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External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
Water Services Regulatory Authority
Centre City Tower
7 Hill Street
Birmingham
B5 4UA
External Review of Sewer Flooding Risk Registers
Report
October 2008
Mott MacDonald
Demeter House
Station Road
Cambridge CB1 2RS
UK
Tel : 44 (0)1223 463500
Fax : 44 (0)1223 461007
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
External Review of Sewer Flooding Risk Registers
Report
Issue and Revision Record
Rev Date Originator
Checker
Approver
Description
A 04/08/08 A Heather M Surendra P Chadwick First issue for comment
B 22/08/08 M Surendra A Heather Revisions and corrections
C 15/10/08 A Heather P Chadwick P Chadwick Final corrections
This document has been prepared for the titled project or named part thereof and should not be relied upon or used for any
other project without an independent check being carried out as to its suitability and prior written authority of Mott
MacDonald being obtained. Mott MacDonald accepts no responsibility or liability for the consequence of this document
being used for a purpose other than the purposes for which it was commissioned. Any person using or relying on the
document for such other purpose agrees, and will by such use or reliance be taken to confirm his agreement to indemnify
Mott MacDonald for all loss or damage resulting therefrom. Mott MacDonald accepts no responsibility or liability for this
document to any party other than the person by whom it was commissioned.
To the extent that this report is based on information supplied by other parties, Mott MacDonald accepts no liability for any
loss or damage suffered by the client, whether contractual or tortious, stemming from any conclusions based on data
supplied by parties other than Mott MacDonald and used by Mott MacDonald in preparing this report.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
List of Contents Page
Chapters
1 Introduction 1-1
2 Findings from Review 2-1
3 Summary of Main Observations 3-1
3.1 Comments from CCWater on ‘At Risk’ Registers 3-1
3.2 Company Understanding of the ‘At Risk’ Registers 3-3 3.2.1 Understanding of the Purpose and Issues 3-3 3.2.2 Understanding and Interpretation of the Nomenclature 3-4
4 Main Process Steps 4-1
4.1 Overview of the process 4-1
4.2 Developing the Register 4-1 4.2.1 On First Contact 4-1 4.2.2 During Clean-up Activity 4-2
4.3 Adding Properties to the Register 4-6 4.3.1 After Initial Investigation 4-6 4.3.2 After Detailed Investigation 4-9 4.3.3 During Investment Planning and Delivery 4-9 4.3.4 Addition of Properties Estimated as being at Risk 4-11
4.4 Keeping the Register Up to Date 4-11 4.4.1 The Link with Information Systems 4-11 4.4.2 Update Frequency 4-12 4.4.3 Recording Changes 4-12
4.5 Removing Properties from the Register 4-13 4.5.1 On First Contact 4-13 4.5.2 During the Clean-up Activity 4-13 4.5.3 After a long time without Flooding 4-13 4.5.4 After Company Action 4-13 4.5.5 As a Result of Better Information 4-13
5 Comments on the At Risk Register Approach 5-1
5.1 Register Name 5-1
5.2 Return Periods, Risk and Sub-registers 5-1
5.3 GSS Payments 5-1
5.4 Assigning Properties to the Register 5-2 5.4.1 First Time Flooding 5-2 5.4.2 Repeat Flooding 5-2
6 Recommended Process 6-1
7 Summary of Recommendations 7-1
7.1 Short-term recommendations 7-1
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
7.2 Longer term recommendations 7-2
Figure 4.1: Proportion of companies asking customers about the severity of flooding, if neighbours are
known to be flooded, and if it is raining. 4-2 Figure 4.2: Adding Additional properties 4-3 Figure 4.3: Numbers of companies having exclusion rules for internal and external flooding. 4-4 Figure 4.4: Approaches to GSS payments and definitions of severe and extreme weather 4-6 Figure 4.5: Choice of registers for first-time flooding is reported, and treatment of severe weather 4-8 Figure 4.6: Proportion of companies claiming late additions as outputs 4-10 Figure 6.7: Link between flowcharts 4 Figure 6.1: Best practice process flow chart – identification 6-1 Figure 6.2: Best practice process flow chart – initial investigation 6-3 Figure 6.3: Best practice process flow chart – GSS payments 6-5 Figure 6.4: Best practice process flow chart – Adding to the register 6-6 Figure 6.5: Best practice process flow chart – detailed investigation 6-8 Figure 6.6: Best practice process flow chart – removal from register 6-9
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
1-1
1 Introduction
Ofwat’s ‘At Risk Registers’ record properties that have flooded from sewers and are at risk of flooding
again. There are separate registers for internal and external flooding. Each register is split showing
the risk of flooding twice in ten years (2:10), once in ten years (1:10) and once in twenty years (1:20).
The 1:20 years register, generally used to record properties flooded for the first time, has been
introduced recently compared with the 2:10 years and 1:10 years registers that have been in place for
many years. Companies target investment to remove properties from the 2:10 years and 1:10 years
registers if there is a cost beneficial case for doing so or to mitigate the risk of flooding1 if not.
Investment is not normally targeted at properties on the 1:20 years register, but if properties suffer
repeat flooding, they are moved to one of the other registers and solutions to resolve the problem are
developed as appropriate. Properties flooded in severe weather (rare events) are recorded by the water
companies but do not go on to the registers.
The At Risk Registers are maintained by each of the ten water and sewerage companies in England
and Wales. Each year the companies report details of the numbers of properties and areas at risk of
sewer flooding, together with the number of incidents that year, to Ofwat through the annual June
return in tables 3 and 3a.
Two major outputs are monitored by Ofwat:
• The reduction in size of the 2:10 and 1:10 internal flooding register (the ‘net reduction’).
• The number of problems resolved.
A further measure is the number of properties subjected to flooding, which affects the wastewater
Overall Performance Assessment (OPA).
Ofwat publishes June Return Guidance which sets out information requirements and definitions of the
data required in each line of the June return tables. The manual aims to standardise reporting
definitions but it has become apparent, from comments by water companies, that information
requirements are interpreted differently by the various companies, and that these differences might
affect the number of properties added to the register.
To help understand the differences in approach and their effect on company At Risk Registers, Ofwat
commissioned Mott MacDonald to conduct a short study of each company’s approach to reporting.
The aims of the study were to:
• Identify and set out the various processes and procedures used to identify flooding, record it,
allocate properties and areas to the appropriate register and eventually remove properties from
the At Risk Registers as the risk of flooding is reduced by company action.
• Establish whether a flood would be allocated to the same At Risk Register irrespective of
which company’s methodology was used.
1 ‘Mitigate the risk of flooding’ means relatively minor investment that reduces the flood risk without providing a permanent
solution. Mitigation is appropriate where a permanent solution is too expensive to justify. It is also used to provide a
temporary solution to severe flooding when a permanent solution will take a long time to deliver. Mitigation does not
necessarily provide full protection from flooding and so properties protected by it are reported as a subset of the At Risk
Register.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
1-2
• Check alignment with reporting guidelines, identify the most common approach, and
recommend amendments to the guidance and/or processes to ensure consistency and promote
good practice.
• Establish the impact on the At Risk Registers if Ofwat adopts any recommendations arising
from the study.
The study took the form of a structured interview with each of the ten water and sewerage companies,
limited to a maximum of one day per company during June 2008. At this time most were compiling
June Return data and preparing business plans for their August 2008 submission to the 2009 periodic
review. Interviews were conducted in water company offices so that information systems and flood
records could be seen.
The authors would like to acknowledge the cooperation of the ten water and sewage companies in this
review, as well as the contribution of the Consumer Council for Water to the review.
It must be stressed that this was not an audit of company procedures. The field work was limited to
one visit per company and although companies presented procedures and individual records of sewer
flooding it was not possible to check that the procedures are followed ‘in the field’. It is also possible
that our understanding of company procedures, and hence our view on differences in approach, might
result from the different roles and knowledge of the people interviewed in different companies.
The discussion topics were developed as the main differences emerged between companies. Therefore
some items were not discussed at the earlier interviews. However, those items are mentioned where
more than one approach was seen, as this provides evidence of differences between companies.
Detailed quantification of the impact of differing approaches on the At Risk Registers was not
undertaken and in many cases would not be feasible. A general indication of whether different
approaches would have an upward or downward impact on the number of properties on the register is
made in the report where appropriate.
In this report, ‘flooding’ means sewer flooding, unless stated otherwise.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
2-1
2 Findings from Review
• All companies understand the At Risk Register and the nomenclature, but in matters of detail
there are some differences of interpretation between companies.
• All companies consider that they are adhering to the June Return reporting requirements, but
there are differences in interpretation of the Reporting Requirements.
• A flooding incident would not necessarily be allocated to the same register irrespective of
which company’s methodology was used. For example the availability and choice of data and
analytical technique influences the assignment of return period and hence the choice of
register. Since solutions are usually implemented for properties on the 2:10 years and 1:10
years registers, selection for a solution may depend on which company’s area the flooded
property is in.
• In most companies, the At Risk Register is compiled by automated reporting directly from job
management systems. In others, it is compiled as an off-line database or spreadsheet, using
more manual approaches to reporting from customer relationship management systems or job
management systems. Effective examples of both approaches were seen. On reliability of the
approaches, companies pointed out that both depend on the source data having a correct
classification of the incident. Hence all companies conduct manual checking of individual
records to check that the classification aligns with descriptions of the findings.
• Clarification of some aspects of the Reporting Requirements would help to improve the
comparability of company registers. Areas for consideration should include:
o The definition of, and approach to identifying, severe weather. Differences in
approach can affect whether flooded properties are added to the register, or excluded
on the grounds of severe weather. There are three aspects to this: clarifying the
definition of severe weather, strengthening the guidance on best practice assessment
of severe weather events, and ensuring that all companies apply best practice methods
of assessment and report using the same definition of severe weather.
o Clarifying the definition of internal flooding, especially for cellars and garages.
o Clarifying the approach to flood return period analysis and the addition of first-time
flooding to the At Risk Registers.
o Clarifying the approach to assessing repeat flooding, especially where some instances
are the result of severe weather and others are not.
o Clarify the approach for recording properties when flooding is discovered at a much
later date (through detailed investigations or anecdotal evidence from customers).
Current practice varies between companies as to whether such properties are added to
the register and count towards outputs.
o Separating the monitoring of properties that have mitigation measures installed, to
better reflect their true risk of flooding.
• There is agreement that the term ‘at risk register’ is misleading and ‘flooding incident
register’ or ‘unresolved flooding incident register’ would be a better description. There is also
support from some companies for the register to record the risk of individual properties
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
2-2
flooding (frequency x severity), compared with the current approach which only records
internal or external flooding in large frequency bands.
• There is general concern from companies about the potential impact of at risk registers on
home information packs (HIPs) and property values. The inclusion of a measure of flood
severity as well as frequency might help alleviate this, as it would enable property owners and
buyers to distinguish between large risks and small ones. It would also enable the benefits of
mitigation measures, as well as network enhancements, to be recognised.
• The approach to dealing with ‘severe weather’ if there is repeat flooding differs between
companies. In some cases, all events are counted once it has been established that a property
floods when the weather is not ‘severe’. In others, severe weather is always excluded. The
former approach is simpler for customers to understand and may better reflect the risk to the
property, while also increasing the frequency recorded for that property.
• Sewer flooding is identified from physical signs at the site, such as the location of flood water,
debris, movement of access covers etc. Customer reports and sometimes photographs also
provide evidence of the origin of the flood water.
• All companies check if neighbouring properties have flooded at the same time as a reported
flood, but the extent of the check varies between companies. The evidence suggests that more
rigorous searches during the initial investigation will identify the greatest number of flooded
properties.
• Flooding is attributed to overloaded sewers if no other cause can be found. Where hydraulic
overload is suspected, the majority of companies conduct CCTV surveys after the incident to
check that there are no hidden obstructions in the pipe. However, the subsequent removals of
properties from the register, on the grounds of better information, suggest that this is not a
precise science.
• Some companies continue to operate exclusion rules despite Ofwat having stated that no
incidents should be excluded (for example the time taken to clean up after external flooding,
or damp patches on cellar walls). Even where a company does not have a formal exclusion
rule, it is clear that at the time of the initial investigation a judgement is made as to whether to
conduct further investigation or to close the case, and hence on whether to report the flooding.
• Properties are removed from the At Risk Register when companies have taken action to
resolve the problem; when it is found that the problem is not associated with water company
assets; or sometimes if the property does not flood again for a specified length of time.
Properties may be transferred between registers or removed when better information about the
cause becomes available, as may happen during detailed network investigation after the flood.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
3-1
3 Summary of Main Observations
3.1 Comments from CCWater on ‘At Risk’ Registers
As part of the review, we met a representative of the Consumer Council for Water (CCW) to obtain its
views on companies’ procedures with respect to sewer flooding and ‘at risk’ registers from a
customer’s perspective. The views of several CCW regions were collated and are summarised below:
(i) Acceptability
CCW research confirms that sewer flooding is not acceptable to customers.
(ii) Customer reporting
There is a perception that customers’ concerns about potential property blight leads them to under-
report sewer flooding incidents. Therefore, CCW expects companies to pay special attention to
identifying flooded properties, so that robust solutions can be specified.
(iii) Internal flooding definition
CCW is concerned that some companies define some types of flooding such as toilet overflows,
flooding of garages with no internal doors and below-floor flooding as external flooding. CCW would
like clearer definitions to ensure that theses are classified as internal flooding events.
(iv) GSS payments
Some companies are going beyond the statutory minimum requirement for GSS payments, which
CCW welcomes. CCW also welcomes prompt payment, provided the company also pays due attention
to preventing flooding.
(v) Hydraulic overloading and risk assessment
CCW feels that severity and frequency might be better measures of flood risk than the nominal
internal/external and banded return-period registers (some companies also suggested this/agreed with
this comment).
The level of risk is not currently meaningful to customers and CCW feels that companies hide behind
the technical classification. It is difficult for customers to understand why some flood events count and
some do not.
(vi) Cost benefit analysis
CCW feels that cost benefit analysis should be used as a prioritisation tool rather than for the ultimate
selection/rejection of a property for a solution.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
3-2
(vii) Mitigation
CCW suggests that if the result from implementing mitigation measures for a property is a lower level
of flood risk than neighbouring properties, then perhaps it does not need to remain on the register.
Companies should count properties where sewer flooding was only prevented by customers’ own
mitigation actions.
(viii) Garages and sheds
CCW believes that if garages and sheds are in regular use and they are flooded, they should be
counted as internal flooding. There is concern that companies are counting too many garages as
external flooding.
(ix) Design standards
CCW notes that different companies have different design standards with respect to designing their
networks to prevent flooding and wonders whether this is this equitable.
(x) Communications
CCW supports regular communication with customers on the register (for example, some companies
send an annual letter). Communications need to be sensitive and show what is being done, with
delivery dates.
(xi) Information flow
CCW contends that there is generally poor information flow between local authorities and water
companies. Customers often report flooding to the wrong agency and information is not always passed
on to the right one
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
3-3
3.2 Company Understanding of the ‘At Risk’ Registers
3.2.1 Understanding of the Purpose and Issues
The At Risk Register enables Ofwat to monitor company performance in managing its network and
informs customers about network capacity issues. However, there were some comments on the
purpose of the register:
• The classes of risk (2:10, 1:10, and 1:20) may be considered as a measure of the need for a
solution, but in most companies, more sophisticated risk assessments or cost-benefit
assessments are now being used. In some companies the three frequency bands and the
differentiation for internal and external flooding are used for reporting purposes only. Some
companies advocate a more continuous risk scale based on an assessment of the frequency,
severity and scale of likely flooding, but not all companies would want to change from the
existing approach.
• The register has been considered a technical device used between the companies and the
regulators. Many companies stated that the introduction of Home Information Packs has made
house sellers and buyers more aware of flooding. They are concerned that reporting of 1:10
and 2:10 flooding through property searches might introduce housing blight. Companies feel
that this changes the emphasis on the At Risk Register and that great care should be exercised
in placing properties on the register. All companies said that they make the decision on what
properties to enter and they do not accept householder requests to be excluded from it, when
the company has evidence that the property is subject to flooding.
• Dealing with properties where mitigation reduces the risk but the properties remain on the
register. Companies generally agreed that there are benefits to maintaining records of which
properties have had mitigation measures installed. However there are situations where the
mitigation may reduce the flood return period to less than that of neighbouring properties
which are not on the register. For example, mitigation measures for a house on the 1:10
register may reduce the flood risk to 1:30, whereas the risk for neighbouring properties is
1:25. Yet searches by a prospective house buyer would show the house was at risk of flooding
– because it is on the 1:10 register – but not the neighbouring house. This could therefore
mislead a house buyer as to the true risk and also blight the value of the house.
• Although the severe weather exemption is a necessary acknowledgment of the design
limitations of the network, it can be confusing for customers, especially when their property is
flooded several times under differing weather conditions.
• Most companies acknowledge the differences in interpretation of the reporting requirements,
and would welcome more prescriptive guidance on reporting flooding.
• Adoption of private sewers: In the event that private sewers are transferred to the water
companies, reporting could become much more complicated. For example, there are currently
three frequency classes for both internal and external flooding, and if private sewer flooding is
reported separately this will result in a total of 12 register categories. This may reduce the
certainty of allocation and thus lead to more changes as better information is gathered during
the asset management process. Companies do, however, acknowledge the need to monitor the
impact of adopting private sewers on network management costs.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
3-4
3.2.2 Understanding and Interpretation of the Nomenclature
There was some debate about the meaning of At Risk Registers. Many companies pointed out that the
registers do not list all properties at risk of flooding, but rather are flooding incident registers, because
they list properties that have flooded. Several companies suggested that the term ‘at risk register’ is
misleading because customers may believe that their property is only at risk of flooding if it is listed
on the register. In practice, the register is mostly restricted to properties that have flooded and which
remain at risk of flooding in the future. Other properties may also be at risk of flooding due to
hydraulic overload as a result of changing demand or weather patterns, but a suitable trigger event
may not yet have happened.
‘Flooding Incident Register’ or ‘Unresolved flooding incident register’ may therefore be a better name
for the At Risk Register, although any description would need to explain that the properties are listed
because they remain at risk.
A further area of confusion relates to the term ‘sewer flooding’. Ofwat’s guidance defines a flooding
incident as an event of flooding from a public sewer, whether foul, combined or surface water. It
should perhaps be clearer that this definition excludes flooding incidents from sewers that are caused
by surface run-off, fluvial flooding, some types of highway drainage or other sources of flooding
where they are the responsibility of other agencies. Ofwat wishes to ensure that water customers only
pay for sewer flooding schemes that address problems arising from flooding that water companies are
clearly responsible for.
Despite this lack of clarity, companies do generally exclude flooding events where the cause is clearly
the responsibility of others, for example flood events arising from private sewers.
The problem arises where the cause of flooding is unclear at the point of flooding (perhaps because
there is a mix of overland flow and sewer surcharge) or the cause is remote from the point of flooding
(for example, where high river levels prevent downstream overflows from operating properly and
sewage backs up in the network). Companies’ assessments of such causes is generally limited to what
the clean-up team can observe at the flooding site, with limited follow-up analysis to confirm the
cause and identify the principal owner of the flooding problem. As companies adopt more holistic
solutions in partnership with other members of the drainage community, the identification and sharing
of flood risks between the responsible agencies will become more important.
Properties are not usually added to the register unless flooding has occurred, although one company
does add adjacent properties that it ‘assesses to be at risk of flooding’, even if they have not flooded at
the time of an incident being investigated.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
3-5
Conclusions and recommendations:
� ‘At risk register’ is confusing terminology and does not describe what the register is.
Although properties on the register remain at risk of further flooding, the ARR is not a
comprehensive list of properties at risk of flooding. Therefore ‘Flooding incident register’ or
‘Unresolved flooding incident register’ better describes it.
� The ‘at risk’ registers record frequency, but not severity of flooding, so they do not record
‘risk’ (probability x consequence). This can make it difficult to distinguish between small
internal flooding problems and large external flooding problems, for example. The impact
extends to home information packs (HIPs), where the presence of a property on a register can
have the same impact irrespective of its flooding ‘risk’, as it is conventionally understood. An
alterative approach that could be considered would be a measure of the frequency, severity
(depth, area) and location of flooding, rather than location and frequency alone. The result
would be a continuous risk scale that would more closely align with the cost:benefit
approaches now in use in most companies, although for reporting the risk would be grouped
into a few (e.g. five) categories.
� A risk-assessment approach also applies to mitigation, where interventions reduce the
frequency of flooding but do not eliminate the risk of flooding. Such properties currently
remain on the register after mitigation has been delivered (although they are reported as
having received mitigation). This can cause anomalies with neighbouring properties that are
not on the At Risk Register but have a greater flood risk than those with mitigation in place. A
risk measure could therefore include mitigation measures, to demonstrate the benefit that they
deliver.
� If properties on the registers had a full risk rating, it would enable a distinction to be made
between frequent small flooding incidents and infrequent large flooding incidents. It would
also enable the benefits of mitigation to be recognised, where a property would remain on the
register, but its risk rating would change. This would also reduce the potential impact of the
register on property value and recognise the efforts made by companies to manage flood risks,
when increasing the capacity of the network is not always cost-beneficial.
� Some companies already record flood severity information, (such as depth and extent of
flooding and duration) to inform their prioritization methods and flooding cost-benefit
assessments. Consistent methods of assessment would need to be agreed if this was taken
forward. We recommend that Ofwat considers reviewing these comments with the industry
and other stakeholders, to establish support for developing full risk registers, and what form
the measures should take. It would be important, for example, that the risk categorisation was
simple and easily understood by customers and property owners, and would require careful
consultation.
� We also recommend that a definition of sewer flooding is provided with a clear statement of
what it includes and what it excludes. This could also be the subject of consultation with the
industry and other organisations with responsibility for drainage. The aim would be to ensure
that all stakeholders in the management of flooding agree on a common definition and
understand the boundaries between water company responsibilities and their own. This could
help initiate or reinforce conversations that companies are already having with stakeholders in
the context of integrated urban drainage.
� Once a definition for sewer flooding is agreed, companies would need to review their
procedures for assessing and recording the cause of flooding and flood ownership. This is
likely to develop as integrated urban drainage becomes established as common practice.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-1
4 Main Process Steps
4.1 Overview of the process
This section reviews the main activities that create and manage the At Risk Register in four sections:
• Developing the register
• Adding properties to the register
• Keeping the register up to date
• Removing properties from the register.
There is an implication of a sequential process but in practice but the various activities are carried out as
they are required for each flooding case.
4.2 Developing the Register
4.2.1 On First Contact
All companies record all customer contacts through contacts centres and central call handling systems.
All companies attempt to identify the main causes of flooding and whether the problem is with a privately
owned sewer. In some companies the call centre staff can see electronic maps of the sewer networks and
this improves their ability to differentiate between company and private sewers. When flooding is caused
by private sewers, the customer is normally told that it is their responsibility, if it is clear that the sewer is
blocked or collapsed. Sometimes a problem with the public sewer – including hydraulic overload –
results in egress from a private sewer. Companies said that if there is any doubt about ownership or the
cause then they will visit the customer.
It is also common to ask customers if it is raining, the severity and location of the flood, and whether
neighbours are affected (see Figure 4.1). This helps in prioritising and allocating the best response. For
example, if a householder feels sure that no other properties are affected and it is not raining, then the
problem is more likely to be blockage or collapse, than if a house floods during heavy rain or several
properties are affected. This is not a definitive guide at this stage and in most companies is used to
inform the operational response rather than for full classification of the event.
Call handling software and the capability to forward information to field staff are variable. Although it is
possible that jobs might not be forwarded at this stage, we found no evidence of this during our visits to
companies. In some companies the call handlers have to learn the questions to ask customers, with scripts
or prompt sheets on their desks. In others ‘case based reasoning’ is used, in which the questions are
presented to the call handler on screen. Subsequent questions are presented by the software, dependent on
the preceding response. This decision-tree approach is highly consistent and results in a specific set of
actions appropriate to the reported incident. It appeared to be efficient and in some companies it is
directly linked to the job management system, GSS payment systems, and At Risk Register reporting.
The use of GIS maps linked to the call handling software allows cal centre staff to see the location of
recent calls, which helps in the early identification of wide area flooding.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-2
Figure 4.1: Proportion of companies asking customers about the severity of flooding, if neighbours are known to be flooded, and if it is raining.
Ask about severity of
flooding
Ask if neighbours affected Ask if it is raining
The uncertainties at the time of the initial call mean that most companies investigate more flooding than
their assets cause. Depending on circumstances they are probably also clearing some blockages of, and in
some cases carrying out minor repairs to, customers’ pipes.
At this stage in the process companies have an unverified list of reported floods and their nature. This is
the list which eventually becomes the basis of the at risk register, after investigation of the problem,
validation of the cause, and estimation of the likely frequency of flooding.
Best practice
� Case-based reasoning offers advantages over manually read or memorised scripts for consistent
and reliable evaluation of customer calls about sewer flooding, where establishing responsibility
for the problem is important.
� Visibility of electronic maps of the sewer networks improves call handlers’ ability to differentiate
between company and private sewers. By showing the location of recent calls it also helps to
identify emerging area-wide problems as calls come in.
� Simple questions about whether it is raining and are neighbours affected give early indication of
the nature of the problem.
� Direct linkage between the call handling software and the job management system and GSS
payment system is an efficient and reliable means of data handling.
4.2.2 During Clean-up Activity
All companies have targets for response times to investigate and clean up flooded properties. During the
visit the details recorded by the customer contact centre are reviewed to confirm that they are correct. All
companies provide for field staff to add comments and correct the details about the scale and location of
the flooding, and ownership of the assets. At this stage it is common to identify blockages and collapses
and to clear such problems immediately. Where no cause can be found it is assumed that the hydraulic
capacity of the sewer has been exceeded.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-3
In demonstrating their quality control procedures, some companies showed examples where field staff
had classified flooding as hydraulic overload, and subsequently noted the removal from the pipe of an
obstruction such as a brick. It might be that the obstruction was seen as resulting in reduced capacity and
hence hydraulic overload or that staff felt under pressure to attribute the problem to underlying capacity
rather than operational problems.
All companies have a quality control step after the initial investigation, in which all entries are reviewed
and their classification is checked. In the example above, the classification would be changed to blockage
or collapse, depending on what other information was available about the source of the brick.
The extent to which neighbouring properties are checked appears to vary significantly between
companies. In some cases leaflets are left with neighbours but in others neighbouring properties are only
checked if the clean-up staff can see evidence that they have flooded. This latter practice is likely to lead
to fewer properties being reported as flooded at this stage, with a potential benefit to companies’ OPA
measures.
Figure 4.2: Adding Additional properties
Search for Neighbouring
Properties
ExtensiveDepending on evidenceMinimal
Addition of Neighbouring
Properties
To current job To new job
All companies were able to show examples of how additional properties are added to their database
following initial clean-up. Some companies add them to special fields in the same job as the original
flood, others create new entries specifically for neighbouring properties. One company mentioned
anecdotal evidence that adding neighbouring properties to the original job requires less work and is
therefore more reliably implemented.
(i) Exclusion Rules
Ofwat’s reporting requirements state that all flooding must be recorded, and all companies do record
details of all incoming calls about flooding. When deciding whether to record flooding, some companies
operate exclusion rules, including a description of ‘surcharge’ if external flooding can be cleaned up
within 15 minutes and the exclusion of ‘weeping manholes not causing a nuisance’. Some companies
reported difficulty in deciding how to handle damp patches on cellar walls where there is insufficient
water to take a sample, and many will not record these as flooding until a sample can be taken.
In practice most companies are likely to use judgement in deciding whether to record a flood, especially
during heavy rain when surface water flooding is prevalent. If all discharges, however small, are reported
then the number of external floods is likely to rise significantly.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-4
The June Return Reporting Requirements already state that all flooding incidents should be recorded and
exclusion rules do not apply. The guidance lacks clarity in its definition of sewer flooding (as described
earlier) and although companies check that the problem arose from a public sewer not a private sewer,
checks on root causes and ownership of the flooding problem are variable.
There are also differences in the interpretation of ‘internal flooding’. Two companies consider garages to
be external if they do not have an internal door connecting them to the property, even when they are
integral to the building’s construction (see Figure 4.3). We consider this to be a misinterpretation of the
reporting guidelines – it is certainly not a common interpretation – which reduces the number of internal
flooding incidents being reported. Although garages tend to have low door sills and therefore be more
likely to flood than other parts of a building, we consider this is unlikely to make a big difference to the
size of register. Classifying integral garage flooding as external might make it more difficult to justify
investment for those properties where they flood.
The June Return Reporting Requirement provides a definition of internal flooding but it appears that two
companies consider internal garages with no connecting door to be external. The guidelines could be
modified to state: “However, garages forming an integral part of a property are classed as part of the
building, even where there is no internal door to the house, and are included, even if their prime purpose
is storage, etc.”
Some companies noted that customers often consider flooding of garages and cellars to be internal
flooding. It is therefore possible that the current June Return definition does not align with customer
preferences. This could be checked by inclusion in future customer perception or stated preference
surveys.
Figure 4.3: Numbers of companies having exclusion rules for internal and external flooding.
Exclusion Rules for External
Flooding
0
2
4
6
8
10
Overw helmed by w ater course
Time to clean
None
Exclusion Rules for Internal
Flooding
0
2
4
6
8
No
internal
door in
garage
Damp
patch on
cellar wall
None
No internal door in garage
Damp patch on cellar w all
None
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-5
Best practice
� All flooding incidents should be recorded, as required by the guidance.
� Adoption of robust quality control procedures is essential to check that the call-centre details are
correct and to check for anomalies in work-order entries that might imply incorrect identification
of the cause of flooding.
� Prioritising the early capture (generally within two weeks of an event occurring) of information
about the number of neighbouring properties affected and other data on the causes and severity of
flooding is important to prevent loss of data. Information captured long after flooding has taken
place is likely to be more anecdotal and less certain of being correct.
� The allocation of flooded neighbouring properties to the same work order as the original flood
ensures a better data audit trail. This may also be achieved by a database link to the original work
order.
� Identification of the root causes and ownership of flooding problems is necessary to establish
responsibility for resolving them. With the exception of identifying ownership of private sewers,
practice in this area is variable.
� Flooding of garages should be categorised as internal flooding where the garage is an integral part
of the building, even if there is no internal door. This aligns better with customers’ perceptions of
internal flooding.
(ii) Link to GSS payments and the Estimation of Flood Return period
Some companies allocate GSS payments regardless of the flood return period, except in cases of wide
area flooding where it is obvious that extreme weather is likely to have contributed. Such companies
make more GSS payments than they are required to. Companies cited the difficulty in obtaining reliable
return period data and benefits of showing goodwill in a difficult situation as the main reasons for this
behaviour.
Some company areas are covered by high resolution rainfall radar and in these areas companies are able
to identify severe weather more quickly, therefore reducing the number of GSS payments. One company
fully models the sewer event return period, even though this might mean missing the GSS payment
deadline and having to make an additional penalty payment. Another company purchases weather
analysis where it considers that unusually high rainfall might have happened, this approach balancing the
cost of acquiring weather data and the cost of overpaying GSS. Anther calculates the return period of
rainfall in each 1km grid square evry night, so return period analysis is available the following day.
In addition to its effect on GSS payments, the assessment of flood return periods determines the register
to which the flood will be allocated. The approach to assessing flood return period varies and this leads to
differences in the allocation of properties to registers (see Figure 4.4.). Furthermore there is some
confusion between the definition of ‘extreme weather’ in the GSS guidelines and ‘severe weather’ in the
June Return reporting requirements for At Risk Registers.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-6
Figure 4.4: Approaches to GSS payments and definitions of severe and extreme weather
Definition of Exceptional
Weather
1:10 yr storm 1:20 yr storm
1:40 yr storm Not know n
Definition of Severe Weather
1:10 yr storm
1:20 yr storm
1:40 yr storm
GSS and severe weather
Normally paid before w eather data av ailable
Get w eather data if sev ere w eather suspected
Alw ay s get w eather data first
Note: “Not known” means that the subject was not discussed with company, or the contact for the study was not involved in the GSS process and was not certain of the company’ approach.
We understand that Ofwat is currently consulting on the definition of exceptional weather for use with
GSS regulations. We recommend that when this is complete Ofwat clarifies the severe weather and
exceptional weather definitions. This would help to ensure that service is described in the same terms for
all customers, regardless of where they live. There might also be potential to align GSS definitions and
entry of properties on the ‘at risk’ registers.
4.3 Adding Properties to the Register
4.3.1 After Initial Investigation
In most companies the initial investigation is carried out at the time of clean-up. It is common for minor
repairs to be carried out at the same time, for example clearing roots or repairing collapsed pipes. Most
companies conduct an initial investigation within a fortnight of the event occurring, except in extreme
cases of wide area flooding.
When the clean-up and initial investigation is complete, companies have a database of reported flooding
events, with follow-up details that confirm which appear to result from hydraulic overload, and which are
from other causes. In many companies this subset of the customer contact or job management database
forms the basis of the At Risk Register, although at this time the return period of the sewer event might
not have been estimated. In some companies the relevant records are extracted and added to a separate
flooding register, in a spreadsheet or local database. Companies having integrated information systems
generally referred to a ‘live’ register, where those with off-line approaches tended to update their registers
monthly or annually.
The definition of ‘severe weather’ is a source of confusion and there are many approaches to determining
if the flooding has resulted from severe weather, in part because of differences in the source data that are
available in different regions. The issues are:
• The determination of flood return periods is complex and requires detailed network data and
modelling.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-7
• Some companies use rainfall return period as a surrogate for flood return period, despite specific
guidance from Ofwat that ‘DG5 measures the frequency of flooding incidents and not the return
period of the storm that causes the flooding’.
• Some companies use the interval between repeat floods as a surrogate for flood return period.
• Some companies use weather radar data, but radar coverage is variable and not universally
available. (Water companies do not operate weather radar systems.)
• Some companies use Environment Agency local rain gauge data, which may also be combined
with data from other sources, but rain gauges are not necessarily near enough to the event being
investigated to give confidence in the results.
• There is some evidence that even Met Office analysts differ on the return periods of individual
rainfall events.
The analysis of return periods is also complicated by the effects of climate change, which means that a
weather event with a 1 in 20 return period today may have a shorter return period (i.e. be more frequent)
in the future. In addition, urban creep has led to a greater proportion of rainfall entering the drainage
system, exacerbating the effects of climate change. Simple methods of analysis are likely to miss these
effects and events assessed as ‘severe’ based on today’s data may become the norm in the future,
increasing the number of emerging flooding incidents. If the ‘at risk’ registers are to be comparable
between companies and geographical areas, a consistent approach is required. That consistency does not
exist at present.
At this stage the list will be classified for internal or external flooding, severity, and an initial assessment
of the return period. It forms the basis for reporting in the At Risk Register although it will be developed
over time as more information becomes available.
At this stage two issues influence the choice of register and lead to differences between companies: the
approach to the assessment of return period and the extent to which repeat flooding takes account of
severe weather.
• The true return period of a flood is not generally evident from a single event. Companies
typically use either the estimated return period of the rainfall event as a surrogate for the flood
return period, or add the property to the 1:20 register. The former approach allows properties to
be added directly to the 2:10 register if the rainfall return period is estimated as less than 2:10
years and tends to increase the relative size of the 2:10 and 1:10 registers. Most companies will
take account of customer statements that previous flooding has not been reported, which may lead
to the property being entered to the 2:10 or 1:10 register instead of the 1:20. One company builds
hydraulic network models to assess the return period of every internal flood event, and chooses
the register on the result of that modelling rather than the rainfall return period.
• The treatment of severe weather in repeat flooding events differs between companies. Flooding
events arising from severe weather are normally excluded from water company assessments, but
this can cause confusion and dissatisfaction among customers for whom flooding is frequent and
also occurs under ‘normal’ weather conditions. One company includes repeat flooding under
‘severe weather’ conditions in its frequency assessment, once it knows that the property has
flooded under ‘normal’ conditions. Another will retrospectively include previous flooding that
happened under severe weather conditions, if a repeat flooding occurs under normal conditions.
Both these approaches probably better reflect the actual flood risk, but they also increase the
reported flood frequency and hence change the register choice, compared with the companies that
do not adopt this approach.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-8
The number of companies adopting each approach to the two issues is illustrated in Figure 4.5
Figure 4.5: Choice of registers for first-time flooding is reported, and treatment of severe weather
Choice of Register on First Time
Flooding
0
2
4
6
1:2
0
un
less
Ra
infa
ll
retu
rn
1:20 unless evidence of previous f looding
1:20 register
Rainfall return period
Modelled
Severe Weather and Choice of
Register
0
2
4
6
8
10
Severe
weath
er
is
Doe
s n
ot
co
unt
seve
re
Severe w eather is counted in repeats
Severe w eather counted w hen previous
f looding under normal conditionDoes not count severe w eather at all
Best practice
� A rapid GSS payment to sewer flooding victims is commended, and whilst the difficulties of
analysing extreme weather events are acknowledged, payments should be made as quickly as
practicable.
� A range of techniques is applied to the assessment of the return period for sewer flooding
incidents. No one method stands out as best practice, as the type and availability of data varies
between companies. There is also inconsistency in the way that climate change and urban creep
(a gradual increase in drained area) are accounted for in the assessments. In the absence of a
consistent approach or clear best practice for identifying return periods, we recommend that
properties should be added to the 1:20 register on first flooding, unless there is credible evidence
that previous flooding has occurred but not been reported. Allocation to 1:10 or 2:10 registers
should be on the basis of observed flood frequency. We recommend that the assessment of return
periods should be subject of an industry-wide study to share information and establish consistent
yet workable methodologies which companies can apply in practice in their areas. This will
enable a consistent approach for allocation to be developed, when companies should then use the
method to assign properties to registers. The benefit will be to ensure that severe weather is
identified consistently, and that the selection of registers for repeat flooding is consistent between
companies.
� We recommend clarification of the meaning and use of ‘severe weather’ and ‘exceptional
weather’. This would enable service to be described in the same terms for all customers,
regardless of where they live.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-9
4.3.2 After Detailed Investigation
The approach to detailed investigation differs between companies and may be formed of one or several
steps. Where hydraulic overload is suspected, it is common to carry out more detailed review and
analysis of the network, including camera surveys and hydraulic modelling. This is commonly done to
determine the most appropriate long-term solution to the problem and leads into capital programming.
Companies reported issues with the use of cost-benefit analysis at this stage. In some cases it is difficult
to justify capital intervention on cost-benefit terms and yet the companies believe they will be subject to
criticism or legal action if a solution is not implemented. Many companies reported that some schemes
are not truly cost-beneficial but that they believe they have a moral obligation to deliver a level of service.
If the level of service is described as a social choice rather than an economic case, it is likely that benefits
have not been expressed at their true value. Some companies commented that this valuation is especially
difficult where willingness to pay is taken into account, as customers do not always value the service in
line with its costs. On a straightforward economic assessment this could lead to an increase in unresolved
sewer flooding until customers valued the service sufficiently to pay for it, but those companies
discussing the subject felt that would be the wrong approach.
The detailed investigation step identifies errors in classification and leads to updating of the At Risk
Register categories, typically reducing the number of floods attributed to hydraulic overload. The
changes result from wider surveys of the network identifying blockages and collapses, and from hydraulic
modelling showing adequate capacity. Some companies carry out more detailed rainfall investigation at
this stage, leading to further exclusions where severe weather caused the hydraulic overload.
Some companies carry out additional investigation of neighbouring properties at this stage (although most
do this either with the initial investigation or with the investment delivery). Searching for additional
flooded properties often results in new additions to the register. Hence the timing of this investigation is
important: if it is carried out early in the process it may affect OPA measures, but at the same time it
improves the prioritisation of the register for investment and increases the benefit in cost-benefit
assessment.
4.3.3 During Investment Planning and Delivery
All companies carry out more detailed assessment in the design of capital solutions to flooding. The
scope of this step depends on the extent of previous investigational steps. At this stage companies
generally check for additional flooded properties in the neighbourhood, and those that have not done a
substantial early search typically find between 20% and 60% additional properties.
During our visits to companies, some commented that flooding events were becoming more localised,
with only a few properties affected each time, rather than the larger groups benefiting from previous big
schemes. This is consistent with a mature flood risk register, where companies have been prioritising the
bigger problems for some years. We have not collected information on the number of properties per
event, so cannot comment on whether this applies to all companies.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-10
If companies are experiencing a diminishing number of properties/event, this makes it more difficult to
make a cost-beneficial case for increasing the capacity of the network and more likely that mitigation
measures will be proposed instead. Mitigation solutions might be effective for less than their design life
if the underlying problem results from gradually increasing drained area or climate change, because
increasing flow may render mitigation ineffective. In such situations more strategic investment would be
indicated in order to prevent the problem returning and possibly affecting more properties. Risk
assessments and the associated cost-benefit analysis therefore need to take account of the rate of change
of ‘demand’ so that appropriate long-term solutions are selected.
The scope of this project did not include a review of company procedures for designing network
investment and hence we are unable to comment on .examples of best practice.
Any additional properties identified during pre-design investigations may be added to the At Risk
Register in the year of delivery of the solution – the same year in which they are removed. They add to
the number of properties removed from the register as a result of company action, but do not support the
output of net reduction in the At Risk Register. One company notes such additions separately and does
not claim them in its outputs (although they would be included in the cost-benefit assessment), preferring
to measure outputs in line with the original quantities assumed in its business plan (see Figure 4.6).
The addition or removal of large numbers of properties from the register at this stage implies a poor
quality of initial investigation and reduces confidence in any year’s register values.
Reporting progress of the properties originally on the register – i.e. not allowing late additions to be
counted within the company’s outputs – might provide a more balanced incentive for better investigation
and addition of all flooded properties at the time of a flooding incident. But it might also encourage
companies to add too many at the initial stage and lead to large numbers of removals through better
information later in the process. Hence we recommend reviewing the links to other incentive mechanisms
before changing the reporting requirements.
Figure 4.6: Proportion of companies claiming late additions as outputs
Adding Neighbouring
Properties
Claim outputs
Don't claim outputs
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-11
4.3.4 Addition of Properties Estimated as being at Risk
Most companies only add properties to the register when flooding has been experienced. One company
adds nearby properties if on investigating a flood, it considers they are at risk of flooding in the future,
even if they have not flooded at the time.
Best practice
� Detailed investigations, including CCTV and network modelling are essential to confirm the
initial investigation, assess the root cause of flooding and flood ownership and to inform
assessment of the flood return period. This needs to be done as soon as possible after the flood, to
ensure that all relevant information is captured whie it is available. Companies having good
coverage of their sewer network with verified models are in a good position to undertake this
quickly.
� The use of cost benefit analysis to justify expenditure on network enhancements to resolve
flooding problems is a relatively new requirement and some companies are questioning it. We
recommend a review of how companies have applied cost benefit analysis to sewer flooding
problems following PR09 to share experience and best practice. This should extend to the use of
CBA for mitigation schemes. If flood severity is introduced, (see above), then the value of
mitigation in reducing flooding risks will be more visible.
� Some companies are reporting a diminishing trend in properties per flood event. It would be
helpful if more facts were available on this to assess if it is an industry-wide experience. Late
additions should be included in the analysis.
� In assessing the benefits of a scheme to enhance network capacity to resolve flooding problems,
account should be taken of the effect of climate change and urban creep on future numbers of
properties affected should nothing be done. It is not clear that this is being done consistently and
thus benefits may be under-stated.
� The addition or removal of large numbers of properties from the register at the design stage
implies a poor quality of initial investigation and reduces confidence in any year’s register values.
� One company notes late additions at the design stage separately and does not claim them in its
outputs (although they are included in its cost-benefit assessment), preferring to measure outputs
in line with the original quantities assumed in its business plan. Extending this practice to the rest
of the industry might provide a more balanced incentive for better investigation and addition of
all flooded properties at the time of a flooding incident, when confidence in the data is more
assured.
4.4 Keeping the Register Up to Date
4.4.1 The Link with Information Systems
The quality and degree of integration of information systems varies significantly between companies. In
some companies the information systems are highly integrated and the current status of the At Risk
Register may be interrogated at any time. In others the systems are not integrated and the register is
compiled off-line.
We observed that companies with well integrated information systems tended to have most confidence in
demonstrating their At Risk Register process, and that the results would compare with those in the June
Return, although some companies with off-line processes also had good data management processes and
archives of the registers. In the companies with integrated systems, the At Risk Register was seen as a
simple report from the database, rather than a separate entity.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-12
There did not appear to be a link between the approach to information management and the number of
‘better information’ changes to the register entries over time. This suggests that these changes are a
function of the quality of the investigation processes rather than the approach to data management.
4.4.2 Update Frequency
Companies with integrated information systems can report the status of individual properties on the
register at any time during the reporting year, although most companies review the status of flooding on a
monthly basis.
Companies without integrated information systems review new flooding that occurs in each month
through internal reports, but the regulatory At Risk Register may be compiled on an annual basis.
Companies generally aim to settle on a confident register allocation by the June Return reporting deadline
for the year, but achieving this depends on the number of floods that occur in the last month of the
reporting year. However, the extent of additions to, and removals from, the registers as a result of ‘better
information’ is significant in some companies, suggesting low confidence in the reported allocations in
those companies. It would be expected that company confidence grades for the At Risk Register reports
should reflect the proportion of retrospective changes that companies are making to their registers.
4.4.3 Recording Changes
Most companies retain a full audit trail of the changes that are made to records of flooding and the
progress of an individual property from entry to the register to ultimate removal. Companies having
information systems that do not allow the full data history to be retained stated they were developing new
systems that will retain a full audit trail.
Best practice
� Integrated information systems provide greater confidence that data has not been lost and is not
subject to human error from manual handling of data between off-line storage systems and
applications and corporate systems.
� The status of the ‘at risk’ registers should be reportable at any time and updated on a monthly
basis at least. A full audit trail should be available for properties added and removed from the
register.
� The extent of additions to and removals from the registers as a result of ‘better information’ is
significant for some companies. It might be expected that the confidence grades for the ‘at risk
register’ reports for these companies would reflect this.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
4-13
4.5 Removing Properties from the Register
4.5.1 On First Contact
If it is clear during the initial customer call that the flooding has arisen on a private pipe, as a result of a
problem with that pipe, then the property will not be entered on the At Risk Register.
4.5.2 During the Clean-up Activity
During the initial visit and clean-up, the causes other than hydraulic overload are commonly identified.
These will be entered on the job record and the property will not be entered onto the At Risk Register.
4.5.3 After a long time without Flooding
The Flood Estimation Handbook provides for removal of properties from the At Risk Register if they
have not flooded for a pre-determined time. All companies were aware of the ‘time-out rule’ but it is not
in widespread use. Its most common use is in companies where properties assessed as being at risk were
added to the register, but have not flooded, and one company uses a shorter approach than the normal
‘95% confidence’ duration. Although there are differences in approach, relatively few properties are
removed from At Risk Registers using the time-out rule.
4.5.4 After Company Action
The majority of properties are removed from At Risk Registers as a result of company action to improve
network capacity.
4.5.5 As a Result of Better Information
All companies remove some properties from the At Risk Register as a result of better information about
the cause of flooding. In some cases this information only becomes available during preparation of
capital solutions. Where large numbers of properties are removed through better information, it may be
inferred that early investigations are insufficient to provide confidence in the entry onto the register.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
5-1
5 Comments on the At Risk Register Approach
Companies were invited to make their own suggestions of ways to improve the At Risk Registers.
5.1 Register Name
The term “At Risk Register” may be associated with other flood risk assessments – e.g. in surface
water management plans, when the issue is a risk of flooding in extreme weather conditions such as
1:100 storms, during which the underground system is effectively irrelevant.
A better title for the ARR might be “Flooding Incident Register” or “Unresolved Flooding Incident
Register”; the description would state that it’s a register of properties that have flooded, and which are
listed because they are at risk of flooding again if the circumstances are repeated.
5.2 Return Periods, Risk and Sub-registers
Some companies commented that the current division of the register is not particularly useful. For
example, there might be an overlap between customer perception of minor internal flooding (e.g.
water coming up in a shower tray but not leaving the shower tray) and severe external flooding.
Likewise the return period is hard to determine accurately and the exclusion of a flooding event for
severe weather for a property that has already flooded in the past under ‘normal’ conditions is
distressing for some customers.
An alternative suggested by some companies would be to list properties according to a measure of
observed risk – i.e. the frequency and severity. This would then be reported in categories such as high,
medium, and low risk. The definitions for property owners would, for example, be that a ‘high risk’
would be liable to flooding that causes significant disruption, on a regular basis. Other companies
disagreed with that approach, suggesting that ‘high’ would effectively be the same as internal 2:10.
When sewer flooding is identified, an emergency job is started. Most companies use field service
contractors and if internal flooding is being investigated the company’s sewerage manager or
supervisor will also attend. Companies stated that this demonstrates ownership to the customer. It was
also mentioned that the company manager provides a point of contact for the customer after the clean-
up is finished.
All companies check the extent of flooding at this stage, and all said that for internal flooding they
knock on neighbours’ doors if they believe the flooding might affect those properties.
Some companies have exclusion rules, which provide guidance for staff on what to class as flooding.
These rules apply to external flooding and include ‘puddle size’, ‘puddle depth’, ‘time to clean up’.
Since Ofwat amended its reporting guidelines to include ‘any escape from sewers’, most companies
have discontinued these exclusions. However, where there is no exclusion rule, companies agreed that
staff have to use their discretion at the time of the visit. The extent of this discretion is likely to
influence the number of external flooding incidents that are reported.
5.3 GSS Payments
All companies reported that except in the case of wide area flooding, it is generally difficult to identify
the flooding return period at the time of the customer call or initial investigation. Therefore GSS
payments tend to be authorised as soon as the site visit confirms that the flooding has taken place.
This means that companies are often paying more in GSS payments than they would have to if they
were able to make a strict application of the rules. Where a strict application of the rules is applied,
companies find it difficult to complete within the 20 day limit.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
5-2
5.4 Assigning Properties to the Register
Companies do not all follow the same approach to assigning properties to registers and this affects the
number that are added to each register. Some of the main differences are described below:
5.4.1 First Time Flooding
Ofwat’s June return reporting guidelines state:
“When a previously unreported property or external area is flooded, it should normally be considered
to be at risk and added to the 1 in 20 category unless:
- investigation clearly shows that it is at risk of flooding more frequently than once in ten
years, when it should be included in the one in ten year category;
- investigation clearly shows that it is at risk of flooding more frequently than twice in ten
years, when it should be included in the twice in ten year category;
- the storm was exceptionally severe and investigation shows that it is clearly not at risk of
flooding as frequently as once in twenty years and the severity of the storm can be
verified (e.g. by the Meteorological Office); or
- the cause was a blockage, etc”
The most common approach used by companies is to add first-time flooding to the 1:10 register, but
one company adds them to the 1:20 register, and one uses the rainfall return period to determine the
register. Another conducts a full hydraulic analysis of the local network response to rainfall and thus
derives a modelled return period for the flood event. These differences affect the proportion of
properties on different registers rather than the total number reported. The use of rainfall data leads to
many additions directly to the 2:10 register. Using rainfall data alone will add more properties directly
to the 2 in 10 register simply because there are many more rainfall events at 2 in 10 year return period
than 1 in 10 year return period. However the effect on the sewer network should be considered when
allocating risk. It is not clear why a sewer network would become overloaded from a 2 in 10 year
return period storm.
5.4.2 Repeat Flooding
All companies review a property’s position on the register when a repeat flooding incident occurs.
The period between flooding incidents is frequently used to determine if a property should be moved
to a ‘higher’ risk category.
Some companies mentioned that it is not clear how they should handle multiple flooding when some
instances are caused by severe weather. Some companies ignore the severity of the weather once it
has been identified that flooding occurs under normal conditions. In other companies, repeat flooding
is not counted if it is the result of severe weather. Companies commented that the latter approach is
difficult to explain to customers and can lead to an underestimate of the risk of flooding.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
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6 Recommended Process
We set out the approaches approach to managing DG5 registers using a common generic
process flow-chart that allows comparisons to be made. From our assessment of company
processes and taking a comparative view, we have suggested a best practice approach in
Figures 6 to 11. The six flow-charts cover the following areas of practice:
• Identification of a flooding event from first contact with the customer.
• Initial investigation of a flooding event, establishing the cause and the number of
properties affected.
• Making decisions on GSS payments.
• Adding properties to the ‘at risk’ registers.
• Undertaking detailed investigations to inform the design of capital solutions.
• Processes for removing properties from the registers.
We have categorised processes as preferred (green), non-preferred (grey) and optional
(yellow). The latter applies particularly to the assessment of flood-event return period, where
a number of techniques are used according to the availability of local data and weather
analysis. We believe that this is an area of developing practice that is worthy of industry-wide
study to share information and establish consistent yet workable methodologies that
companies can apply in practice in their areas.
Commentary on the recommended processes and the potential impact of their adoption on the
‘at risk’ registers is given after each figure.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
6-2
Figure 6.7: Link between flowcharts
Chart B: Initial InvestigationChart B: Initial Investigation
Chart D: Adding to RegisterChart D: Adding to Register
Chart E: Detailed InvestigationChart E: Detailed Investigation
Chart F: Removal from RegisterChart F: Removal from Register
Chart A: IdentificationChart A: Identification
Chart C: GSSChart C: GSS
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
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Figure 6.1: Best practice process flow chart – identification
Ask about severity of flooding
Can see asset register
Automatic creation of work order Manual creation of work order
Chart A: Identification
Dispatch investigation/clean-up team
Ask about location of flooding
Can see GIS maps
Ask if neighbours affected
Check if it is raining
Check if private sewer
Manual Scripting
Call Handling
Record details of all calls
No
YesEnd
Check if repeat
flooding
Preferred process
Optional processes
Non-preferred process
At same time as first contactTelephone customer to confirm visit time
Case Based Reasoning
Legend
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
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Principal points:
• Case-based reasoning preferred to manual scripts.
• Access to GIS and asset register helps call-handlers identify private sewers.
• Simple questions establish nature and extent of problem early.
• Automatic set-up of work-order using integrated IT preferred to manual methods.
• Inform customer of expected time of arrival of clean-up team on first contact.
Impact on ARRs:
• Limited direct impact, but integrated systems likely to lead to fewer data losses and
improved audit trail for subsequent processes.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
6-5
Figure 6.2: Best practice process flow chart – initial investigation
Telephone customer to confirm visit time
Clean up and repair damage
Supervisor or
engineer attends
as well as field contractors
Identify cause
Complete Work Order (WO)
CCTV
Leave forms if unoccupied
Exclusions
(none)
Update WMS Update via paper form
Write to customer
Pay GSS2 2 = For companies who pay GSS without reference to severe weather analysis
Check neighbours only if evidence of flooding
Confirm not private sewer
Internal External
Blockage/Collapse Hydraulic Overload Equipment failure
Exclude
surcharges
Exclude non-
sewer flooding
Exclude non-integral
garages
Exclude1 damp
patches (internal)
Additional properties added to same WOAdditional properties added to new WO
Validate WO entries
Check all neighbours
potentially flooded
EndEnd
Unless already done on first contact
1= Where samples cannot be taken and patches may arise
from groundwater
Chart B: Initial investigation
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
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Principal points:
• Supervisor attends as well as clean-up team/contractors to ensure that procedures are
followed.
• CCTV undertaken for all flooding events unless information already available.
• Exclude non-sewer flooding (eg surface-water flooding) where responsibility for resolving
the problem lies wholly with others.
• Exclude damp patches if samples cannot be taken and cause could be groundwater.
• Undertake detailed investigation of neighbouring properties affected, including those in the
area that are unoccupied. The intention is to capture and report this information once and it is
not expected to be amended months later through better information.
• Add any properties subsequently confirmed to have been affected to the same work order
record to ensure clean data audit.
• Validate work-order entries through robust quality control procedures and update digital
records (not via paper forms).
• Make GSS payments where they apply, unless it is an exceptional wide-area flooding event
clearly resulting from severe weather.
• Inform customer in writing of the findings from the initial investigation and the company’s
proposed actions.
Impact on ARRs:
• Reporting all sewer-flooding incidents may increase the number of properties on the
registers for those companies applying exclusion rules in the past.
• Emphasising the need to capture information on properties affected at the initial
investigation stage should reduce the number of late additions through better information and
provide more confidence that the ARRs are correct and up to date.
• Validating work-order entries through quality control procedures and application of
integrated IT will ensure improved data audit trails. This should be reflected in improved
data confidence grades.
• Early payment of GSS payments is encouraged as a goodwill gesture, accepting that some
customers may be ‘over-paid’ by the strict interpretation of the extreme-weather rules. Some
companies already do this.
• It is only proper that companies write to customers to inform them of the findings from the
initial investigation and the company’s proposed actions to reassure them that their problem
is being addressed. It is acknowledged that this may raise expectations that a capital solution
or mitigation measure will be delivered when the company may not always be able to make a
cost-beneficial case for doing so.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
6-7
Figure 6.3: Best practice process flow chart – GSS payments
Flood confirmed by field team
Chart C: GSS
Check if ‘extreme’
weather
Automatic ID GSS Manual ID GSS
Confirm GSS payment
Check GSS paid (audit)
Write to customer confirming actions
High resolution radar data
Weather
Analysis
Met Office data
Pay if < 1:20
Pay if < 1:40
Pay if < 1:10
Aligned with ‘severe’ weather
Principal points:
• Automatic identification of customers potentially eligible for GSS payments linked to the
flooding incident report and work management systems ensures that eligible customers are
not missed, which is a risk with manual systems.
• A range of techniques for estimating the return period of the flooding event can be applied,
according to available data. Best practice in this area needs to be defined through separate
study.
• The criterion for ‘extreme’ weather should be reviewed to consider alignment with that for
‘severe’ weather used to assess the inclusion of properties on the ‘at risk’ register. The
criterion proposed is that adopted by most companies, which is 1 in 20 years return period.
GSS payments do not apply if the flooding event is more extreme than this. Companies that
have adopted a higher standard than this in the past (e.g. 1 in 40 years) may opt to continue
to apply this to GSS payments (though not to the ARR, to ensure national consistency).
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
6-8
Impact on ARRs:
• Clarification of the severe weather definition to rarer than 1 in 20 years will affect the
number of properties on registers for those companies that have adopted a different standard
in the past.
Figure 6.4: Best practice process flow chart – Adding to the register
Does customer report previous flooding?Ignore
Rainfall intensity
determines register
Consistent rules
determine register
Event specific network analysis
determines register
Interval
determines register
Rainfall
intensity determines
register
Severe weather is counted
in repeats
Severe weather not counted
in repeats
Keep record of severe
weather floodingAdd neighbouring properties assessed as being “at risk”
NO
YES
Repeat
flooding
Accept
Check if severe weather
Anecdotal evidence from
customers flooded in the past informs
register
Model output for standard
storm events informs register
Anecdotal
evidence from customers
flooded in the
past informs register
Model output
for standard storm events
informs
register
Event specific
network analysis determine register
1:20 1:40
First time
flooding
If confirmed by
company records
If not
confirmed
From results of initial investigations
Add to register
2:10 1:10 1:20
Chart D: Adding to Register
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
6-9
Principal points:
• A >1 in 20 year return period is used to check for severe weather, in accordance with
Ofwat’s guidance.
• Information from customers about previous flooding events can be accepted although there
should ideally be evidence to corroborate the information. It should be clear that it resulted
from hydraulic overload and not other causes. Anecdotal evidence from customers of past
flooding should not otherwise be considered in determining the appropriate register to place
the property.
• A consistent set of rules should be applied when assessing the return period of a flooding
event to decide which register to record the affected properties against. This should not be
determined by the return period of the storm alone.
• The decision may be informed by the results from verified network models from simulating
standard storm events. Such information may already be readily available. Companies may
also wish to model the storm event using verified models to try to replicate the flooding
event. This is more onerous and may not be justified for all flooding events.
• Most first-time flooding events are added to the 1 in 20 years register, unless there is
compelling reason why it should be added to the other registers.
• Records should be kept of severe weather flooding events, even though the effected
properties are not added to the register.
• For repeat flooding, the interval between flooding events informs which register to allocate
the properties to. This may be supported with network modelling.
• Where one of the flooding events results from severe weather, the severe weather event
should count towards the assessment of the register allocation. This represents a change to
Ofwat’s guidance (which excludes all events arising from severe weather) but aligns better
with customer perceptions.
Impact on ARRs:
• The use by all companies of a 1 in 20 year return period for assessing the occurrence of
severe weather will bring consistency in the number of properties excluded from the ARR
for severe weather.
• Disallowing anecdotal evidence of past flooding from customers unless company records
support it should reduce the number of new additions for some companies; effectively, if
customer claims it was flooded, but there is no record on the DG5 register of past event, then
assume it must be due to ‘other causes’ or it never happened.
• Focusing on the return period of the flooding event, rather than that of the storm, will
probably result in a reduction in the number of first-time flooding events to the 1 in 20 years
and other registers for some companies.
• Including severe weather events in the assessment of repeat flooding may increase the
number of properties on the 2 in 10 and 1 in 10 registers slightly for some companies, though
by definition, severe weather events are rare.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
6-10
Figure 6.5: Best practice process flow chart – detailed investigation
Chart E: Detailed investigation
Detailed rainfall
analysisCheck severe weather
Check neighbouring properties – door to door
Claim additional properties in
outputs
Update register
Check neighbouring
properties – letter survey
CCTV
Hydraulic modelling
To confirm analysis undertaken when properties were added to the register and to assess future weather patterns
To obtain additional information to that gathered for the initial investigation where required
To confirm analysis undertaken when properties were added to the register and to assess the impact of future weather patterns on solution design
Further checking of neighbouring properties
optional. This should have been established at the
initial investigation stage
Confirm cause
Principal points: • The purpose of the detailed investigations is to capture additional information to that
captured at the initial investigation stage to inform the development of capital solutions to
flooding problems.
• Further assessment of severe weather determines future weather patterns as well as current to
inform the design and cost-benefit assessment.
• Additional CCTV surveys (if required) and network modelling studies are undertaken to
confirm the initial analysis and to assess the impact of future weather patterns and growth in
flood volumes on proposed solutions.
• Further checking for neighbouring properties affected can be undertaken at this stage, but
this is expected to have been completed at the initial investigation stage.
• Late additions should not be allowed in outputs, which should reflect the original register
values, unless there are compelling reasons to the contrary.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
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Impact on ARRs:
• The emphasis on identifying all affected properties at the initial investigation stage and only
allowing the claiming of late additions in outputs by exception should reduce the number of
late additions through better information.
Figure 6.6: Best practice process flow chart – removal from register
Chart F Removal
Separate register for mitigation
Remove as a result of company action
Demote via time-out rule
Remove via time-out rule
Better Information
Full audit trail of decision to remove
End
Remove if strong evidence that analysis undertaken when properties were added to the register was inaccurate (egdiscovered to be other causes). This should only occur on rare occasions
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
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Principal points:
• Full audit trail of decisions to remove properties from the register.
• Generally, properties are only removed as a result of company action. Removal for better
information remains possible, but should only occur on rare occasions.
• Separate register kept for properties protected through mitigation measures.
• Large numbers of properties would not normally be removed through time-out rules.
Impact on ARRs:
• A separate register for mitigation measures will not affect numbers on the ARR, but will
record the benefits that such measures bring.
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
7-1
7 Summary of Recommendations
7.1 Short-term recommendations
• Clarification of some aspects of the Reporting Requirements would help to improve the
comparability of company registers. Areas for consideration should include:
o The definition of, and approach to identifying, severe weather. Differences in
approach can affect whether flooded properties are added to the register, or excluded
on the grounds of severe weather. There are three aspects to this; clarifying the
definition of severe weather, strengthening the guidance on best practice methods of
assessing severe weather events and ensuring that all companies report using the same
definition and applying best practice methods of assessment.
o Clarifying the definition of internal flooding, especially for cellars and garages.
o Clarifying the approach to flood return period analysis and the addition of first-time
flooding to the At Risk Registers.
o Clarifying the approach to assessing repeat flooding, especially where some instances
are the result of severe weather and others are not.
o Clarifying the approach for recording properties that are discovered at a much later
date, (through detailed investigations or anecdotal evidence from customers), that are
likely to have been flooded. Current practice varies between companies as to whether
such properties are added to the register and count towards outputs.
o Separating the monitoring of properties that have mitigation measures installed, to
better reflect their true risk of flooding.
• The name At risk register’ is confusing and it should be named the ‘Flooding incident register’
or ‘\Unresolved flooding incident register’.
• The definitions of ‘severe weather’ (sewer flooding) and ‘extreme weather’ (GSS) should be
aligned, to a 1:20 year return period.
• When a property floods under ‘severe weather’ and ‘normal weather’ conditions, the severe
weather flooding should be included in cost-benefit analysis (if it has a return period that
should be met by normal design standards), as this better reflects the customer experience and
hence the benefit of investment.
• The definition of sewer flooding should be strengthened with guidance on what is included
and excluded. This would reduce the range of interpretation and help to provide a more
consistent service between sewage company regions.
• Companies should not exclude ‘minor’ flooding from the sewer flooding register (for example
through the use of exclusion rules or judgement). Proper checks should be in place to ensure
that all flooding is recorded.
• Full checks for flooding of neighbouring properties should be made at the time of the initial
investigation. Although it is appropriate to add more neighbouring properties through better
information at the time of investment, the addition of significant numbers of properties at that
External Review of Sewer Flooding Risk Registers Mott MacDonald
Water Services Regulatory Authority
247338/01/C - 15 October 2008
7-2
stage suggests that companies are not making adequate checks at the time the flooding occurs,
and may indicate poor service to some customers who experience flooding.
A common approach to the assessment of flood return periods (or flood frequency if a full risk scale is
used) is required. His should ideally be developed by an industry-wide study or by consensus of the
sewerage companies.
When assigning June Return confidence grades, companies should take account of the number of
additions to and removals from the ARR. (Where there is a significant addition or removal rate
through better information, companies cannot have high confidence in the original data.)
7.2 Longer term recommendations
The flood frequency (2:10, 1:10, 1:20) and severity (internal, external, other flooded areas) categories
should be replaced with a risk scale taking account of the frequency and severity of flooding. This
would be better aligned with the benefit-cost analysis now common in planning solutions. For
reporting purposes, risk would be reported in categories.
If a risk scale is adopted, the impact of mitigation on the flood risk should then be taken into account
and used to re-assess the risk once mitigation is in place. This would provide clearer information to
customers on the impact of mitigation.