SIST EN 16933-2:2017

SLOVENSKI STANDARD
01-november-2017
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Drain and sewer systems outside buildings - Design - Part 2: Hydraulic design
Entwässerungssysteme außerhalb von Gebäuden - Planung - Teil 2: Hydraulische
Berechnung
Réseaux d'évacuation et d'assainissement à l'extérieur des bâtiments - Conception -
Partie 2 : Conception hydraulique
Ta slovenski standard je istoveten z: EN 16933-2
ICS:
93.030 Zunanji sistemi za odpadno External sewage systems
vodo
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 16933-2
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2017
EUROPÄISCHE NORM
ICS 93.030
English Version
Drain and sewer systems outside buildings - Design - Part
2: Hydraulic design
Réseaux d'évacuation et d'assainissement à l'extérieur Entwässerungssysteme außerhalb von Gebäuden -
des bâtiments - Conception - Partie 2 : Conception Planung - Teil 2: Hydraulische Planung
hydraulique
This European Standard was approved by CEN on 30 July 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 16933-2 E
worldwide for CEN national Members.

Contents Page
European foreword . 5
Introduction . 6
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Symbols and units . 10
5 General . 11
6 Design criteria . 12
7 Hydraulic capacity of pipelines . 12
7.1 Introduction . 12
7.2 Pipeline headlosses . 12
7.2.1 General . 12
7.2.2 The Colebrook-White formula. 12
7.2.3 The Manning formula . 13
7.2.4 Pipeline roughness values . 13
7.2.5 Pipeline headloss values . 13
7.3 Local headlosses . 13
7.4 Total headlosses . 13
8 Incoming flows . 14
8.1 Foul wastewater flows . 14
8.1.1 General . 14
8.1.2 Calculation of foul wastewater flow rates based on the appliances connected . 14
8.1.3 Calculation of foul wastewater flow rates from population and average flows . 14
8.2 Rainfall and runoff from precipitation . 15
8.2.1 Rainfall . 15
8.2.2 Runoff . 15
8.3 Extraneous flows . 16
9 Hydraulic calculation of drain and sewer systems . 17
9.1 General . 17
9.2 Flow in sewer systems . 17
9.3 Flow simulation methods . 19
9.3.1 Introduction . 19
9.3.2 Simple/empirical methods . 19
9.3.3 Other simplified methods . 19
9.3.4 Dynamic wave methods . 19
9.3.5 Selection of calculation method . 19
9.4 Surface flood routing . 20
9.5 Validation of models . 20
10 Hydraulic design . 21
10.1 Capacity of drains and sewers . 21
10.1.1 General . 21
10.1.2 Foul drains and sewers . 21
10.1.3 Surface water drains and sewers . 21
10.1.4 Combined drains and sewers . 22
10.2 Design for self-cleansing . 22
10.2.1 Sediment transport . 22
10.2.2 Minimization of blockages . 22
10.3 Sewers with steep gradients . 23
10.4 Surface water inlets . 23
10.5 Separators, screens and pretreatment devices . 23
10.5.1 General . 23
10.5.2 Control of sediments. 24
10.5.3 Control of grease and fats . 24
10.5.4 Control of light liquids . 25
10.5.5 Treatment of surface water to remove dissolved pollutants . 25
10.5.6 Screening . 26
10.6 Infiltration drainage systems . 26
10.7 Evaporation systems . 26
10.8 Manholes and inspection chambers . 26
10.9 Combined sewer overflows . 27
10.10 Tanks and ponds . 27
10.10.1 General . 27
10.10.2 Detention tanks . 27
10.10.3 Ponds . 28
10.11 Outfalls . 28
11 Sources of additional information . 28
Annex A (informative) Sources of additional information . 29
A.1 National Standards Bodies . 29
A.2 Austria . 29
A.2.1 Regulatory Bodies . 29
A.2.2 Other organisations . 29
A.3 Denmark . 29
A.3.1 Regulatory Bodies . 29
A.3.2 Other organisations . 30
A.4 France . 31
A.4.1 Regulatory Bodies . 31
A.4.2 Other organisations . 31
A.5 Germany . 31
A.5.1 Regulatory Bodies . 31
A.5.2 Other organisations . 32
A.6 Ireland . 32
A.6.1 Regulatory Bodies . 32
A.7 Italy . 33
A.7.1 Regulatory Bodies . 33
A.7.2 Other organisations . 33
A.8 The Netherlands . 33
A.8.1 Regulatory Bodies . 33
A.8.2 Other organisations . 33
A.9 Norway . 34
A.9.1 Regulatory Bodies . 34
A.9.2 Other organisations . 34
A.10 Portugal . 34
A.10.1 Regulatory Bodies . 34
A.10.2 Other organisations . 35
A.11 Sweden . 35
A.11.1 Regulatory Bodies . 35
A.11.2 Other organisations . 35
A.12 Switzerland . 35
A.12.1 Regulatory Bodies . 35
A.12.2 Other organisations . 36
A.13 United Kingdom . 36
A.13.1 Regulatory Bodies . 36
A.13.1.1 General . 36
A.13.1.2 England . 36
A.13.1.3 Wales . 37
A.13.1.4 Scotland . 37
A.13.1.5 Northern Ireland . 38
A.13.2 Other organisations . 38
Bibliography . 40
European foreword
This document (EN 16933-2:2017) has been prepared by Technical Committee CEN/TC 165
“Wastewater engineering”, the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by March 2018 and conflicting national standards shall be
withdrawn at the latest by March 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent
rights.
This European Standard EN 16933, Drain and sewer systems outside buildings — Design, contains the
following parts:
1)
— Part 1: Physical design
— Part 2: Hydraulic design.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

1)
Currently in preparation.
Introduction
Drain and sewer systems are part of the overall wastewater system that provides a service to the
community. This can be briefly described as:
— removal of wastewater from premises for public health and hygienic reasons;
— prevention of flooding in urbanised areas;
— protection of the environment.
The overall wastewater system has four successive functions:
— collection;
— transport;
— treatment;
— discharge.
Collection and transport of wastewater is provided by drain and sewer systems.
EN 752:2017 provides a framework for the design, construction, maintenance, operation and
rehabilitation of drain and sewer systems outside buildings. This is illustrated in the upper part of the
diagram in Figure 1. EN 752:2017 is supported by more detailed standards for the investigation, design,
construction, organization and control of drain and sewer systems.
Investigation and assessment standards include:
— EN 13508, Investigation and assessment of drain and sewer systems outside buildings.
Design and construction standards include:
2)
— EN 16932 , Drain and sewer systems outside buildings — Pumping systems;
3)
— EN 16933 , Drain and sewer systems outside buildings — Design;
4)
— EN 1295 , Structural design of buried pipelines under various conditions of loading;
— EN 1610, Construction and testing of drains and sewers;
— EN 12889, Trenchless construction and testing of drains and sewers;
— EN 15885, Classification and characteristics of techniques for renovation, repair and replacement of
drains and sewers.
Management and control standards include:
— EN 14654, Management and control of operational activities in drain and sewer systems outside
buildings.
2)
Currently in preparation.
3)
Currently in preparation.
4)
Currently in preparation.
Figure 1 — Relationship to EN 752:2017 and other drain and sewer standards [Source
EN 752:2017]
1 Scope
This European Standard specifies requirements for the design of drain and sewer systems outside
buildings.
It is applicable to drain and sewer systems from the point where the wastewater leaves a building, roof
drainage system, or paved area, to a point where it is discharged into a wastewater treatment plant or
receiving water body.
This document specifies requirements for the hydraulic design of drain and sewer systems and the
assessment of the capacity of existing drain and sewer systems.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
EN 858-1, Separator systems for light liquids (e.g. oil and petrol) — Part 1: Principles of product design,
performance and testing, marking and quality control
EN 858-2:2003, Separator systems for light liquids (e.g. oil and petrol) — Part 2: Selection of nominal size,
installation, operation and maintenance
EN 1825-1, Grease separators — Part 1: Principles of design, performance and testing, marking and
quality control
EN 1825-2:2002, Grease separators — Part 2: Selection of nominal size, installation, operation and
maintenance
EN 16323:2014, Glossary of wastewater engineering terms
EN 752, Drain and sewer systems outside buildings — Sewer system management
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 16323, EN 752 and the
following apply.
NOTE 1 Certain key definitions from EN 16323:2014 have been repeated below for clarity. The following
additional terms used in this document are defined in EN 16323:
backdrop manhole; catchment area; combined sewer overflow;
combined system; detention tank; domestic wastewater;
drain; dry weather flow; extraneous flow;
foul wastewater; gradient; gravity system;
hydro-biological stress; industrial wastewater; infiltration (see Figure 2);
inspection chamber; inverted syphon; maintenance;
manhole; outfall; ramp manhole;
receiving water body; rehabilitation; relevant authority;
renovation; repair; replacement;
rising main; runoff coefficient; self-cleansing;
self-purifying capacity; separate system; septic wastewater;
sewer; sewer system; surcharge; tank sewer;
time of concentration; time of flow; wastewater treatment plant.
NOTE 2 The following additional terms used in this document are defined in EN 752:2017:
expected frequency
flooding
rainfall intensity
3.1
depression storage
precipitation retained in surface hollows that does not contribute to runoff
3.2
rainwater
water arising from atmospheric precipitation, which has not yet collected matter from the surface
Note 1 to entry: See Figure 2.
[SOURCE: EN 16323:2014, 2.1.1.1]
3.3
runoff
water from precipitation that flows off a surface to reach a drain, sewer or receiving water
Note 1 to entry: See Figure 2.
[SOURCE: EN 16323:2014, 2.1.1.2]
3.4
sub-critical flow
state of flow when the water velocity is less than the velocity of the small surface wave with water
levels tending to be stable
3.5
super-critical flow
state of flow when the water velocity is greater than the velocity of the small surface wave with violent
fluctuations in water level being possible
3.6
surface receiving water body
receiving water body that is on the surface of the ground (e.g. river, lake or sea)
Note 1 to entry: See Figure 2.
[SOURCE: EN 16323:2014, 2.1.3.7]
3.7
surface water
water from precipitation, which has not seeped into the ground and is discharged to the drain or sewer
system directly from the ground or from exterior building surfaces (see Figure 2)
[SOURCE: EN 16323:2014, 2.1.1.3]
Key
1 rain water
2 runoff
3 surface water
4 infiltration
5 surface receiving water body
Figure 2 — Terminology for flows derived from rain water [Source EN 16323:2014]
3.8
vortex manhole
circular manhole within which a large difference in level is accommodated by the wastewater entering
tangentially and descending helically
3.9
wastewater
water composed of any combination of water discharged from domestic, industrial or commercial
premises, surface run-off and accidentally any sewer infiltration water
[SOURCE: EN 16323:2014, 2.3.10.65]
4 Symbols and units
For the purposes of this document, the following symbols and units apply.
A is the area receiving rainfall (measured horizontally), in hectares [ha]
c
A is the flow cross-section perpendicular to the invert of the drain or sewer, in metres squared
f
[m ]
C is the runoff coefficient (between 0,0 and 1,0), dimensionless [-]
s
c is the factor with inclusion of additional losses, dimensionless [-]
D is the internal diameter of the pipe (bore), in metres [m]
g 2
is the acceleration due to gravity, in metres per second squared [m/s ]
h is the local headloss in bends, valves and other fittings, in metres [m]
f
h the pipeline head loss due to friction, in the pipe in metres [m]
p
i is the rainfall intensity, in litres per second and hectare [l/s/ha]
J is the hydraulic gradient (energy loss per unit length), dimensionless [-]
E
J is the friction gradient, dimensionless [-]
F
J is the gradient of the invert of the drain or sewer (with open channel possibly not constant),
s
dimensionless [-]
K 1/3
is the Manning coefficient, in metres raised to the power one third per second [m /s]
k is the headloss coefficient, dimensionless [-]
f
k is the hydraulic pipeline roughness, in metres [m]
s
L Is the length of the pipeline, in metres [m]
Q 3
is the flow, in metres raised to the power of three per second [m /s]
Q is the peak flow rate, in litres per second [l/s]
pk
q is the lateral inflow per unit of length in the direction of the flow (assumed steady-state), in
metres cubed per second and metre [m /(s m)]
R is the hydraulic radius, in metres [m]
h
t is the time coordinate, in seconds [s]
v is the velocity in the direction of flow averaged across the flow cross-section, in metres per
second [m/s]
x is the path coordinate in direction of flow, in metres [m]
y is filling height in profile or depth of water (perpendicular to invert) or the pressure head in
completely filled drains at the invert of the pipe or profile, in metres [m]
λ is the friction coefficient for the pipe, dimensionless [-]
ν 2
k is the kinematic viscosity of fluid, in metres squared per second [m /s]
5 General
EN 752 specifies objectives and functional requirements and the principles for design of drain and
sewer systems outside buildings.
Hydraulic design can affect the functional requirements, in particular the following:
— protection from sewer flooding;
— protection of surface receiving water bodies;
— prevention of odours and toxic, explosive and corrosive gases;
— maintaining the flow.
Drains and sewers systems shall be designed to provide sufficient capacity for the design flows. In
selecting the diameter and/or gradient of the pipe consideration shall also be taken of the need to
minimize build-up of sediments and to minimize the risk of blockages (see 7.1).
6 Design criteria
Except where they are specified by national or local regulations or the relevant authority, design
criteria shall be determined in accordance with EN 752:2017, 5.3.2.
Design criteria relating to hydraulic design can include:
a) the expected frequency at which no surcharge occurs in the drain and sewer system;
b) the expected frequency at which a specified amount of surcharge occurs in the drain and sewers
system;
c) the expected frequency of sewer or surface water flooding (this can occur without surcharge);
d) the impact of the flows from surface water outfalls on river flooding from surface receiving water
bodies or on groundwater flooding; and
e) the impact of flows on the operation of the wastewater treatment plant.
7 Hydraulic capacity of pipelines
7.1 Introduction
The basis for design is that flows in drains and sewers are turbulent. Two formulae are recommended
5) 6)
for use in calculating turbulent flows in drains and sewers: Colebrook-White and Manning .
7.2 Pipeline headlosses
7.2.1 General
When using recommended hydraulic pipeline roughness values it is necessary to establish whether
allowance has been included for local headlosses. The hydraulic pipeline roughness (k ) or the Manning
s
flow coefficient (K) should allow for headlosses due to pipe material taking account of other factors
including, the internal profile of the pipe, losses due to discontinuities at the joints and biofilms that
grow on the pipe surface below the water level.
The effect of the biofilm can be more significant than any difference in the roughness of the material
without the biofilm. A single value, regardless of pipe material is therefore often used.
7.2.2 The Colebrook-White formula
For circular pipes flowing full, the velocity of flow is given by Formula (1):

k 2,51⋅ν

s k
v=−⋅22⋅ g⋅ D⋅ J ⋅ log + (1)
( )
E 10
3,71⋅ D
D⋅ 2⋅⋅gD⋅ J
( )
E

For partially running full pipes or pipes with non-circular cross-sections the velocity of flow is given by
Formula (1) by replacing D by 4 R where R is the hydraulic radius (flow cross-sectional area divided
h h
by the wetted perimeter).
5)
This formula is named Colebrook in the French version and Prandtl-Colebrook in the German version.
6)
This formula is named Manning-Strickler in the French and German versions.
7.2.3 The Manning formula
For both circular and non-circular cross-sections whether running full or partially full, the velocity of
flow is given by the Formula (2):
2//3 1 2
v= KR J (2)
h E
7.2.4 Pipeline roughness values
When using recommended hydraulic pipeline roughness values it is necessary to establish whether
allowance has been included for local headlosses. Values currently in use differ from country to country,
1/3 −1 1/3 −1
and range from 0,1 mm to 3,0 mm for k and 70 m s to 90 m s for K.
s
Approximate comparisons of velocity estimates using Formulae (1) and (2) above can be made using
the following Formula (3):
 

32 3,71⋅ D
K=4⋅⋅g ⋅ log   (3)

 
Dk

 s 
7.2.5 Pipeline headloss values
To calculate the head loss in the pipe for given flow conditions, the friction coefficient can be calculated
from Formula (4):
2 ⋅⋅gD⋅ J
E
λ= (4)
v
The pipeline headloss in the pipe can then be calculated from Formula (5):
λ⋅ L
h = (5)
p
D
7.3 Local headlosses
Headlosses, in addition to those mentioned in 7.2, occur at junctions, changes of cross-section,
manholes, bends and other fittings. If direct calculations are to be made, the following Formula (6) shall
be used:
kv⋅
f
h = (6)
f
2⋅ g
7.4 Total headlosses
Two methods of calculating total headlosses are:
— adding local headlosses (see 7.3) to the pipeline headlosses (see 7.2);
— accounting for local headlosses by assuming a higher value of hydraulic pipeline roughness in the
calculation of pipeline headloss.
8 Incoming flows
8.1 Foul wastewater flows
8.1.1 General
For drains and sewers serving small populations, the capacity of the pipe is often determined by the
minimum pipe diameter specified by the relevant authority.
In gravity drains and sewers the ratio between the peak flow and the average dry weather flow reduces
as the flow moves downstream.
Where the flow passes through the collection tank of a pumping system, attenuation occurs and the
ratio between the peak flow and the average dry weather flow used to design the pumps is reduced
accordingly. The peak flow downstream of the pumping station is determined by the capacity of the
pumps.
8.1.2 Calculation of foul wastewater flow rates based on the appliances connected
The design of drains and sewers to serve individual or groups of buildings where discharges from
individual appliances give relatively high flows of an intermittent and irregular nature shall use a peak
rate of flow derived from the number and type of appliances connected.
The rates of flow in the drains from the buildings or premises, calculated using EN 12056-2, should be
used in the design of downstream drain systems.
Flow rates for individual appliances and factors to be applied can be specified by national or local
regulations or the relevant authority. Industrial wastewater flows shall be calculated separately.
This method may be used for the design of gravity foul drains and sewers where the upstream
population is less than approximately 1 000.
8.1.3 Calculation of foul wastewater flow rates from population and average flows
Where the dry weather flow is continuous and large enough that it is not significantly increased by the
input from single appliances, the design foul wastewater flow rates for domestic wastewater can be
calculated as follows. Flow rates can be based on either: the population and a measured rate of flow per
head or, for new developments where such data are not available, on the planning criteria for the
population or the type and number of dwellings. For a new development and for an upgrading scheme
on an existing development, the estimates used shall be appropriate for the specified planning horizon.
This method may be used for the design of gravity foul drains and sewers where the upstream
population is greater than approximately 500.
Existing water supply statistics can be helpful to derive future water supply consumption and hence
domestic wastewater flows. Flow patterns for daily consumption and anticipated variations between
different types of development can also be established. Consumer water usage that does not enter the
drain and sewer system and distribution leakage are of particular importance in assessing domestic
wastewater flows. The rate of flow per head can be based on local water supply statistics allowing for
consumption that does not result in discharge to the sewers and, where appropriate meters are not
available, distribution losses. Typical discharge figures for developments similar to those under
consideration may also be used.
The peak design foul wastewater flow rate shall take into account the ratio between the peak foul
wastewater flow rate and the average value. For existing systems, where there is continuous flow this
should be based on measured values where these are available. Where measured values are not
available design peak flow factors should be estimated by comparison with similar catchments.
Design values for the peak factor or the peak flow rates can be specified in national documents.
8.2 Rainfall and runoff from precipitation
8.2.1 Rainfall
National or local regulations or the relevant authority can specify design rainfall values to be used.
Design rainfall intensities reflecting the local conditions can be in one of the following forms:
a) local or regional constant rate rainfall values;
b) constant rate rainfall calculated from intensity-duration-frequency curves;
c) single event rainfall profiles;
d) rainfall time series.
Single even rainfall events or rainfall time series can either be taken from observed historic rainfall
data, or be formed from artificial profiles based on statistical analysis of rainfall data.
8.2.2 Runoff
8.2.2.1 General
Runoff shall be calculated taking into account a number of factors including:
a) design rainfall;
b) area that could drain to the inlets connected to the system:
1) extent of impermeable area;
2) extent of permeable area;
c) likely losses of runoff due to:
1) infiltration of rainfall into the ground;
2) interception, i.e. rainfall retained by vegetation or other objects that prevents raindrops falling
onto the ground;
d) likely above ground flows from adjacent pervious surfaces;
e) likely increases in connected area.
The possible impact of climate change should also be considered.
A simple method of calculating the runoff from small areas is included in 8.2.2.2.
8.2.2.2 Methods of calculating runoff from small development schemes
In the absence of a method specified by national or local regulations or the relevant authority, a simple
method of estimating the peak rate of discharge of surface water, applicable for areas of up to 200 ha or
times of concentration up to 15 min and assuming a uniform rate of rainfall intensity, may be used. The
rainfall intensity to be adopted depends on factors such as time of concentration of the contributing
area and the analysis of local rainfall data. Peak flow rate is given by Formula (7):
Q C⋅⋅i A (7)
pk s c
=
Appropriate values for (C ) are given in the Table 1.
s
Table 1 — Runoff coefficients for calculating runoff from small development schemes
Nature of connected area Runoff Coefficient C Comments
s
Impermeable areas and steeply 0,9 to 1,0 Depending on depression
a storage
sloping roofs
Large flat roofs b 2
0,7 to 1,0 Over 10 000 m
Small flat roofs 1,0 2
Less than 100 m
Permeable areas 0,0 to 0,3 Depending on ground slope and
cover
a
Vertical surfaces should be considered as appropriate.
b
Depending on type of roof material (gravel at low end glass or metal at top).
8.2.2.3 More complex runoff models
Where sewer flow simulation models are being used consideration should be given to other factors that
can affect the runoff. These include:
a) the movement of rainwater from paved areas to adjacent permeable areas;
b) the infiltration of rainwater through cracks or impermeable areas in paved surfaces;
c) the storage of rainwater in depressions in the surfaces;
d) the evaporation of rainwater from surfaces;
e) the movement of runoff from permeable surfaces onto paved surfaces, for example when:
1) the rate of rainfall exceeds the infiltration capacity of the permeable surface;
2) the soil is so saturated that rainwater cannot infiltrate into the ground;
3) the soil has become frozen; or
4) the soil has become temporarily impermeable due to a long period of hot and dry weather.
Various runoff models are included in available sewer flow modelling software to model these
processes.
8.3 Extraneous flows
If the risk of extraneous water entering drains and sewers is considered to be unacceptable,
investigations shall be carried out to determine the extent of this risk. Sources of extraneous flow can
include:
— misconnections of surface water to foul drains and sewers;
— seepage of runoff through joints in manhole covers;
— infiltration of water from the ground through defective joints or other defects in drains and sewers
and other associated structures (e.g. manholes and inspection chambers).
An allowance for extraneous flows may be made by increasing the design dry weather flows (see 8.1).
When estimating likely extraneous flows account should be taken of local factors including:
— the groundwater levels;
— the standards of construction including the amount of supervision and control of drain and sewer
construction.
9 Hydraulic calculation of drain and sewer systems
9.1 General
A variety of methods have been developed to assist in the design of drain and sewer systems. In all
cases the runoff process has been simplified to enable the design parameters to be estimated cost
effectively. This document reviews the range of methods available and gives guidance where they
should be used.
9.2 Flow in sewer systems
Flow in drain and sewer systems is unsteady gradually varied flow. The flow conditions can be
calculated by applicat
...