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Urban Drainage

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Table of contents
1.

Introduction
1.1 Urban drainage and sewerage
1.2 History of urban drainage and sewerage
1.3 Purpose of urban drainage systems

2.

Approaches to urban drainage

2.1 Piped or natural systems
2.2 Combined and separate systems
2.3 Improved combined and improved separate sewer systems
2.4 Pressurised and vacuum systems
2.5 Components of sewer systems
2.6 Concepts, definitions and abbreviations

3. Inflows into urban drainage systems

3.1 Amount of wastewater

3.1.1 Domestic wastewater

3.1.2 Industrial wastewater


3.1.3 Infiltration and exfiltration

3.1.4 Drainage water


3.1.5 Extraneous water

3.2 Amount of stormwater

3.2.1 Amount of precipitation and runoff behaviour

3.2.2 Rainfall intensity duration frequency and rainfall mass curves

3.2.3 Using rainfall mass curves

3.2.4 Run-off processes

4. Environmental requirements for sewer design

4.1 Development of environmental guidelines for sewer systems

4.2 Basic effort/Basisinspanning

4.3 European Framework Directive

4.4 Storage and stormwater pumping capacity

4.5 Hydrodynamic evaluation of environmental impacts of sewer systems

4.6 References
5. Hydraulics for sewer design

5.1 Uniform channel flow in partially filled sewer pipelines

5.2 Hydraulic resistance in components of sewer systems


5.2.1 Sewer overflow weirs

5.2.2 Inlets, outlets and manholes

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1.

Introduction

1.1

Urban drainage and sewerage

Drainage is defined in the Cambridge Online

Dictionary as the process of water or waste liquids
flowing away from somewhere into the ground
or down pipes. In the case of urban drainage,
somewhere is the urban environment, mainly
constituted of roads, houses and green spaces.
Urban drainage may be used to describe the
process of collecting and transporting wastewater,
rainwater/stormwater or a combination of both.
Sewage is water-carried wastes, in either solution
or suspension, that is intended to flow away from
a community. Sewage and Sewerage may be
used interchangeably in the USA but elsewhere
they retain separate and different meanings sewage being the liquid material and sewerage
being the pipes, pumps and infrastructure through
which sewage flows. Similar to urban drainage,
sewerage may refer to systems collecting
wastewater or rainwater or a combination of both.

1.2 History of urban drainage and

sewerage
Artificial drainage systems were developed
as soon as humans attempted to control their
environment. Archaeological evidence reveals that
drainage was provided to the buildings of many
ancient civilisation such as the Mespotamians, the
Minoans (Crete) and the Greeks (Athens).

Romans
The Romans are well known for their public
health engineering achievements, particularly the
impressive aquaducts bringing water into the city.
Equally vital were the artificial drains they built, of
which the most famous is the cloaca maxima, built
to drain the Forum Romanum (and still in use today)
(Butler and Davies, 2004). The Romans were
proud of their rooms of easement (i.e., latrines).
Public baths included such rooms -- adjacent to
gardens. There Roman officials would sometimes
continue discussions with visiting dignitaries while
sitting on the latrines. Elongated rectangular
platforms with several adjacent seats were utilized

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(some with privacy partitions, but most without).

These latrine rooms were often co-ed, as were the
baths. Public latrines were used by many people,
but for the most part, human wastes were thrown
into the street. Rome had extensive street washing
programs (with water supplied by aqueducts, the
first being built in 312 BCE). Only a few homes had
water piped directly from the aqueducts; the vast
majority of the people came to fountains to gather
their water. Even though not many homes were
directly plumbed into the sewers, when the wastes
were thrown into the street, the street washing
resulted in most of the human wastes ending up
in the sewers anyway. Direct connection of homes
to the sewers was not mandated until nearly 100
CE (cost was a factor; also mandating such a
connection was then considered an invasion of
privacy). Sewage resulting from the public baths
and the included latrines was discharged into
sewers and eventually to the Tiber River. It is
worth noting that the Romans recognized the value
of their water (which had been transported to the
city via aqueducts, often over a distance of 20-30
miles); as such, any wastewater from the public
bath facilities was often re-used, frequently as the
flushing water that flowed continuously through the
public latrine facilities. From the latrines, it flowed
to a point of discharge into the sewer system.
Middle Ages to 20th century
The Roman Empire fell in early CE along with the
concepts of baths, basic sanitation, aqueducts,
engineered water or sewage systems, etc.
Sanitation reverted back to the basics: very
primitive. During the so-called Dark Ages,
there arose a brotherhood among men noted
for skill in combat. There also evolved a creed
that uncleanliness was next to godliness.
As such, bathing/sanitation became quite
uncommon; homes, towns, and streams became
filthy. Diseases were commonplace; epidemics
decimated towns and villages. Twenty-five percent
(or more) of the ancient European population
died of disease (cholera, plague, etc.). The major
transmitter of the plague was rats (actually bacteria
conveyed from rats to people via flea bites). The rat
population thrived amongst the mess and stench
commonplace in medieval times.

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Living conditions aggravated most seriously in

large cities like London and Paris. In the beginning,
natural streams were used as sewers. As cities
developed, these natural drains were structurally
covered. Early on, these sewers were used
primarily for storm waters. For instance in Paris,
the Menilmontant sewer, first noted in the early
1400s, was initially an open wash and later a
closed conduit. It intercepted surface flows from
Paris north slope area (i.e., that area lying on the
right bank of the Seine River). It was called the
Great Drain (grand gout or gout de ceinture).
Prior to the wide use of cesspools in Paris, cesspits
(ones that percolate) were widely used. Their use
in combination with the large growing population,
however, resulted in the subsoil of Paris becoming
putrid. Cesspools, instead, were then encouraged.
However, they required periodic/routine cleaning,
which the city couldnt adequately provide. Another
stinky mess arose. A Nite Soil program started
to facilitate the collection and disposal (elsewhere)
of the wastes (in community cesspools, rivers,
vegetable gardens). The problem was that all of
the people could not afford the service.
In the 1830s a series of cholera epidemics
started in Paris. To combat the epidemics, new
and bigger sewers (called Les egouts ) began
to be constructed in the 1840s-1890s. They
became the pride of Paris. The design father
of the complex system of sewers under Paris
was Eugne Belguard. The construction of this
newer/larger system started in 1850, on borrowed
money. By 1870, over 500 km of new sewers were
either in service or under construction. By 1930,
the entire system (a combined system) was
finished: One sewer for each street. From these
times, Sewerman became a profession. Tours
of the sewers were given by the sewermen on
weekends. Some of the sludge found in the sewers
was removed through manholes. Most of it was
moved downstream via boats (with wings) to the
discharge point of the sewer into the river -- where
the sludge was pushed onto barges, from whence
it was transported to various places of reuse or
disposal.

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Londons oldest sewer, known as the Ludgate

Hill Sewer, was constructed in 1668. (Initially, it
was an open channel fed by springs, big enough
to be used by boats. It was covered in 1732.)
Early sewers (initially, natural watercourses that
had been covered) started in the London area
in the 1730s -- primarily for storm water. Privies/
cesspools were used to collect home wastes;
some of these facilities also collected the
methane generated by the decaying waste. The
result was often explosions/fires ... and death.
1858-59 were the years of the Big Stink in
London. The Thames River received wastes
of thousands of people who lived upstream of
Parliament. Many of the sewers tributary to the
Thames River could only physically drain during
low tide. The problem was that at low tide, the
river did not have enough flow to carry the waste
downstream and out to sea. The incoming tide
pushed the waste upstream. This cycle resulted
in the river becoming virtually a wide-open-tothe-sunlight cesspool for the excrement of nearly
three million people! Parliament had to shut down
often in summer months. This situation created
an even greater problem: the Thames was also
the source of water for a large portion of London!
During these years, various ways to minimize
sewer odors were tried, including the addition to
the sewers (especially in warm weather) of large
quantities of lime or chloride of lime.
Sir Joseph William Bazalgette, CB (28 March
1819 15 March 1891) was chief engineer of
Londons Metropolitan Board of Works and his
major achievement was the creation in of a
sewer network for central London which was
instrumental in relieving the city from the Great
Stink. Bazalgettes solution was to construct
1,100 miles (1,800 km) of underground brick main
sewers to intercept sewage outflows, and 1,100
miles (1,800 km) of street sewers, to intercept
the raw sewage which up until then flowed freely
through the streets and thoroughfares of London.
The outflows were diverted downstream where
they were dumped, untreated, into the Thames.
The scheme involved major pumping stations
on the north and south sides of the Thames.

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Bazalgettes foresight may be seen in the diameter

of the sewers. When planning the network he took
the densest population, gave every person the
most generous allowance of sewage production
and came up with a diameter of pipe needed. He
then said Well, were only going to do this once
and theres always the unforeseen. and doubled
the diameter to be used. As it is the sewers are
still in use to this day.
The unintended consequence of the new sewer
system was to eliminate cholera not only in
places that no longer stank, but wherever water
supplies ceased to be contaminated by sewage.
This result was unintentional, since disease was
believed to be transferred by bad odours. It was
not until 1854 that Dr. Snow made the connection
between human wastes (from over-loaded privies)
and water supplies (wells) within the Broad
Street Neighborhood: he found that a well at 40
Broad Street was found to be contaminated with
sewage from a nearby overloaded/flowing privy;
the well was removed from service and the cholera
outbreak ended. In the mid-1800s Louis Pasteur
proved disease could be caused by germs. The
link between bacteria and infectious diseases was
beginning to be understood.

1.3

Purpose of urban drainage systems

The four objectives of drain and sewer systems

are (NEN-EN752:2008):
Public health and safety;
Environmental protection;
Sustainable development;
Occupational health and safety.
Drain and Sewer systems are provided in order
to prevent spread of disease by contact with
faecal and other waterborne waste, to protect
drinking water sources from contamination
by waterborne waste and to carry runoff and
surface water away while minimising hazards
to the public. Additionally, the impact of drain
and sewer systems on the receiving waters shall
meet the requirements of any national or local
regulations or the relevant authority. Finally,
sewer systems should be designed, constructed,
operated, maintained and rehabilitated at the

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best environmental, social and economical

costs so that it uses materials that minimise the
depletion of finite resources, can be operated
with the minimum practicable use of energy and
can be constructed, operated and, at the end
of their life, decommissioned with the minimum
practicable impact on the environment. To protect
the health of sewer workers occupational health
and safety risks likely to arise during installation,
operation, maintenance, and rehabilitation should
be minimised.

2.

Approaches to urban drainage

2.1

Piped or natural systems

Historical developments of urban drainage

systems has been from natural towards piped
systems: natural channels were used to collect
rainwater and as cities developed these were
structurally covered to create additional space
for urban developments and to contain the bad
smells arising from the waste collection channels.
The urban drainage systems that were designed
in the 19th century and onwards consisted mainly
of underground pipes, because they had to be
incorporated into existing, densely built cities.
The recent trend, that started in the 1970s, has
been to move towards a more natural means of
drainage, using infiltration and storage properties
of semi-natural features such as constructed
wetlands, ponds and permeable pavements.
The movement towards increased use of natural
drainage mechanisms has been termed differently
in different countries. In the US, these techniques
are usually called best management practices
(BMPs). In the UK they are called Sustainable
Urban Drainage Solutions (SUDS). In Australia the
term water sensitive urban design is often used
to refer to the incorporation of natural drainage
mechanisms in urban areas.
This series of lectures is mainly dedicated to
piped systems that constitute the majority of urban
drainage systems in existing urban areas in the
Netherlands and many other western European
countries.

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Figure 2.1 Principle combined system

2.2

Combined and separate systems

Urban drainage systems collect and transport

to types of flows: wastewater and stormwater.
In combined systems these flows are drained
through one and the same system (see figure 2.1),
in separate systems wastewater and stormwater
are drained through separate pipes. In western
Europe, most older systems are combined
systems. In the Netherlands about 70% of the
population is connected to combined systems,
while a little over 25% is connected to separate
systems. The same percentages apply to the UK,
France and Germany.
Combined sewers systems
During dry weather, combined systems carry only
wastewater flow. During rainfall, the flow increases
as a result of the inflow of stormwater. The
combined flow of wastewater and stormwater is
transported towards a wastewater treatment plant.

Figure 2.2 Principle separate system

The stormwater flow exceeds the wastewater flow

even under light rainfall conditions. During heavy
rainfall, stormwater flow exceeds wastewater
flow by a factor 100 to 1000 or more. It is not
economically feasible to provide capacity for the
total flow under these conditions. Therefore, in
combined systems so-called combined sewer
overflows are installed to discharge excess water
that cannot be transported towards the wastewater
treatment plant, to surface water. The overflow
water is a mixture of wastewater and stormwater
and as a result the quality of the surface water, in
most cases temporarily, is extremely reduced. The
dying off of fish, foul odours and visual pollution
is often the result.
Separate systems do not have this drawback
of mixing relatively clean stormwater with
wastewater. A separate sewer system consists of
two sewer pipelines, one for wastewater and one
for the drainage of stormwater (See figure 2.2).
Some cities, like the city of Amsterdam, started

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constructing separate systems early on, yet in

most cities in the Netherlands separate systems
started to be constructed from the 1970s onwards,
as the drawbacks of combined systems become
more and more apparent.
In theory, in separate systems stormwater does
not become mixed with wastewater. It happens
from time to time, that domestic sewer connections
accidentally become connected to stormwater
pipes. This puts a constant strain on the quality of
the receiving surface water. Also the opposite can
happen; the stormwater pipes become connected
to the wastewater system. This can result in
overloading of the wastewater system. In both
cases one speaks of faulty or illegal connections.
Initially, the assumption was that stormwater that
was drained through stormwater systems was
uncontaminated, since during drainage it did
not get mixed or come in contact with domestic
wastewater or other wastewater. However,
research has shown that this assumption does
not hold true. During runoff over urban surfaces
rainwater takes up all sorts of contaminated
substances. These substances settle on the
surface as a result of traffic, human activities
and direct deposition from the atmosphere.
Stormwater systems drain towards surface
water more than 50 times per year on average,
during each precipitation of importance, which
implies that polluted stormwater is discharged to
surface water frequently. On the other combined
systems overflow less than 10 times annually, on
average. The result is that the annual pollution
load to surface water from separate systems and
combined systems can be equally large.
Separate systems have the additional disadvantage
that construction and maintenance costs of
separate sewer systems are in most cases higher
than in combined systems. Only in in cases where
a large amount of surface water is present to
locate stormwater oulets, so transport distances
to surface water are short, separate systems can
be cheaper than a combined system.

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2.3 Improved combined and improved

separate sewer systems

As the drawbacks of combined and separate

systems became apparent, attempts have been
made to overcome the disadvantages to both
combined systems and separate systems.
Improved c ombined systems include so called storage -and-settlement basins
(bergbezinkbassins, in Dutch) that are installed at
sewer overflows to reduce the amount of water that
is spilled through the combined sewer overflows
(See figure 2.3). During heavy precipitation the
overflowing water is retained in the basin. The
amount of overflowing water decreases as a
result of this and with that the contamination of
the surface water is decreased as well. Once the
basin is full the on-coming water is discharged to
the surface water. Before the water reaches the
(external) combined sewer overflow, it is partially
stripped of pollutants, thanks to sedimentation in
the basin.
A storage and settlement basin, therefore, reduces
the amount of combined sewer overflow water
and in addition the overflowing water is less
contaminated. However, one condition to this is of
course that the design of the basin must be such
that it allows sedimentation to occur.
Combined sewer systems can also be improved
through installing vortex overflows or by adjusting
the combined sewer overflow itself (improved
overflow drain). The idea behind these special
receiving
water

WWTP
storage and settlement tank

1 = internal combined sewer overflow

2 = extreme combined sewer overflow
supply pipe

Figure 2.3 Improved combined system

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WWTP
rain water pump
combined
sewer overflow
receiving
water

wastewater
sewer

storm water
sewer

Figure 2.4 Improved separate system

structures is that they remove pollutants from the

overflow water by containing settleable material
that many of the pollutants adhere to.
Improved separate systems are devised to
overcome the effect of faulty connections in
separate sewer systems. This done by connecting
the stormwater system with a wastewater sewer in
a suitable place (See figure 2.4). The wastewater
flow that undesirably goes through the storm water
sewer is in this way led to the wastewater system
and eventually to the wastewater treatment plant.
A weir is installed at the outlet of the stormwater
system to contain stormwater inside the system
during small precipitation and transport it towards
the wastewater system. The installation of a
weir makes sure that a part of the contaminated
rainwater (because of faulty connections) also
undergoes treatment in the wastewater treatment
plant.
This solution does not entirely remove the
harmful effects caused by connecting rainwater
pipes to the wastewater system. However, due
to the installation of extra pumping capacity for
the transport of stormwater from the stormwater
system, this capacity is also available to transport
stormwater that was directly (and erroneously)
connected to the wastewater system. This is indeed
taken into account in the design of the wastewater
treatment plant that should allow the treatment of
an limited amount of rainwater from the improved
separate system. The amount of stormwater that
ends up in the wastewater treatment plant this way
depends on the distribution of rainfall over small

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and large rainfall events: stormwater from most of

the small events (up to about 5mm) are transported
towards the wastewater treatment plant and only a
part of the stormwater from larger events (> 5mm)
is discharged to surface water.
One other possibility to reduce the adverse effects
of sewer overflows is the enlargement of the
capacity of sewer pumping stations that pump
sewer water towards the wastewater treatment
plant. The disadvantage of this is, that a larger
amount of relatively clean water must be treated
in the wastewater treatment plant and that the
hydraulic capacity of the wastewater treatment
plant must be enlarged to cope with the extra
water (unless the existing capacity is sufficient).
The efficiency of wastewater treatment plants is
lower for less concentrated influent waters, so by
transporting more stormwater to the treatment
plant, its efficiency is reduced and fewer pollutants
are removed during treatment. In the end, it comes
down to finding a balance between the prevention
of combined sewer overflows, reduction of polluted
stormwater flows and maintenance of treatment
efficiency at the wastewater treatment plant.
This balance is studied in optimization studies
for wastewater systems (in Dutch: Optimalisatie
AfvalwaterSysteem, OAS).
Furthermore, the amount of overflow water
can be reduced by minimising the amount of
runoff water that is connected to the urban
drainage system. The reduction of the amount of
connected runoff-surface is called disconnection
of impermeable surfaces (in Dutch: afkoppelen).
Of course disconnection can only take place when
the stormwater can be transported to alternative
facilities such as infiltration systems or open
drainage channels and ponds.

2.4

Pressurised and vacuum systems

In conventional sewerage methods wastewater

is collected through a gravity-flow sewer, using
gravity to transport wastewater. If the sewage
cannot be transported under gravity, because
ground level variations are small and transport
distance are long, pressurized or vacuum

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systems can be applied. These systems are

general installed in areas where a gravity flow
sewer system is too expensive, such as detached
buildings that are spread far apart.
In general pressurized and vacuum system only
transport wastewater and no stormwater, because
the latter would necessitate a larger pipe diameter
and pumping capacity and therefore reduce the
financial advantages of a pressurized systems
compared to gravity systems.
Pressurised sewer systems consist of a few
(small) pumping stations and pressurised mains.
Wastewater from a few detached houses is led by
a gravity-flow line to a pump chamber. From there
the water is pumped into the pressurized main
to the next pump chamber. One or more houses
can be connected to these pump chambers.
This method allows the wastewater from a few
detached houses to be collected. Finally the
wastewater is led to the gravity sewer systems
that eventually transports it to the wastewater
treatment plant (See figure 2.5).
Many pressurized sewers in the Netherlands
were constructed with state subsidy. The intention
behind distributing the subsidy was to get as
many buildings as possible connected to a sewer
system. Currently 3.6% of the population in the
Netherlands is connected through pressurized
sewers.
There are disadvantages to pressur ised
sewer systems. The most obvious ones are, in

Figure 2.5 Example of a pressurized sewer system

comparison to the gravity-flow sewer system, high

maintenance and replacement costs. Foul odours
occur as wastewater is transported over large
distances as a result of anaerobic degradation
of the transported wastewater. This results in the
production of sulphuric gases (H2S) that lead
to severe corrosion of concrete sewers where
pressurised sewers are connected to gravity
sewers.
In vacuum sewers residential wastewater is led
through vacuum pipelines to a pump chamber.
From this pump chamber the following transport
of the wastewater takes place under pressure.
(See figure 2.6)
Vacuum sewer systems have been put to little
use in the Netherlands. Although the operational
safety is comparable to pressurised sewer
systems, its higher installation costs prevent its
widespread usage. One disadvantage of vacuum
sewer systems in relation to pressurised sewer

Figure 2.6 Principle diagram vacuum sewerage system for a house boat

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systems is also that vacuum sewer systems can

barely overcome slight vertical obstacles. This is
because the negative pressure (vacuum) in the
system itself is limited.

2.5

Components of sewer systems

A sewer system consists of a network of sewer

pipelines that are connected through manholes.
The sewer pipelines are made up of pipes, which
can be circular shaped, rectangular or oval. In old
sewers large rectangular pipes are sometimes
covered with a barrel vaulting.
The pipes (Dutch: rioolleidingen) are made of
concrete, cast iron, PVC, HDPE, PE, glazed
stoneware (in Dutch: gres) or brickwork. Most
sewer pipes in the Netherlands (72% of the
total sewer length (RIONED, 2010) are made of
concrete.
Manholes (Dutch: rioolputten) are constructed
where more than two sewer connect and at
regular distances to allow inspection of the sewer
system. Manholes need to be accessible so that
it is possible enter materials for inspection and
maintenance of sewer systems. This means that
the maximum distance between manholes is of
the order of 40 to 60 m. Manholes in systems of
concrete sewers are usually made of concrete,
sometimes of brickwork. Manholes in systems of
PVC sewers are made of PVC. The manholes are
covered with a manhole cover. These are most
often made of cast iron, as is the frame in which
the cover is encased.

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sewer overflows take place, thus combined sewer

overflows occur less frequently and surface water
is less polluted.
Combined sewer overflows (Dutch: gemengde
overstorten) are implemented in combined sewer
systems to relieve the pressure on the system
during precipitation.
O u t l et s (Dutc h: regenwater uitlaten) are
implemented in separate stormwater systems
to allow outflow of stormwater towards surface
water. Outlets in separate stormwater systems
have no weirs; there is a constant open connection
between surface water and the stormwater
system.
Emergency outlets (Dutch: nooduitlaten) are
installed in the pumping stations collection
chambers. Emergency outlets are supposed to
work exclusively in case the pumping station is
broken down for a long period.
Combined sewer over f lows, out falls and
emergency outfalls are usually made of concrete,
and occasionally of brickwork.
Gully pots or sewer inlets (Dutch: kolken) collect
water from roads and transport it to the sewer
system.

Sealed covers (Dutch: geknevelde putdeksels)

can be applied at locations where excess pressure
sometimes causes water to push off the manhole
cover as water flows out of the manhole, creating
a dangerous traffic situation.

House connections (Dutch: huisaansluitingen)

collect water from households and transport it to
the sewer system. Combined house connections
collect wastewater and stormwater and transport
it to a combined sewer system; separate house
connections collect wastewater or stormwater and
transport it respectively to a separate wastewater
or stormwater system. House connections are
predominantly made of PVC. Old connections are
usually made of glazened stoneware.

Internal weirs (Dutch: interne stuwen) are

installed in combined sewer systems with the aim
to optimise in-sewer storage, especially when
different part of the sewer systems have different
bottom levels. As a result of this more water is
stored inside the sewer system before combined

Pumping stations (Dutch: pompstations) are

indispensable in flat areas like the western and
northern parts of the Netherlands. Sewer pumping
stations have capacities anywhere between a
few m3/h to a few thousand m3/h, depending on
the size of the system that drains to the pumping

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station. The design, building and installation of

bigger sewage pumping stations, is a matter for
specialists.

contain pollutants and limit the frequency and

pollution content of combined sewer overflows.
With careful design the tanks are able to contribute
nicely towards improving the surface quality.
Storage and Settling Basins are usually designed
as closed basins, especially if they are installed
close to buildings. The basins are usually made
of concrete.

Shutting down pumps creates shockwaves in

the pressurised mains. Pumping stations with a
large capacity that are connected to pressurised
mains that are a few kilometres long may need
to buffer the shockwaves by mounting so-called
water hammer provisions. These are usually
surge towers or pressure vessels. Even the
dimensioning of water shock provisions is a matter
for specialists.

2.6 C o n c e p t s , d e f i n i t i o n s a n d
abbreviations

In this paragraph a list is given with the most

important concepts and abbreviations in the
sewerage field. The terms in bold are defined just
as in the NEN 3300: Buitenriolering; termen en
definities (Drains and sewers outside buildings;
terms and definitions). In addition, a list of these
with the technical terms found in English, French
and German literature is provided.

Pressurised mains (Dutch: drukleidingen) are

components of pressurised sewer systems.
Small small pressurised systems (Dutch:
drukriolering) connect detached houses at long
distances from a gravity system to the gravity
system. Large pressurised systems (Dutch:
persleidingen) consist of pressurised pipelines
of large capacity that transport water from a
gravity system to a wastewater treatment plant
or between subcatchments of a gravity system.
The pressurised mains are predominantly made
of HDPE or PE.

Abbreviations
WWTP Wastewater Treatment Plant
BOD
Biochemical Oxygen Demand
COD Chemical Oxygen Demand
DWF Dry Weather Flow
TKN
Total Kjeldahl Nitrogen
OF
Overflow Frequency
POC
Pump Over-Capacity
WWF Wet Weather Flow

Storage and Settling Basins or Tanks (Dutch:

bergbezinkbassins) are placed behind combined
sewer overflows to provide extra storage capacity,
Nederlands

Engels

Duits

Frans

afvalwater

wastewater

Schmutzwasser

eaux uses

afvloeiingscofficint

runoff coefficient

Abflubeiwert

coefficient de ruisselement

afvoerend oppervlak

catchment area

Einzugsgebiet

bassin versant

droogweerafvoer

dry weather flow

Trockenwetterabflu

dbit de te,ps sec

gemengd rioolstelsel

combined system

Mischsystem

rseau unitaire

gescheiden rioolstelsel

separate system

Trennsystem

rseau (de type) sparatif

grondwater

groundwater

Grundwasser

eaux souterrain

huisaansluiting

house sewer system

Grundstck entwsserung

drainage domestique

huishoudelijk afvalwater

domestic sewage

Husliches Schmutzwasser

eaux uses domestiques

industrieel afvalwater

trade effluent

Betriebliches Schmutzwasser

effluent industriel

lekwater

infiltration/exfiltration

Infiltration/Exfiltration

infiltration

afstromend hemelwater

stormwater

Regenwasser

eaux de ruissement

regenwaterafvoer

rain discharge

Regenabflu

coulement de pluie

riolering

sewer system

Kanalisation

rseau dassainissement

riool

sewer

Abwasserkanal

missionaire

rioolwater

sewage

Abwasser

effluent

11

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3.
Inflows into urban drainage
systems

water gardens to fill pools and more water is used

to bathe compared to colder climates.

Sewer water consists of wastewater or rainwater

or a mix of both. Wastewater is broken down into
domestic wastewater and industrial wastewater,
infiltration and drainage water and extraneous
water. The wastewater flow is also known as dry
weather flow; wet weather flow refers to the flow
under rainfall conditions.

In the Netherlands the average drinking water

consumption is 125 to 135 liters per person per
day. The drinking water consumption per person
can be subdivided into various domestic activities,
such as washing clothes, cooking bathing, toilet
flushing and washing dishes. Less than circa 40 %
of the supplied drinking water is used for personal
hygiene, drinking and cooking. The other 60%
of the drinking water is used for purposes that
do not require drinking water quality. In recent
years drinking water companies have investigated
possibilities to produce second, lower quality
water for such purposes. Application of second
quality water in households requires a second
distribution network for this water, which in most
cases does not make such solutions cost-efficient.
For industrial purposes, requiring large volumes of
second quality water, the production of separate
lowe quality water can be beneficial.

3.1

Amount of wastewater

3.1.1
Domestic wastewater
The amount of daily domestic wastewater depends
on the drinking-water consumption per person,
loss and the number of individuals consuming
drinking water. When designing wastewater
systems future developments must be taken into
account, such as an expected population growth
or changes in the drinking-water consumption per
person. Additionally the amount of wastewater
fluctuations throughout the day must also be taken
into account.
Drinking water consumption per person
Average drinking-water consumption depends
on both prosperity and climate conditions. In
many poor areas, toilet flushing does not occur.
In prosperous areas in the tropics, subtropics and
arid areas, a lot of drinking water is consumed to

Figure 3.1 Relationship between drinking water supply

and wastewater drainage

12

Losses
A part of the supplied drinking water is lost during
the consumption process and does not reach
the sewer system. This refers for instance to
water used for watering gardens and water that
evaporates, for example from laundry. In the
Netherlands these losses amount to nearly 10%
of the drinking water consumption, so that the
amount of wastewater is around 115 to 120 liters
per person.
Where the drinking-water supply is less than 20
liters per inhabitant per day (only one faucet in
the house) nothing gets drained to the sewer.
With a drinking-water supply of circa 80 liters per
inhabitant per day virtually all the supplied drinking
water is drained through the sewer. Above that
amount, increasing degrees of water usage does
not reach the sewer. See figure 3.1.
Leakage losses that occur in the distribution
network also play a large part in explaining the
difference between drinking water consumption
and wastewater production. These losses can
make up 10 to 50% of drinking-water production
in some countries. In the Netherlands leakage

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urban drainage

losses amount to less than 5% of production. In

table 3.1 drinking-water supply (read: drinking
water production) and the amount of wastewater
for a few places is shown.
In many areas the amount of wastewater is
substantially smaller than the amount of supplied
drinking water. In Amsterdam and the Grand
Rapids wastewater flow is however larger than
drinking water supply. In Amsterdam this can be
attributed to the infiltration of groundwater into the
sewer network. Presumably something similar is
going on in Grand Rapids.
The presented figures demonstrate that when
dimensioning a wastewater system based on the
amount of drinking-water production it can lead
to either over or under designing of wastewater
system capacity.
Distribution over the day
Wastewater is not produced evenly throughout the
day; at night there is barely any wastewater flow.
The largest amount of wastewater is produced
during about 10 hours of the day. Wastewater
production in most systems shows a peak in the
morning and evening. See figure 3.2.
In the Netherlands it is common practice to
take into account a domestic wastewater flow
equal to 12 l/(inhabitant.h) in the design of
sewer systems. Hereby is assumed that the total
amount of domestic wastewater of 120 l/(inh.
day) is discharged in 10 hours. The peak factor
employed in the Netherlands for wastewater flow
then becomes:
peak drainage is: 120/10 = 12 l/(inh.h)
average drainage is: 120/24 = 5 l/(inh.h)
peak factor is:
12/5 = 2,4

Table 3.1 Drinking water supply and wastewater

drainage
city

supply

drainage

(lpppd)

(lpppd)

Las Vegas (US)

1.560

760

Little Rock (US)

190

190

Wyoming (US)

570

300

Boston (US)

550

530

Caro (Egypt)

800

150

Amsterdam

130

209

670

720

(part of the city)

Grand Rapids (US)

Table 3.2 Peak factors. Peak factors calculated for

France are limited between 1.5 and 3.
Number of

France

USA

inhabitants

The
Netherlands

100

3,0

4,2

2,4

1.000

3,0

3,8

2,4

100.000

1,7

2,0

2,4

In the United States the following various formulas

are used: For Desmoines, for example, the
following formula applies:

P=

18 + l
4+ l

(3.2)

where I is the population times thousand.

In table 3.2 the peak factor is given for varying
populations.
The previous information shows that the magnitude
of the applied peak factors from country to country
differs. This is explained by the empirical character
of these peak factors.

In France the following peak factor is applied:

P = 1 .5 +

2 .5
qm

P = peak factor (1,5 < P < 3)

qm = drinking water supply

(3.1)

l/s
Figure 3.2 Daily dry weather flow fluctuations

13

urban drainage

When it comes to appropriate design of wastewater

systems, the prevention of rainwater connections
to a separate wastewater system is of much
greater importance than the value of the applied
peak factor. This is illustrated by the following:
The peak wastewater production per inhabitant is
12 l/h in the Netherlands. The amount of impervious
surface connected to a sewer system is about 60
m2 per person. This amounts to a wastewater flow
of 0.2 mm/hour (12l/60m2). Wastewater sewer
systems are designed for a filling rate of 50%.
The filling rate is equal to the water depth in the
sewer divided by the diameter. The full capacity
of the sewers is for a filling rate of 100%, double
the design inflow. This margin is kept to allow for
future changes, expected or unexpected, such
as the connection of new urban developments to
the system and especially faulty connections of
stormwater to the wastewater system. When the
design capacity of a wastewater system equals
0.2 mm/h, faulty connections of stormwater from
1/100th of the total impervious surface fill up the
wastewater system to full capacity for a rainfall
intensity of 20 mm/h. Larger rainfall intensities and
larger amounts of impervious surfaces connected
through faulty connections lead to overloading
of the wastewater system. This may result in
wastewater flowing back up into the houses
through the house connections.
3.1.2
Industrial wastewater
The amount of industrial drinking-water consumed
varies per type of company. When a sewer system
needs to be designed for a new neighbourhood
an estimation of the industrial wastewater
production is made based on average drinking
water consumption figures for the expected type
of industries. If it is not known which industry will
establish itself an assumption of a load of 2 l/
(s.ha of industrial area) is often made. This load is
derived from the gross impervious surface, since
the net impervious surface is not yet known at the
moment the design is made.
If the plan is for an existing neighbourhood then
the amount of wastewater can be calculated based
on data from the drinking water company and an

14

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estimation of losses during the industrial process.

It must be realized however that some industries
acting in their own interest supply their own water
by abstracting ground water. In that case a permit
for groundwater abstraction must be obtained
from the local authorities (the province). The water
consumption from such industry can be derived
from the text on the permit.
Sometimes certain industries (the soft drink
industries, breweries) do not drain their water into
the sewerage system. This must be taken into
account in setting up the design for the sewer
system.
3.1.3
Infiltration and exfiltration
Sewer systems are constructed to be watertight.
However, in older systems, particularly in areas
with poor soil conditions, this is often not the case.
Joints between sewer pipes become leaky as a
result of degradation of the rubber sealing rings
or because pipes move due to ground settlement.
Therefore, in sewer system design a leak flow rate
of 0.2 m3/(km/h) sewer per hour is often taken into
account for sewers below the groundwater table
(in the United States up to 3 m3/(km/h)).
When sewers are constructed above the
groundwater table, leakage of wastewater out
of the system can give rise to groundwater
contamination.
3.1.4
Drainage water
Drains are often laid in areas with high groundwater
tables to keep the groundwater table at a minimum
depth below roads and building foundations.
When the drainage water cannot be discharges
to surface water because there no water courses
nearby, the drainage water is often discharged by
connecting the drains to the sewer system. Water
quality managers are generally not in favour of
transporting comparatively clean drainage water
to sewers and to a wastewater treatment plant.
Therefore drainage water, if connected to sewers
if preferably connected to separate stormwater
sewers.

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If the expected amount of drainage water is known,

this can be taken into account in the design of
the sewer system. If not, the drainage water is
assumed to be incorporated in the leakage water.
3.1.5
Extraneous water
Brooks or covered waterways can also make up
part of sewer systems, particularly in the eastern
and southern parts of the Netherlands. The
term extraneous water applies to the flow that
is transported from a brook or waterway into the
sewer system. In the dimensioning of a sewer
system or in control calculations for existing
systems this flow must be properly taken into
account.

3.2

Amount of stormwater

One of the design requirements for urban drainage

systems is that houses and buildings may not
become flooded during precipitation of any
intensity and volume. Damages of flood events
caused by heavy rainfall are reported in the press
at regular intervals. To prevent such damages
it is of great importance to have good insight
into the possible expectations of the amounts
of precipitation and the runoff behavior over the
lifetime of a system.
3.2.1
Amount of precipitation and runoff
behaviour
In the Hydrology course fundamental concepts of
rainfall events were explained, which are briefly
repeated here:
Every rainfall event or storm event can be described
by its temporal and spatial characteristics:
Rainfall volume, mostly expressed in terms of
rainfall depth (d) on the surface [mm]
Duration (t) of the rainfall event [hours]
Rainfall intensity (P or I): amount of rainfall per
unit of time [mm/h]
Frequency: frequency of occurrence, usually
expressed in terms of a frequency scale T
(i.e. the parameter under consideration occurs
once every T years)
Area: geographical scale of rainfall [m2 or km2]

urban drainage

Rainfall depth, duration and intensity in one

station (point measurement) have the following
relationship:
t

d = Pdt

(3.3)

[L]

d
t

And the average rainfall intensity P is:

=

P=

d
t

[LT-1]

(3.4)

A mass curve presents the accumulation of rainfall

with time. A hyetograph presents the variation of
rainfall intensity with time.
Total rainfall depth (d), rainfall duration (t) and
frequency scale (T) are also called the external
statistics of a storm event and can be described
by probabilistic distributions. The internal statistics
of a storm event refer to the distribution of rainfall
intensity with time, during the storm event. This
distribution is often depicted in the form of a
histogram.
The distribution of rainfall intensity with time is
important for urban drainage, because run-off
processes in urban areas are fast and peak
rainfall intensities within a storm event determine
whether system capacity gets overloaded and
flooding occurs.
3.2.2
Rainfall intensity duration frequency
and rainfall mass curves
Use of intensity duration frequency curves (IDFcurves) and mass curves is widespread in the
design and analysis of urban drainage systems,
sewer systems as well as open channels systems,
retention basins and storage and settlement tanks.
IDF-curves and mass curves provide information
on the occurrence of rainfall intensities over a
given period of time for certain return periods.
Mass curves and IDF-curves, based on available
precipitation information, can be compiled in two
ways:
partial series
extreme values series
With partial series all the peaks that are above
a certain threshold are taken into account in

15

urban drainage

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KNMI and Meteoconsult (Wijngaard et al., 2004;

Malda et al., 2006).
Braaks curves are compiled from 37 years of
analysed rainfall values. Recent curves by KNMI
are based on hourly rainfall data for one location,
the Bilt, for a period of 97 years (1906-2003).
Meteoconsult has extended this analysis by
including rainfall data from 23 rainfall stations in
the Netherlands, including the Bilt, for 10 years of
rainfall data per station. The analysis for the Bilt
has been extended by Buishand and Wijngaarden
(2007) to include short duration rainfall down to a
5 minute time-step.
Figure 3.3 - Return periods for exceedance frequencies
of once per 1, 10, 100, 250 and 1000 years voor rainfall
duration between 5 and 120 minutes (Buishand and
Wijngaarden, 2007).

statistical analysis. In extreme value series only

the maximum occurrences that take place within a
certain period (a year for example) are taken into
account. In the latter case high precipitation that
is nevertheless lower than the annual maximum
precipitation is not included in statistical analysis.
This precipitation can however be higher than the
maximum that occurs in one of the other years in
consideration. As a result not all the information is
processed and some information gets lost.
An example of a mass curve that has been
frequently used in urban drainage design in the
Netherlands are Braaks curves, dating from 1933.
In recent year, these curves have been updated by

It is clear that the differences in the length of the

examined periods and the data time step will result
in variations between rainfall duration curves. The
reliability of the various rainfall duration curves
depends on the period examined and the applied
method.
Figure 3.3 is extracted from the publication by
Buishand and Wijngaarden, KNMI technical report
TR-295.
The return period and rainfall duration that is to
be used in analysis and design depends on the
type of system. Urban drainage systems typically
require rainfall data of the order of several minutes
up to hours. In rural areas like polders, run-off
processes are slower and rainfall over a period of
several days is critical to determine the required
water system capacities. The return period that
is used in design depends mainly on economic
factors such as expected damage to buildings or
agricultural products.
Influence of area size
In the previous text rainfall was described as
a point measurement. In reality, storms have
spatial dimensions and rainfall intensities usually
decrease with distance from the center of the
storm. This implies that average rainfall over a
certain area decreases with the size of the area
(see figure 3.4).

Figure 3.4 - Influence of area size on average rainfall

depth over area

16

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Urban drainage systems usually drain small areas

compared to surface water systems in rural areas.
This means that high intensity rainfall over short
periods of time is of importance for the design
of system capacity. It also means that system
transport capacity must be large compared to
that of rural water systems. This would lead to
very large pipe dimensions if the same return
periods were applied to rural and urban systems.
In the Netherlands, relatively low return periods
are applied for design calculations, mostly T=2
years. Urban drainage system are designed to
get overloaded at a relatively high frequency, in
which case streets take over a part of the transport
and storage capacity of underground systems.
This is not a problem as long as buildings are
located sufficiently high above street level. While
in some locations flooding occurs more frequently
as a result of increase in impervious areas that
were connected to urban drainage systems over
the past decades, the use of street storage to
compensate for a lack of transport capacity in
sewer systems is a debated topic. In the end,
economic considerations are likely to end the
discussion in favour of street storage instead
of having to increase pipe capacities in existing
systems.
3.2.3
Using rainfall mass curves
The recorded values in the rainfall duration curves
refer to the amount of precipitation that falls within
a certain period of time, or the external storm event
statistics. The internal storm event statistics for a

urban drainage

rainfall depth of for instance 20 mm over a duration

of 60 minutes can be composed in many different
ways, for example:
1. 5 mm of steady rainfall in the first 50 minutes,
followed by the remaining 15 mm in the next
10 minutes;
2. 15 mm steady rainfall in the first 10 minutes,
followed by the remaining amount in the next
50 minutes;
3. steady precipitation over the whole period of
60 minutes.
Suppose that a storage settlement tank must
be dimensioned for precipitation that falls in 60
minutes and for a return period of 5 years. From
figure 3.3 it can be read that the designer for T=5
must take a rainfall volume of about 20 mm into
account. The capacity of the pumping station that
empties the tank is 0.2 mm per minute (12 mm/h).
The required tank volume is determined by the
greatest difference between the supply (rainfall)
and drainage (pumping) that can occur at any
moment. Figure 3.5 shows that for the first and
second storm characteristics the required storage
capacity must be 13 mm, while a straightforward
use of the rainfall mass curves yields a required
storage capacity of 8 mm. This example clearly
shows that using rainfall mass curves can cause
under-designing in sewerage systems or its
components.
The vast majority of rain showers do not fall
linearly. This means that the amount of combined

Figure 3.5 Relationship of distribution precipitation necessary storage

17

urban drainage

sanitary engineering

Table 3.3 Runoff coefficient

Surface of the earth

runoff coefficient

slate roofs

0,95

tiled roofs

0,90

flat roofs

0,50 - 0,70

asphalt roads

0,85 - 0,90

tile paths

0,75 - 0,85

cobblestone pavement

0,25 - 0,60

gravel roads

0,15 - 0,30

bare surfaces

0,10 - 0,20

parks, strips of land

0,05 - 0,10

Runoff
coefficient

Relative
proportion

Proportion

Table 3.4 Estimation compounded runoff coefficient

22,5%

0,90

0,20

7,5%

0,50

0,08

Pitched roofs

10,0%

0,90

0,09

Flat roofs

10,0%

0,70

0,07

Unpaved/pervious runoff

25,0%

0,15

0,04

Impervious runoff

25,0%

0,00

0,00

Total

100

Asphalt roads
Cobblestone roads

0,48

storm water overflows and storm water overflow

frequencies will be greater than what is derived by
the calculation based on rainfall duration curves. It
is also important to consider that a previous storm
can lead to filling of (a part of) available system
storage, thus influencing starting conditions for
the following storm.
This means that under-designing of retention
basins can occur. Other important factors can
conversely lead to over-designing. The most
important are:
Not taking rainfall losses into account
(evaporation, surface wetting, retention on
the surface, interception by vegetation etc.);
Overestimation of the runoff coefficient (see
following paraghraph; runoff coefficients
in the Dutch Guidelines for Sewer Design
(Leidraad Riolering) are on the high side, so
the calculated runoff load to the stormwater
system is higher than what occurs in practice.);
Not taking storage in drains and house
connections into account;

18

- ct3420

Not taking runoff delay into account (in general

this has little influence on the design of sewer
systems, since runoff processes are fast).
3.2.4
Run-off processes
Part of the precipitation does not run off to the
sewer system, thus does not add to inflow into
the system. Careful estimation of the amount of
precipitation that actually does run off to the sewer
system is important to prevent under-design or
over-design of the sewer system. Runoff behaviour
depends on the following processes:
interception
evapotranspiration
infiltration
retention through ponding
Interception is the process in which part of the
precipitation on the surface is absorbed and
does not run off. Evapotranspiration is constituted
of rainwater that directly evaporates from the
ground, plants and building as well as water that
evaporates indirectly through plants. The term
infiltration refers to water that sinks into the soil
through pervious surfaces. Retention through
ponding occurs on uneven surfaces, where water
does not flow towards a sewer inlet.
Runoff behaviour is taken into account with the
help of a runoff coefficient in the design of sewer
systems. In table 3.3 a few typical values are
shown.
In general in the Netherlands values used in
sewer design as a first approximation are: runoff
coefficient C=1 for impervious areas (roofs and
roads) and C=0 for pervious areas (parks and
gardens).
The runoff coefficient shows which part of the
precipitation runs off the surface and comes into
the sewer system. The record shows that the
runoff coefficient amounts to 48% of the total built
up area. This is 2% less than the percentage of
impervious surfaces that is present in the built up
area. This is nearly the case in each built up area.
This is why when examining the working of sewer
systems the drainage is assumed exclusively for

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impervious surfaces. The runoff coefficient, which

in that case is applied, naturally has the value 1!
Traditionally, based on years of experience,
Dutch sewer systems were designed to be able
to process a constant rainfall intensity of 60 l/s/ha.
For hilly areas sometimes 90 l/s/ha was chosen
in order to promote better flood prevention. The
hectares included relate to the total impervious
surface. Most older sewer systems, up to the
1970s were designed this way. In the 1980s
computers started to be used for sewer design
and more advanced calculations could be made,
taking rainfall intensity variations into account.
More details on rainfall characteristics and sewer
design are explained in the Fundamentals of
Urban Drainage course (CT4491).

urban drainage

Question:
1. Given a residential urban area of 1 ha, 100
houses and 250 inhabitants.
a. Calculate the average annual stormwater flow
and the average annual wastewater flow for
this area.
b. Calculate the peak stormwater flow and the
peak wastewater flow for this area
Answer:
a.
Annual stormwater flow: assume annual rainfall:
800 mm; annual stormwater f low: annual
rainfall*area: 0.8x104 = 8000 m3
Annual wastewater flow: assume daily wastewater
production: 120 l/p/day; annual wastewater flow:
daily ww prod*nr of inhab*days/year: 250x120x365
= 10,950 m3
b.
Assume peak stormwater flow: 60 l/s/ha (or
90 or 120 l/s/ha); peak stormwater flow: peak
flow*area: 60x104 /1000 = 600 l/s
Assume peak wastewater flow: 1.5x120 l/p/
day; peak wastewater flow: peak flow*nr of
inhab*conversion days to seconds: 1.5*120*250
/ 24/3600 = 0.5 l/s

19

urban drainage

4.
Environmental requirements for
sewer design
4.1 Development of environmental
guidelines for sewer systems

In the beginning of the 20th century, surface

waters in many urban areas became heavily
polluted as a result of increased, untreated
overflows from sewer systems and industries.
Wastewater treatment plants began to be built
near large cities to collect and treat polluted water
and control the pollution of urban surface waters.
In the Netherlands, in 1970, the Surface Water
Pollution Act (Wet Verontreiniging Oppervlaktewateren, WVO) was adopted, aiming at the
reduction and prevention of polluted overflows to
surface water. It enforced treatment of polluted
overflows to surface waters based on a system
of pollution permits. This Act has been replaced
by the Dutch Water Law (Waterwet) in December
2009.
In the Netherlands, in 1970, the Surface Water
Pollution Act (Wet Verontreiniging Oppervlaktewateren, WVO) was adopted, aiming at the
reduction and prevention of polluted overflows to
surface water. It enforced treatment of polluted
overflows to surface waters based on a system
of pollution permits. This Act has been replaced
by the Dutch Water Law (Waterwet) in December
2009.

Figure 4.1 - Polluted spills to surface waters

20

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In 1986 a fire at the Santoz chemical plant in

Switzerland had disastrous consequences for the
Rhine. Thousands of gallons of toxic chemicals
were washed into the river and millions of fish
and other wildlife were killed. There was a public
outcry and politicians from all the Rhine countries
agreed that action had to be taken. The result was
the Rhine Action Programme of 1987.
The Rhine Action Programme stated that by 1995:
D ischarge of the most important noxious
substances should be cut by 50% compared
with 1985.
Safety norms in industrial plants should be
tightened.
Weirs must be fitted with fish passages to
allow the fish to travel upstream and spawning
grounds must be restored in the upper
tributaries.
The riverside environment should be restored
to allow the return of plants and animals typical
of the Rhine.
The first point has been an important trigger for
reduction of (combined) overflows from urban
drainage systems. Still there are approximately
13,0 0 0 over f low structures of (improved)
combined sewer systems in the Netherlands.
During heavy rainfall, sewage water diluted
with stormwater is discharged, together with
disturbed sewage sludge, from the combined

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sewer systems via the overflow structures into

the surface water. In separate systems, collected
stormwater is discharged to surface water together
with pollutions washed down from impervious
surfaces. Many overflow structures are situated
on small(semi-)stagnant watercourses. As a
result, the influence of discharges from overflow
structures on surface water quality, sediments and
the aquatic ecosystem is considerable.
Initially, requirements for granting discharge
permits based on the Surface Water Pollution Act,
mainly concerned the average frequency at which
overflows from combined sewer systems can
occur. No permit was necessary for stormwater
outlets of separate systems. Meanwhile research
has shown that the over flow frequency is
representative to a limited extent only for the
pollution load and the effects on surface water.
From 1983 to 1990 the National Sewerage
and Water Quality Working Group (Nationale
Werkgroep Riolering en Waterkwaliteit, NWRW)
conducted thorough research on combined sewer
overflows from sewer systems and its effects on
surface water quality.
Intermezzo: Conclusions of the National
Sewerag e and Wat er Q uali t y Wor king
Group (Nationale Werkgroep Riolering en
Waterkwaliteit, NWRW)
The most important conclusions are:
The annual pollution loads from combined and
from separate sewer systems is of the same
order of magnitude;
Improved separate system produce the lowest
pollution loads, compared to (improved)
combined systems and (non-improved)
separate systems;
Combined sewer overflows and stormwater
outlets should preferably be located at large,
non-stagnant waters. Discharge to small
brooks, canals and isolated ponds should be
avoided.
Only stormwater that runs off from roads with
low traffic intensities can be directly drained to
surface water;
The most efficient method to reduce overflow
loads from combined sewer systems is to

urban drainage

add a storage and settling tank between the

combined sewer overflow and surface water
(this implies installing an improved combined
system).

4.2

Basic effort/Basisinspanning

Based on the results of NWRW research, the

CUWVO (Committee for execution of the Surface
Water Pollution Act) recommended a basic
effort (in Dutch: Basisinspanning) to reduce the
adverse effects on surface water quality caused
by overflows from sewer systems in their report
(CUWVO, 1992). This basic effort in principle
applies to all sewer systems. The recommended
basic effort is based on the principle of best
practicable means and has been defined so as
to act as a reference effort corresponding with
a certain expected type and amount of pollution
emission.
Depending on local circumstances, a solution may
be selected based on lowest costs , provided that
the expected pollution load corresponds with that
of the defined basic effort. The basic effort has
been defined in the form of reference systems for
three different circumstances:
for sewer systems to be newly built: the
preferred system is an improved separate
sewer system;
for existing combined sewer systems: systems
should have an in-sewer storage capacity
equivalent to 7 mm of rainfall, a stormwater
pumping capacity (Dutch: pompovercapaciteit)
of 0.7 mm/h and additional storage capacity
equivalent to 2 mm in storage and settling
tanks;
for existing separate sewer systems: systems
should be converted to improved separate
sewer systems. Treatment of stormwater at
stormwater outlets should only be implemented
in cases of highly polluted impervious surfaces
connected to the storm water system and large
expected adverse effects on the receiving
water.
The recommended storage capacities and
stormwater pumping capacity for combined

21

urban drainage

sewer systems were based on the NWRW

results for annual pollution loads and an analysis
of combined sewer overflow frequencies. The
defined capacities would correspond with an
overflow frequency of about 10 times per year
and an annual pollution load (according to NWRW
results) of 350 kgCOD/ha/yr.
It is furthermore of importance that the maintenance
condition and lay-out of sewer systems are such
that sedimentation is kept to a minimum in order
to prevent pollutions adhering to sediments from
being discharged to surface water. Discharges
to sensitive surface waters should be especially
prevented. In the CUWVO 1992 report, the year
1998 was in principle taken as a final date for
realisation of the recommended basic effort. In
the execution of improvement measures, priority
should be given to situations where adverse
effects on surface water quality are most severe.
I n 2 0 01, t h e C o m m i t t e e f o r I nt e g r a t e d
Watermanagement (CIW, successor of CUWVO),
issued a new report (CIW, 2001) that provided a
further elaboration of the basic effort definition,
which upon practical application had proved
to be open for different interpretations. This
report states that The reference system defined
by CUWVO is taken as the starting point for
establishing the criterion for average annual
emissions of pollutants, which is expressed in
kilograms of organic material (COD) per hectare of
relevant impervious surface per year. The report
establishes the following criterion: the criterion for
maximum annual pollution is a maximum load of
50 kg of organic material (COD) per hectare per
year, totaled over all combined systems within a
municipality. The criterion applies to the total area
of impervious surface connected to the sewerage
system within the drainage area.
The evaluation is based on the following principles:
The average annual emission of pollutants from
existing combined systems must not exceed
the criterion.
The average annual emission of pollutants from
combined systems should be met across the
entire municipality.

22

sanitary engineering

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Over flow volumes should be calculated

in accordance with Module C2100 of the
Sewerage Guidelines for a time series
corresponding to the De Bilt weather centres
10 -year rainfall time series 1955 -1964
(hydrodynamic calculations for rainfall time
series will be explained later in this chapter).
The average concentration of organic material
(COD) during overflows should be 250 mg/l.
Average storm water sedimentation tank
performance should be 45%.
The results of measurements may only be
used in relation to the determination of the
basic effort in specific circumstances, following
consultation with the water management
authority.

4.3

European Framework Directive

The Water Framework Directive (more formally the

Directive 2000/60/EC of the European Parliament
and of the Council of 23 October 2000 establishing
a framework for Community action in the field of
water policy) is a European Union directive which
commits European Union member states to
achieve good qualitative and quantitative status
of all water bodies (including marine waters up to
kilometer from shore) by 2015. It is a framework
in the sense that it prescribes steps to reach
the common goal rather than adopting the more
traditional limit value approach.
The directive defines surface water status as
the general expression of the status of a body
of surface water, determined by the poorer of its
ecological status and its chemical status. Thus,
to achieve good surface water status both the
ecological status and the chemical status of a
surface water body need to be at least good.
Ecological status refers to the quality of the
structure and functioning of aquatic ecosystems
of the surface waters. Water is an important facet
of all life and the water framework directive sets
standards which ensure the safe access of this
resource.
The Directive requires the production of a number
of key documents over six year planning cycles.

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Most important among these is the River Basin

Management Plans, to be published in 2009,
2015 and 2021. Draft River Basin Management
Plans are published for consultation at least one
year prior.
Good ecological status is defined locally as being
lower than a theoretical reference point of pristine
conditions, i.e. in the absence of anthropogenic
influence. In areas under heavy anthropological
influence, such as most areas in the Netherlands,
the theoretical reference point is replaced by a
self-defined reference point that water authorities
can decide upon, based on an analysis of current
ecological status and anthropogenic influences.
Aditionally, limit values have been defined for 33
priority substances and groups of substances.
Pollution control measures should be implemented
to meet those limit values in all surface waters.
In March 2005 the risk analysis reports for the
four river catchments that the Netherlands are
involved in, Rhine, Meuse, Scheldt and Ems
catchments, were delivered to the EU, reporting
on expected achievements with regard to meeting
the Directives objectives in 2015.

4.4 Storage and stormwater pumping

capacity: traditional approach to evaluate
environmental impacts of combined sewer
systems
The following definitions are important in the
understanding and evaluation of environmental
impacts of combined sewer overflows:

urban drainage

up of an energy gradient that is required for

transport towards the overflow. Dynamic storage
is estimated to be of the order of 0.2 to 0.3 mm.
Stormwater pumping capacity (Dutch: pompovercapaciteit, poc): average pumping capacity
that is available after the end of a storm to empty
stored stormwater in a sewer system. This
capacity is usually calculated as the gros pumping
capacity minus dry weather flow.
Overflow frequency (Dutch: Overstortingsfrequentie): average number of combined sewer
overflows per unit of time, usually number of
overflows per year.
Traditionally, evaluation of combined sewer
overflows was based on expected overflow
frequencies. Calculation of the overflow frequency
was based on available storage and stormwater
pumping capacity. This method is based on the
concept of reference systems: a reference system
is defined that corresponds with an acceptable
of combined sewer overflow frequency. The
reference system is defined in terms of storage
capacity and stormwater pumping capacity. The
method then compares the capacities of a sewer
system to those of a reference system to decide
whether system capacities are sufficient.
The calculation method is called Kuipers method
and provides an easy and straightforward way
to evaluate expected pollution from combined
sewer overflows. The sewer system is modeled
as a basin with a single overflow weir level and a
single, continuous stormwater pumping capacity

Storage: in-sewer storage of combined sewer

systems is defined as the available storage volume
in sewer pipelines that is situated below the lowest
overflow weir in the system. This storage volume
is often referred to as below-weir-storage (Dutch:
onderdrempelberging). Storage in manholes was
traditionally not taken into account, nor was the
volume of dry weather flow.
Dynamic storage: available storage volume
above the lowest overflow weir in the system,
water can be stored here as a result of the building

Figure 4.2 - Schematization of combined sewer system

for overflow frequency calculation

23

urban drainage

sanitary engineering

(figure 4.2). The storage capacity and stormwater

pumping capacity are calculated by dividing the
total storage capacity in a sewer system and the
total stormwater pumping capacity in a system
by the impervious area connected to the sewer
system.
The Kuipers method is based on a so-called
dot-graph (Dutch: stippengrafiek), see figure 4.3.
The dot-graph contains rainfall events events for
the period 1926 to 1962. The Kuipers method
evaluates the capacities of a system by a drawing
a line in the graph that corresponds with the given
storage and stormwater pumping capacities. The
number of storm events that by volume would
have exceeded the given storage and pumping
capacities can directly be read from the graph.
The fraction of this count and the length of the dot
graph period (37 years) gives an estimate of the
overflow frequency for this system.
The Veldkamp graph was developed based on
the Kuipers dot graph to further facilitate overflow
frequency evaluation: in the Veldkamp graph,
the expected overflow frequency can directly be
read from the graph, based on a storage capacity

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expressed in mm and a stormwater pumping

capacity expressed in mm/u.
The disadvantage of the Kuipers method is that
it neglects dynamic effects that occur in reality
during stormwater flow. Especially in larger sewer
systems and systems with different overflow weir
levels, this approach is not realistic.

4.5 H yd r o d y n a m i c eva l u a t i o n o f
environmental impacts of sewer systems

The NWRW research results showed that overflow

frequency is unsuitable as a criterion for evaluation
of pollution loads from combined sewer systems.
Still, the Kuipers method was applied to design
and evaluate sewer systems until about 19951998.
From 1998 onwards a new method was developed
in the Netherlands to design and evaluate
combined sewer overflows. This method is based
on hydrodynamic calculations with a simulation
model for an extended period of 10 to 25 years.
This results in a series of overflow quantities for all
combined sewer overflow locations in the sewer

60

rainfall depth (mm)

50

40

30

20

10

0
0

300

600

900

1200

1500

1800

duration (minutes)

Figure 4.3 - The Kuipers dot graph: every dot represents a storm event a given duration and rainfall volume or rainfall
depth.

24

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system. These results are used to evaluate the

performance of the combined system based on:
Combined sewer overflow frequency per
overflow location
Total combined sewer overflow frequency for
the entire system
Combined sewer over flow volumes per
overflow and for the entire system (m3)
Combined sewer overflow quantities (m3/h)
In this way, dynamic properties of flow in sewer
systems are taken into account and a more
complete evaluation of combined sewer overflows

urban drainage

and their potential effect on receiving waters

can be conducted. It is important to include dry
periods between storm events in the calculation,
because these provide information on the starting
conditions of the storm event: especially whether
the storage capacity of the system still partly filled
by the previous event. Similarly, dry weather flow
variations during the calculation period should
be properly taken into account, because these
influence the amount of storage available for
stormwater inflow.

70
mm

rainfall volume

56
42
28
14
0
1800

1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

minuten

overflow durations

1440
1080
720
360
0
12000

1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

m3

overflow volumes

9600
7200
4800

590416

2400
0

1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

Figure 4.4 - Graphs depicting daily rainfall volume (top), overflow durations (middle) and overflow volumes (bottom)
for the period 1955 to 1980 (25 years). Taken from: Module C2100 of the Sewerage Guidelines, RIONED, 2004.

25

urban drainage

Dynamic calculations for extended periods of

time, as described in this method, require a lot of
computational power capacity, especially for larger
sewer systems of several hundreds of kilometers
of sewer length. Therefore large sewer systems
are schematized to a more simplified form, yet still
representative of system characteristics, to reduce
calculation time.
The following results of rainfall series calculations
for combined sewer overflow evaluation are
usually reported.
For every sewer overflow location:
The total combined sewer overflow volume for
the calculation period;
The average yearly overflow volume;
The total number of overflow events;
The average number of overflow events per
year;
The overflow volume at this location as a
percentage of the total overflow volume of the
system.
For every sewer overflow event (overflow event is
defined by a separation of at least 24 hours with
the next overflow event).
Start and end time of the event;
Total and net overflow event duration (net
duration = total duration dry period of less
than 24 hours)
Event overflow volume;
Maximum overflow volume per 5 minutes
Maximum overflow volume per 15 minutes
Maximum per 30 minutes
Figure 4.4 gives an example of the results of an
combined overflow series calculation. These
results and the abovementioned reports are
used to date to evaluate environmental impacts
of sewer systems. In addition to this evaluation,
the expected effects on surface water quality are
analysed separately in a so-called water quality
track (Dutch: waterkwaliteitsspoor) approach.
The water quality track defines measures
to obtain acceptable water quality conditions.
Measures can apply to source control, reduction
of sewer overflows and changes in water system
characteristics (such as vegetated embankments,

26

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minimum flow). The water quality track is important

in those cases where conventional measures to
reduce emissions from sewer systems are not
sufficient. The impact of overflows on surface
water quality are leading in the establishment of
additional measures for water quality improvement.
The targets as defined in the European Water
Framework serve as a reference. STOWA (the
Dutch Organisation for applied research on
water systems) recently released a document
with guidelines on how to evaluate water quality
conditions. (STOWA, 2010). The assessment
of impacts of sewer overflows on surface water
quality is often difficult, because many influencing
factors play a role. The guideline document aims
to help water authorities in this assessment. It also
strongly recommended establishment of surface
water quality monitoring programs, since in most
cases insufficient data are available for proper
water quality assessment.

4.6

References

CUW VO, 19 92. Cordinatiec ommissie

Uit voering Wet Verontreiniging Oppervlaktewateren, Werkgroep VI, Overstortingen
uit rioolstelsels en regenwaterlozingen.
A anbevelingen voor het beleid en de
vergunningverlening. April 1992.
CIW, 2001, Riooloverstorten. Deel 2: Eenduidige
basisinspanning. Nadere uitwerking van de
definitie van de basisinspanning.
EU, 2000. Directive 2000/60/EC of the
European Parliament and of the Council of
23 october 2000 establishing a framework for
Community action in the field of water policy.
STOWA , 2 010. R a p p o r t K n e l p u nte n beoordelingsmethode waterkwaliteitsspoor
(STOWA 2010-17)

sanitary engineering

5.

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urban drainage

Hydraulics for sewer design

5.1 Uniform channel flow in partially

filled sewer pipelines

In sewer pipelines, flow experiences significant

friction and is often turbulent (Re>2000), so the
Bernoulli equation does not apply. Instead, friction
effects lead to a continual reduction in the value
of the Bernoulli constant (equation 5.1).
(5.1)

Continuity equation (mass balance):

y Q
+
=0
t
x

2 : Convective acceleration

x A

( y + z ) : Gravity force
x

: Friction

Instead, the De Saint-Venant equations ((5.2)

and 5.3)) are used to describe the flow of water
through sewer systems. The equations consist of
2 components: the continuity equation (5.2) and
the equation of motion (5.3), under the assumption
that components of velocity in the y and z direction
are negligible in comparison to the component of
velocity in the x direction (uy=uz<<ux).

Q : Advective acceleration

g A

p u
+
+ z = constant
g 2g

The equation of motion (eq.5.3) consists of 4

components:

(5.2)

Equation of motion (momentum balance):

Q Q 2
( y + z )
+
+ = 0
+ g A
t x A
x

(5.3)

Various simplifications from the complete 5.1 and

5.2 equations are applied in practice. Depending
on the type of hydraulic conditions, parts of the
equation can be neglected. Simplified equations
are applied to either speed up the numerical
simulation process or to enable analytical
solutions. Term I and II (acceleration terms) can
for example be neglected for stationary, uniform
flow at normal depth in channels and partially filled
pipes: in those cases flow does not vary with time
and gravity forces are at equilibrium with friction
forces. We will see later that the same applies for
full pipe flow.
Under stationary conditions and uniform flow
at normal depth in partially filled pipes, gravity
forces are at equilibrium with friction forces (see
figure 5.1)

Where:
Q
A
g
z
y
t
t
r
W
B

flow rate (m3/s)

flow area or wet area (m2)
gravity acceleration (~9.813 m/s2)
bottom elevation (m)
water depth (in part-full flow) or pressure
head (in pipe flow) (m)
time (s)
wall shear stress
density (kg/m3)
wet perimeter (m)
surface width (m)
Figure 5.1 - Flow in partially filled pipeline, friction force
due to shear stress along the pipe wall.

27

urban drainage

sanitary engineering

and the momentum equation (5.3) simplifies to:

g A

( y + z )
+ = 0
x

(5.4)

For flow at normal depth, the friction slope equals

the bottom slope:

Sfr = Sb =
(bottom slope):

y + z
x

g A Sb =

(5.5)

Ratio A/W is the hydraulic radius Rh, so:

= g Rh Sb

(5.6)

W: wet perimeter:

u
8

B = 2R sin ; for full

For full pipes under stationary conditions, gravity

forces and friction forces are at equilibrium and the
same equation (5.5) applies as for partially filled
pipeline flow at normal depth. Yet for full pipe flow,
the friction slope does not necessarily equal the
bottom slope:

Sfr =

y + z
Sb
x

In the formula of Darcy Weisbach (equation 5.9),

for full pipes, the hydraulic diameter Dh equals the
pipe diameter D:

dHfr =

L u2

D 2g

formula of Darcy Weisbach for full pipes

(5.7)

C: Chzy coefficient C [L1/2 T-1]

= 2R

B: width of water surface:

pipe sin a=0, B=0

Wall shear stress (t) is approximately proportional

to the velocity squared (u2); common expressions
for t are based on a friction factor:

gu 2
=
C2

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In full pipes the wet area A and hydraulic radius

Rh are calculated as follows:
A: wet area: A = r 2

2
(5.8)

B
(r y ) ;
2

for full pipe a=p, B=0 and A=p r 2 or, since r=1/2D:
A=0.25p D2

l: White-Colebrook friction factor l [-]

For a pipe section of length L, friction losses dHfr
due to wall shear stress are given by:

L u2
dHfr =

Dh 2g

(5.9)

formula of Darcy Weisbach

Where:

R: hydraulic radius: Rh=A/=0.25D

The value of the friction coefficient l can be
calculated according to White-Colebrooks
formula:

= 2 log(0.27

k
2 .5
+
)
D Re

(5.10)

Dh: hydraulic diameter:

Where:
Re Reynolds number [-]
k
wall roughness (mm)
D
diameter (m)
l
friction coefficient [-]

a: angle between the vertical and the centre-towaterline radius at a given filling rate (see also
later in this chapter)

Examples of the value of k, the pipe wall

roughness, are given in table 5.1 for various kinds
of sewer pipe materials.

4A
or Dh = 4Rh

B
A: wet area: A = r 2 (r y )
2

28

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urban drainage

Table 5.1 k-value pipeline materials

Material

k-value in mm

Brickwork

1-5

Cement

0.5 - 2

Plastic

0.2 - 0.5

in the design load of a sewer system, and 0.25 m

< D < 1 m: 0.25106 Re 106 and

k
2 .5

0.27 D >>

Re

The k-value that is applied for gravity sewers

systems is an averaged k-value; the resistance of
pipe joints, sediments and other rough elements is
accounted for in the general k-values mentioned
in table 5.1. In pressurised pipelines the k-value of
the material itself is used, since added roughness
of joints and sediments is assumed to be of little
influence. Friction losses due to valves, pipe
curves are accounted for separately.
The Reynolds number Re is calculated by:

Re =
u
n

uD

(5.11)

pipe flow velocity (m/s)

kinematic viscosity of water (m/s)

(5.12)

Then, the value of the friction factor l based can

be calculated by:

A 1
y B
y

= cos1 1 1
Af
R D R

(5.13)

The value of l can also be read from the Moody

diagram (figure 5.2) that charts l (vertical axis,
left) as a function of the Reynolds number Re
(horizontal axis) and k/D (vertical axis, right). The
left part of the diagram shows a linear relation
between l and Re that is applicable for laminar
for flow conditions. For transitional turbulence
conditions, l depends on both Re and k/D, while
for rough turbulence conditions l depends on
k/D only.

The value of (EQ) depends on the temperature

and the type of wastewater to be transported. For
wastewater of nearly 18C (EQ) is around 10 -6 m/s.
For a flow velocity of 1 m/s, that frequently occurs

Figure 5.2 - Moody Diagram for friction factor l

29

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sanitary engineering

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Partially filled versus full pipe geometric

relations
The relations between geometric formulae for
partially filled and full pipelines are expressed in
terms of the filling rate y/R that is calculated at
follows:

Figure 5.4 - Ratios for wet area, wetted perimeter,

hydraulic ratio and width water surface/diameter (B/D)
for varying filling rates y/R of pipelines (subscript f
applies to full pipes)

Figure 5.3 - Partially filled pipe with filling depth y, pipe

radius R, angle a and wetted area A.

The following relations apply for the ratios of

partially filled versus full pipes (subscript f applies
to full pipes):
Wet area ratio

A 1
y B
y

= cos1 1 1
Af
R D R

(5.14)

Figure 5.5 - Ratios for the Reynolds number, discharge

and flow velocities for varying filling rates y/R in
pipelines (subscript f applies to full pipes)

The width at the water surface is maximum and

equal to the pipe diameter for half filled pipes (y/
R=1); it is 0 for empty and for full pipes.

Wetted perimeter ratio

1
y

= cos1 1
f
R

(5.15)

Hydraulic radius ratio

Rh
A / Af
=
Rh,f / f

(5.16)

In figure 5.4 the ratios for wet area, wetted

perimeter, hydraulic ratio and width water surface/
diameter (B/D) are depicted for varying filling
rates y/R. The figure shows that wet area and
wetted perimeter ratios are more or less linear,
the hydraulic ratio radius increases towards a
maximum above 1 at about 75% filling rate (y/
R1.7), then decreases to 1 for full pipes (y/R=2).

30

Figure 5.5 shows the ratios for the Reynolds

number, discharge and flow velocities for varying
filling rates y/R (subscript f applies to full pipes).
The maximum pipe discharge occurs for a filling
rate just below 1. In calculations for sewer pipes,
full-pipe discharges are used, since these apply
for full pipe flow as well as for nearly filled pipes (y/
R1.9) , while higher discharges occur for a limited
range of filling rates. The figure shows that flow
velocities are equal for half full pipes (y/R=1) and
for full pipes. The Reynold number ratios reaches
a maximum for a filling rate y/R of about 1.6.

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urban drainage

5.2 Hydraulic resistance in components

of sewer systems
In this paragraph local friction losses for various
sewer system components are discussed.

5.2.1 Sewer overflow weirs

Sewer overflow weirs are typically walls of
concrete or brickwork situated in a manhole or at
the end of a pipe. Therefore, in most cases, they
can be considered as sharp-crested and the flow
over these weirs as a case of rapidly varied flow.
Water backs up before the weir so that in flowing
over the weir the water goes through critical depth.
Figure 5.6 illustrates flow over a sharp crested
weir. The following equation applies for the flow
over the weir crest if friction losses are neglected.

uc
gh2

Figure 5.6 - Free flow over a sharp crested weir, with

energy level H1 upstream of the weir, energy level H2
downstream of the weir and h2 the water level above
the weir.

= 1 (critical flow above weir crest)

(5.17)

And:

uc2 uc2 uc2 3uc2

H1 = h2 +
=
+
=
2g g 2g 2g

(5.18)

Also:

h2 = 32 H1

(5.19)
Figure 5.7 - Submerged flow over a sharp crested weir,
with energy level H1 upstream of the weir, energy level
H2 downstream of the weir and hd the downstream
water level.

Therefore:

Q = uc h2B = 32 BH1

2
3

gH1

(5.20)

Or:

The value of the weir crest m depends on the

shape of the crest; the value of m varies roughly
from 1.5 to 3. For a reliable calculation of the
discharge over a weir, the weir coefficient should
be calibrated by in-situ measurements.

Q = 1.7BH12
Where:
uc
critical flow velocity above weir
B
weir crest width
The streamlines in the water flow above the crest
are not parallel or normal to the area in the plane,
so in reality friction losses do occur. To account for
this effect, the constant 1.7 in the flow equation is
replaced by a weir coefficient m (5.21).
3

Q = mBH 2

(5.21)

Figure 5.6 and equations 5.17 to 5.21 apply for free

flow over the weir: flow above the weir is critical
and the downstream water level does not influence
the flow. This is true when the downstream water
hd level is less than 2/3 of the upstream water
level H1 (fig. 5.7).
In practice, weir parameters and flow conditions
depend not only on the shape of the weir crest,

31

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For outflow of a pipe into a reservoir, A1/A 2 goes

to zero and hL equals u2/2g.
When a sewer discharges under water the outflow
losses equal a the velocity head times a loss factor
k (5.23). The value of k varies between 0 and 1.

u2
hL = k
2g
Figure 5.8 - Flow over weir influenced by downstream
maintenance condition.

but also on maintenance conditions of the inflow

and outflow pipes and the weir itself, as figure 5.8
illustrates.
5.2.2 Inlets, outlets and manholes
Figure 5.9 gives a schematic representation of flow
through a weir opening. As water flows through
the opening, local deceleration losses occur. The
following equation applies for the calculation of the
head loss due to flow through a narrow opening
(5.22):

A1 u12
hL = 1

A2 2g

(5.22)

Where:
h L
local head loss
A1
cross-section of weir opening
A 2
cross-section of downstream pipe; in case
of a reservoir A 2 (infinity)
u1
upstream flow velocity

(5.23)

The same equation (5.23) is used to calculate

head losses in manholes. The value of k for
local losses in manholes depends on the height
of the water level in the manhole. Figure 5.10
gives values of k for varying water heights for two
different manhole bottom shapes.
In the Netherlands, the bottom manhole shape in
figure 5.10 top is usually applied. Measurements
have shown that when the filling height of the
manhole is at least 1 time the pipe diameter, the
value of k lies between 0.7 and 0.9.
When more than two pipes meet in a manhole, the
local head loss is usually calculated based on the
flow velocity of the outflow discharges. The total
head loss in the manhole is the sum of the head
losses for the outgoing pipes.
In practice, it is often assumed that the local losses
in manholes are negligible compared to the friction
losses along the pipe length.

Figure 5.9 Flow through a weir opening. Situation on the left show flow through opening; situation on the right
shows flow through an outlet pipe in the weir.

32

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urban drainage

Figure 5.10 Local loss coefficient k for varying water

level heights in manholes.

33