Flood warnings from behind the barricades
A new 2-D modelling approach has been developed for predicting the potential inundation of low lying land behind coastal and river defences, as George Mitchell, principal engineer at the UK’s Atkins Water explains.
The floods of autumn 2000 left their mark on many communities all over the
UK. Some of these floods were the worst since records began, with a number of
areas suffering unprecedented inundation more than once. Evidently, residential
properties have been built on highrisk flood plains.
Atkins has been involved in all aspects of flood related work for many
years, including surveying, flood risk assessment, flood defence asset surveying,
hydrology, marine modelling and particularly flood forecasting and inundation
modelling. Many projects have involved the sole use of onedimensional models.
These models are ideally suited to rivers flowing in banks, but when floods
occur and the river bursts its banks, or a tidal flood occurs at the coast,
the situation is very different. Instead of the flow being relatively fast and
onedimensional it is often more sluggish and twodimensional. Simulating this
with a onedimensional model requires a priori assumptions about where
the flow paths are. This is less than ideal and may not always be entirely objective
For these situations Atkins developed and now uses a bespoke two dimensional
modelling system. A typical local scale application of the two dimensional model
has been to support an application for development within a low-lying area behind
tidal defences. We simulated the consequences of different tidal defence failure
scenarios and predicted the flood wave evolution. The work showed conclusively
that a secondary flood defence wall actually protected the proposed development
area from flooding due to failure of the tidal defences. With the two-dimensional
approach no assumptions needed to be made about flow paths – the model itself
On a larger scale, the Environment Agency in England has commissioned Atkins
to map the inundation that would result from the 1000-year combined high tide
and surge around the entire English and Welsh coastlines. The level of detail
demanded requires developing about 80 models.
The Atkins modelling system has been designed to generate each complete model
automatically including the boundary conditions that drive the simulation. This
demands a comprehensive data set covering the land topography, marine bathymetry
and tidal and surge conditions. The final output is a flood envelope inland
from the coast showing the extreme flood outline. The results will be used for
high level planning.
The same modelling techniques can be used for real time forecasting and Atkins,
in partnership with the Danish Hydraulics Institute, has developed and installed
a large real time forecasting system for the East Anglian region.
This system provides forecasts over the complete range of flows although most
importance is attached to the higher flows which may cause flooding. This system
is based on a set of one dimensional models. There is now sufficient computing
power available to include two dimensional models in real time forecasting systems
which will provide information which can be used to plan evacuation in the
event of catastrophic flood defence failure or overtopping.
Numerical solutions have become well developed in the last 25 years but it
is only in the last ten we have had the computing power and software to develop
modelling systems that are really powerful, flexible and easily applied. A modelling
system is nothing without the data to represent the topography, structures
and flood defence features accurately.
The change from ground based survey to aerial laser (e.g. LIDAR) based surveys
and the production of digital elevation models (DEMs) has revolutionised inundation
modelling. DEMs coupled with conventional information about flood defence
features provide a basis for rapid model development and a more faithful representation
of the real world than was possible up until a few years ago.
With these techniques developers and the Environment Agency will be in better
position to advise of flood risk, rather than just the potential consequences.
Only 5% of people living in a flood risk area say they take precautions. Picture
courtesy of the Environment Agency