Environmental Engineering Reference
In-Depth Information
results were available for the full extent of bound-
ary conditions generated from the extremes anal-
ysiswith climate change uplifts (fromDefra 2006).
Two illustrative examples of structure functions
are shown in Figures 23.3 and 23.4.
previous reports (e.g. Halcrow 1988). In essence
the calculation involves two main stages; firstly
using the 1D ISIS model (www.halcrow.com/isis)
to estimate a matrix of water levels at each of the
selected locations along the Estuary (known as
structure functions), and secondly using a statis-
tical model to calculate the design water level at
each of those points, for a selection of return
periods.
Calculating return period water levels
This complex process (developed by CEH Wall-
ingford; Halcrow1988), calculates level-frequency
results for a number of return periods and loca-
tions along the Estuary. The method uses records
of sea level at Southend and river flows at Kingston
(upstream of Teddington), along with the struc-
ture functionsas inputstotheir statistical analysis.
The analysis uses the data to calculate the
probability of a given Southend water level occur-
ring with a given fluvial flow, considering also the
likelihood of a Barrier closure for that event. These
probability data are calculated for neap-to-neap
cycles and then converted to an annual maximum
form, so that level frequency diagrams can be
drawn for each location and water levels for a
particular return period can be reported. To pro-
vide an indication of potential climate change
impacts on river flows, the statistical modelling
Calculating structure functions
Structure functions report a water level at a point
along the Estuary as a function of a given sea level
and river flow, and are determined by hydrody-
namic modelling, which takes many factors into
account (e.g. channel geometry, Thames Barrier
operation, tidal propagation, etc.). Structure func-
tions have been calculated for several points along
the Estuary. Design tides (plus surge) at Southend
were modelled at 0.5-m intervals and ranged from
1.5m AOD (Above Ordnance Datum) to 5.5m
AOD for present day, 1.8m AOD to 5.8m AOD
for 2052, and 2.1m AOD to 6.1m AOD for 2102.
Fluvial flows ranged from2m
3
/s to 1200m
3
/s. The
ranges in levels and flows were set to ensure that
Fig. 23.3
Example structure function for Teddington (ISIS model node 2.1). (See the colour version of the figure in
Colour Plate section.)
