Reporting guidance – Risk of sewer flooding in a storm
| Date | 04 April 2019 |
1
Reporting guidance – Risk of sewer flooding in
a storm
Overview
This guidance seeks to enable all water and wastewater companies (companies) to
report on the risk of sewer flooding during an extreme wet weather event for the
defined year with confidence, at a reasonable level of accuracy and with a common
approach. Companies shall where possible apply consistent and robust methods and
common assumptions. This will facilitate the comparison of performance across
companies by customers, regulators and other companies with reasonable
confidence.
This metric has been designed to measure the resilience of sewerage undertakers in
respect of their drainage systems. The metric is a single hazard, single consequence
measure and has been designed with a view to assessing existing and future
resilience to an extreme wet weather event causing sewers to flood.
The metric has been developed with extensive engagement of a Project Steering
Group comprising the ten water and wastewater companies of England and Wales,
Scottish Water, Ofwat, the Environment Agency, Defra, CCWater and WaterUK.
This definition is derived from the original report Developing and trialling wastewater
resilience metrics (Atkins), to enable further changes that have been suggested by
the ten water and wastewater companies of England and Wales.
Key principles
The metric is a hybrid, with elements that incorporate engineering judgement and
modelled outputs. The proposed metric:
Utilises a proportionate and pragmatic approach in the development of a
baseline position.
Ensures that all catchments that are characterised as medium to high
vulnerability undergo more detailed assessment whether or not a model
currently exists for the catchment.
Provides a risk-based, but predominantly subjective, approach to assessing
vulnerability in those catchments where no models currently exist.
Final reporting guidance for PR19
2
Utilises nodes (manholes) predicted to flood as the modelled measure as it is
considered to better reflect the risks being assessed and is likely to be more
relevant to customers than a measure based on surcharge.
Is aimed at driving positive behaviours from companies; while improving
knowledge is a resilience intervention, and is likely to be a key source of
change in the metric in the initial stage of implementation, ultimately the metric
is about recognising practical efforts to reduce the probability that customers
will be impacted and the severity of that impact. It is acknowledged that the
metric cannot recognise all forms of resilience intervention as the outputs are
limited in what they can show (in particular modelled outputs); however, they
do recognise key interventions to reduce, in particular, surface water inputs to
the system whether undertaken by the companies alone or in partnership with
other stakeholders
Provides a means for companies to engage with their customers in respect of
sewer flooding arising from extreme wet weather events.
Provides for companies that are able to utilise more advanced 2D models to
understand risk.
Developing a robust baseline against which change can be measured will be key to
the effectiveness of the metric.
Benefits
The metric is a single hazard, single consequence measure; however, it is designed
to reflect companies’ performance against an impact that is of significant concern to
customers. In addition to measuring outcomes, the metric is aimed at providing
companies with a means to prioritise investment, engage more extensively in
partnership working (to derive better value to customers) and with customers, and,
importantly, to focus the development of long-term planning strategies with a view to
reducing the chances that residential and business customers will be flooded in
future.
To continue reading
Request your trial