Environmental Engineering Reference
In-Depth Information
.
5.75m (with a 0.4m increase in surge magni-
tude); or
.
6.0m (with no increase in surge magnitude).
Fortunately, a sea level rise of 5.25m by 2100 is
higher than that postulated for any of the climate
change scenarios currently in use by the TE2100
project (e.g. the 'High
รพรพ
' scenario allows for
3.2m of mean sea level rise combined with 1m
increase in 1:1000 year surge by 2100).
It should be noted that this work did not explore
the economic, social and environmental 'costs'
associated with the implementation of the re-
sponses. The 'of some of the responses explored
may indeed be prohibitive, thus narrowing the
envelope of sea level rise that could be adapted to
with engineering or structural responses alone.
A portfolio of responses - both structural and
non-structural - will have to be employed in the
Thames Estuary in order to adapt to future flood
risk.
The use of a 1D hydrodynamic model was the
appropriate method to enable many hundreds
of simulations to be undertaken within a short
project programme. As flood risk (flooding prob-
ability
consequences) was not required to be
calculated the 'glass-walled' 1D model did not
need to extend onto the floodplain. It should be
noted, however, that in this high-level study the
application of engineering judgement was an
equally important element as the modelling.
.
Fully hydrodynamic but relatively low-resolu-
tion broad-scale 2D modelling of floodplain
flow (dynamically linked to the 1D model of the
river).
.
Treatment of breaching through a combination
of breach factors and embayment-scale TUFLOW
2D breach models.
.
Simulation of deterioration of defences over
time.
.
Explicit consideration of the main source of
uncertainty (Southend extreme water level).
.
Likelihood of defence failure included as a func-
tion of defence type and water level in the river
(represented by fragility curves).
.
Inclusion of the additional risk due to potential
failure to operate the Thames Barrier (and pro-
posed barriers).
.
Calculation of flood depth probabilities result-
ing from defence breaching, overtopping and
barrier failure.
.
Direct property damages (including residual
risk) calculated to derive annual average damage
(AAD) and present value (PV) damages.
.
Estimation of annual risk to life based on the
method described in Defra (2008).
Two baselines ('walk away' and 'maintain ex-
isting') and seven strategic option sets were sim-
ulated for a range of epochs up to 2170 for two
climate change scenarios.
The use of broad-scale 2D modelling of the
floodplain was selected as appropriate at it was
essential to estimate floodplain flood depths - the
use of detailed 2D modelling would have resulted
in unacceptable run times given the project
programme (many hundreds of simulations were
required). The use of non-fully hydrodynamic
modelling of the floodplain was also considered
but rejected as it was not considered sufficiently
accurate for appraisal.
Appraisal of options
The TE2100 project included the formal appraisal
of a set of strategic flood risk management op-
tions for the Thames Estuary. The appraisal re-
quired a range of flood risk metrics to be
calculated including direct property flood da-
mages, 'risk to life' estimates and a large set of
floodplain depth-probability grids (Wicks
et al. 2009). The depth-probability grids are used
in subsequent analysis to informwider indicators
of social, economic and environmental impact.
Themain components of themethod used for the
modelling were:
.
Fully hydrodynamic ISIS 1D modelling of the
tidal Thames for a range of tidal and fluvial events.
Conclusions
As described above, a range of flood modelling
methods has been found to be necessary to meet
the needs of the Environment Agency on the
Thames Estuary:
