1.
Dr.Ir.J.A.E.tenVeldhuis
CIE4491 Fundamentals of Urban Drainage
Design Assignment
Olofsbuurt
February 2013
2.
Table of Contents
1. The Assignment 4
1.1. Introduction 4
1.2. Design requirements 4
1.3. Preliminary design 4
1.4. Detailed design, manual calculation 4
1.5. Detailed design, hydrodynamic computer model calculations 5
1.6. Evaluation of design calculations 5
1.7. Final report 5
2. Project Description 6
2.1 Project Area 6
2.2 Catchment area and characteristics 6
2.3 Surface Water 7
2.4 Ground levels and ground water levels 7
2.5 Pumping station 8
2.6 Drinking water consumption 8
2.7 Specific design requirements 8
2.8 Intensity Duration Frequency Curves 8
3. Calculations 9
3.1. Wastewater production (dry weather flow) 9
3.2. Stormwater discharge 9
3.3. The catchment areas 12
3.4. Hydraulic calculations 13
3.5. Design of the overflow construction 14
4. Layout and Design 15
4.1. The preliminary layout 15
4.2. Dimensioning of the sewer sections 15
4.3. Final layout 15
5. Hydrodynamic modelling of sewer systems 18
5.1. SOBEK workshop 18
5.2. SOBEK modeling in design assignment 18
5.3. Reporting of hydraulic modelling results 19
Appendix 20
3.
CIE4491
Fundamentals of Urban Drainage
Design Assignment
Responsible chair: Urban Drainage
Principal tutor: Dr.Ir. J.A.E ten Veldhuis
E-Mail: J.A.E.tenVeldhuis@TUDelft.nl
Room: HG 4.65
Case: Olofsbuurt
Design objective: Ensuring a proper urban drainage situation in a new or
redevelopment area by designing an efficient and robust
system for collection and transport of wastewater and storm
water and for control of groundwater levels, taking into
account environmental, social and economic requirements.
Learning objectives: After successful finishing of the design assignment the
student should be able to:
-- apply acquired knowledge and skills to design an urban
drainage system for a new or a rede¬velopment area;
-- make manual design calculations for an urban drainage
system based on population data, rainfall data, urban
characteristics and application of the Rational method;
-- make design calculations for an urban drainage system
using a hydrodynamic computer model ;
-- check performance of an urban drainage system under
varying conditions of rainfall input and degradation of
the underground network;
-- write a comprehensible report about the design process
that explains the choices made, clarifies design calcula-
tions and presents the final outcomes.
4.
4
CIE4491 Fundamentals of Urban Drainage
1. The Assignment
1.1. Introduction
This design assignment is an integral part of the MSc course on Fundamentals of Urban Drainage
(CIE4491). The lecture series of this course aim to provide the student with the necessary theoretical
background to design and analyse an urban drainage system. The acquired theoretical knowledge will
be applied in the assignment by designing an urban drainage system for a realistic case, based on real-
life data. The student chooses one out of three cases that have been selected in the city of Delft: the
‘Westerkwartier’ residential area, the Olofsbuurt residential area and the ‘Poptahof’ commercial centre
and surrounding redevelopment area.
The workload for the design assignment is an estimated 56 hours (2 ECTS). The design process follows
five basic steps:
-- preparation of a detailed list of design requirements for the urban drainage system, including an ac-
ceptable return period for the occurrence of flooding;
-- preliminary design: draft layouts of the wastewater and stormwater systems (or of the combined
system) and calculation of design parameters (wastewater and stormwater flows);
-- detailed design of the urban drainage system, manual calculation: final layout and dimensioning of
the wastewater collection system and of the stormwater system applying the rational method;
-- hydrodynamic model calculations using Sobek software; analysis of calculation results for various
rainfall inputs and conditions of the underground network;
-- reporting of the design steps and results.
1.2. Design requirement
A detailed list of design requirements must be prepared that will serve as a starting point for the design
of the urban drainage system. Design requirements and criteria stated in this manual (see also ap-
pendix) should be taken into consideration and choices in the design should be justified in the design
report. Furthermore, additional basic data required for the design may be collected in the field or taken
from textbooks.
The required return period for flooding is to be decided upon by the student; the chosen return period
must be motivated in the final design report.
1.3. Preliminary design
The student will first prepare a preliminary design for the given design situation, taking into account
design requirements and aspects such as res¬toration of natural water flows and minimum energy con-
sumption. The preliminary design includes a description of system principles and main components and
a draft lay-out on the map of the case study area.
Design parameters will also be calculated in this step, in particular values will be quantified for waste-
water flows and stormwater flows. The latter will be based on realistic estimates of runoff parameters
and application of the rational method.
The preliminary design will be discussed with one of the supervisors in week 3 of the course (see also
course schedule on BB).
5.
5
Design Assignment Olofsbuurt
1.4. Detailed design, manual calculation
A detailed design will be developed, including a final layout of the urban drainage network, dimen-
sions of sewer pipes, channels and other facilities. Hydraulic calculations will be done to check that no
flooding occurs in the designed system for rainfall conditions associated with the chosen return period.
1.5. Detailed design, hydrodynamic computer model calculations
The student will develop a model of the designed stormwater (or combined) system in Sobek. Various
hydrodynamic calculations will be made to check the design and compare performance of the system
under varying rainfall conditions and varying conditions of network as a result of degradation. Results
of Sobek model calculations The design calculations will be discussed with one of the supervisors in
week 6 of the course period (see course schedule).
1.6. Evaluation of design calculations
The results of manual design calculations and of the hydrodynamic model calculations will be analysed
and discussed in the final report. Topics to include in the discussion are differences between stationary
and dynamic rainfall input, differences between degradation conditions and assumptions made with
regard to Rational method calculations and Sobek model input (infiltration, runoff coefficients, rough-
ness factor etc.).
1.7. Final report
The end product of the design assignment is a final design report and a poster summarising main char-
acteristics and results of the design. The poster will presented and discussed during a mini-symposium
taking place at the end of the course period.
The final report should at least include the following items:
-- list of design requirements and criteria with respect to the future situation;
-- motivated choice of a return period to be applied for the design calculations;
-- calculation of design parameters needed for the design (wastewater production, distribution of
catchment areas, runoff parameters, stormwater flows);
-- layout of the designed system on the city map including: main wastewater and stormwater pipes
and/or channels, location of the pumping station for wastewater, location of outflows or overflows
to surface water, locations for stormwater storage, direction of flow in the pipes under design rainfall
conditions (for stationary rainfall), indication of surface areas connected to the stormwater pipes or
channels;
-- design calculations for dimensions of the system (e.g. pipe diameters), hydraulic calculations for the
manual design;
-- results of Sobek calculations;
-- discussion of results for the manual and hydrodynamic computer calculations for the detailed design
and discussion of possible flooding and capacity problems;
-- conclusions and recommendations.
6.
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CIE4491 Fundamentals of Urban Drainage
2. Olofsbuurt
Design a new system to replace the existing combined sewer system with a focus on innovative ideas
given the lack of (underground) space in the area.
2.1. Project Area
The ‘Olofsbuurt’ in Delft is a mainly residential area of about 21 hectares in one of the older parts of
Delft and located just behind the main train station. The number of inhabitants is 3298 and the area is
characterized by old buildings and narrow streets.
The project area is enclosed by the railway on the North-East side; by the Buitenwatersloot on the
South-East side; by the Westplatsoen on the South-West side; and by the Van Gaalenlaan, Eliza Dorus-
traat, Van der Heimstraat and Adriaan Pauwstraat on the North-West side. The boundaries of the project
area are drawn in Figure 1.
In the current situation, combined sewer overflows are located at the northwestern and southeastern
end of the area, see blue spots in Figure 1, and the receiving surface water flows through the entire city
before reaching larger surface waters. This is an undesirable situation. The area has high groundwater
tables and limited discharge capacity for stormwater, the combination of which causes frequent flooding.
The new situation should provide solutions for these problems.
2.2. Catchment area and characteristics
The catchment mainly consists of impervious area, i.e. roofs and streets. The pervious areas are mainly
small back gardens. A detailed map with all impervious and pervious areas is can be found on Black-
board (filename: ‘catchment characteristics.pdf’). A snapshot from this maps can be seen in Figure 2
on the next page.
Figure 1: Map of the Olofsbuurt area. (Source: Google Maps)
7.
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Design Assignment Olofsbuurt
2.3. Surface Water
The Olofsbuurt area is part of the so-called Van Houtenstraat polder area with a target water level at
NAP -1.50 m and a maximum water level variation of 0.40 m above target level.
The future water levels in the Buitenwatersloot and future Phoenixstraat canals will be NAP -0.43 m.
These canals function as ‘boezem’ and have a higher surface water level compared to the polder water
levels.
2.4. Ground levels and ground water levels
Ground levels can be found in the Autocad file as provided on blackboard. In this file the ground levels
are given next to the manholes (G-numbers in file).
In this area groundwater levels have been monitored at various locations. Graphs showing the ground-
water fluctuations can be found on Blackboard. An example of the Van de Spiegelstraat is given below
in Figure 3.
Figure 2: snapshot of map with impervious surfaces (not north-southorientated!)
Figure 3: 2 years series of groundwater levels at the Van de Spiegelstraat.
8.
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CIE4491 Fundamentals of Urban Drainage
2.5. Pumping station
Wastewater and stormwater from the combined sewer system in the Van Houtstraat area is currently
transported to the Van Houtstraat pumping station which has a capacity of 0.306 m3
/s. It can be as-
sumed that the pumping station capacity is sufficient to receive water from the Westerkwartier area.
2.6. Drinking water consumption
For the calculation of the wastewater production it can be assumed that the water consumption per
inhabitant is 140 litres per day, which is consumed over a period of 10 hours. The water losses amount
to 20 litres per day per inhabitant.
2.7. Specific design requirements
-- The available capacity for storage of stormwater that falls on the area needs to be sufficient to
storage at least 36 mm of rainfall (equivalent to the volume of a T=10 yrs storm). This capacity is
needed to store stormwater before it is being transported out of the area to prevent overloading of
downstream areas during heavy rainfall;
-- Flooding frequency needs to be brought down to at least once per 2 years;
-- Buitenwatersloot quays serve as flood defence structures (dikes, as it where) and cannot be inter-
sected by pipelines or overflow constructions;
-- Maximum groundwater level 0.5m below ground level;
-- Stormwater infiltration cannot be applied in this area; groundwater fluctuations may disturb the shal-
low foundations of the buildings.
2.8. Intensity Duration Frequency Curves
The sewage system needs to be designed for a certain return period of rainfall, which is decided upon
by the designer. Corresponding rainfall intensities are to be derived from the IDF-curves (see Chapter 3).
9.
9
Design Assignment Olofsbuurt
3. Calculations
3.1. Wastewater production (dry weather flow)
The wastewater production can be calculated based on the number of inhabitants and the drinking
water consumption, taking into consideration water losses and possible infiltration of ground water. The
urban drinking water consumption comprises both domestic and industrial usage. Water losses (leak-
age) into underground pipes occur when drinking water does not end up in the sewage system, e.g.
water used to wash cars or to water the garden.
The wastewater production is subject to a diurnal pattern, as can be seen in Figure 4. In order to ac-
count for this effect, a so-called peak factor can be estimated as:
av
2.5
p 1.5
Q
= + (1)
with:
p = peak factor
Qav
= average daily waste water production in l/s
peak factor:
av
2.5
p 1.5
Q
= +
with:
p = peak factor
Qav
= average waste water production in l/s
Using table B2 in appendix B the wastewater production can be determined for the design of the s
system.
Figure 4 Wastewater production in 24 hours
3.4 Storm water quantities
The precipitation data is based on measurements in Lelystad from 1970 through 1984. Based on
measurements partial series have been derived for rain periods of 5, 15, 30, 60 and 120 minutes.
The sewage system needs to be designed for the transportation of an amount of water that has a
return period, for example once every year or once every two years (see § 3.6 for further informat
Not all water is actually being discharged towards the sewage system. Storm water that falls at un
areas, such as public gardens, private gardens, etc, is supposed to infiltrate into the ground. Wa
falls at paved areas, such as roofs and roads, is supposed to fully flow into the sewage system. T
centage of water that flows into the sewage system therefore depends upon the percentage of
area in a neighbourhood, often expressed as a run-off coefficient between 0 and 1 (0=100% un
1=100% paved)
The discharge of water in a downstream section of a sewage system is the sum of discharges of
tions situated upstream of this section. This principle of adding values is the basis of the so-called r
method. The rational method can be used to calculate discharges in a branched sewage system. Fo
complex systems, computer modelling software has been developed (e.g. Sobek, Infoworks).
The following relation between discharge and rain intensity is assumed:
n
n m m
m 1
Q i ( F )
=
= × ψ ×∑
with:
Qn
= the discharge in the system at a location with n upstream sections in l/s
DESIGN AND CALCULATASSIGNMENT CT4490
Qav
Figure 4: Typical diurnal pattern of wastewater production.
Alternatively, it is often assumed that the daily wastewater production takes place within ten hours of
the day. This is equivalent to a peak factor of p = 2.4.
3.2. Stormwater discharge
Generation of stormwater discharge depends on the characteristics of the runoff surface. Stormwater
that falls on pervious areas, such as public gardens, private gardens, etc., is assumed to infiltrate into
the ground. Water that falls on impervious areas, such as roofs and roads, largely runs off to a sewer
system. The percentage of water that flows into the sewage system therefore depends upon the per-
centage of impervious area in a neighbourhood, often expressed as a run-off coefficient between 0 and
1 (0=100% pervious, 1=100% impervious).
Rational Method
The discharge of water in a downstream section of a sewage system is the sum of discharges of all
sections situated upstream of this section. This principle of adding values is the basis of the so-called
rational method. The rational method can be used to calculate discharges in a branched sewage system.
For more complex systems, computer modelling software has been developed (e.g. Sobek, Infoworks).
The following relation between discharge and rain intensity is assumed:
10.
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CIE4491 Fundamentals of Urban Drainage
n
n m m
m 1
Q i ( F )
=
= × ψ ×∑ (2)
with:
Qn
= the discharge in the system at a location with n upstream sections in l/s
i = the precipitation intensity in l/(s·ha)
Ψm
= the run-off coefficient of the catchment area that discharges towards sewer section ‘m’
Fm
= the catchment area (impervious area) that discharges towards sewer section ‘m in ha.
The rational method is based on the following assumptions:
-- The rain is spread evenly over the catchment area, in other words the rain intensity is constant over
the catchment area.
-- The intensity is constant for the duration of the rainfall.
-- The maximum discharge at a random location P in the sewer system (see Figure 5) is a function of
the average precipitation intensity that is associated with the time needed for a rain drop to travel
from the furthest location in the system to location P, in combination with the time needed for a rain
drop to get into the sewer system via the earth surface.
Fm
= the catchment area (paved area) that discharges towards sewer section m in ha.
The rational method is based on the following assumptions:
- The rain is spread evenly over the catchment area, in other words the rain intensity is consta
the catchment area;
- The intensity is constant for the duration of the rainfall;
- The maximum discharge at a random location P in the sewer system (see figure 5) is a functio
average precipitation intensity that is associated with the time needed for a rain drop to travel f
furthest location in the system to location P, in combination with the time needed for a rain drop to
the sewer system via the earth surface.
Furthest location
hallo The above-mentioned amount of time is called ‘conce
time’ (tc):
tc
= to
+ td
to
= The amount of time it takes a raindrop to get into the
system via the surface. For this assignment it is assumed
= 5 minutes and remains constant.
td
= The amount of time it takes a drop of rain to travel f
furthest location in a sewer system to the location under con
tion. This amount depends upon the velocity of flow in the
and therefore upon the diameters of the pipes. The rain i
used in the calculations of the critical flow depends upon t
centration time.
Rainfall depth-duration-frequency curves are frequently u
the design of sewer systems (see figure 6) when apply
rational method. These curves are of the form:
R = a . (tr
)b
with:
R = rainfall in mm
a = constant with for each return period a specific value
b = constant, 0b1
tr
= duration of rainfall in h
Rainfall depth-duration-frequency curves do not represent actual rain storms !
They exclusively give information on the frequency of a certain amount of rain (R) during a certain
of time (tr
). This frequency is always expressed as a return period, e.g. once every two years (T=2
return period is associated with a different rainfall depth-duration curve.
For this assignment it is necessary to know the intensities, which occur with a certain return per
for a certain duration of rainfall. These intensities can be derived from the rainfal
duration-frequency curves:
with:
iT=n
= the precipitation intensity in mm/h, for return period T
RT=n
= the amount of precipitation in mm, for return period T
tr
= the rainfall duration in h
n = the return period in years
Depending on the function and development of the catchment area, a sewer system is designe
intensity that occurs with an acceptable return period. A sewer system will never be designed for
Figure 5 -Travel route
T n
T n
r
R
i
t
=
=
=
DESIGN AND CALCULATIOASSIGNMENT CT4490
Figure 5: Travel route.
The above-mentioned amount of time is called ‘concentration time’ (tc
):
c 0 dt t t= + (3)
with:
t0
= The amount of time it takes a raindrop to get into the sewer system via the surface. For this
assignment it is assumed that t0
= 5 minutes and remains constant.
td
= The amount of time it takes a drop of rain to travel from the furthest location in a sewer system
to the location under consideration. This amount depends upon the velocity of flow in the sew-
ers and therefore upon the diameters of the pipes. The rain intensity used in the calculations of
the critical flow depends upon the concentration time.
Intensity-Duration-Frequency (IDF) Curve
The occurrence of rainfall intensities for different return periods can be derived from a statistical analysis
of rainfall series. This information is summarized in an Intensity Duration Frequency curve (IDF curve).
IDF curves are used in the rational method to find a design intensity for a given concentration time and
return period. The choice for a return period is the result of an optimisation process between the costs
11.
11
Design Assignment Olofsbuurt
involved in constructing and maintaining a sewer system versus economic and social benefits like the
prevention of damage and nuisance. Three examples of IDF curves for different locations in the world
are presented in Figure 6, Figure 7 and Figure 8.
Figure 6: IDF curve based on rainfall data collected from location 1.
Figure 7: IDF curve based on rainfall data collected from location 2.
12.
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CIE4491 Fundamentals of Urban Drainage
The three IDF curves are from areas with a temperate maritime climate, Mediterranean climate, and a
tropical climate (in random order).
-- Based on the functions and the type of development, a return period must be chosen and motivated
by the student.
-- A rainfall intensity is then derived from one of the IDF curves, and is used as hydraulic load for the
rational method calculations. The choice for the IDF curve used should be motivated.
3.3. The catchment areas
The amount of stormwater that is discharged into the sewer system depends on the size and charac-
teristics of the catchment. Stormwater that falls on pervious areas largely infiltrates into the ground,
whereas stormwater that falls on impervious areas (streets, side-walks, roofs) runs off and predomi-
nantly enters the sewer system.
For hydraulic calculations it needs to be known which part of a catchment area runs off towards which
section of the sewer system. An example of a method of allocating catchment areas to a specific sewer
section is shown in Figure 9.
Figure 2 -Standard representation of sewer information
3.2 The catchment areas
The amount of storm water that is discharged into the sewage system depends on the size and cha
acteristics of the catchment area. Storm water that falls on unpaved areas largely infiltrates into the
ground, whereas storm water that falls on paved areas (streets, side-walks, roofs) runs off and pred
inantly enters the sewage system.
For hydraulic calculations it needs to be known which part of a catchment area runs off towards wh
section of a sewer pipe. An example of a method of allocation of catchment areas to a specific sew
section is shown in figure 3.
Figure 3 -Allocation of catchment area
3.3 Wastewater production
The wastewater production can be calculated based on the number of inhabitants and the drinking w
consumption, taking into consideration water losses and possible infiltration of ground water.
The urban drinking water consumption comprises both the usage by companies and industries and b
population. Water losses occur when drinking water does not end up in the sewage system, e.g. w
used to wash cars or to water the garden.
A laundry using large amounts of water is located at the Grote Markt. Besides the normal wastew
production, an extra discharge of wastewater flows here into the sewage system, spread out evenly
DESIGN AND CALCULATIOASSIGNMENT CT4490
Figure 8: IDF curve based on rainfall data collected from location 3.
Figure 9: One method of allocating catchment
areas to specific sewer sections.
13.
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Design Assignment Olofsbuurt
3.4. Hydraulic calculations
Hydraulic calculations are useful to understand and evaluate the system’s behaviour given the load ap-
plied.
The diameters for wastewater sewers are determined by required capacity to transport wastewater
flow, while the diameters of combined and separate stormwater sewers are determined by the required
stormwater transport capacity.
When pipes are partially filled, as is the case of most wastewater sewers, hydraulic calculations for grav-
ity flow conditions apply. When pipes are fully filled, as is the case of many combined and stormwater
sewers in flat areas like the Netherlands, hydraulic calculations for pressurised flow apply.
The head loss for flow through full pipes can be described using the Darcy-Weisbach formula:
(4)
with:
Δh = head loss in m water column
λ = friction factor (dimensionless)
L = length of pipe, m
Dh
= diameter of pipe, m
u = velocity of flow, m/s
g = acceleration of gravity, m/s2
The friction factor λ can be calculated using the formula of White-Colebrook:
1 2.51 k
2log
3.71DRe
=− +
λ λ
(5)
with:
Re = the Reynolds number = u·D/ν
ν = kinematic viscosity
k = wall roughness, m
Equation (5) can be used for any kind of pipe, that is, for all wall roughnesses. The first term between
the brackets relates to hydraulically smooth pipes, the second one to hydraulically rough pipes. Concrete
sewer pipes (k = 1.5 mm) can be considered hydraulically rough, so that the first term can be neglected
(the maximum error that is introduced is 2.5%).
Hence, the friction factor for hydraulically rough pipes can be calculated as:
1 3.71
2log
k /D
=
λ
(6)
Rewriting the formula of Darcy-Weisbach (4) as function of the flow velocity yields:
1
u 2gDI=
λ
(7)
with:
I = the hydraulic gradient = Δh/L
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CIE4491 Fundamentals of Urban Drainage
Substitution of λ and Re= u·D/ν yields the formula for the velocity of flow:
3.71
u 2 2gDI log
k /D
=
(8)
The discharge can be calculated for full pipes using:
21
4
Q u D= × π (9)
For each combination of pipe diameter D and gradient I, the discharge Q and velocity of flow u can be
calculated using above-mentioned formulae for a completely filled pipe.
The kinematic viscosity depends on the temperature and is defined as:
6
1.5
497 10
(T 42.5)
−
⋅
ν =
+
(10)
with:
T = temperature in °C
Based on above-mentioned formulae, pairs of values for Q and u were calculated for a water tempera-
ture of 10 °C and a wall roughness of 1.5 mm. These values are provided in Appendix D.
3.5. Design of the overflow construction
An overflow construction can be implemented as a rectangular overflow weir. For this construction the
following formula applies:
3 / 2
Q 1.86Bh= (11)
with:
Q = overflowing discharge, m3
/s
B = width of the weir crest, m
h = overflow height, m
For alternative forms of an overflow construction, please refer to literature for the correct discharge
formulae.
15.
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Design Assignment Olofsbuurt
4. Layout and Design
4.1. The preliminary layout
Wastewater from the development area needs to be transported to the pumping station(s). From there
it will be pumped towards a wastewater treatment plant. In this assignment all wastewater has to be
transported towards the pumping station(s) under free-flow conditions.
For this assignment it is mandatory that the sewer system is branched.
Respecting these conditions, a draft layout can be designed using the following procedure:
-- Determine the direction of flow of storm water runoff over land using the information about ground
levels in the area. Determine the direction of flows in the sewer pipes towards a pumping station,
stormwater storage facility, combined sewer overflows or stormwater outflows;
-- Indicate the direction of flow in the sewer pipes on the map;
-- Indicate the manholes in the main sewer pipe;
-- Determine which surface areas discharge into the which sections of the main sewer.
4.2. Dimensioning of the sewer sections
After the preliminary layout of the system has been drafted, the dimensions of each section of the main
sewer pipe can be calculated starting at the location of the most upstream situated manhole. The fol-
lowing steps should be taken into account:
-- future dry weather flow;
-- intensity-duration-frequency curve for rainfall;
-- catchment area per section;
-- run-off coefficients per area;
-- infiltration of ground water;
-- available pipe diameters;
-- the ground cover should at least be 1 m to protect the sewer pipes against traffic loads and to allow
for house connections;
-- the average flow velocity in the pipes cannot exceed 2 m/s;
Appendix provides examples of hydraulic calculation tables. An examplary hydraulic calculation for the
layout in Figure 11 can be found at the end of Appendix B for a storm water system.
The example tables include calculations of the piezometric levels at the upstream and downstream ends
of pipe segments. Compare these levels with the ground level at this points and check whether manhole
flooding (i.e. piezometric level above ground level) occurs. If it does, the design should be changed so
that no flooding occurs. The piezometric levels also indicate whether a pipe is full or partially filled. The
corresponding calculation formulas can be used accordingly.
4.3. Final layout
In the final layout, all details needed for a complete design will have to be given in the drawing.
These are:
-- The serial numbers and sizes of the catchment areas;
-- The serial numbers of the manholes (only the ones that are considered in calculations) including their
ground level and invert level;
-- The lengths and diameters of the calculated sewer sections of the main sewer.
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CIE4491 Fundamentals of Urban Drainage
Figure 10 shows an example of the standard representation of above-mentioned parameters in the
layout drawing.
Figure 2 -Standard representation of sewer information
3.2 The catchment areas
The amount of storm water that is discharged into the sewage system depends on the size and char-
acteristics of the catchment area. Storm water that falls on unpaved areas largely infiltrates into the
ground, whereas storm water that falls on paved areas (streets, side-walks, roofs) runs off and predom-
inantly enters the sewage system.
For hydraulic calculations it needs to be known which part of a catchment area runs off towards which
section of a sewer pipe. An example of a method of allocation of catchment areas to a specific sewer
section is shown in figure 3.
Figure 3 -Allocation of catchment area
3.3 Wastewater production
The wastewater production can be calculated based on the number of inhabitants and the drinking water
consumption, taking into consideration water losses and possible infiltration of ground water.
The urban drinking water consumption comprises both the usage by companies and industries and by the
population. Water losses occur when drinking water does not end up in the sewage system, e.g. water
used to wash cars or to water the garden.
A laundry using large amounts of water is located at the Grote Markt. Besides the normal wastewater
production, an extra discharge of wastewater flows here into the sewage system, spread out evenly over
10 hours each day.
Leakage out of the sewage system is negligible. In those sewers with an invert level below 1.50m+NAP
(the average ground water table) infiltration of ground water occurs.
In appendix B an average wastewater production is calculated in l/s per hectare catchment area. However,
the wastewater production varies significantly over a time-span of one day, as can be seen in figure 4 In
order to take this effect into account the average production is multiplied by a so-called
DESIGN AND CALCULATIONSASSIGNMENT CT4490
Catchment
charateristics
Manhole
charateristics
Pipe
charateristics
Figure 10: Standard representation of sewer information.
An example of a layout is given in Figure 11 and an exemplary longitudinal section is shown in Figure 12.
After the layout of the system has been determined, the dimensions of each section of the main sewer pipe
can be calculated, using the tables in appendices E or F and starting at the location of the most upstream
situated manhole. The following should take into account in the use of the tables:
- future dry weather flow;
- intensity-duration-frequency curve for rainfall;
- catchment area per section, taking into account different wastewater productions per area
(see § 3.5);
- run-off coefficients, per area;
- infiltration of ground water;
- available pipe diameters and required minimum sewer pipe gradients;
- a minimum depth of cover of 1,0 meter to protect the sewer pipes against traffic loads and to enable
house connections;
- a maximum excavation depth of 4.5 m;
- the elevation at both ends of the sections.
The hydraulic tables should be filled out row by row. For the layout in figure 9 an accompanying hydraulic
calculation is presented in table 1 for a storm water system (not including the hydraulic control).
When designing the system, some restrictions have to be taken into account:
- The excavation depth cannot exceed 4.50 m.
- The ground cover should at least be 1.00 m.
- The average flow velocity in the pipes cannot exceed 3 m/s.
An example of a longitudinal section of (a downstream part of) the storm water system in figure 9 is shown
in figure 10.
Figure 9 -Example of a layout
6,70
1
8,00
6,23
2
7,80
5,85
3
7,50
5,50
4
7,30
5,00
6
7,00
6,60
7
8,00
6,30
8
7,60
5,60
9
7,10
6,40
10
7,80
6,00
11
7,50
5,68
12
7,30
5,60
13
7,00
I A
1 1,48
I C
2 1,44
II C
3 0,66
IIIB
A
4 0,52
IIIC
5 0,40
I B
6 1,85
II A
7 3,00
I A
8 2,00
I A
9 2,20
II C
100,68
IIIB
111,32
IIIB
122,65
5
7.10
5.30
5.10
L=200
D=0.4L=200 D=0.5
L=175
D=0.3L=145
D=0.6
L=80
D=0.6
L=80
D=0.8
L=60
D=1.0
L=240D=0.5
L=240D=0.4
L=160D=0.4
L=160
L=160
D=0.4
L=90
L=90D=0.6
17
DESIGN AND CALCULATIONSASSIGNMENT CT4490
Figure 11: Example of a layout.
17.
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Design Assignment Olofsbuurt
Manhole number
Ground level
Invert level
Sewer gradient ‰
Distance
section
total
Diameter
Figure 10 -Longitudinal section of a storm water system
18
DESIGN AND CALCULATIONSASSIGNMENT CT4490
Figure 12: Longitudinal section of a storm water system.
18.
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CIE4491 Fundamentals of Urban Drainage
5. Hydrodynamic modelling of sewer systems
Hydrodynamic modelling is applied to analyse the conveyance of water through the sewer system in
more detail, under dynamic conditions. For this purpose, the SOBEK-Urban modelling software is used,
which is made available by TU Delft and Deltares. SOBEK-Urban consists of two combined modules: a
rainfall runoff component to simulate rainfall runoff processes, and a hydrodynamic component to simu-
late water flow through the pipes and other system components. Based on the detailed design, a SOBEK
model of the system is to be built.
5.1. SOBEK workshop
A SOBEK workshop will be organized in week 4 of the course period (see course schedule) to get familiar
with the SOBEK software. During the workshop, you will individually build a SOBEK model and analyse
the results, which have to be written down in a report. This SOBEK workshop report determines 10 %
of the overall course grade.
5.2. SOBEK modeling in design assignment
The SOBEK modelling consists of the following steps.
Implement design in SOBEK
The detailed design of the sewer system by rational method is implemented in SOBEK. The following
steps guide you through this.
Setting up a new network in Sobek
Open program
Select: New Project, Select new Case
Go to: Task block ‘Settings’
Select modules SOBEK-Urban 1DFLOW and RR modules
Go to: Task block ‘Import network’
The Import network window will pop up: select ‘Start from Scratch’
Go to: Task block ‘Meteorological data’
Select a precipitation event (any event; this can later be changed before you start the simulation)
Go to: Task block ‘Schematisation’
Select ‘Edit model’
Go to: Edit, select Network.
The ‘Edit network” Toolbar will now appear in the Toolbar section of the screen.
From dropdown menu, choose: ‘Nodes’ (several options for adding, moving, connecting nodes)
Add attributes to nodes and connections (select node/connection, right mouse button, model data etc.).
Note: the standard connection type is a trapezium channel, change into pipe for sewers.
Running the SOBEK model
Set up your model by implementing all sewer pipe diameters as calculated by the rational method. Cre-
ate a stationary rainfall event in SOBEK that is representative for the design conditions you applied in
the rational method calculation. Then, run the SOBEK model and analyze your results. After that, change
your pipe diameters to a minimum, without flooding occurring.
19.
19
Design Assignment Olofsbuurt
Run the model again, but now by implementing other rainfall events, which are included in the SOBEK
software. The rainfall events to implement are STNBUI08 (T=2 yrs), STNBUI09 (T=5 yrs) and STN-
BUI10 (T=10 yrs). Analyse your results after each model run.
Changes in the SOBEK model
Sewer systems deteriorate due to aging, overloading, misuse and mismanagement. In order to ensure
sewer service availability, such systems have to be properly maintained.
In this step, the effects of different defects commonly found in sewer systems are evaluated. Sewer
conditions are determined by visual inspection – closed circuit television (CCTV). The defects are regis-
tered according to the visual inspection coding Standard NEN-EN 13508-2, while the Standard NEN 3399
is used to assign a level of severity to each defect - condition assessment. Table 5.1 shows inspection
results from 2013 from your area. Locations of defects in the system will be selected by the supervisor
when the detailed design is implemented in SOBEK.
Implement all changes at once in your SOBEK model. Run the model and analyse the results. After that,
implement the changes again one by one, in order to assess which of the defects has the most influence
on the occurrence of flooding.
Suggest a maintenance strategy after analysing all results. Motivate your choices in your final report.
Code Description Class SOBEK calculation changes
BAF surface damage 3
4
4
5
k = 1.7 mm
k = 3 mm
k = 4.5 mm
k = 6 mm
BBB attached deposits 3
3
4
4
pipe diameter decrease 15%
pipe diameter decrease 20%
pipe diameter decrease 35%
pipe diameter decrease 45%
- measured slope
(settlement)
- slope is 0.0
slope decrease 35%
slope decrease 55%
slope decrease 70%
5.3. Reporting of hydraulic modelling results
In the final report, comment (for the SOBEK calculations) on the differences between rainfall inputs, at
what locations flooding occurs and what are the effects of changes to the system to increase the return
period of flooding. Next to that, comment on the differences between results of the rational method
calculations and SOBEK calculations. Furthermore, discuss the model output after changing parameters
related to defects, and comment on the maintenance strategy that you suggest.
Table 1: CCTV inspection results.
20.
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CIE4491 Fundamentals of Urban Drainage
Appendix A. Demands of the future network manager
A.1. General requirements
-- Minimal diameter of 315 mm for non-pressurised pipes.
-- Minimal diameter of 63 mm for pressurised pipes.
-- Note that for PVC pipes nominal diameters are external diameters. For all other pipe types nominal
diameters are internal diameters. This must be taken into account for the hydraulic calculations.
-- Maximum flow velocity in pressurised pipes of 2.0 m/s.
-- Maximum filling of non-pressurised pipes of 50%.
-- Pipe material: PVC.
-- Minimal cover of the pipes of 1.00 m for residential streets.
-- Minimal cover of the pipes of 1.60 m for main streets.
-- Maximal length of pipe section between manholes of 70 m.
-- Restricted number of different pipe types/diameters.
-- The return period for flooding should be chosen at an acceptable level: please motivate your choice.
-- A reservation should be made for a future installation of a rainwater treatment facility.
-- If possible, rain water pipes must be located above waste water pipes.
-- No height jumps in the sewer system.
-- Minimal gradient of 1:250 in the first section and of 1:500 in all other sections.
-- Sewers that are constructed on piles must be made from reinforced concrete. PVC is not allowed.
A.2. Intersection of wastewater and stormwater pipes
For the intersection of wastewater and stormwater pipes there are two possible design options:
-- The rainwater pipe is conducted via an inverted siphon below the wastewater pipe;
-- The wastewater pipe crosses a manhole in the rainwater collection system: rainwater is flowing
through the manhole whereas the wastewater pipe crosses the manhole.
A.3. Intersection of water collection pipes and surface water bodies
For the intersection of waste- or stormwater pipes with ditches there are two possible design options:
-- The whole waste- or storm water collection system is located at least one metre below the bottom
level of the surface water body;
-- An inverted siphon is used locally at the intersection with the ditch.
N.B.: In the municipality of Delft there are about 100 inverted siphons in the wastewater collection
system and about 200 to 300 inverted siphons in the separate stormwater collection systems.
Inverted siphons and pipes crossing manholes, however, are prone to blockage and accumulation
of debris.
A.4. Ground water leakage
The pipes in the municipality of Delft are for the most part located below the groundwater table. As a
consequence, the dryweather flow can consist of up to 50% groundwater. This fact must be taken into
account when designing the wastewater collection system. As a rough estimate, ground water infiltra-
tion in this assignment can be assumed to amount to approximately 0.3 litre per second per hectare.
21.
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Design Assignment Olofsbuurt
A.5. Connection of pressurised and non-pressurised pipes
The most important requirement in relation to the connection of a pressurised main to a gravity sewer
main is that the pressurised main should always release its discharge under water to avoid odour nui-
sance. Wastewater from pressurised mains suffers from oxygen depletion, especially for longer mains.
As a result, hydrogen sulphide and methane gases are formed. These gases are released as the water
enters from the pressurised main into a gravity sewer. Discharging water from the pressurised main
under water prevents most of the gas from rising to the urban surface, where they would cause odour
nuisance.
This requirement can be fulfilled by connecting the pressurised main to a manhole that is always filled
with water. This is realised by placing weir in the manhole where it connects to the gravity sewer.
22.
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CIE4491 Fundamentals of Urban Drainage
Appendix B. Hydraulic Calculations123456789101112131516171819
SectionOriginDRYWEATHERFLOWRAINFALLRUNOFFTOTALDISCHARGE
from
man
hole
to
man
hole
surface
branch
company
Catchmentarea
Domesticandindustrial
wastewater
InfiltrationPeak
discharge
CatchmentareaConcentration
time
RainintensityDischarge
added
A
cum.
ΣA
discharge
q
discharge
qA
cum.
ΣqA
addedcum.Qp
8+10
runoffcoef
Ψ
added
ΨA
cum.
ΣΨA
tc
iQr=i.AQp+Qr
nonohahal/s.hal/sl/sl/sl/sl/shahaminl/s/hal/sl/s
Appendix: Hydraulic calculations
combined sewage system
Page 1: Calculation of discharge
25
APPENDICESASSIGNMENT CT4490
23.
23
Design Assignment Olofsbuurt
20212223242526272829303132333435363738
SectionPROPOSEDCONDUITHYDRAULICCONTROLREMARKS
from
man
hole
to
man
hole
ElevationLengthSlopeSizeFullpipePeak
dryweatherflow
PartiallyfilledpipePiezometriclevel
GroundlevelInvertlevelDis.
capacity
VelocityQp/QoVelocityQr/QoVelocityVelocityhUpper
end
Lower
end
Actual
concen
tration
time
updownupdownQovo
nonomNAPmNAPmNAPmNAPm‰ml/sm/s%m/s%m/sm/smmNAPmNAPmin
Appendix: Hydraulic calculations
combined sewage system
Page 2: Proposed conduit
26
APPENDICESASSIGNMENT CT4490
24.
24
CIE4491 Fundamentals of Urban Drainage
27
ASSIGNMENT CT4490 APPENDICES
1234567891011121314151617181920212223
SectionOriginDRYWEATHERFLOWPROPOSEDCONDUITREMARKS
from
man
hole
to
man
hole
Catchment
area
Domesticandindustrial
wastewater
InfiltrationPeak
discharge
ElevationLengthSlopeSizeFullpipePeakdischarge
GroundlevelInvertlevel
surface
branch
company
added
A
cum.
ΣA
discharge
q
discharge
qA
cum.
ΣqA
addedcum.Qp
8+10
updownupdownLICap.
Qo
VelocityQp/QoVelocity
nonohahal/s.hal/sl/sl/sl/sl/sm
NAP
m
NAP
m
NAP
mNAPm‰ml/sm/s%m/s
Appendix: Hydraulic calculations
separate sewage system
Wastewater system
25.
25
Design Assignment Olofsbuurt
123456789101112131415161718192021222324252829
SectionOriginRAINFALLRUNOFFPROPOSEDCONDUITHYDRAULICCONTROLREMARKS
from
man
hole
to
man
hole
surface
branch
CatchmentareaConcen
tration
time
Rain
intensity
Dischar
ge
ElevationLengthSlopeSizeFullpipePartiallyfilled
pipe
PiezometriclevelActual
concen
tration
time
arearunoff
coef.
addedcum.tciQr
GroundlevelInvertlevelDis.
capac.
VelocityQr/Qo
VelocityVelocityhUpper
end
Lower
end
AΨΨAΣΨAupdownupdownQovo
nonohahahaminl/s.hal/smNAPmNAPmNAPmNAPm‰ml/sm/s%m/sm/smmNAPmNAPmin
Appendix: Hydraulic calculations
separate sewage system
Storm water system
28
ASSIGNMENT CT4490 APPENDICES
26.
26
CIE4491 Fundamentals of Urban Drainage
20
Exampleofdesigncalculationforaseparatesewersystem
123456789101112131415161718192021222324252829
SectionofRAINFALLRUNOFFPROPOSEDCONDUITHYDRAULICCONTROLREMARKS
from
man
hole
to
man
hole
AreaCatchmentareaConcen
tration
time
Rain
intensity
Dischar
ge
ElevationLengthSlopeSizeFullpipePartiallyfilled
pipe
PiezometriclevelActual
conc.
time
Bran
ch
AreaRun-
off
coef
addedcum.tciQr
GroundlevelInvertlevelDischar
ge
capacity
VelocityQr/Qo
VelocityVelocit
y
Hydr.
Grad.
Upper
end
Lower
end
AΨΨAΣΨAUpDownUpDownQovo
nrnrhahahaminl/s.hal/smNAPmNAPmNAPmNAPm‰ml/sm/s%m/sm/s‰mNAPmNAPmin
1211,480,350,520,521092488.007.806.706.231752,70,30510,72940,763,8
2321,440,751,08
7-21,850,500,932,5310922337.807.506.235.851452,280,602901,04801,116,1
3430,660,750,50
8-33,00,351,054,0810923757.507.305.855.50804,40,604061,44921,337,2
4540,520,500,26
12-41,986,3210925817.307.105.505.30802,50,806531,30891,398,2
5650,400,750,30
9-52,650,501,33
13-51,320,500,668,6110927927.107.005.105.00601,71,009681,23821,33
7261,850,500,931092868.007.806.606.231602,30,401000,80860,86
8373,00,351,051092977.707.506.305.752402,30,401000,80970,83
101182,00,350,701092647.807.506.406.002002,00,40940,74680,784,3
111292,20,350,771,4710921357.507.306.005.682001,60,501510,77900,828,3
124100,680,750,511,9810921827.307.305.685.56901,350,602240,79810,8510,1
95122,650,51,3310921227.107.105.605.202401,610,501510,77810,83
135111,320,50,661092617.007.105.605.281602,00,40940,75650,79Appendix:
Table separate sewage system
Storm water system
2929
APPENDICESASSIGNMENT CT4490
27.
27
Design Assignment Olofsbuurt
Appendix C. Hydraulic Properties of Partially Filled Circu-
lar Pipes
C.1. Graphical representation
Figure 13 shows the relation between water depth in the pipe and the flow velocity and the discharge,
respectively.
H = water depth
D = pipe diameter
v = flow velocity in a partially filled pipe
Q = discharge in a partially filled pipe
Vfull
= full pipe velocity
Qfull
= full pipe discharge
Q/Qfull
v/vfull
Figure 13: Velocity and discharge in partially filled pipes.
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