Solomon Seyoum

Learning objectives Upon successful completion of this lecture, the

participants will be able to: Describe and perform the required step for designing

sewer system networks

Outline Design philosophy Constraints and assumptions Design steps Design criteria Design example

Design philosophy A sewer system is a network of pipes used to convey

storm runoff and/or wastewater in an area.

The design of sewer system involves the determination of diameters,

slopes, and

crown or invert elevations for each pipe in the system

Constraints and assumptions Free surface flow exits for the design discharges;

that is, the sewer system is designed for “gravity flow”;

pumping stations and pressurized sewers should be avoided as much as possible (are not considered here)

The sewers are of commercially available circular sizes

The design diameter is the smallest commercially available pipe having flow capacity equal to or greater than the design discharge and satisfying all the appropriate constraints

Constraints and assumptions Sewers must be placed at a depth such that they

will not be susceptible to frost, will be able to drain basements, and will have sufficient cushioning to prevent breakage due

to ground surface loading. To these ends, minimum cover depths must be specified.

• The sewers are joined at junctions such that the crown elevation of the upstream sewer is no lower that of the downstream sewer

Constraints and assumptions To prevent or reduce excessive deposition of solid material in

the sewers, a minimum permissible flow velocity at design discharge or at barely full-pipe gravity flow is specified

To prevent scour and other undesirable effects of high-velocity flow, a maximum permissible flow velocity is also specified

At any junction or manhole, the downstream sewer cannot be smaller than any of the upstream sewers at that junction

The sewer system is a dendritic, or branching, network converging in the downstream direction without closed loops

Design Steps Step 1 - Topographical map

Obtain or develop a map of the contributing area Add location and level of existing or proposed details

such as: Contours physical features (e.g. rivers) road layout Buildings sewers and other services outfall point (e.g. near lowest point, next to receiving water

body)

Design Steps Step 2 - Preliminary horizontal layout

Sketch preliminary system layout (horizontal alignment): locate pipes so all potential users can readily connect into the

system try to locate pipes perpendicular to contours try to follow natural drainage patterns locate manholes in readily-accessible positions

Design Steps Step 3- Preliminary sewer sizing

Establish preliminary pipe sizes and gradients Step 4 - Preliminary vertical layout

Draw preliminary longitudinal profiles (vertical alignment): ensure pipes are deep enough so all users can connect into the

system try to locate pipes parallel to the ground surface ensure pipes arrive above outfall level avoid pumping if possible

Design Steps Step 5 - Revise layout

Revise the horizontal and/or vertical alignment to minimise system cost by reducing pipe: Lengths Sizes depths

Design Criteria The following criteria need to be formulated for design of

sewer systems: peak rates of dry weather flow (wastewater + groundwater

infiltration) heavy producers of wastewater allowance for illicit rain water connections to sanitary sewers design storm runoff coefficient Pipe profiles (and materials) hydraulic friction constants minimum slopes of sewers outlet levels (maximum water level, invert for storm water)

Design Criteria For a large urban area the runoff factor and the wastewater

production are related to the unit area and classified into a number of classes Dry weather flow production rate

Heavy producers of wastewater - Determine design flow rate

of heavy sewage producers

District/ Area

Population density

Water consumption

Water loses

Wastewater production

Average Peak factor Maximum

p/ha l/p/d l/p/d l/p/d l/s/ha l/s/ha

1 2 3 Total

Design Criteria Infiltration to sewer pipes

Assume specific rate of groundwater infiltration (in l/s/ ha) for sewers with their invert located below the groundwater table

Allowance for illicit inflow Compile available sewer sizes

Design Criteria Storm water quantities

The amount of storm water to be transported is determined with the rational method. Indicate what design frequency (return period) is used Determine the rainfall intensity - duration curve for the

required frequency Indicate runoff coefficients

Design Criteria Hydraulic criteria

Steady and uniform flow conditions are assumed Usually Colebrook-White formula is used for the

design of circular conduits:

102.512 2 log

3.7 2s

ff

kV gS DD D gS D

where ks pipe roughness (m) Sf hydraulic gradient or friction slope, hf /L (m/m) ν kinematic viscosity (m2/s)

Design Criteria Non-circular profiles (open channels, box profiles) are

designed with the Manning formula or any other experimental formula

Manning: where: n is roughness factor

2 3 1 21v R Sn

=

Design Criteria Determine the hydraulic performance of selected

profiles Establish partial flow diagrams if necessary

Design Criteria

sinT D θ=

( )1 cos2Dy θ= −

P Dθ=

( )2

2 sin 28

DA θ θ= −

sin 214 2hDR θ

θ = −

Design Criteria Minimum slopes of sewers

To assure that sewers will carry suspended sediment, two approaches have been used: the minimum (or self-cleansing) velocity and the minimum boundary shear stress method, also called the

“tractive force”

self-cleansing - a full-pipe velocity of at least 0.6 m/s

Tractive force

The required minimum tractive force of the flow should

be larger than the resistance of the sediments (τmin) or the critical tractive force which is given by the following formula;

where d = selected specific diameter of sediment (grit) (from the sieve analysis) f = a constant called Shields parameter, for sewers f=0.056

Design Criteria

hgR s

min ( )g wfgdτ ρ ρ= −

Design Criteria Criteria for discharge -Maximum discharge levels

(invert level of the outlet pipe)

Design period Select suitable design period: • population and industrial growth rate • water consumption growth rate.

Sanitary sewers

Design Storm Select suitable design storm: • return period • intensity • duration.

Storm sewers

Contributing area Quantify: • domestic population • unit water consumption • commercial/industrial output • infiltration.

Dry weather flows. Select design method- Calculate: • dry weather flows • peak flow-rates.

Contributing area Quantify: • catchment area • surface types • imperviousness.

Runoff flows Select design method - Calculate: • peak flow-rates and/or • hydrographs.

Hydraulic design Establish hydraulic constraints: • pipe roughness • velocities • depths. Calculate pipe: • sizes • gradients • depth.

Calculation tables The design of sewers can be accomplished by using

design tables and steps provided in the lecture note

Example Design the a storm drain network for the arae shown

in the figure below for a rain fall intensity of 1 year return period given by the following equation. Use inlet time of 5 min and minimum concentration time of 10 min. The design criteria are given in Table .

0.708

195it

=

Example

Class

Runoff coefficient C

A 0.10

B 0.35

C 0.65

D 0.85

The runoff coefficient classes are as follows;

Diameter Minimum slope ‰

Full capacity

Flow (m3/s) Velocity (m/s)

0.25 4.0 0.038 0 78 0.30 3.3 0.056 0.79 0.40 2.5 0.105 0.83 0.50 2.0 0.169 0.86 0.60 1.7 0.252 0.89 0.70 1.4 0.343 0.89 0.80 1.25 0.461 0.92 0.90 1.11 0.595 0.93 1.00 1.00 0.741 0.94

Design criteria

key

A B

C

A = Drainage sub-area number B = Area in hectares C = Class of runoff coefficient

E = Manhole number F = Ground level G = Invert level upstream sewer H = Invert level downstream sewer

E F G H

L=100m Φ0.40 L=100m Φ0.40

L=100m Φ0.40 L=100m Φ0.40

L=100m Φ0.30 L=100m Φ0.50 L=100m Φ0.90 L=100m Φ0.90

L=100m Φ0.60

L=100m Φ0.80

L=100m Φ0.25

L=100m Φ0.30

1 10.00

8.60 2 9.80

8.35

3 9.00 7.60 7.40

4 9.00 7.23 7.03

5 9.20 6.90 6.80

6 9.00 6.69

7 9.30 7.90

8 9.10 7.65

9 9.20 7.90

10 8.80 7.37

11 9.80 7.55

12 8.70 7.15 7.10

13 9.00 6.43

1 1.5

C

6 1.5

B

8 1.5

B

2 1.0

C

7 0.75

C

9 0.88

D

3 0.88

C

4 0.62

D

5 0.75

D

10 1.5

A

11 0.75

B

12 0.38

B

End