerage design manuals for reading ka, kr, a/A, r/R, and also v/V
and q/Q for different d/D values

Hydraulic radius (r) = area of flow / wetted
perimeter
d/D for simplified sewerage is 0.2-0.8
<0.2 do not ensure sufficient velocity for preventing
solids deposition in the sewer
>0.8 do not allow sufficient ventilation
For any known d/D, angle of flow can be found
From angle of flow, area of flow, hydraulic radius
and breadth of flow can be found
For d/D=0.2, Ka and Kr values are 0.1118 and
0.1206 respectively
For d/D=0.8, Ka and Kr values are 0.6736 and
0.3042 respectively
Properties of circular sewer section








SinD
r 1
4
2
DKa a
DKr r
  SinKa 
8
1








Sin
Kr 1
4
1
2
DP 

Manning’s equation
2
1
3
21
ir
n
V 
‘v’ is velocity (m/s)
‘n’ is Manning’s roughness coefficient
‘r’ is hydraulic radius
‘i’ is sewer gradient
‘a’ is flow cross sectional area (m2)
‘q’ is flow rate (m3/s)
Flow velocity is proportional to hydraulic radius which in turn to d/D
2
1
3
21
ir
n
aaVq 
From writing Manning’s equation for partial flow and full flow
taking ratio one can obtain v/V and q/Q as
  3
2
3
2 sin
1 








R
r
V
v      3
2
3
2 sin
1
2
sin











 





R
r
A
a
Q
q
 is a function of d/D
v/V and q/Q vary with d/D
v=V for d/D=0.5
v/V is maximum (1.14) when d/D is around 0.81
q/Q is maximum (1.07) when d/D is around 0.94

Gauckler-Manning Equation
• V is flow velocity (m/sec.)
• n is roughness coefficient, taken as 0.013
for PVC, vitrified clay and even for
concrete sewers
• The bacterial slime layer makes the
roughness almost same for all the
materials
• ‘i’ is sewer slope or gradient
• ‘q’ is sewage flow rate (m3/sec.)
2
1
3
21
ir
n
v 
2
1
3
21
iar
n
vaq 
2
1
3
2
2
)(
1
iDKDK
n
q ra
  22
.
8
1
DSinDKa a  
D
Sin
DKr r .1
4
1















 
D
d
21cos2 1

8
3
2
1
4
1
8
3
8
3










i
q
KKnD ra
2
3
8
3
2 








DKK
nq
i
ra

Tractive Tension (boundary shear stress)
Tangential force exerted by the flowing
sewage per unit wetted boundary
area
Denoted by  and units are N/m2 or
Pascals, Pa
Obtained by dividing weight component
of the flowing sewage in the flow
direction by the wetted boundary
area of the sewer
LP
SingaL
LP
SinW
.
.
.
. 
 
DigKgrSin r 
iKg
D
r
1









2
1
3
2
2
)(
1
iDKDK
n
q ra
6
133
8
21 







 i
g
KK
n
q ra


13
613
16
13
6
21 













 q
g
KK
n
i ra


W is weight of sewage
L is sewer length
 is density of sewage
a is area of flow
 is sewer inclination angle
since  is very small sin =tan 
tan  is the sewer slope (i)

Design of the sewer
Find initial and final (at the start and at the end of the design
period) peak sewage flow rates
If the flow is <1.5 L/Sec., then use 1.5 L/Sec. as peak flow
Using the initial peak sewage flow rate, for the minimum tractive
tension required, find minimum slope required
Ka and Kr should correspond to d/D = 0.2 at which tractive tension
is minimum
Required tractive tension for simplified sewers is 1 Pa
For sanitary sewers it is 1-2 Pa and for storm sewers and combined
sewers it is 3-4 Pa
PWkkq 21
13
613
16
13
6
21 













 q
g
KK
n
i ra



Design of the sewer
Find sewer diameter using the Gauckler-Manning equation
Here final peak sewage flow rate is taken as q
Ka and Kr values corresponding to d/D=0.8 are considered
The sewer diameters calculated may not be always commercially
available – then chose the next larger diameter sewer
commercially available
Minimum sewer diameter considered in simplified sewerage is 100
mm
8
3
2
1
4
1
8
3
8
3










i
q
KKnD ra

Surface Material Manning's - n -
Asbestos cement 0.011
Asphalt 0.016
Brass 0.011
Brickwork 0.015
Cast-iron, new 0.012
Clay tile 0.014
Concrete - steel forms 0.011
Concrete – finished 0.012
Concrete - wooden forms 0.015
Concrete - centrifugally spun 0.013
Galvanized iron 0.016
Glass 0.010
Gravel 0.029
Masonry 0.025
Metal – corrugated 0.022
Plastic 0.009
Polyethylene PE - Corrugated with smooth inner walls 0.009 - 0.015
Polyethylene PE - Corrugated with corrugated inner walls 0.018 - 0.025
Polyvinyl Chloride PVC - with smooth inner walls 0.009 - 0.011
Steel - Coal-tar enamel 0.010
Steel - smooth 0.012

Hydraulic elements graph for circular sewers

Self cleansing velocity
• In a sewer sufficient velocity (self cleansing velocity) should be developed
on a regular basis ensuring self cleansing
• Self-cleansing velocity can be found by Camp’s formula
• SG is specific gravity of the particle
• dp is particle size
• Ks is constant and its value is taken as 0.8
• Recommended self-cleansing velocity is 0.6 m/sec.
• Ensures transport of sand particles of 0.09 mm size and 2.65 specific
gravity without allowing settling
• For preventing deposition of sand and gravel 0.75 m/sec. velocity is
recommended
• Self-cleansing velocity of 0.8 m/sec. at design peak flow and 0.6
m/sec. at present peak flow are often suggested
• Velocity in the sewer is recommended not to exceed 3 m/sec. for avoiding
damage to sewers from erosion
• Flow velocity for the present peak flow should be >0.6 m/sec. and for the
design peak flow it should be <3.0 m/sec.
   2
1
6
1
1
1
pS dSGKR
n
V 

Slope and diameter of sewers
• For sewers running partially full for a given flow and slope,
flow velocity is little influenced by pipe diameter
• Slope of sewer is first fixed for the present peak flow, then
pipe diameter is decided on the basis of design peak flow and
permissible depth of flow
• For ensuring a minimum velocity of 0.6 m/sec., slope of the
sewer can be
• Minimum practical slope considered for construction is 1 in
1250
S.No. Present peak flow (LPS) Slope
1. 2 6 in 1000
2. 3 4 in 1000
3. 5 3.1 in 1000
4. 10 2 in 1000
5. 15 1.3 in 1000
6. 20 1.2 in 1000
7. 30 1.0in 1000

Sewer ventilation
• Sewers are preferably run partially full (d/D <0.8) for
facilitating ventilation
• Sewer ventilation is needed to avoid
– Dangers of asphyxiation of maintenance personnel
– Buildup of odorous gases
– Development of explosive mixture of sewer gases
(methane and oxygen)
• Hydrogen sulfide can be generated in the sewer
specially when laid at minimum slope
– H2S can cause odour problems, precipitates trace metals
as sulfides, and deteriorates cement containing materials

Design computations
• Accurate and detailed map of the area to be covered by the
sewerage system
– Scale of the map can be 25 m = 1 cm (maps of 5 m = 1 cm are
also often needed
– Location of streets, alleys, highways, railroads, public buildings,
parks, streams/drains, ditches, etc., features should be
identified on the map
– Accurate elevations of street center lines at every 15 m distance
and elevation at all locations of abrupt surface slope changes
• Decide on the layout of the sewer line
– Draw the sewer map
– Identify, locate and number the manholes on the sewer map
and code the sewers
• Locations of change of direction, sewer junctions, and upper ends
of the sewers can have manholes
• Manholes can be provided at regular distances (30 m -120 m)

Design computations
– Find surface elevation of the upstream and downstream
ends of each of the sewer
– Identify the local tributary area for each of the sewer
– Find the present and the design population equivalents of
the local tributary areas
– Find present and design average and peak sewage flow for
the local tributary area
• Collect additional information for the right of the way of the
sewer line
– Profiles of all existing and proposed streets, alleys and
potential right-of-ways
– Location of surface and subsurface utilities like water
mains, electrical conduits, communication lines, and other
underground structures
– Soil data upto 1.5 m below the bottom of proposed sewer
• Prepare sewer design computation table

Sewer design computation table
• This can be an excel worksheet and include
• Columns identifying the sewers and summerizing basic data
– Sewer code and upstream and downstream manhole numbers
– Sewer length
– Local (tributary) area, its present and design population, and its
present and design average and peak sewage flow
– Present and design average and peak flows from commercial,
institutional and industrial activities of the local (tributary) area
– Infiltration allowance for the sewer length
– Surface elevation at the upstream and downstream sewer ends
• Columns showing cumulative present average flow and peak
flow and average and peak flow at the end of design period
• Columns showing computed slope and diameter of the sewer
and Qfull

Sewer design computation table
• Columns showing hydraulic elements for the present and the
design peak flows when Manning’s n is variable
– d/D corresponding to the qPFP and to the qPFD
– Flow velocity at qPFP and at qPFD
• Columns showing sewer layout data (invert elevations at the
upstream and at the downstream ends of the sewer)
• Corrected invert elevations of the sewer on the basis of
– Sewer pipe thickness and crown cover required
– In case of a sewer junction, invert elevation of the outlet sewer
is fixed by the lowest inlet sewer’s invert elevation
– If sewer size increases crowns of the sewer in question should
be matched with that of the upstream sewer at the manhole

Urban Storm Water Drainage
System

Urban Storm-water Drainage System
Urban Drainage Design Manual; National Highway Institute, 3 rd
edition (2009) – revision (2013)
– Determine runoff
– Gutters, inlets and catch basins
– Storm water drains
– Outfalls
Determine runoff.
• Watershed characteristics and their changes with future
developments
– Urban development increases both peak runoff and total runoff
• Land use of the catchments
– residential , commercial, industrial, agricultural, parks and
undeveloped lands
– Streets, parking lots, bridges
• Rainfall and runoff calculations

Urban Stormwater Drainage System
Urban development fragments watersheds and disturbs natural
drainage
– Necessitates development of new drainage system to collect
storm water from each of the fragments and convey out
Gutters, inlets and catch basins from each of the fragments
Storm sewer map and storm sewers (may also include culverts)
mostly with gravity flow
– Underground conduits or open surface drains designed for flow
velocities 0.6 to 3.6 m/sec.
– Only a few rainfall events can result in self-cleansing velocities in
the storm sewer
All sized to collect and convey peak flows (probability of local
ponding is usually limited to ≤50%)
– Combined sewers and storm sewers also collecting and
conveying grey water

Peak flows of storm water
• Forms basis for the design of storm water drainage system
– Design storm events with 2 year return period
– Catchments characteristics after the planned development
• Rational method is usually used to assess the peak flows
– Considered appropriate for catchments of <80 hectares size
– Peak flow is believed to occur when entire catchment
contributes (time of concentration)
– Rainfall intensity is taken as same for the entire area over the
time of concentration (tc)
uK
CIA
Q 
Q peak flow in m3/sec.
C runoff coefficient
I Rainfall intensity in mm/hr
A drainage area in hectares
Ku units conversion factor (360)

Runoff Coefficient (C)


i
ii
weighted
A
AC
C
Composite Runoff Coefficient
should be used
Runoff coefficient taken should
be for the design storm event (2
year return period!)
Baringo curve

Rain fall intensity
Usually read from the regional RDF curves
• Intensity corresponding to the time of concentration for the
catchment/watershed for a specified return period is used
Regional IDF (intensity-duration-frequency) curves
• Developed through frequency analysis of rainfall events monitored
at multitude of rain gages
• Relate storm duration and exceedence probability (frequency) with
the rainfall intensity

Regional RDF (Rainfall intensity – Duration – Frequency) Curve

Time of concentration (tc)
• Time required for the entire catchment to contribute to
runoff at the point of interest for hydraulic design
• time taken for the most hydraulically remote point of the
catchment to contribute storm water to the outlet
• tc of 10 to 300 minutes is acceptable for application in the
rational method
• for tc <10 min., the rainfall intensity is unacceptably high
• for tc >300, the assumption of steady rainfall is less valid
• Factors affecting the tc
• Ponding, surface roughness and catchment slope
• Fraction of impervious area and fraction of area directly
connected to flow
• Flow path length, channel slope, channel shape and flow
pattern
• Urbanization decreases tc

Time of concentration (tc)
• Methods of estimation of Tc
• Kinematic wave method
• Kirpich and Kerby methods and Kerby-Kirpich method
• NRCS (National Resources Conservation Service) lag method
and NRCS travel time method
• Both NRCS and Kerby-Kirpich methods estimate tc as the sum
of travel times for discrete flow regimes.
• Stormwater is considered to move first as sheet flow, then as
shallow concentrated flow and finally as open channel flow
• tc is taken as sum of travel time of all the three types of flows

Estimation of time of concentration
Kerby-Hatheway method
Used for small watersheds with overland flow predominance
N is Kerby roughness factor (0.02-0.8)
467.0
67.0







S
NL
tc
Morgali and Lisely method
Used for planar small urban areas of drainage area <10-20 acres
 
3.04.0
6.0
.94.0
Si
Ln
tc 
i is intensity of rainfall (inch/hr)
n is Mannings surface roughness factor
L is length of flow (ft)
S is slope and tc is time of concentration
Calculation involves iteration
Adhoc method for time of concentration
areadrainagetc 
tc is in hours and
drainage area is in square miles

Estimation of time of concentration
Kirpich method (1940)
Used for <200 acres size basins with channel flow predomination
L is length of the main channel (ft)
h is relief along the main channel (ft)
385.03
0078.0 






h
L
tc
47.0
3
2







S
nL
tc
tc is time of concentration (min.)
L is flow path length (ft.)
S is mean slope of the basin
n is Manning’s roughness coefficient
(taken as 0.02 for smooth surface & 0.8 for grass
overland)
Hatheway formula

Generalized terrain description
Dimensionless retardance
coefficient (N)
Pavement 0.02
Smooth, bare, packed soil 0.10
Poor grass, cultivated row crops, or
moderately rough packed surfaces
0.20
Pasture, average grass 0.40
Deciduous forest 0.60
Dense grass, coniferous forest, or
deciduous forest with deep litter
0.80
The Kerby method for tc

Estimation of time of concentration (tc)
Kirpich method: Used for channel flow component
385.077.0 
 SKLtch
K: units conversion coefficient (0.0195 for SI units)
tch is in minutes
L is channel length in meters
S is slope
cholfc ttt 
The Kerby-Kirpich method: Applicable to watersheds of 0.25
to 150 square mile area, main channel lengths of 1 to 50 miles
and main channel slopes of 0.002 to 0.02
Main channel slope: Change in elevation from watershed divide
to watershed outlet divided by the channels curvi-lenier length
tch is in minutes as per Kirpich method
tolf is in in minutes as per Kerby method
  235.0467.0 
 SLNKtolf
K is units conversion coefficient (1.44 for SI units)
L is overland flow length in meters (<366)
N is dimensionless retardance coefficient
S is slope
Kerby method for tc
Useful for smaller watersheds with overland flow predominance

Time of Concentration
• Time of Concentration (tc) is taken as sum of the following:
– Sheet flow travel time
– Shallow concentrated flow travel time (flow in rills then in
gullies)
– Open channel flow/pipe flow travel time
• Storm water flow in the catchments/drainage basins
– In a catchment surface runoff starts as a sheet flow
– after relatively a short distance travel, the sheet flow transforms
into shallow concentrated flow
– The shallow concentrated flow enters open channels/sewers
and conveyed out
• Sheet flow travel time







S
nL
I
K
T u
ti 4.0
Tti is sheet flow travel time in minutes
Ku is empirical coefficient (its value is 6.92)
I is rainfall intensity in mm/hr (depends on
the tc to be calculated)
L is flow length in meters
S is slope (catchment slope)
N is roughness coefficient

Time of Concentration (tc)
• Shallow concentrated flow
– Shallow concentrated flow velocity is estimated first and used
for estimating the shallow concentrated flow travel time
V
L
T
KSKV
ti
pu
60
5.0


V is velocity in m/sec.
Ku is taken as 1.0
K is intercept coefficient (depends on land cover/ flow
regime)
Sp is slope percent
Tti is shallow concentrated flow travel time (in minutes.)
L is flow length

Time of Concentration (tc)
• Open channel/pipe flow
– Here also flow velocity is estimated first and used for estimating
the open channel/pipe flow travel time
V
L
T
SR
n
K
V
ti
u
60
5.03
2


‘n’ is roughness coefficient
R is hydraulic radius (for open channels with
width >10 times depth taken as depth)
Ku is unit conversion factor (taken as 1.0)
S is slope
V is velocity in m/sec.
L is flow length in meters
Tti is travel time in ith segment in minutes
flowpipeorchannelopentflowedconcentratshallowtflowsheettc TTTt 

USGS Regression Equations for Peak Flow
Assessments
• Used in ungauged sites for estimating peak flows
• US geological survey has developed and compiled these
regression equations (for the return periods 2, 5, 10, 25, 50,
100 and 500 years)
• Developed from the urban run off data obtined from 269
basins in 56 cities of 31 states
• These regrassion equations include the following 7
parameters
1. AS is the contributing drainage area in square miles
2. SL is the main channel slope in ft/mile (measured between
10% and 85% of the main channel upstream of the outlet)
3. RI2 is rainfall amount in inches in 2 hours for a rainfall event of
2 years return period
4. ST is basin storage as percent of the total area occupied by
lakes, reservoirs, swamps and wetlands.
      47.015.032.065.004.217.041.0
2 21383235.2 RQIABDFSTRISLAUQ SS



USGS Regression Equations for Peak Flow
Assessments
5. BDF is basin development factor (a measure of hydraulic
efficiency of the basin) - estimated through
– division of the basin into upper, middle and lower thirds,
and
– rating each of the third on 0 – 1 scale against the
following four parameters
a) Channel improvements
b) Channel lining (prevalence of impervious surface lining)
c) Storm drains/ sewers
d) Curb and gutter streets
6. IA is percent area of the basin occupied by impervious
surfaces
7. RQ2 is 2-year rural peak flow – calculated by using the
following regression equation

Even after the necessary verifications, magnititude of error in
the assessment is 35-50% of the actual field measurements

SCS (NRCS) Peak Flow Method
Peak flow is estimated as
Dkup QAqq 
qp is peak flow in m3/sec.
qu is unit peak flow in m3/sec./km2
Ak is basin area in km2
QD is runoff depth in mm
Unit peak flow is calculated by
    2
210 loglog
10 cc tCtCC
uu kq 

qu is unit peak flow in m3/sec./km2
ku is units conversion factor and its value is 0.000431
C0, C1 and C2 are coefficients whose value depends on
Rainfall distribution type
Ia/p (Ia is initial abstraction in mm and given as Ia=0.2SR)
The coefficients values are read from the table available
tc is time of concentration

Direct runoff depth (QD)
 










10
100
8.0
2.0
2
CN
kS
Sp
Sp
Q
uR
R
R
D
QD is direct runoff depth in mm
p is depth of 24 hr precipitation in mm
SR is retention in mm
SR (retention) is obtained from
Ku is units conversion factor (value is 25.4)
CN is runoff Curve Number
Its value depends on
Soil type
Land cover
Antecedent moisture
its value is read from tables
For multiple land use/soil type combinations within a basin, area
weighted CN is used

If ponding/swampy areas occur in the basin and retain
considerable runoff (as temporary storage), then the peak flow
requires adjustment as
ppa Fqq 
qa is adjusted peak flow
qp is calculated peak flow
Fp is adjustment factor (values are given below)
The method is acceptable if
the basin is fairly homogenousand has CN >40
the basin has a single main channel or branches with nearly equal tc
tc is within 0.1 to 10 hrs range
Ia/p is within 0.1 to 0.5 range
The ponds/swamps are not in the tc flow path

Sewerage Types
Based on collection
• Combined sewerage
• Separate sewerage
– Sanitary sewerage
– Storm water sewerage
Based on transport
• Conventional sewerage
• Simplified sewerage
• Solids free sewerage
• Pressurized sewerage
• Vacuum sewerage
• Sewerage of open channels and drains

Combined Sewerage
• Underground network of pipes collecting and conveying
domestic sewage, industrial wastewater and storm water
• Requires no on-site pre-treatment and storage
• Sewers are laid beneath roads at >1.0 to 3 m depth (avoids
damage from traffic loads on roads)
• Manholes at regular intervals
• Designed for gravity flow
• Designed to maintain self cleansing velocity (0.6 to 0.75
m/sec.) during peak flows
• Pumping stations are used when sewers become too deep
• Higher capital cost (higher than simplified sewerage)
• Maintenance is costly and requires trained personnel and
involves inspection, unblocking and repair
• Extension is both costly and difficult

Separate Sewerage
• Sewage (by sanitary sewerage) and storm water (by storm
water sewerage) are collected and conveyed separately
• Good for areas of irregular heavy rainfall
• Capital cost is higher than that of combined sewerage
• Operational costs are moderate
• Can provide higher level of hygiene and comfort
• Facilitates separate management of sewage and storm water,
and reuse of storm water
• Defects in pipes and manholes and illegal connections can
result in storm water flow into sanitary sewerage

Simplified (Condominal) Sewerage
• Uses smaller diameter pipes (min. dia. 100mm not 150 mm)
• Laid at shallower depths (crown cover reduced to 0.4 – 0.5 m)
– Sewers are not beneath the central roads, but within the
property boundaries and beneath the sidewalks
• Laid at flatter gradient than conventional sewerage – slope is
determined by tractive tension (not by minimum velocity)
• Expensive manholes are replaced by simple inspection
chambers or flushing points
• Needs more periodical removal of blockages and flushing
– To avoid frequent clogging, scum and heavier solids and garbage
are often removed prior to entry into the sewer
• Capital cost is lower (50-80% lesser than conventional
sewerage) and operating costs are lower
• Easily extendable
• Suitable if interceptor tanks/ septic tanks/ other on-site
pretreatment systems already exist

Solids Free Sewerage
• Wastewater is settled for solids removal prior to entry into
sewer
• Pre-settling units (interceptor tanks) require maintenance and
frequent emptying
– Removed sludge may require proper treatment and disposal
• Very small sewer diameter - lower gradient (even negative
slope), fewer pumps, and pipes at shallower depth
– Self cleansing velocity may not be needed
– May require fewer inspection points or manholes – but requires
more frequent repairs and removal of blockages than
conventional systems – may also require annual flushing
• Capital investment is lower and operating cost is lower
• Appropriate for areas where soak pits are inappropriate (lack
of space, ground water being sensitive)
• Easily extendable as the community changes and grows

Pressurized Sewerage
• Pumps rather than gravity is used to transport sewage
– Electrical power input is must for these systems
• Sewage is collected into a collection tank and ground prior to
allowing into the sewer
– May require lesser water for the excreta transportation
• Relatively smaller diameter pipes built in shallow trenches are
used
• System is independent of the topography
• All system components require regular servicing
• Appropriate for rocky and hilly areas and for areas with high
ground water tables
• Cost is comparable to a gravity sewerage or lesser

Vacuum Sewerage
• It is a high tech system and requires well instructed workers
for its operation and maintenance
– Unsuitable for self-help.
• A central vacuum source is used to convey sewage from
individual points of generation to a central collection station
• Wastewater is carried by gravity first to a collection chamber
• Once water level in the chamber reaches a set value, a valve
will open to create vacuum and suck out the wastewater
– Power (constant energy) is required to create the vacuum
– Flexible pipelines are used
• Pumping costs are lower
• Large amounts of flushing water can be saved
• May be appropriate for the areas short of water supply and
for areas with obstacles for gravity flow

Open channels and drains
• Have free water surface
• Less expensive, but land requirements are reasonably high
– Locally available materials can be used in the construction
• May prove a simple solution for storm water drainage
• Often, in steep terrains, provisions are made to slow down the
flow
• Bear many risks to health and environment
– Illegal discharge of wastewater and solid waste is a risk
– Can be breeding grounds for pests and insects
– Can have spillover and flooding risks, and may require regular
cleaning
• May be used as a secondary drainage system

Sewerage System

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