Flood management
Abstract
Flood engineering has a long history. For example, evidence of engineering dates back 4,000 years in
the Middle East and China. The aim of flood defence is to reduce the risk of flooding. Two broad
categories of flood defence exist:

Hard engineering structures, such as dams, levées and channel modification.

Soft engineering techniques, such as afforestation, terracing of slopes and the re-creation of
floodplains.
Hard engineering structures are often point-specific, whereas soft structures are more extensive in area.
Introduction
As early as 1864 George Perkins Marsh highlighted the problem of flash floods and accelerated soil
erosion in the Alps caused by deforestation. Deforestation in lower Provence, France, in the fifteenth to
seventeenth century had resulted in erosion of good farmland and the deposition of coarse load on top
of it.
Today, there is growing awareness that methods of flood engineering should have an environmentally
sensitive approach, linking the channel and the drainage basin/floodplain, and not just dealing with the
channel.
Hard engineering options
Channelising has taken place on 25% of the main rivers in England and Wales. Channel widening
increases the channel capacity but may lead to increased siltation. In urban areas, land may not be
available for widening. Channel deepening can be achieved through dredging or narrowing the channel.
It may, however, lead to increased bank instability.
Channel straightening removes meanders in rivers, steepening the gradient and increasing velocity, and
therefore reducing the flood stage. In contrast, sinuous channels have low channel efficiency. The most
famous straightening was that of the Mississippi, where 16 cut-offs reduced the length of the river by
240km.
Flood relief channels are used to by-pass existing urban areas where straightening is not an option
(Figure 1), and are used as flood overflow channels. Large concrete channels, such as at the Thames in
London, are expensive to build, damage the river ecology and are visually unattractive, especially at
low flow. Bank protection may be provided by concrete wooden piles or groynes, or even by
vegetation. Willow trees are particularly fast growing and take in large volumes of water.
Figure 1 – Relief channel.
Levées increase the carrying capacity of the channel but may increase the risk of flooding downstream.
They may also increase sedimentation in the channel after the flood peaks have passed. In extreme
conditions the level of the riverbed may rise so much that the riverbed level is higher than the
floodplain. Levées are expensive, so are used to protect high-value property and vulnerable
communities.
Levée position is important. If placed on the edge of the channel, it is more likely to be breached. If
placed in the floodplain, the amount of water that can be stored is greatly increased.
Levées can fail for a number of reasons:

Overtopping

Piping

Saturation flow between the base of the levée and the ground

Under seepage.
Research in Scotland has shown that levées on the outside bend of the river and overlying old river
courses are most vulnerable to erosion. In addition, small-scale scour around posts at the top of the
levée can destroy sections of levée. Moreover, the construction of new levées upstream may have
impacts on ones downstream and destabilise them. Levées need to be maintained, but in countries
experiencing rapid social change, such as LEDCs, such maintenance might not happen. In Vietnam,
however, people used to be required to devote 30 days of labour per year to flood mitigation in their
local area. That requirement is now down to 10 days.
Various types of dam can be used to control and regulate water flow:

Retarding dams are simple structures that have an outlet at the base, allowing water to drain out
at a set rate. They cannot hold back water permanently.

Detention basins are used to store water in times of floods. They are used for farming (pastoral)
and other low-intensity purposes, such as recreation, but not for buildings or other valuable
purposes.

Storage dams have gates or valves to store and regulate water flow. These dams may be part of a
multi-purpose scheme, such as the Three Gorges Dam on the Yangtze. Such schemes may need
to keep the dam quite full for a variety of uses, such as HEP or irrigation. Single-purpose dams,
used for flood control, can empty the dam of water following a flood and thereby be fully
prepared to receive floodwater from the next flood. The location of a dam is important – it needs
to be located near to the area to be protected.
Some of the disadvantages of dams include dam failure and flooding of upstream locations. In addition,
the storage capacity of a dam decreases with time, as sedimentation fills the area behind it. With a
reducing capacity rate of 0.5% p.a., 25% of the storage is lost in 50 years. In central India, the
Nizamsager Reservoir lost over 60% of its capacity in 40 years, and has been predicted to lose the rest
of its storage capacity within a further 30 years.
Environmental and ecological impacts of hard engineering
Natural rivers have highly variable flow regimes with a variety of pools, riffles, meanders, innercliffs
and slip-off slopes (Figure 2). River engineering produces a more uniform condition and a consequent
reduction in biodiversity (Figure 3).
Figure 2 – Meandering section of the River Cole.
Figure 3 – Straight section of the River Cole.
A historical pattern has been observed in the engineering of European and Asian rivers. Levées are
built at first, increasing downstream floods and reducing the number of low flows. Later the
construction of dams leads to decreased floods and more low flows. This can lead to channel
aggradation (deposition) and degradation (erosion). Aggradation occurs as a result of erosion in the
drainage basin. The riverbed, trapped between the levées, rises above the surrounding floodplain. In
contrast, with increased sedimentation behind the dam following its construction, downstream
‘clearwater’ erosion occurs.
In recent decades there has been increased awareness of allowing a river to interact with its floodplain.
‘River restoration’ refers to the return of rivers to a natural state, complete with meanders and
floodplains. ‘Rehabilitation’, in contrast, refers to a partial improvement in river quality. Remediation
leads to an improvement in the ecology of a stream, although the end result may not necessarily
resemble the original stream.
Soft engineering options
Flood abatement tends to be focused on headwater streams (upper course) where a combination of
high-intensity storms and farming practices increases the risk of flooding. High rainfall and snow melt
also lead to increased soil erosion, thus any attempt at flood abatement also has an impact on reducing
soil erosion. Flood abatement aims to tackle the flood problem as near to its source as possible. This
means that a large area needs to be managed. In general, there is limited flood abatement in urban
areas.
Topographic manipulation
Topographic manipulation involves altering the length of the slopes. A long slope produces more runoff and erosion than a shorter one, since water velocities increase downslope. Topographic
manipulation techniques include:

Terraces – artificial vertical layers on a slope designed to reduce water movement. They
increase infiltration and sediment deposition.

Contour ploughing – this involves the ploughing of surfaces along the contours to reduce
overland run-off.

Strip cropping – alternate strips of different crops are grown to produce variations in
interception and run-off.
Surface and underground water storage
Much of the surface and underground water storage is natural, such as in small lakes and swamps, but
some may be artificial such as ponds.
Water spreading occurs mainly in arid and semi-arid areas. This is a process whereby surface flows
from flash floods are diverted by dykes to increase infiltration and protect areas from flooding.
Gully control
In many steep mountainous areas dams are built to reduce the flow of water. These dams are cheap to
build, often being made of loose rock. In flatter areas they can help to spread water.
Vegetation cover management
In general, a complete vegetation cover helps to reduce flooding through increased interception,
increased evaporation and evapotranspiration, increased infiltration and reduced run-off. In addition to
providing above-ground protection from rain-drop impact, vegetation adds organic matter to the soil
and binds the soil particles together. Changes in flood characteristics occur after forestry. Following
forest destruction by fire in the Yarrangobilly basin of the Australian Alps, peak flows increased from
60–80m3 sec-1 to 370m3 sec-1, whilst land cleared for tea cultivation in Kenya registered an increase in
flows from 0.6m3 sec-1 to 27m3 sec-1.
Afforestation should reduce peak flows through increased interception, evaporation,
evapotranspiration and infiltration, and reduce overland run-off. However, in early years, the density of
vegetation cover is low and so the impacts are not high. As the forest grows, there is increased
interception, evaporation, evapotranspiration and infiltration, and so flood peaks are lower. Evidence
from New York State has shown that reforestation of 58% of a watershed reduced winter and spring
peak flows by between 16 and 66 %.
Flood proofing
Flood proofing can be achieved through a variety of structural and non-structural adjustments to the
design, use or contribution of buildings.
Raising properties above the flood level – known as elevation – is a common response, especially when
the properties are detached. Examples include traditional long-houses in the Malaysian rainforest, and
new properties in Maidenhead, England where the level of the door is above the 1947 flood limit.
Dry flood proofing involves the sealing of a property so that flood waters cannot enter. It is only
feasible for areas of low-level floods (<1m).
Wet flood proofing allows flood water to enter buildings but cause minimal damage. Ground-level
floors may be tiled or concreted and hooks in the ceiling hoist small items up when a flood is
imminent. This tends not to be used widely in MEDCs.
In a survey of 1500 homeowners in Louisiana, Illinois and Wisconsin (Figure 4), the majority of
homeowners adopted some form of dry flood proofing. Most homeowners only used a method once
they were flooded – preventative methods were not common. In addition, most used specialist
contractors to do the work, suggesting that greater technical expertise would lead to better results.
Figure 4 – Types of flood proofing retrofitted in Louisiana, Illinois and Wisconsin (survey of 1500
homes).
Type
Wet flood proofing
Raised furnace/heater/appliances
Raised wiring/fuse box
Stopped using basement
Sub-total
Percentage
10
4
7
21
Dry flood proofing of a basement
Glass-bricked windows
Protected basement openings
Installed sump pump
Water-proofed basement walls
Added dirt fill next to house
Sub-total
4
6
16
11
11
50
Prevention of sewer back-up
Installed back-up valve
Installed standpipe or plug
Sub-total
8
8
16
Protection from surface water
Built wall around house
Built levée or berm
Improved drainage next to house
Sub-total
2
2
9
13
Total percentage
Number of responses
100
1538
Land use management
Increased physical protection offered by structural defence has led to increased floodplain
development. This is known as the ‘levée’ or ‘escalator’ effect. Studies of floodplains in England and
Wales show that increased floodplain development following flood defence projects is widespread
(Figure 5).
Figure 5 – Profile of floodplain development trends in six urban locations in England and Wales.
Estimated number of
properties in the 1:100
floodplain
Location
River Thames,
Maidenhead
River Twyi,
Carmarthen
Date of
completion of
flood defence
project
1960–66
1984
Nature of flood defence
project
Channel improvements
to Maidenhead
Ditch/Cut
Flood wall
improvements; new
flood walls
Flood
defence
design
standard
1:10/20
Pre-project
start (date)
1560 (1947)
Post-project
end (date)
3303 (1989)
1:100
0
476
Black Brook,
Loughborough
1968
1979
River Tone,
Taunton
River Stort,
Bishop’s Stortford
1969
1979
Channel realignment;
embankments;
embankment/drainage
improvement
Channel improvement;
new weirs
Channel improvements;
new weirs
1:100
476 (1968)
672 (1988)
1:70/100
795 (1964)
912 (1989)
1:70
129 (1976)
210 (1989)
Living with floods
In the USA they believed, formerly, that hard engineering could deliver all of the answers.
Comprehensive flood management recognises that this is not the case and that nature has a role to play.
Even in the Mississippi, comprehensive planning is being developed (Figure 6) using a combination
of levées, upstream reservoirs, land use planning, relocation of vulnerable premises, and turning
frequently flooded areas into parks. It is proposed that key facilities and infrastructure should be
located on higher ground and farmers in low-lying areas should convert to pastoral farming. All this
would require a redirection of existing subsidies away from purely hard engineering to forms of soft
engineering such as afforestation, wetlands restoration and wetland re-creation. Dwellings built on
stilts represent another form of protection.
Figure 6 – An artist’s impression of future comprehensive flood management around the Mississippi.
Floods in Bangladesh
The risk of flooding in Bangladesh is well known. Some 80% of the country is floodplain and 700
rivers make up a total river length of over 22,000km. The rivers carry about 2.5 x 10 9 tonnes of
sediment, and most of the coastal region is at risk of flooding in the monsoon season. Moreover, the
Bangladeshi population is very poor, with an average GNP of less than $500, the country has high rates
of population growth (2.3% pa) and low levels of literacy (40–50%).
By 1990 Bangladesh had over 8,000 hydraulic structures and 7500km of levées protecting
approximately 2.5% of the land. These included:

Levées among the main rivers, such as the 217km-long Bramaputra Right Embankment

Freshwater polders – multi-purpose schemes designed for flood protection and farming

Submersible levées – small levées to project the paddy rice in the pre-monsoon flood period

Levées along small rivers – mainly in upland areas to protect against the floods

Coastal-zone polders to prevent seawater intrusion and to protect against storm surges.
In the floods of 1987 and 1988 these defences proved inadequate. For example, in 1987 over 1200km
of levées failed, leading to losses of US$0.5 billion; in 1988 the figures were almost 2000km and
US$1.3 billion.
River engineering in Bangladesh will always be difficult since 90% of the water originates outside the
country. The levées prevent the river from depositing in the floodplain, therefore the river deposits its
load in the river bed. In addition, as the floodplain is not being replenished with sediment, and as sea
levels rise, they will flood more of the floodplain with the result of the submergence of large areas of
the country. Levées will also increase the flood risk downstream.
Nevertheless, the scope for non-engineering schemes is limited, owing to the scarcity of flood-free
land, and the widespread poverty and illiteracy. Land zoning is also difficult owing to the very high
population densities. However, some alternative measures exist, such as:

More raised flood shelters with health services, water and sanitation

Improved storage facilities to store cooking equipment and tools

Post-disaster employment for flood victims repairing levées

More boats for use in emergencies

Flood-resistant infrastructure

Flood-resistant housing.
Protection against flooding in Bangladesh is bound up with development. Poverty and illiteracy are
major problems to be overcome. Only the literate population appear in favour of flood adjustment – the
poor tend to be more fatalistic. In addition, it is the richer (less vulnerable) who are able to move to
safer areas – the poor have limited choice but to remain.
The notion of ‘living with floods’ seeks to gain benefits from the annual floods whilst reducing the
risks during the hazardous events.
Managing the flood risk in Oxford
Flooding occurs in Oxford for a number of reasons:

The rivers drain a large area.

It is relatively flat and low lying.

The bedrock is impermeable clay.

Two main rivers converge in Oxford.

Many bridges hold back water.

Floodplain development has increased the amount of impermeable surface area.

Agricultural practices have led to a reduction in tree cover in the catchment area and an increase
in the amount of water flowing over the surface and into the rivers.
The main ways in which the flood risk at Oxford has been managed has been through a mix of land-use
zoning and flood-relief channels. In addition, levées, channel scour and straightening have all been
used. However, according to the Environment Agency (EA) they might not be enough and there are
major plans to overhaul and improve Oxford’s flood defences.
Managing flooding in Oxford
In December 2000 the Thames overspilled its banks and damaged 160 houses built on low-lying land.
A similar flood in January 2003 damaged 250 homes (Figure 7). After years of study, the Environment
Agency believes that the most effective solution is an 8km-long, 25m wide, flood relief channel. The
£100m flood relief scheme could see Oxford bypassed by a water channel of similar size to the River
Thames. A 25m-wide channel running from the Godstow Lock near Wolvercote to Sandford Lock
south of the city is the centrepiece of a new flood defence strategy. The proposed channel would make
full use of all the weirs, tributaries and streams as escape routes for floodwater. The scheme would also
see the widening of the Bulstake, Hinksey, Seacourt and Weirs Mill streams (Figure 8).
Figure 7 – Impacts of flooding on St George’s Meadow, which is part of Oxford’s floodplain.
Figure 8 – Flood relief channels in Oxford.
However, any scheme incorporating a flood relief channel through the western corridor has the
potential for impacts upon the Oxford Meadows Special Area of Conservation and Iffley Meadows Site
of Special Scientific Interest. The potential impacts tend to be greater to the north, where valuable
instream and marginal habitats could be lost because of the widening works. However, a similar
scheme at Maidenhead is said to have saved at least 1300 homes from flooding during periods of high
risk. Construction is likely to begin in 2010.
In addition, new pumps have been installed in a well at Osney Island. The pumps automatically switch
on when the water level reaches a pre-set level, then water drains away from the well through three
new gullies in Earl Street and a path between Earl Street and Duke Street. There is also a new filter
drain along the path to help control the high water in times of flooding.
The EA has considered other possible solutions, for example building above-ground reservoirs
upstream of Oxford, but dismissed the idea as expensive and unworkable. Another option is to hold
water back, not in reservoirs, but in wetlands and floodplains. For decades, the EA and its predecessors
have been energetic drainers of farmland, with the channels of rivers and streams having been
deepened and straightened. But, in this era of surplus farm produce, critics believe that instead of
moving water downstream as fast as possible, the EA should try to hold it back in wetlands, and restore
rivers to a more natural state, with their former meanders, rapids, riffles and gravel beds.
Changes in land use to reduce run-off would offer major potential benefits that would be catchmentwide. However, the cost would be very high, and benefits would be achieved only over a long period of
time. But this action ignores EU agricultural reforms that are looking to remove production subsidies
and providing extra funds for agri-environmental programmes.
Yet a small number of pioneering projects backed by the EA are showing the way forward. At
Sherborne, Gloucestershire, the National Trust has restored extensive riverside water meadows, with
the help of agri-environment payments from the Environmentally Sensitive Area scheme, while the EA
has reinstated bends and spawning beds on adjacent stretches of the River Windrush. Likewise, on
Otmoor, near Oxford, the RSPB has raised water levels on 267 hectares of formerly arable land
purchased in 1997, and also created a 22-hectare reedbed reservoir with the help of the EA. The reserve
now has nationally important numbers of teal, pintail and shoveler ducks, and dragonfly, and holds
back upwards of half a million cubic metres of winter rain. Unfortunately, such initiatives have always
been under-funded relative to hard-engineered drainage and flood relief schemes.
Managing the Lavant
In 1991, the government sanctioned the diversion of the River Lavant to prevent Chichester from
being swamped by floods. The River Lavant, so insignificant in summer that it dries up, becomes a
torrent in winter and devastates Chichester, West Sussex. A £4.7 million scheme to divert the river on
a new 8km overflow route was approved by the government and the Environment Agency. It is part
of £51 million worth of anti-flooding schemes.
The Lavant caused the flooding of 750 homes, shops and factories and cut off the A27, the east-west
main road, in 1993–94 (Figure 9). In 2001, the EA realised it was about to happen again and called in
the army. They pumped water away and built emergency flood channels.
Figure 9 – Map showing area flooded by the River Lavant in 1994.
A combination of climate change and changed farming practices in the hills above Chichester, which
makes the volume of water greater and the run-off faster, means that the EA estimates the flooding will
occur once every 12 years if nothing is done. Although the scheme makes the river 9km longer, it will
return it to the ancient riverbed that medieval people diverted it from. Flooding is now expected to
occur only once in 75 years. Whether the original diversion was to beautify the city, flush the sewage
away or provide a drinking water supply is not known. Either way, the river was prevented from
following the river valleys down to Pagham Harbour and made to turn west through the city to
Fishborne Creek.
The problem of the current River Lavant in Chichester is that for part of its length the river is
channelled into two culverts, which at times of peak flow cannot cope with the water. Therefore a grant
of £3 million towards work was made by the government. Most of the money will be spent on building
tunnels and bridges to take cars and pedestrians above the new flood river, which will operate as soon
as the normal River Lavant flow is exceeded.
Between Christmas Day 1993 and 10 January 1994 over 30cm of rain fell on Chichester. The result
was a flow on the River Lavant four times larger than normal. Floods can last for many days because
the river is fed by natural chalk aquifers, which overflow following heavy rain. Apart from the risk of
the culverts not being able to take the volume of water, there is the potential for debris washed down in
the flood to block them, leaving the water no escape, except through the crowded city centre. The new
channel should overcome these problems.
Conclusion
Rivers are a vital part of modern society and there are increasing risks of flooding as more people live
in vulnerable locations. Methods of river management are varied and may even increase the risk of
flooding in some areas. The risks are not confined to LEDCs such as Bangladesh, but are just as
important in local situations in MEDCs. The examples of Oxford and Chichester illustrate the local
nature of the flood risk problem in the UK, compared with Bangladesh for example, and also just how
expensive it is to manage rivers. Moreover, there are increasing calls for river engineering to adopt a
holistic floodplain approach (comprehensive planning), which integrates methods of soft engineering
with some of the more traditional hard engineering approaches.