Download Future changes to river flood flows in England and Wales

UK CLIMATE
PROJECTIONS
Future changes to river flood flows
in England and Wales
Author/Organisation
Bill Donovan, Environment Agency, Karl Hardy, Defra and Nick Reynard, Centre
for Ecology and Hydrology, Wallingford
What is the overall aim of the work?
Current Defra/EA guidance requires all flood management plans to allow for
climate change by incorporating within a sensitivity analysis an increase in
river flows of up 20% over the next 100 years. Although the 20% figure is a
memorable precautionary target, there is the risk that it leads to a significant
under- or over-estimating of the future risk. The objective of this research is to
use the latest evidence, including UKCP09, to re-consider the existing guidance
and provide evidence for policy makers to help set new allowances depending
on catchment vulnerability.
Which data source(s) of UKCIP08 were used?
• 11-member RCM
What dummy outputs were used?
• Observed climate datasets
• 11-member RCM
Why were these outputs used?
The 11 member RCM output was used as this is the first available UKCP09 dataset.
It allowed us to consider both changes to monthly mean rainfall, but also the
potential changes to the structure of rainfall e.g. changes to the number of
rainfall days or changes to the intensity of rainfall in the future.
How were the outputs used?
The project took a different approach to the normal climate change impact
assessment. Rather than taking data from the climate projections to drive a local
impact model to determine changes to future flood flows, we explored how
arbitary climate changes affected a large number of catchments’ response. This
provides us with sensitivity surfaces describing the climate/catchment dynamic
in a ‘scenario neutral’ way. This approach is helping us to evaluate which
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UKCIP08?: Worked examples – Assess adaptation measures
catchments are most vulnerable to change and if there are similar patterns in
different catchments’ responses.
We have then taken the 11 member RCM data and considered where on the
sensitivity surface for each catchment these different climate model realisations
occur. This helps us understand the likely range of change that these climate
models are suggesting. As well as the 11 member RCM data we have also
considered all the Global Climate Model (GCM) data from the IPCC 4AR approved
model set to evaluate what change they indicate with respect to the sensitivity
surfaces. This helps us determine if the potential range of change and where
the 11 member RCM set sit compared to the GCM set. This approach of working
from arbitary change through the Hadley RCM set to the widest set of different
GCM models gives us a broad picture of the vulnerability of a catchment and the
potential impact on river flows and some idea of the likelihood of that change
occuring. This will provide us with important evidence to reconsider our current
guidance and climate change allowances.
• Location chosen — 150 catchments across England, Wales and Scotland
broadly representative of the different catchments types and locations across
these countries.
• Climate variable chosen — rainfall and temperature to drive calibrated
hydrological models and determine changes to river flows
• Averaging period (eg Summer, January) — mean monthly changes, but we
also considered the changes to the intra-monthly pattern of rainfall
• Emission scenario chosen — RCM ensemble only provides one emission
scenarios, but we have also considered four emissions scenarios from the
wider IPCC 4AR GCM data set.
• Time Period chosen — historic data and future data especially for 2050s and
2080s periods.
Describe any difficulties or limitations associated with using probabilistic information such as UKCIP08.
The lack of potential evapotranspiration projections in the probabilistic
projections is a limiting factor
We are not clear yet how we will make best use of the weather generator data in
comparison to the RCM data, because of the large spatial information we need
to consider, e.g. rainfall over a large catchment
How would you communicate the results to your target audience?
We are using a graphical representation of the sensitivity space for each
catchment with RCM information overlaid to illustrate the potential changes
river flood flows. See diagram opposite.
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UKCIP08?: Worked examples – Assess adaptation measures
Change to the 1:20 year river flow for the River Trent. The plot shows changes
to flows resulting to changes to mean annaul precipitation (y-axis) against
seasonality — ratio of winter rainfall to summer rainfall (x-axis). The coloured
bands are change to river flow in 10% increments, grey is no change or reduction.
The dots represent the current projections from all IPCC 4AR models (black),
UKCIP02 (red) and UKCP09 (blue).
What other UKCIP08 information (if any) not included in the dummy ouputs would you have
liked to have used?
We intend to consider rainfall data from the weather generator when this is
available.
Links to sources of more information
http://www.defra.gov.uk/Environ/Fcd/Research/
Contact Details
Bill Donovan, Environment Agency: [email protected]
Karl Hardy, Defra Flood Management: [email protected]
Nick Reynard, CEH: [email protected]
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