Supplementary memorandum submitted by
the Natural Environment Research Council's Centre for Ecology
and Hydrology (FL 112a)
1. The Centre for Ecology and Hydrology
(CEH) welcomes the invitation to submit additional evidence following
the oral session on 12 December 2007.
2. Paragraph 37 of the Natural Environment
Research Council's original written evidence briefly outlines
relevant research directions at CEH addressing future flood risk.
Given the issues of interest discussed at the session in December,
it appears appropriate to enhance this to indicate the major capabilities
of hydrological research which do already and can be further expected
to inform flood risk assessment under changing environmental conditions.
Scope of hydrological modelling
3. For given climate predictions used as
input variables, the hydrological response of the landscape can
be modelled: a variety of methods is available with different
degrees of complexity of interpretation of the hydrological domain.
Because this hydrological domain of the rural and urban landscape,
both surface and subsurface, is complex and both spatially and
temporally variable, it is not a straightforward matter to assess,
even in the past, the separate effects of land use, land management,
river regulation and climate variability on flood risk. Assumptions
are necessarily made in encapsulating real-world behaviour in
tractable hydrological models: good modelling practice makes these
assumptions clear and it also tests model performance against
such observations from the past as exist, before moving to predictions
of the future. It is appropriate also to offer some quantification
of error or, as is frequently said, uncertainty relating to the
results. Hydrological modelling of river catchments under expected
changed climate scenarios was the basis of Defra's current guidance
for climate change allowances in scheme design.
4. Global temperature rises are generally-accepted
features of climate change. The future flood risk in the UK is
a function of how this translates into regional and local climate
which drives the catchment hydrological response, particularly
in the case of precipitationnot only with respect to totals
but also intensities, areal extents and sequences. These aspects
are far less well known, particularly the extreme aspects of rainfall.
Note, though, that extreme flooding occurs with or without climate
change: it is the relative frequency of floods of particular magnitudes
which may change. Outputs from hydrological models include river
flow regimes, which can be tested against past observations and,
in the case of some models, aspects of soil and aquifer (water-bearing
rock) water content, for which there are far fewer observations
to test performance.
5. Given the current state of knowledge
and the considerations above, the most compelling hydrological
research to inform flood risk management includes:
a) Analysis of performance of operational
real-time hydrological modelling procedures in the light of data
gathered from the 2007 floods and assessment of any necessary
procedural modifications /enhancements.
b) Downscaling of most recent outputs from
regional and global climate models for driving spatially-distributed
catchment hydrological models for river flow regimes (and catchment
wetness where appropriate) together with associated error levels.
c) Elucidation of the hydrological behaviour
of urban and suburban areas for rainfall events above the design
level of drainage systems, together with management options which
avoid increased risk in a wider catchment context.
d) Assessment of the risk of concurrent spatially-extensive
flooding, given its greater impact on emergency services, whether
pluvial (direct from extreme precipitation), river, groundwater
and/or coastal flooding.
e) The inclusion of tested urban (see "c)"
above) and groundwater flooding models/procedures (see, inter
alia, NERC written evidence paragraphs 21, 35) in a regional
and countrywide system of overall flood risk assessment, for example,
RASP ("Risk Assessment of Flood and Coastal Defence for Strategic
Planning"): the latter system was used in Floods Foresight
but at that time included river and coastal flooding only.
f) If an updated Foresight process is re-run
in the future for the long-term (30-100 year) view, it would be
advantageous, as well as covering "c)" and "d)"
above, to have more than a single group determining expert weightings
on risk factors, given that much emphasis rests on these weightings
in determining the end results.
g) Given the Making Space for Water
philosophy, together with the view that land use is likely to
affect flood risk only at local scales and for floods at lower
recurrence intervals (rather than regional scale large floods),
a nation-wide assessment of potential flood water storage volume
available from a wide range of sources, including soft-engineered
options, within catchments would be informative, bearing in mind
the need for flood conveyance as well as storage.
Centre for Ecology and HydrologyNatural Environment