Environmental Engineering Reference
In-Depth Information
.
Calculation of extreme (in-channel) water levels
required the use of a 1D model.
.
Production of flood maps required detailed 2D
modelling of the floodplain with boundary condi-
tions provided by a non-linked 1D model of the
channel.
.
For flood forecasting a 2D model of the sea and
outer Estuary is linked to a 1D model of the
Estuary upriver of Southend. Where the potential
area impacted following a breach is required to be
forecast, then a detailed 2Dmodel of thefloodplain
is provided as part of the Themis inundation
modelling system.
.
For analysis of strategic flood risk management
options a 1Dmodel has been used to assess thresh-
old of adaptation. For appraisal of options a
dynamically linked 1D (channel) and broad-scale
2D (floodplain) has been used to enable property
damages and other impacts to be estimated.
The selection of the most appropriate model-
ling methods is not necessarily easy and requires
experienced modellers to consider a range of
criteria, including: flow mechanisms, required
accuracy, availability of existing models, time
available for modelling, outputs required and
availability of suitably skilled modellers. Even
when the most appropriate method is used, there
will remain key assumptions to make and uncer-
tainties to understand and communicate to end
users. Some key uncertainties that apply to much
of the modelling described in this chapter are
discussed below.
Whilst river water levels can be predicted with
some confidence (for given boundary conditions),
the prediction of maximum flood extents, flood-
plain velocities and floodplain water depths has
much lower confidence (both because the pro-
cesses are harder to simulate and because there is
a lack of observed data to confirm accuracy). In
addition, there are many parameters and phe-
nomena that we include in our models over
which we need to make informed judgements or
limiting assumptions in order to include, for
example:
.
how or when a defence might fail;
.
the forecast shape and coincident timing of a
surge with a high astronomical tide level;
.
the probability of a particular flow occurring
with any particular tide;
.
future extreme water levels (sea level rise and
increased surge size).
There are many parameters and phenomena
that we do not include in our hydraulic models
(e.g. due to lack of data, lack of mathematical
representations or lack of deterministic knowl-
edge), including:
.
non-standard human behaviour effects on oper-
ation of river structures;
.
river bed sedimentation/erosion interactions
with hydraulics;
.
unpredictable weather effects;
.
extremes or failure that we have not experienced
or have been unable to conceptualize.
Acknowledgements
This chapter references material produced by
Halcrow for the Environment Agency. The latter's
help and encouragement is gratefully acknowl-
edged although the views and opinions expressed
do not necessarily reflect those of the Environ-
ment Agency. The most up-to-date data on
Thames Estuary flood risk should be obtained
directly from the Environment Agency.
References
Defra (2006) FCDPAG3 Supplementary Note to Operat-
ing Authorities - Climate Change Impacts, October
2006. Defra, London.
Defra (2008) FCDPAG Supplementary Note on Asses-
sing and Valuing the Risk to Life from Flooding
for Use in Appraisal of Risk Management Measures,
May 2008. Defra, London.
Gilbert, S. and Horner, R. (1984) The Thames Barrier.
Thomas Telford Ltd.
Halcrow (for Thames Water) (1988) Tidal Thames
Defence Levels - Final Report. June 1988. Halcrow
Group Ltd.
Halcrow (for the Environment Agency) (2005) Tidal
Thames Extreme Water Levels - Reassessment of
Joint Probability Analysis. August 2005. Halcrow
Group Ltd.
