Supplementary memorandum submitted by
Norwich Union Insurance (FL 006b)
1. The differences between the Environment
Agency flood mapping and the mapping Norwich Union has developed
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| Norwich Union flood map | Environment Agency map
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| — Can differentiate between different flood risks within a postcode and assign a unique flood risk to individual properties, ie goes down to address level.
| — Assesses flood risk in 100m squares—not at an individual property level
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| — Uses height data that is accurate +/- 0.5m in the South east and +/- 1m for the rest of the UK. This is the best national height model available.
| — Uses the same height data as the Norwich Union data, but does not produce the same results as flood risk is mapped in generic squares, and it does not incorporate claims data.
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| — Was specifically designed for use in the insurance industry and benefits from the use of claims data
| — Designed to be a first step for insurance purposes, but is too generalised to assign flood risk to individual properties.
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| — Using the NU flood map, we believe there to be 241,000 properties at "significant" risk (worse than 1 in 75). This figure is significantly lower than the EA's estimate, due to the fact that we are more accurately able to assess risk in individual areas (ie not 100m squares).
| — Recent EA estimates suggest that there are 518,000 properties at "significant" risk
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A visual example of the difference between the two maps is
included below.
Environment Agency National Flood Risk Assessment (NaFRA) data
Environment Agency maps split the flood risk into 100m squares
assigning each square low, moderate or significant risk. This
method assigns all properties in each square the same risk, which
is not always realistic.
Norwich Union Flood Risk map
Norwich Union's flood map assigns flood risk to individual
properties. In the below map, 75 = significant, 100 = moderate,
250 = low.
Both maps . . ..
— Can only assess flooding from natural watercourses,
ie from rivers and the sea. Much of the flooding that occurred
in Summer 2007 was due to intense rainfall over urban areas (ie
pluvial flooding) which neither the Environment Agency or Norwich
Union maps account for.
— Do not provide information on flood depth, speed
or volume of flow. Local Environment Agency offices may have more
detailed local knowledge that is not fed into national data.
2. The extent to which Norwich Union's flood maps incorporate
meteorological information such as possible rainfall intensity
Norwich Union's flood map does not directly incorporate meteorological
data; instead we use data which gives us the flow in each river,
for various return periods (eg a 1 in 75 return period). This
is the same method as used in the Environment Agency models.
Our maps show which areas will flood, given a certain volume
of flow in the river. If we were to include meteorological data
to model river flooding, it is unlikely it would make much difference
to the areas that we predict are at risk of river flooding.
However, what may change in the future, if we see more of
these extreme, intense downpours as a result of climate change,
is the frequency of flooding. If these rainfall events become
more frequent, we may find for example, that what was a 1 in 100
year event becomes a 1 in 50 year event, but the area at risk
will remain the same.
This year's flooding was caused by some quite unusual meteorological
conditions, leading to intense rainfall over many areas of the
country at once, rather than the more usual isolated downpours
that can lead to flooding. It is notoriously difficult to assign
a return period to a particular flood event, but from information
we have seen we think the recent events are probably nowhere near
as extreme as a 1 in 1000 year event.
Norwich Union Insurance
January 2008
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