CIV 364 - Lecture 2ADesign of Sanitary Sewers

Kodwo Beedu Keelson Msc Env Eng

MODULE CONTENTS

General Design Procedure for Sewers

Critical Design Parameters for Sanitary Sewers

Computer Aided Design of Sanitary Sewers

General Design Procedure for Sewers

Sequence of Design Activities

Acquisition of a topographical map

Preliminary horizontal alignment

Preliminary vertical alignment

Preliminary sewer sizing

Revise layout

Sketch Preliminary Horizontal Alignment

Locate pipes so that all potential users can readilyconnect into the system

Try to locate pipes perpendicular to contours

Try to follow the natural drainage pattern

Locate manholes in readily accessible positions

Locate outfall point (e.g. near the lowest point, next toreceiving water body, treatment works )

Draw Preliminary Longitudinal Profiles

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

Preliminary Sewer Sizing

Establish pipe sizes

Establish pipe gradients based on pipe (excavation)

depths

Revision of Layout

Revise the horizontal and/or vertical alignment to

minimise system costs by:

reducing pipe lengths

reducing pipe sizes

reducing excavation depths

Computer modelling is recommended for this activity

Critical Design Parameters for Sanitary Sewers

List of Design Parameters

Design period (2050 years)

Contributing area

Dry weather flows

Hydraulic design parameters

Select a Suitable Design Period

Estimate population and industrial growth rate

Estimate water consumption growth rate

Quantify the Contributing Area

Estimate the domestic population

Estimate the unit water consumption

Estimate commercial/industrial output

Estimate infiltration

Determining Dry Weather Flow (DWF)

Estimate average discharge from various sources

within the contributing area e.g. domestic, industrial etc

DWF = PxG + I + E

Estimate peak flow Qp based on average DWF and

peak factor

Qp = Peak factor x DWF

Hydraulic Design Parameters

Pipe roughness - dependent on Qp rather than pipe

material (0.6 -1.5 mm)

Flow velocities (0.75 < v < 3.0) m/s

Flow depth (d < 0.75D)

Pipe slope (> 0.001m/m or 0.1%)

Computer Aided Design of Sewers

Computer Models for Collection Systems

Main uses of computer models -

Design of new sewer systems

Analysis of existing systems

Design tends to be concerned with peak flows to determineif there is sufficient capacity

Analysis tends to determine if the system needs to beimproved and if so how it can be done

Hydraulics and water quality can be modelled

List of Computer Packages

SWMM (US EPA)

HydroWorks (Wallingford, UK)

MIKE URBAN (Danish Hydraulic Institute)

WinDes/WinDap (MicroDrainage,UK)

SWMM 5

Free/ Non-propriety software

Training courses are not free

Not very user friendly (pre- and post processing)

Sanitary Sewer Workbook is a pre-processor

Non-propriety GIS tools can also be used for pre- andpost processing tasks

User interface for SWMM 5

Modeling with SWMM 5

Sewer components modelled as either nodes or links

SWMM 5 Model Components

Nodes

Junctions (Manholes)

Outfalls

Storage Units

Pipes

Pumps

Weirs

Orifices

Links

SWMM 5 Model Gravity Mains

Manhole surcharging possible

SWMM 5 Model Force Mains

Manhole surcharging not possible

Questions?