Environmental Engineering Reference
In-Depth Information
flood risk management in the Thames Estuary for
the next 100 years. The production of the plan was
supported by a programme of modelling designed
to improve understanding of the baseline behav-
iour of the system and predict future behaviours
both with and without changes in flood risk man-
agement. Hydraulic modelling included both 1D
and 2D modelling: the Estuary was modelled in
both 2D (Telemac-2D) and 1D (ISIS), with the
floodplainmodelled using 1D flood cells (ISIS) and
2D (TUFLOW). In this section two examples of the
TE2100 flood modelling are described:
.
1D modelling to help understand the 'limits of
engineering adaption';
.
linked 1D and 2D modelling to help appraise
options.
.
Threshold 5: the point at which it is considered
impractical to intervene further to manage flood
risk through engineering (i.e. the overall engineer-
ing limit to adaptation).
The approach taken for the study was to use
extensive 1D hydrodynamic modelling together
with application of engineering judgement. Each
engineering response was tested against a range
of sea level rise scenarios ranging up to a maxi-
mum of 8m above the current mean sea level.
The engineering responses explored were con-
strained to: (i) raising of defence walls and em-
bankments; (ii) modifying the Thames Barrier; and
(iii) construction of new throttles, barriers and
barrages. ISIS models representing different future
flood risk management responses were run with
a series of extreme tides at Southend to calculate
in-bankwater levels at a number of locations along
the Thames. These water levels were then com-
pared with defence data (i.e. information on cur-
rent and future crest levels), with an allowance for
freeboard.
The 1D models were schematized as in-bank,
meaning water could not spill out onto the flood-
plain. Thus the results for each scenario could be
reused to define overtopping thresholds for differ-
ent defence levels. Once the simulations were
completed, the maximum water levels from each
design event were extracted and stored in a spread-
sheet, which was then used to facilitate analysis.
Various overtopping thresholds were then calcu-
lated for each response, both with defences at
current levels, and with defences raised by 1m.
A range of assumptions and simplifications
were needed to ensure the study remained tracta-
ble; these included: ignoring increases in fluvial
flows, using a representative surge shape, simpli-
fying the barrier closure rules and using an in-bank
1D model.
The study indicated that, assuming that the
requirement is to maintain a 1:1000-year standard
of protection for the urbanized embayments along
the Thames Estuary, the absolute maximum rise
in mean sea level that the potential engineering
adaptations tested in this study could accommo-
date is:
.
5.25m(for a 1-mincrease in surgemagnitude); or
Limits of engineering Adaptation
The 'Limits of engineering adaptation' study was
initiated by the TE2100 team to gain an early
appreciation of the likely limits of large-scale
'hard engineering-biased' flood risk management
options against sea level rise and future increases
in storm surge events. One of the main aims of the
study was to define the various points, in terms
of sea level rise, at which engineering responses
would face a critical threshold that would force a
further adaptive change in the system. The study
thus defined a number of key adaptation thresh-
olds, as follows:
.
Threshold 1: the point at which the freeboard
allowance within the existing flood defences is
eroded by a given surge event, or by a future spring
tide.
.
Threshold 2: the point at which the height of
existing downriver defences and the crest level of
the existing Thames Barrier would need to be
raised.
.
Threshold 3: the point at which the existing
Thames Barrier and associated walls and embank-
ments cannot be adapted further, leading to the
possible move to an outer estuary barrier (e.g. at
Southend).
.
Threshold 4: the point at which it is necessary
to modify the structure at Southend into a
barrage.
