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 highvelocity 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 District/ Area 1 2 3 Total 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 Heavy producers of wastewater - Determine design flow rate of heavy sewage producers
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: V ks 2.51 2 2gS f D log 10 3.7D D 2gS f D where k s pipe roughness (m) S f hydraulic gradient or friction slope, hf /L (m/m) ν kinematic viscosity (m 2 /s)
Design Criteria Non-circular profiles (open channels, box profiles) are designed with the Manning formula or any other experimental formula Manning: 1 23 12 v= R S n where: n is roughness factor
Design Criteria Determine the hydraulic performance of selected profiles Establish partial flow diagrams if necessary
Design Criteria P = Dθ T = Dsinθ y D = 1 cosθ 2 ( ) 2 D A = 2θ sin 2θ 8 R h ( ) D sin 2θ = 1 4 2θ
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
Design Criteria Tractive force gr s h The required minimum tractive force of the flow should be larger than the resistance of the sediments (τ min ) or the critical tractive force which is given by the following formula; τmin = fgd( ρg ρw) 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 Criteria for discharge -Maximum discharge levels (invert level of the outlet pipe)
Sanitary sewers Design period Select suitable design period: population and industrial growth rate water consumption growth rate. Storm sewers Design Storm Select suitable design storm: return period intensity duration. Contributing area Quantify: domestic population unit water consumption commercial/industrial output infiltration. Contributing area Quantify: catchment area surface types imperviousness. Dry weather flows. Select design method- Calculate: dry weather flows peak flow-rates. 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. i = 195 0.708 t
Example The runoff coefficient classes are as follows; Clas Runoff s coefficient C A 0.10 B 0.35 C 0.65 D 0.85 Diameter Design criteria Minimum slope Flow (m 3 /s) Full capacity 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
key A C B A = Drainage sub-area number B = Area in hectares C = Class of runoff coefficient E F G H E = Manhole number F = Ground level G = Invert level upstream sewer H = Invert level downstream sewer
13 9.00 6.43 10 1.5 A 11 0.75 B L=100m Φ0.25 L=100m Φ0.30 11 12 9.80 7.55 8.70 7.15 7.10 4 3 0.88 C 9.00 7.23 7.03 4 0.62 D 3 9.00 7.60 7.40 L=100m Φ0.60 L=100m Φ0.80 2 1.0 C L=100m Φ0.40 7 0.75 C L=100m Φ0.40 9 0.88 D 8 2 9.80 8.35 9.10 7.65 L=100m Φ0.40 L=100m Φ0.40 1 7 1 1.5 C 10.00 8.60 6 1.5 B 9.30 7.90 8 1.5 B L=100m Φ0.90 L=100m Φ0.90 L=100m Φ0.50 L=100m Φ0.30 12 0.38 B 6 9.00 6.69 5 0.75 D 5 9.20 6.90 6.80 10 8.80 7.37 9 9.20 7.90
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