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.

Model limitations

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

predicts this.

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

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