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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?