Embed Size (px)
Citation preview
2 Regional overview This chapter describes the main physical and environmental characteristics of the plan area. It also
explains how these factors may help or hinder how we manage flood risk.
We look at the topography, soils, geology, land use, drainage system and hydrology of Thames region.
These characteristics provide the background to help understand what causes floods. We then describe
other regional characteristics and explain why and how they impact upon flooding.
In this Chapter we have presented a high level overview of the physical and human characteristics at a
regional or basin scale. Where a particular feature of the existing risk as been important in deciding what
approaches to adopt to manage future flood risk and select policy, this is drawn out in more detail at a
policy unit scale in Chapter Six.
The Thames region is one of eight regions of the Environment Agency. Located in the south-east of
England, the region covers less than 10% of England and Wales (figure 2.1). Although comparatively
small by area, in terms of population and economics, it is one of the most important areas of England and
Wales. Nearly a quarter of the population of England and Wales lives and works in this region, producing
more than a quarter of the Gross National Product1. This is emphasised by the statistics in table 2.1.
Flooding, which affects the population that live and work in this region, can also have a major impact effect
on the whole economy of England and Wales.
Figure 2.1. Thames Region location plan
1 Gross National Product (GNP) – GNP measures the value of goods and services that a country's citizens produce. GNP is one measure of the economic condition of a country, under the assumption that a higher GNP leads to a higher quality of living, all other things being equal.
24
Statistic England and Wales
Thames Region Thames as a % of national total
Total area (km²) 151,200 12,900 8.5
Area of 0.1% AEP2 floodplain (km²) 18,000 1,200 6.7
Properties within 0.1% AEP floodplain (tidal and fluvial)
2,228,400 705,700 31.7
Population at risk from a 0.1% AEP flood (tidal and fluvial)
5,014,000 1,588,000 31.7
Area of internationally designated sites within the 0.1% AEP floodplain (km²)
14
Length of main river (km) 40,100 5,400 13.5
Length of fluvial raised defences (km) 350
Length of tidal raised defences (km) 221
Annual expenditure on flood risk management (£)
425,646,000 (England only)
85,230,000 20 (England only)
Table 2.1. National and Regional Comparison
When compared with other Environment Agency regions, the Thames region has the smallest area of
floodplain for a 0.1% AEP flood. However, due to the large number of properties within this floodplain, it
contains the highest density of properties at risk from fluvial flooding in England and Wales (560
properties/km²).
Away from London, there are a number of densely populated urban areas. These include Oxford, Swindon
and Reading to the west, Luton and Stevenage to the north and Guildford and Crawley to the south. The
locations of these are shown on figure 2.2, along with the major infrastructure.
2 AEP = Annual Exceedance Probability (This is the likelihood of a particular flood event occurring over a period of one year, expressed as a percentage)
25
The Thames CFMP covers almost the entire area of the Environment Agency Thames region. This is
made up of the River Thames basin and its tributaries, most notably the River Lee (figure 2.3). The River
Thames consists of a non-tidal and a tidal part (the tideway and the estuary). However, the Thames
CFMP only covers the fluvial and non-tidal part of Thames region. Our Thames Estuary 2100 (TE2100)
project is looking at the long-term flood management policies for the tidal part of the River Thames. Figure
2.3 shows the tidal part of the Thames region.
2.1 Definition and extent of the Thames CFMP
Figure 2.2 Location of major urban areas in Thames region
• contains the largest number of properties at risk of fluvial flooding
• has the smallest area of 1% AEP floodplain
Thames region covers approximately 10% of the total area of England and Wales. Compared to other Environment Agency regions, Thames region:
26
Figure 2.3 Overview of topography and the three river basins in Thames region
27
2.2 Topography The topography of Thames region is strongly influenced by the structure of the underlying geology, the
effects of glaciation and the fluvial drainage pattern. The region is bounded in the west by the Cotswolds,
with the steep limestone scarp slope on the western side and the long gentle dip slope to clay valley on
the eastern side. In the South East, the region is bounded by the North Downs, with the steep chalk scarp
slope on the south side and the long gentle dip slope to clay valley on the northern side. The Berkshire
Downs and Chilterns form a higher area positioned as a diagonal crossing the region. These higher areas
can be described as rolling chalk or limestone hills. The lower areas consist of wide, flat and clay river
valleys. Figure 2.3 shows a pictorial representation of this topography, indicating the major features.
Elevation is shown in figure 2.4. London and the lower Thames are very low-lying areas (less then 25m
above ordnance datum). Elsewhere, the land rises up to over 300m, for example in the Chilterns,
Berkshire Downs and Cotswolds. The relatively flat and low-lying areas of floodplain are clearly visible.
Figure 2.4 Thames region elevation (height above ordnance datum – 10m contours)
Locally, there are some steeper parts of the region, especially in the Chilterns, Berkshire Downs and some
areas of south London. This results in steep river profiles in these parts of the region. In Figure 2.5, river
gradients across Thames region are compared. It shows the height (in metres above ordnance datum) of
28
the rivers from their source, to their confluence with the River Thames. The headwaters of the Cherwell
(with its source in the Cotswolds), the Kennet (with its source in the Berkshire Downs) and the Wandle
have very steep channel gradients compared with most of the other rivers. Gradient is an important factor
in determining the hydrological response and in steeper catchments water levels can rise quickly after
rainfall, with little advance warning. Gradient is not the only factor, however. The Kennet and the Cherwell
have similar gradients, but the Kennet will tend to react slower to rainfall because the baseflow is largely
dependent upon groundwater levels. The Wandle is a very steep catchment, but reacts very quickly to
rainfall because it is also heavily urbanised. All of these characteristics act as a constraint and affect the
way risk could be managed.
0
20
40
60
80
100
120
140
160
180
200
0 25 50 75 100 125 150 175 200 225 250 275
Distance (km)
Hei
ght (
m)
Thames Cherwell Thame Kennet Loddon Colne Wey Mole Wandle Lee Roding
Figure 2.5 River gradients in Thames Region
The geology and topography of Thames region strongly influences its hydrological system. In general, it is a low energy system, with the rolling hills and wide, flat river floodplains making the hydrological response slow. In chalk areas, river flows are generally low, filled slowly by groundwater. At the regional scale it is a long time before rainfall has an effect on water levels in the river. This affects the way we manage flood risk Exceptions are in the steeper parts of the tributaries where water levels can rise more quickly.
29
2.3 Geology and hydrogeology Geology can have a great influence on how certain catchments respond to rainfall and also how flood risk
can be managed. Thames region sits upon a mixed but distinctive geology. The following key geological
categories are all found within the region: chalk, limestone, gravel, sand and clay. Each has different
characteristics that have an effect on the water cycle. The regional variations in geology type are shown in
figure 2.6
The characteristics of chalk (Berkshire Downs, Chilterns, Hampshire Downs, Hog’s Back and North
Downs) and limestone (Cotswold) areas mean that water can infiltrate quickly, and move within and
through these rocks. Rainfall in these areas (termed aquifers) becomes part of the major groundwater
resources of Thames region. The groundwater from the chalk and limestone provides a significant
baseflow component to the rivers in Thames region. Water flows slowly through the aquifers and is
released at a slow rate into the rivers. Within these chalk and limestone areas, the impact of rainfall will be
spread out over a relatively long period of time.
The valleys are mainly formed by clay and sand sediments, which are not very permeable. To the west of
the Berkshire and Chiltern diagonal, the valleys consist of clay in a mixture with sand and gravel. To the
east of the Berkshire and Chiltern diagonal, the valleys consist of gravel and sand. The London basin and
the estuary are made up of mainly clay. Water does not infiltrate quickly in these areas. A higher
percentage of rainfall will run off directly into watercourses. Rainfall can quickly affect water levels and
generate short, high peaks in river levels. Highly urbanised areas, especially London, also have these
characteristics. Impermeable man-made surfaces can make the hydrological system respond even faster.
There will be less time for flood warning in these areas than in the chalk and limestone areas.
The underlying geology also determines the characteristics of groundwater flows. Groundwater can occur
in low-lying areas a long way from any watercourse. The on-set of flooding from this source can be linked
to fluvial events, but can also occur independently. Water movements of this type are important factors
when considering floodplain defences and physical barriers.
30
Figure 2.6 Thames region bedrock geology
© Crown copyright [2007]. All rights reserved. Licence number 100026380
The geology of the region strongly influences the characteristics of its hydrological system and as a result, the nature of flooding. In general, impermeable catchments are more responsive to rainfall and the water reaches the river system much faster. In permeable catchments, a higher percentage of the rainfall is retained by the soils and released into the rivers at a slower rate. This has implications for the way that we manage flood risk in Thames region.
2.4 Fluvial geomorphology Fluvial geomorphology is the study of the erosion and sedimentation within the river catchment and river
channel. Erosion is caused by a combination of factors and can be natural or man-made. Natural factors
cannot be changed, but man-made factors possibly can. We are interested in the sediment that reaches
the rivers and can affect flood risk management. In a national context the Thames region has a low level
of erosion. Information provided by the Environment Agency land quality policy team has been used to
produce a regional overview of relative erosion levels (figure 2.7).
Natural sedimentation reaching watercourses is calculated using the following parameters: 1) the
vulnerability of land to rill and gully erosion. 2) the vulnerability of land to run off and soil wash. 3) the
sediment transport to rivers. In the Hampshire Downs, North Downs, Berkshire and Marlborough Downs
and in the south parts of the Chilterns and Cotswold low levels of natural erosion reaches rivers. Relatively
31
high levels of sediment reaching rivers are found in the Weald, Northamptonshire Uplands and the
southern part of the Thames Basin Heaths.
The contribution of man made erosion is calculated by the density of crops associated with increased
erosion and the type and density of livestock. Due to the small amount of agricultural land, there is little
man made erosion reaching the rivers from the city of London. As you move to the west of the region, the
amount of man made erosion increases.
We are interested in the combination of natural and man made erosion affecting rivers. In London the
levels of erosion are generally low. Spreading to the west of the region, the chance of sediment reaching
the rivers as a result of erosion becomes higher. The areas of highest erosion pressure are in the
Northamptonshire Uplands, Upper Thames Clay Vales, Thames Basin Heaths and Weald.
Figure 2.7 Indicative map of combined erosion pressures (natural & man-made) in Thames region
The level of erosion determines the amount of sediment that is deposited in watercourses. Sedimentation
can reduce the capacity of the channel in certain locations. This reduces the channel capacity and hence
the flow within the channel, which, in turn, increases the likelihood of flooding. The process of erosion,
transport and deposition and the effect this has, can influence the way we manage flood risk. Reducing
the level of sediment in the rivers to maintain the channel capacity could result in high maintenance costs
32
or damage to the environment. However, reducing erosion in the catchment and preventing sediments
entering the watercourses can be advantageous. It can reduce localised flood risk and maintenance costs,
and also deliver wider environmental and social benefits.
Within the watercourses we need to consider the impact of human activity. The management of the river
corridor and channel network by regulating the flow regime and changing the channel morphology has an
impact on flood risk management.
2.4.1 Channel morphology
The Water Framework Directive (WFD) updates all existing European water legislation and promotes a
new approach to water management through river basin planning.
It requires all inland and coastal water bodies to be at ‘good ecological status’ by 2015. Within England
and Wales, we have statutory duties to ensure that objectives are met by this time. A ‘good’ ecological
status depends on a number of elements, including hydromorphology. This refers to the physical structure
of a river (channel morphology), the flow and the water level.
We carried out an initial broad brush assessment of the current pressures and impacts on the rivers in
Thames region to characterise the unique nature of each catchment. The results were published in River
Basin Characterisation reports in 2004. In this CFMP we are interested in the morphological pressures
which are those that affect the physical modification of a water body and can include flood defences,
hydropower and changes due to navigation. These changes can alter water and sediment movements,
and can also affect natural habitats.
Each watercourse has been classed as having high, moderate or low levels of morphological pressure.
This is shown in figure 2.8. Watercourses with high or moderate levels of morphological pressure may
have fast flows or a reduced channel capacity. Both characteristics have an impact for flood risk
management. 36% of the rivers in Thames region are at high risk of failing to achieve a ‘good ecological
status’ in relation to channel morphology under the WFD. Small tributaries in the upper parts of
catchments generally have lower levels of morphological pressure than major rivers or rivers in urbanised
areas. For example in London, the rivers have been heavily modified for flood risk management purposes.
In the south London catchments, almost 50% of the watercourses have been modified and over 20% are
in culverts. As a result, only 1% of the rivers in London are at low risk of failing to achieve a ‘good
ecological status’ in relation to channel morphology under the WFD.
33
Figure 2.8 Map of the level of morphological pressure in Thames region
Physical alterations to a river can alter water and sediment movements, and can also affect natural habitats. One of the objectives of the Water Framework Directive is to restore natural processes and fluvial environments. This will become a major consideration in future river and floodplain activity. In a national context, Thames region has a low level of erosion. The highest combined levels of both natural and man-made erosion generally occur to the west of the region, particularly in the Northamptonshire Uplands and the Upper Thames Clay Vales.
2.5 Soils Soil can reduce the risk of flooding by slowing the passage of heavy rainfall to surface waters. The type of
soil is related to the underlying geology. Chalk and sandstone areas with lithomorphic (shallow soils
formed over bedrock) and brown soils (widespread, found mainly on permeable materials, at elevations
below about 300m and mostly in agricultural use) have a high infiltration rate, due to loosely packed soil
particles. Rainwater can easily infiltrate into the soil and recharge the groundwater layer. Groundwater will
be released slowly into the rivers. These areas respond relatively slowly to a rainfall event. Figure 2.9
shows the major soil types in Thames region.
34
The areas with mainly clay soils (raw gley soils, that occur in mineral material that has remained
waterlogged since deposition and surface water gley soils, that are seasonally waterlogged slowly
permeable soils), especially the London Basin, have a low infiltration rate, due to the densely packed
layers of clay. Less rainwater will be absorbed by the soil, and, as a result, these areas respond faster to
flooding. This has important implications for flood risk management.
Erosion can reduce the topsoil layer, which means it is unable to retain as much water. When high levels
of erosion occur in the chalk and sandstone areas, they will respond faster to rainfall and any subsequent
flooding will occur more quickly.
Figure 2.9 Indicative map of major soil types within Thames region
35
Soil characteristics are related to underlying geology and can affect the movement of water through a catchment. Chalk and sandstone soils have a higher infiltration rate than clays and therefore these areas will respond slower to rainfall. The predominant soil types in Thames region are surface-water gley, brown and lithomorphic soils. At a local scale, adopting specific soil management practices can have a positive impact on rates and quantities of runoff.
2.6 Land use and land management Land use within the floodplain is an important consideration for flood risk management. Changes in the
way the land is used within the floodplain could affect both flooding and flood risk management measures.
The landscape of the Thames region varies considerably. The western parts of the region are mainly rural,
with a majority of arable land, combined with grassland, woodland and some dispersed urban areas. The
north and the South East also have expansive areas of rural land, with relatively large amounts of forest
and woodlands in the south-east part of the region. However, urban land use is increasing due to urban
expansion and new development. The land in the north is mainly arable, with some urban areas. In the
eastern part, the heavily urbanised Greater London dominates the land use, constrained by the Green Belt
(an area of rural land use). This regional variation is shown in figure 2.10.
Figure 2.10 Land use across Thames region (Land Cover Map 2000)
36
The land use within the region is constantly changing, often as a result of human pressures. From the late
nineteenth century, arable land decreased, as grasslands became more prominent. However, when there
was a need to increase food production during World War II, grassland was transformed into arable land.
Post war urbanisation led to less land being used as arable and grassland, but rural land use has
subsequently become more intensive. Throughout all the changes of the last 40 years the woodland area
has increased. The continuing intensification of farm management, together with increased development
pressure will influence land use change in the future.
The Agricultural Land Classification (ALC) provides a method for assessing the quality of farmland in
England and Wales. It classifies land between Grade 1 (excellent) and Grade 5 (very poor) and is used by
Defra and other organisations, primarily for planning purposes. The classification gives an indication of the
long-term potential quality of the land, although it does not necessarily relate to the current use of the land.
Factors affecting the grade are local climate, gradient of the land, underlying soil type and risk of flooding.
Just over 60% of the land in Thames region is classified as the ‘best and most versatile’ land (ALC grade
1, 2 and 3a). This is considered to be the most flexible, productive and efficient land which can best
deliver future crops for food and non-food uses. Only 5% of the total area of the ‘best and most versatile’
land is within the 0.1% AEP flood extent.
The floodplain in Thames region is mainly natural. Almost 70% of the 0.1% AEP fluvial floodplain is arable,
grassland or woodland, and this is mainly in the northern and western parts of the region. However, 10%
of the floodplain is suburban or rural development. Approximately 15% of the floodplain area is continuous
urban land use, mainly located in the Greater London area. Figure 2.11 provides an overview of the
current land use within Thames region as a whole and also within the 0.1% AEP floodplain.
Regional Land Use Land Use (0.1% AEP Floodplain)
WoodlandArableGrasslandInland WaterBare GroundSuburbanUrbanOther
Figure 2.11 Overview of regional land use and land use within the floodplain.
37
The changes in land use, together with intensification in land management, have resulted in increased
rainfall run off in rural areas. This has also occurred where land has been developed. Flooding will occur
quicker with high peaks and less time for flood warning. With the increase in urban land use, not only has
the time to peak decreased, there are now also more people at risk of flooding.
The high percentage of arable, grassland or woodland in the upper and middle parts of the region could
potentially provide both environmental improvements and flood risk management opportunities. For
example, the regular flooding of grassland can create important wetland habitat. All unimproved grassland
within the floodplain falls into this category (just under 35% of the total area of floodplain in Thames
region). Improved and semi-improved grasslands could possibly become functioning wet grasslands again
by altering water levels, through, for example, flood risk management activities.
2.6.1 Land management Land management is the process of managing the use and development of land resources. We know that
certain ways of managing the land can significantly reduce the amount of rainfall that finds its way into a
catchment’s river network as local surface runoff. Examples include: using cover crops, minimum tillage,
cultivating and planting across slope, targeted use of grass strips and restricting the grazing period.
However, some of these practices may have a negative impact on crop yields and none are likely to be
successful in all situations. Most practices need to be carefully targeted to areas of specific topographic,
soil, cropping and climatic conditions.
A number of funding schemes and initiatives have been developed that include options to change land
management practices. Some of these may help to reduce localised low-order flood events. The recent
reform of the Common Agriculture Policy (CAP) and associated environmental stewardship scheme,
provide opportunities for this kind of land use management change. In urban areas, Sustainable Urban
Drainage Systems (SUDS) can help to reduce and control the amount of run off.
Almost 70% of the 0.1% AEP fluvial floodplain in the plan area is arable, grassland or woodland. This large proportion of undeveloped floodplain offers many opportunities for extensive flood risk management options that can also provide environmental enhancements, for example wetland creation and river restoration. Changes in land management can reduce the amount of surface runoff at a local scale. Within the built environment local drainage systems (for example Sustainable Urban Drainage) can have a positive impact on the quantity, quality and timing of runoff entering the river system. In rural areas initiatives such as the Defra environmental stewardship scheme, encourage landowners to adopt practices that benefit the environment as well as reducing localised flooding. Decisions about the use and management of land have the potential to radically change the consequences of flooding.
38
2.7 Hydrology Hydrology is one of the most important characteristics in understanding fluvial flooding. Effective and
sustainable flood risk management needs to be based on an understanding of the source of flooding, the
responses of the river system, interaction of the tributaries and scales of impact. All of these aspects stem
from the catchment hydrology.
2.7.1 Sources In Thames region, rainfall is the cause of most fluvial floods. Over the period 1961 to 1990, the long-term
average (LTA) rainfall for the region was 696 mm per year, compared with the national average of 897
mm. This makes Thames one of the driest regions of England and Wales. The amount of rainfall does not
vary much throughout the year, fluctuating between 46 and 71 mm per month. This is shown in blue in
figure 2.12 below.
Figure 2.12 Long term average (LTA) annual rainfall, soil moisture deficit and effective rainfall in Thames region
The average annual rainfall varies across Thames region (see figure 2.13). The south and west parts of
the region, together with the Chilterns, receive on average, more rainfall each year than the eastern parts
or central London. The Thames basin has an annual average rainfall of approximately 719 mm, compared
with 637mm in the Lee basin. These variations in the amount of rainfall, together with the effective rainfall
information, are important considerations for flood risk management.
0
20
40
60
80
100
120
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
mm
LTA rainfall LTA SMD LTA effective rainfall
39
Figure 2.13 Average annual rainfall in Thames region (1961 – 1990)
40
The characteristics of a rainfall event (for example the amount, duration and timing of the rainfall) have an
important influence on the likelihood and extent of flooding in a particular catchment. The basin-wide
floods of Autumn 2000 and New Year 2003 were mainly due to widespread and prolonged rainfall.
However a period of intense, widespread rainfall in April 1998 only led to flooding on the Cherwell.
The catchment characteristics are also an important factor in determining the nature a flood event, for
example geology, soil type and land use. They may affect flood risk by varying a catchment’s capacity to
attenuate rainfall and therefore, the amount of runoff that occurs (runoff is the proportion of rainfall that
finds its way into a catchment’s rivers and streams). Figure 2.14 shows the variation in the level of
potential runoff generation across Thames region. The map was produced using a simple classification of
three variables that affect runoff generation – topography, geology and land use. Low-lying, woodland or
grassland areas overlaying sand or chalk would have a low score, as this combination of factors would
produce a small amount of runoff. For example in the areas to the east and south east of Reading
(Blackwater Valley). In comparison, urban areas with clay geology would produce high levels of runoff, for
example in the lower Lee basin and in London.
41
Figure 2.14 Potential levels of runoff generation across Thames region
42
The size, slope and shape of a river basin are particularly important. A larger basin will tend to catch more
rainfall, but the time lag between rainfall and runoff peaks (time to peak Tp) and the duration will tend to
be longer. A steeper basin will tend to respond quickly to rainfall because the runoff is likely to occur
faster. This leads to a peaked hydrograph.
A compact basin will generally result in a highly peaked hydrograph, compared with an elongated basin
that is likely to have a lower, slower response. This is because water draining from across a compact
basin travels a similar distance and is likely to arrive at a similar time. In an elongated basin water travels
far further from the extremities of the basin than from the core and hence arrives over a longer period of
time.
Finally the drainage network will have an impact on the resulting hydrograph. Increasing channel length
with a given basin will generally increase the collection and transport of runoff. This is often referred to as
the drainage density, with greater density tending to result in greater efficiency of drainage. The impact of
all these factors on a catchment’s flood hydrograph is illustrated in figure 2.15 below.
Figure 2.15 The impact of catchment characteristics on flood hydrographs
In addition, the condition of a catchment prior to rainfall can have a crucial impact on how rivers react. A
moderate rainfall event on a saturated catchment can cause more flooding than prolonged rainfall on a dry
catchment. This variable is known as the soil moisture deficit (SMD). It measures how much water can
infiltrate into the soil before it becomes saturated. Once the soil becomes saturated, excess runoff starts to
occur. In the summer when the soil is relatively dry, the SMD is high and rain can infiltrate into the soils. In
certain conditions, the rain might be completely absorbed by the soil. In winter when the soil is wet, and
may be already fully saturated, the SMD is low. As a result, only small amounts of water can infiltrate into
the soil. This occurred in January 2003 when heavy rain fell onto catchments that were already saturated
43
from weeks of prolonged rainfall. This caused subsequent flooding along the length of the River Thames
and on a number of its tributaries. The seasonal variation in SMD is shown in green in figure 2.11.
The ‘effective rainfall’ is the amount of rainwater that actually runs-off or passes through the soil. In the
Thames region during the summer, most of the rain will infiltrate into the topsoil, so the amount of effective
rainfall is low. During the winter months, some of the rain will infiltrate into the topsoil, but most will either
pass into the ground or form run-off (see figure 2.12). In the Thames basin, 34% of the annual average
rainfall forms runoff. In permeable catchments, such as the chalks of the Kennet and the Upper Lee, the
rain will pass through the soil, recharging the groundwater, and water levels in rivers will increase slightly.
Permeable catchments therefore respond slowly to rainfall. In impermeable catchments, the effective
rainfall will run-off directly into the rivers, producing a much faster response to a rainfall event. For
example, in the urban areas of the Lower Lee, catchment run-off is approximately 70%. These areas are
liable to sudden flooding during the winter, especially after storm events.
There can also be occasions when permeable catchments can act in a similar way to impermeable ones.
For example, in 1947, heavy rain fell on frozen (and therefore impermeable) catchments, resulting in high
runoff rates and a major flood event. Runoff can also increase locally in rural areas after a long, hot period
of weather or if the ground has been compacted, as a result of land management practices for example.
This makes it harder for the rain to infiltrate through the soil. Therefore, a catchment’s hydrological
response can be independent of the SMD.
Flooding can occur in Thames region from the following sources; fluvial, tidal, pluvial, groundwater, stormwater drainage (including highways), sewer systems and failure or overtopping of water control structures. The CFMP focuses on fluvial flood events, resulting mainly from rainfall. The nature of the rainfall and the preceding catchment conditions can have a large influence on the hydrological response of the river system and the extent of the flooding that occurs. However, fluvial flooding rarely happens in isolation. Many developed areas (and London in particular) have sites where the river is not the only source of flood risk. Urban drainage can be a major source and this includes surface water drainage, sewer networks and highways. Understanding and tackling these sources will require a collaborative approach between responsible organisations.
2.7.2 The river system of Thames region The River Thames is the major river in Thames region. The region is made up of three basins; the River
Thames basin, the River Lee basin, and ‘the London Rivers’ (see figure 2.3). The major river basins
provide a useful division with which to analyse and report the flood risk. Figure 2.16 provides more detail
on the river system and identifies the major tributaries.
44
We will now look at the hydrology of each of the three river basins in turn. The analysis will give us an
understanding of the hydrological characteristics of the major tributaries and the interaction of these rivers
with the major watercourse in each basin (the Thames or the Lee). In turn, this will give us an insight into
how a chosen policy may impact on an individual tributary and also the whole basin.
To do this we will look at data from previous flood events and also results from the Thames and Lee Broad
Scale Models (BSMs) and other selected hydrological models. The BSM was undertaken to improve our
understanding of catchment processes and to test the effectiveness of possible strategic solutions for
managing flood risk in the region. It allowed us to assess the potential impact of future changes in
parameters such as urbanisation and climate change and the effect of different flood risk management
options on flow. The Thames BSM was run for a 10% and 1% AEP flood event and the Lee BSM was run
for a 1% AEP flood event.
It was constructed using the iSIS hydraulic routing software. The Flood Estimation Handbook (FEH)
rainfall-runoff method was used to provide inflows to the model. The three main parameters in the FEH
rainfall-runoff model (time to peak, standard percentage runoff and the baseflow) were calibrated using
observed rainfall and flow data. The FEH rainfall-runoff method assumes static storms and uniform rainfall
over the catchment. Some additional fine-tuning and calibration was required to account for the varied
geology and landuse of the region1.
1 For further information on the Thames and Lee BSM, please refer to Appendix F or the full reports (‘Thames Broad Scale Modelling for Catchment Flood Management Plans’ and ‘River Lee Broad Scale Model Report’) which are listed in the references section at the end of this document.
45
Figure 2.16 The river system in Thames region
46
Thames basin The Thames basin covers an area of approximately 9,950 km2 down to its normal tidal limit at Teddington
weir (see figure 2.3). The River Thames is one of the most intensely used and managed rivers in Europe.
It rises in the Cotswold Hills, with its traditional source at Thames Head near Cirencester, 104 m Above
Ordnance Datum (AOD) and flows 237 km downstream to Teddington, the normal tidal limit. Along the
way to Teddington, six main tributaries, the River Kennet, River Cherwell, River Loddon, River Thame,
River Colne and the River Wey join the Thames River (see figure 2.16).
From the upstream limit for navigation at Lechlade to Teddington, the average fall of the Thames is 0.34
metres per kilometre. The river width varies considerably along its length and is about 18 m wide at
Lechlade and about 100 m wide at Teddington.
The River Thames between Lechlade and Teddington is heavily controlled by a series of weirs, sluices
and locks. During times of normal flow, the Thames acts like a series of ponds that are fed via upstream
locks, with water levels controlled by downstream structures. For bankfull flows, the sluice gates on the
Thames are fully open and the water surface slope becomes closer to the natural channel bed slope. At
times of high flow, the Thames floods its large rural floodplain and the water surface is determined by the
floodplain flow.
At Kingston (just upstream of Teddington, see figure 2.16) the average flow is approximately 77 m3/s.
However, the Thames flow varies considerably throughout the year. During the summer, the flow can be
far lower than this average. To protect the environment, the flow is maintained at a minimum of 9.3 m3/s,
but in times of severe drought the flow may be reduced to 3.5 m3/s. In the winter, the flow may exceed 350
m3/s. The maximum flow recorded at Kingston was 800 m³/s on the 18th November 1894. By comparison,
the peak flow observed during the January 2003 flood was 472 m3/s.
The black line in figure 2.17 shows the variation in the daily average flow of the River Thames dating back
to 1883. The red areas indicate when the actual flow is lower than the daily average flow, for instance in
1976 when there was a major drought. The blue areas show when the actual flows are higher than the
daily average flows, for instance during the winter of 2000 and 2001 when flooding occurred in parts of the
Thames catchment. This figure shows at a glance the main flood events. It indicates that recent flood
events are not an exception, and floods of a similar magnitude have happened before.
47
Figure 2.17 Historical overview of daily mean flow of the River Thames at Kingston
48
Hydrological characteristics of the Thames basin (a) Analysis of previous flood events Figures 2.19 to 2.21 illustrate how the magnitude, concentration, intensity, location and timing
of a rainfall event across the Thames basin can affect the timing and volume of flow on the
Thames and its tributaries. Depending on the characteristics of this flow - the extent of
flooding that this can lead to is different. The flood events of 1998, 2000 and 2003 are used
as examples. It is important to understand the factors that caused particular flood events in
order to appraise potential flood risk management options.
Figure 2.18 shows the location of selected flow gauges and the corresponding rain gauges.
These were chosen to show how the different rivers across the basin responded to varying
levels of rainfall. In some cases, the selection was restricted by the availability of good quality
data for a particular flood event. Not all gauges are used for all three events.
The data recorded at each gauge during the flood event is shown in figures 2.19 to 2.21. For
each catchment, the rainfall plot is shown on the left, and the corresponding hydrograph is on
the right. The first set of graphs are the furthest upstream and all graphs cover the same time
period.
Easter 1998 There was widespread rainfall across the Thames basin, with each catchment receiving a
similar total for April. However, the graphs for the Cherwell and Windrush (figure 2.19) clearly
show a period of very concentrated, heavy rainfall in the early hours on the 9th April. The
resulting peak in flows on the Cherwell at Banbury is shown on the hydrograph
(approximately 55m³/s). This occurs just under 20 hours after the peak in rainfall. However
the peak doesn’t reach Windsor until another 100 hours later.
This led to a 1% AEP flood event on the Cherwell that inundated large areas of developed
and rural floodplain throughout the catchment. Over 200 properties were affected in Banbury
and up to 150 in Kidlington.
All the other tributaries in the Thames basin experienced relatively small peaks in flow and
there was no flooding. High flows were recorded in the lower Thames (almost 200m³/s at
Windsor) but they were contained within the channel and no flooding occurred. This volume of
flow occurs most winters at Windsor and is not unusual.
49
Figure 2.18 Location of selected flow gauging stations and corresponding rain gauges in the Thames basin
50
Figure 2.19 Rainfall data and hydrographs for selected locations in the Thames basin, during theEaster 1998 flood event
0
25
50
75
100
125
150
175
200
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
Time (hours)Fl
ow (m
3/s)
Thames (Farmoor) Windrush (Worsham) Cherwell (Banbury)
0
25
50
75
100
125
150
175
200
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
Time (hours)
Flow
(m3/
s)
Thame (Wheatley) Kennet (Newbury) Loddon (Twyford)
0
25
50
75
100
125
150
175
200
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
Time (hours)
Flow
(m3/
s)
Thames (Windsor) Colne (Denham) Wey (Tilford)
51
Therefore, this analysis shows that serious flooding in one catchment had a limited impact on
the Thames basin as a whole and no flooding was experienced further downstream, despite
the widespread rainfall across all catchments.
Autumn 2000 Unlike the event in 1998, during late October and early November 2000, the rain fell in five
major events (as illustrated in figure 2.20). The first three (28th/29th October, 1st/2nd and 5th/6th
November) caused flooding problems over a large extent of the Thames basin. Rainfall totals
were high across all catchments, especially in the south east of the region. Between 20th
October and 11th November, 220 mm of rain fell at Leatherhead on the Mole and 213 mm was
recorded at Farnham on the Wey. The lowest total was 132 mm at Grimsbury on the
Cherwell.
Each of the first three rainfall events led to a marked increase in flow on all the rivers in the
Thames basin (see figure 2.20). The greatest increase followed the first rainfall event on the
28th October. Flows at Kingston rose from approximately 45 m³/s on the 28th October to over
350 m³/s on the 1st November. They remained high and rose again to over 460 m³/s on the 7th
November. These peaks were exacerbated by high flows from the tributaries downstream of
Windsor.
The flows on both the Wey and the Mole were the highest since 1968 and approximately 300
and 70 properties were flooded in each catchment respectively. The peaks coincided with
those at Kingston, as shown in figure 2.20. Flows and levels to the west of the region and on
the River Thames itself although high, were not exceptional. It was the duration of the rainfall
event that caused the flooding in the upper Thames (all catchments upstream of Oxford). 160
properties were flooded in Oxford alone and the flood event was estimated to have an AEP of
7%.
Therefore, in a larger catchment event, the timing of the rainfall and peak flows on the
tributaries is an important factor in the level of flooding that occurs along the River Thames. In
2000, the peaks were all close together, maintaining the high flows on the Thames for a long
period of time.
New Year 2003 From the 21st to the 31st of December 2002, 86mm of rain fell across the region, more than
the average rainfall for the whole month. By late December 2003, the entire Thames
catchment was saturated and most rivers were flowing bank full.
As a result of heavy rain from the 26th December 2002 to the 1st January 2003 (see figure
2.21), river levels in upstream catchments including the Windrush, Thame, Kennet and
52
Figure 2.20 Rainfall data and hydrographs for selected locations in the Thames basin, during theAutumn 2000 flood event
53
54
Figure 2.21 Rainfall data and hydrographs for selected locations in the Thames basin, during theNew Year 2003 flood event
55
56
Cherwell were exceptionally high (even higher than in Autumn 2000). The upper tributaries
peaked into the Thames within a relatively short space of time. This, coupled with the already
high levels within the Thames contributed to its quick response and the subsequent flooding
along its entire length. The middle part of the River Thames experienced the biggest flood
since 1947.
All the catchments were saturated making them very responsive to rainfall. There were a
number of rapid increases in flow on both the Wey and Mole between 250 and 500 hours.
This led to an early peak at Kingston at 520 hours (some 60 hours earlier than the peak
upstream at Windsor).
In the upstream catchments, a rapid increase in flows on the Thame and the Kennet just after
500 hours resulted in a steady increase in flows at Windsor up to its peak at around 580
hours. Over 120 properties were flooded in Wraysbury (just downstream of Windsor)
Exceptionally high flows on the Cherwell and Thames led to approximately 120 properties
being flooded in Oxford. Here the event was estimated to have a 5% AEP.
As demonstrated for the 2000 event, the interaction between the major tributaries and the
River Thames was a significant factor in the cause of the flooding that occurred in 2003.
(b) Thames Broad Scale Model One of the applications of the Thames BSM is to understand catchment interaction,
supplementing what we have learnt from the analysis of the flood events described above. It
is especially useful as there has not been a recorded 1% AEP flood event across the whole
basin.
The methodology used to generate the design hydrographs is based on the rainfall-runoff
approach detailed in the Flood Estimation Handbook (FEH). This method assumes static
storms and uniform rainfall over the catchment - there are no temporal or spatial variations in
rainfall. A storm duration of 180 hours was used for the design scenario over the whole
catchment, using an FEH winter storm profile.
The hydrograph below (figure 2.22) shows the flows for the 1% AEP design event at four
major receptors on the River Thames (see figure 2.16 for the BSM node locations).
Flows on the River Thames increase with distance downstream. The peak flow at Oxford is
just over 300m³/s, whereas at Kingston, flows increase to over 700m³/s. This is a result of the
inputs from the tributaries. The timing of the peaks at Oxford, Reading at Windsor follow in
57
succession, however they all occur later than the peak at Kingston. This is due to the timing of
the peak flows on the tributaries downstream of Windsor (Colne, Wey and Mole). This is
partly because of the size of the catchments – these three make up approx. 25% of the
Thames basin. This interaction will now be described in more detail.
Figure 2.22 Modelled 1% AEP flows on the River Thames
Interaction between the River Thames and its tributaries Looking at the hydrographs for the tributaries in comparison with those for the Thames, we
can begin to explain their characteristics and the relationships between them. For a region-
wide rainfall event, figures 2.23 and 2.24 show how the level and timing of flows on the
tributaries, contributes to the flows recorded on the Thames (1% AEP).
58
Figure 2.23 1% AEP flows on the River Thames (at Oxford and Reading) and its major tributaries (upstream of Reading only). Please note that the flows for Oxford and Reading relate to the scale on the left-hand axis. The flows for the Cherwell, Thame and Kennet are plotted against the right-hand axis.
59
Figure 2.24 1% AEP flows on the River Thames (at Windsor and Kingston) and its major tributaries. Please note that the flows for Windsor and Kingston relate to the scale on the left-hand axis. The flows for the tributaries are plotted against the right-hand axis.
60
Of all the tributaries, the Wey has the highest flows (approximately 140 m³/s) and this is also
one of the most responsive catchments (peaking just before 150 hours). This correlates with
the rapid increase in flows at Kingston at this time. The Colne also peaks at a similar time,
however the flows are much smaller (approximately 65 m³/s).
Figure 2.25 shows that the Wey has the greatest percentage contribution to the total volume
at Kingston for both the 10% and 1% AEP events (approximately 11%). This is however a
proportionate contribution in relation to the area of the catchment. For each of the tributaries,
the volume contribution is very similar for both the 10% and 1% AEP event.
Figure 2.25 Catchment area (as a percentage of the total BSM area) plotted against contribution to volume at Kingston (as a percentage of the total volume) for the 1% AEP event
Figures 2.26 and 2.27 show how the volume contributions from the Upper Thames and the
major tributaries, to the total volume at Kingston, change over the course of a 1% AEP flood
event. For the first 150 hours, the majority of the volume is from the River Thames
downstream of Buscot, smaller tributaries (including the Windrush, Evenlode, Pang and Ock)
and the Mole (all shown as ‘Other’ on figure 2.26 and 2.27). However, as the event reaches
its peak, the contribution from the tributaries becomes more dominant. Between 150 and 200
hours, the Wey and Colne contribute to 30% of the total volume at Kingston compared to 40%
from the Upper Thames and Mole. This coincides with the peak flow at Kingston.
61
Figure 2.26 Breakdown by source of volume at Kingston for a 1% AEP modelled event
Figure 2.27 Maps showing the contribution of the catchments to 1% AEP modelled flood volumes at Kingston for the four main time intervals
62
Between 200 and 250 hours, as the peak begins to decline, the volumes from the Middle
Thames increase (16%, 14% and 9% of the total from the Kennet, Thame and Loddon
respectively). This results in a peak at Windsor at just over 200 hours into the event and also
maintains the high flows at Kingston (it doesn’t fall below 600 m³/s until after 260 hours).
When we compare the modelled event with data from observed events, there are some
similarities but also a number of differences. The Thames at Kingston peaks before Windsor
in all cases. The timing of the peaks on the tributaries is less defined in the observed events,
mainly due to the timing, duration, location and number of the associated rainfall events. It is
also unlikely, for the same reason that there will be a single peak in the flow on each river.
The amount of flow on one tributary compared to another and the volume they each
contribute to the total at Kingston, will also differ depending on both the preceding catchment
conditions and the nature of the rainfall event. Factors including the catchment geology, soil
type, land management, land use, topography, soil moisture deficit, combined with the type of
precipitation that occurs, its timing, duration, intensity, total amount, location, direction and the
number of rainfall events, can all affect the nature of the flood event. They all have a different
effect on the hydrology of the basin and therefore, it is hard to predict how a particular
catchment will behave in flood events of different magnitudes. No two observed flood events
have been exactly the same. For example, flows in the upper catchments in 2003 exceeded
those of 2000, despite the rainfall totals being lower.
In conclusion:
• Each tributary only contributes a relatively small volume to the flows in the lower part of
the River Thames. This is illustrated in figure 2.27. Even when the greatest volumes are
being discharged by the Wey (between 150 and 200 hours), its percentage contribution to
the total volume at Kingston is 21% compared to 40% from the Thames itself.
• A very large flood on one tributary (for example the 1% AEP flood in April 1998 on the
Cherwell) can cause significant property flooding in that particular catchment but does not
necessarily result in flooding elsewhere on the River Thames.
• The catchment conditions that precede a rainfall event, have an important influence on
flows and the character and pattern of flooding that can occur. For example, in December
2003, all the catchments were saturated making them very responsive to the heavy
rainfall that followed at the end of the month. Baseflows were already high and as a
result, exceptional flows, especially in the upstream catchments, were recorded. In
comparison, in April 1998, the catchments were much drier and the widespread rainfall
produced some much-needed percolation and groundwater recharge. Flooding only
63
happened in one catchment, and this was mainly due to the high intensity of the rainfall in
that area.
• Flooding in the lower Thames is generally a product of high baseflows and a sustained
period of widespread rainfall or a number of successive, intense rainfall events, with high
flows from all the tributaries.
• Flows in the Mole, Wey and Thame are quite responsive to rainfall. Rapid increases and
falls in flow were witnessed during both the 2000 and 2003 events and are evident in the
BSM analysis.
• On the Kennet and the Loddon, although there are peaks in flow, they are much flatter in
comparison and there are longer periods of sustained high flows (as shown in the
hydrographs for the 2003 event in figure 2.21). This is not reflected in the BSM results for
the Kennet, due to the catchment geology. Because of the highly permeable nature of the
underlying chalk, the flooding mechanism for the Kennet is essentially groundwater
flooding with some surface water flooding in the lower reaches. It is very difficult to
generate a groundwater design flood hydrograph and the FEH rainfall-runoff method is
widely considered to poorly represent chalk catchments.
• In the upper reaches of the River Thames, once the flows have reached their peak, only
moderate amounts of rainfall are required to sustain the high flows over a long period of
time. This is illustrated by the hydrograph for Farmoor during the 2000 and 2003 events
(figure 2.21 and 2.22). Therefore, flooding in the upper Thames, for example at Oxford
during Autumn 2000, is mainly a result of the slow response of the catchments and the
time that it takes for the flows to decline after a rainfall event.
• We have demonstrated that in order for a Thames basin-wide event to occur, widespread
and prolonged rainfall (several waves usually) is required. Catchment-scale events can
occur with less extreme conditions, but are less likely to result in flooding downstream on
the Thames.
By looking at the hydrology of the Thames basin in detail, we wanted to determine whether it
would be possible to reduce flood risk using the natural characteristics of the catchment to
control the timing and volume of flows entering the River Thames from the tributaries. We
also wanted to examine whether this would be feasible in the tributary catchments as well as
on the River Thames.
It is clear from the information and conclusions above that this is not feasible at a basin scale.
There are too many variables (for example rainfall pattern, rainfall intensity and duration and
preceding catchment conditions) within each individual catchment and in combination with the
Thames, that can effect the overall hydrological characteristics of the Thames basin during a
flood event (see Chapter 6.2.1 for further discussion).
64
As a result of this, although we may be able to attenuate water on a particular tributary,
whether this reduces flows and therefore flooding in the lower Thames will depend on the
unique character of each flood event that occurs. On the tributaries, the range of critical flood
conditions is smaller and in some cases, this could enable some form of within-catchment
management. However, in combination with the Thames itself, there are too many variables
to try and predict how this complex system will react.
65
River Lee basin
The River Lee basin covers an area of approximately 1,420 km2 to the north of London. The
source of the River Lee is in Bedfordshire at 150m AOD. From here it flows 85km through an
increasingly urbanised environment, before joining the tidal River Thames just downstream of
Stratford in central London, where the elevation is below 10m AOD. The downstream fluvial
limit is at Lea Bridge (see figure 2.28). Downstream of Lea Bridge, the tidal defences on the
Lower Lee have a standard of protection of 0.1% AEP. The confluence of the River Lee and
River Thames is upstream of the Thames barrier.
The major tributaries are shown in figure 2.28. The catchments upstream of Feildes Weir in
the upper Lee (Mimram, Beane, Rib, Ash and Stort) are larger in area and more rural than
those downstream of Feildes Weir in the lower Lee (namely the Turkey Brook, Cobbins
Brook, Salmons Brook, Pymmes Brook and Ching).
The upper Lee is mainly rural, and includes the built-up areas of Luton, Wheathampstead,
Hertford and Ware. The river character changes below Hertford, where it becomes navigable
and significantly increases in size with the inflows from the main tributaries (Rivers Stort, Ash,
Rib and Beane).
The lower Lee (downstream of Feildes Weir, report location 2 in figure 2.29) includes the
urban areas of Waltham Abbey, Enfield, Walthamstow and Hackney and is a heavily modified
system. The main stem consists of three principal channels - the Old River Lee, the Flood
Relief Channel (FRC) and the Lee Navigation. The tributaries on the east of the basin
discharge directly into the FRC.Those on the west discharge directly into the Old River Lee or
the Navigation Channel (see figure 2.29), from which flows are distributed to the FRC.
The FRC and its associated structures (sluice gates, radial gates and weirs) are critical to the
management of flood risk in the lower Lee. In addition, water levels are managed for water
supply, environmental and navigation purposes. Many abstraction points and a significant
number of sewage and industrial discharge locations within the Lee basin also have an effect
on the hydrological regime.
66
Figure 2.28 The river system of the Lee basin and figure 2.29 Schematic diagram of the River Lee system (from River Lee Broad Scale Model report)
67
Hydrological characteristics of the Lee basin (a) Analysis of previous flood events Figures 2.31 to 2.33 illustrate how the magnitude, concentration and timing of a rainfall event
across the Lee basin can affect the timing and volume of flow on the Lee and its major
tributaries. Depending on the characteristics of this flow - the extent of flooding that this can
lead to is different. The flood events of 1993, 2000 and 2001 are used as examples. It is
important to understand the factors that caused particular flood events in order to appraise
potential flood risk management options.
Figure 2.30 shows the location of selected flow gauges and some corresponding rain gauges.
These were chosen to show how the different rivers across the basin responded to varying
levels of rainfall. In some cases, the selection was restricted by the availability of good quality
data for a particular flood event. Not all gauges are used for all three events.
The data recorded at each gauge during the flood event is shown in figures 2.31 to 2.33. For
each area of the Lee basin, the rainfall plot is shown on the left, and the corresponding
hydrographs are on the right. Due to the density of the rain gauge network, rainfall data was
not available for each individual river, especially in the lower Lee. The first set of graphs are
the furthest upstream and all graphs cover the same time period.
October 1993 Prolonged rainfall at the start of October, followed by a short period of heavy rain overnight on
the 12th/13th of the month (see figure 2.31), resulted in flooding in the Rib, Beane, Stort and
Ash catchments. The rainfall totals for October were similar across the whole of the Lee basin
(approximately 100 mm) but were highest in the middle Lee area (114 mm at Epping and
Darnicle Hill).
The flooding was most extensive on the Stort (see figure 2.30), with mainly rural areas
affected. Approximately 10 properties were flooded in this catchment. On the Ash, the flood
event was estimated to have an AEP of approximately 14%. The downstream flows on the
River Lee were contained within the RLFRC and there was no flooding on any of the
downstream tributaries (Cobbins Brook, Turkey Brook, Pymmes Brook, and Salmons Brook).
All of the major tributaries (especially in the lower Lee) responded rapidly to the rainfall, with
the exception of the Stort. Pymmes Brook was the first river to peak at 277 hours, followed by
Salmons Brook at 280 hours. This was under 15 hours after the intense rainfall event. Flows
on Pymmes Brook were much higher than on Salmons Brook (33m³/s and 10m³/s
respectively).
68
Figure 2.30 Location of selected flow gauging stations and corresponding rain gauges in the Lee basin
69
Figure 2.31 Rainfall data and hydrographs for selected locations in the Lee basin, during theOctober 1993 flood event
70
The initial peak downstream at Low Hall was later (at 295 hours) although the increase was
very rapid. The flows increased by approximately 90 m³/s in 20 hours, up to a maximum of
106m³/s. As this gauging station is on the RLFRC, it is not affected by the Pymmes Brook or
Salmons Brook which both discharge into the navigation channel. The increase in flow at Low
Hall was a result of the flows arriving from the RLFRC further upstream, Cobbins Brook and
to a lesser extent, Turkey Brook1. These two tributaries peaked at 292 and 284 hours
respectively. There was a second peak at Low Hall after 306 hours as a result of the peaks on
the Beane, Rib and Stort arriving downstream.
The catchments in the upper Lee were slower to respond to the heavy rain and did not peak
until approximately 300 hours. The Stort was the last river to peak at 304 hours. The flows on
the Stort also took longer to fall after the peak. They stayed above 20m³/s for over 35 hours.
On the River Lee upstream near Luton, the peak in flows was very small and the flows did not
increase over 4 m³/s.
Autumn 2000 During late October and early November 2000, the rain fell in five major events (as illustrated
in figure 2.32). The first three (28th/29th October, 1st/2nd and 5th/6th November) caused flooding
on the lower reaches of the River Lee, Cobbins Brook, Salmons Brook, Stort and the River
Lee upstream of Hertford. Rainfall totals for the month (20th October to the 20th November)
were similar to those received over this period in the Thames basin, with just over 200 mm
recorded at Epping.
All of the hydrographs in figure 2.32 show a similar pattern of successive peaks and troughs
after each rainfall event. The river system of the Lee basin is a lot more responsive than the
Thames basin. The flows on the River Lee and its tributaries rise and fall very rapidly. In
comparison, in the Thames basin, especially on the River Thames itself, each rainfall event
increases and sustains the high flows. For example at Windsor, the flows remain above 200
m³/s for over 350 hours. Flows on the RLFRC at Low Hall rise from 50 m³/s to over 145 m³/s
and back to under 50 m³/s in 40 hours after the high amounts of rainfall on the 29th October.
This second event on the 29th October produced the largest flows on all of the rivers,
especially on the RLFRC at Low Hall and Cobbins Brook. In response to the intense rainfall
(up to 4mm in 15 minutes at some gauges), all of the rivers peaked rapidly between 230 and
250 hours (between approximately 10 and 30 hours after the start of the rainfall event).
On the tributaries, Cobbins Brook was very quick to respond to the rainfall, especially on the
29th October, when the peak flows reached much higher levels than during the 1993 event.
1 The Turkey Brook is a tributary of the Small River Lee and its flows are split between the navigation channel and an overflow channel that feeds directly into the RLFRC. The volume of flow that enters this
71
Figure 2.32 Rainfall data and hydrographs for selected locations in the Lee basin, during theAutumn 2000 flood event
72
Flows increased to 76 m³/s after 236 hours, but after 249 hours, they had already fallen again
to below 10 m³/s. In comparison, the flows on the other tributaries did not increase much over
25 m³/s.
The 2000 floods were the most recent event on both Cobbins Brook and Salmons Brook. At
Waltham Abbey on Cobbins Brook, almost 100 properties were affected by flooding and the
event had an estimated AEP of between 5 and 3%. Approximately 190 properties in the
London Borough of Enfield were affected by flooding from Salmons Brook. Almost 50
properties were affected in the Stort catchment and 30 on the Ash. The RLFRC prevented
flooding on the River Lee although the river almost reached bankfull levels.
October 2001 Each hydrograph in figure 2.33 has two clear peaks reflecting one rainfall event at the start of
October (7th) and a second one towards the end of the month (21st). However, the peaks were
much smaller on the upstream tributaries, than on the Pymmes Brook and the RLFRC at Low
Hall.
There was a greater amount of rainfall over a shorter period of time on the 21st October
compared with the 7th October, thus causing higher peaks in flow. All catchments received
similar amounts of rainfall over the month (between 102 and 120 mm), except at Hadley Road
in the Stort catchment where 160 mm of rain was recorded. The monthly rainfall totals were
similar to the 1993 event but lower than in October 2000.
The flows on the RLFRC were the lowest of all three flood events (peaking at 80 m³/s,
compared to 106 m³/s in 1993 and 146 m³/s in 2000). However, flows on some of the
upstream tributaries were higher than during the two previous events. Flows peaked at 29
m³/s on the Rib (compared to 23 m³/s in 1993 and 18 m³/s in 2000) and 40 m³/s on the Stort
(compared to 29 m³/s in 1993. No data was available for the Stort for the 2000 event).
These high flows led to flooding on the Rib, Ash, Beane and Stort in the upper Lee. The flood
event on the lower Stort was estimated to have a 3% AEP. Over 30 properties were affected
by flooding in the Ash catchment and the flood event had an estimated AEP of between 3 and
2.5%.
overflow channel will depend on the scale of the flood event, but it occurs quite regularly.
73
Figure 2.33 Rainfall data and hydrographs for selected locations in the Lee basin, during theOctober 2001 flood event
74
With regard to the second rainfall event, the Pymmes Brook was the first river to peak at 484
hours, closely followed by Salmons Brook and Cobbins Brook. The flows remained fairly low
on these downstream tributaries and no flooding occurred.
The Beane and Rib both peaked at around 500 hours, resulting in an increase in flows on the
River Lee at Rye Bridge. The Stort was the last to peak at 513 hours. The RLFRC at Low Hall
was late to peak (at 512 hours) due to the slower increase in flows in the upper catchments. (b) Lee Broad scale model One of the applications of the BSM is to understand catchment interaction, supplementing
what we have learnt from the analysis of the flood events described above. It is especially
useful as there has not been a recorded 1% AEP flood event across the whole basin. The Lee
BSM was not run for the 10% AEP event.
The methodology used to generate the design hydrographs is based on the rainfall-runoff
approach detailed in the Flood Estimation Handbook (FEH). This method assumes static
storms and uniform rainfall over a specified area - there are no temporal or spatial variations
in rainfall. For the Lee BSM, a single storm was considered across the whole of the
catchment upstream of Feildes Weir (see figure 2.29). A single winter storm duration of 25
hours was applied and the rainfall return periods that were chosen were based on the degree
of urbanisation across the whole catchment.
The hydrograph below (figure 2.34) shows the flows for the 1% AEP design event at five
locations on the River Lee (see figure 2.28 for the BSM node locations). In order to compare
the flows along the length of the River Lee, all the nodes are situated on the flood relief
channel (RLFRC). The only exception is Lea Bridge, which is on the Lee Navigation Channel.
Pymmes Brook and Salmons Brook both discharge into the navigation channel upstream of
this point and not directly into the RLFRC.
The first point of interest is that the Lee is a much more responsive system than the Thames.
The first peak is at Lea Bridge after 20 hours, shortly followed by the RLFRC at Walthamstow
after 26 hours. Flows at Lea Bridge fall back to below 20 m³/s after 65 hours and after 75
hours at Walthamstow. In comparison, the peak at Kingston on the River Thames does not
occur until over 150 hours after the start of the flood event and flows remain above 200 m³/s
for almost another 200 hours.
The hydrographs in figure 2.34 do not appear to be any more responsive than the River
Thames (figure 2.22), however the graph for the River Lee is plotted over a much shorter
75
timeframe. The 1% AEP event in the Lee basin lasts around 100 hours, whereas in the
Thames basin, it takes up to 400 hours for the flows to return to baseflow levels.
The flows are also much lower on the RLFRC than on the River Thames (note the differences
in y-axis scales). The highest flows on the RLFRC reach approximately 140 m³/s, just
upstream of the confluence with the Cobbins Brook. In comparison, the flows downstream on
the River Thames at Kingston increase to over 700 m³/s.
Figure 2.34 1% AEP design event flows on the Lee flood relief channel and navigation channel
The hydrographs for the major tributaries of the River Lee for a 1% AEP design flood event,
are shown in figure 2.35 (note the different scale on the y-axis). There is a clear distinction
between the response of the catchments in the upper Lee (upstream of Feildes weir) and
those in the lower Lee.
76
Figure 2.35 Flows on the major tributaries in the Lee basin for a 1% AEP design flood event
The downstream tributaries (Pymmes Brook, Salmons Brook and Ching) drain smaller,
steeper and more urbanised catchments and are therefore very responsive to rainfall events.
These rivers all peak after approximately 17 hours, followed by a rapid fall in flows. However,
the shape of the hydrographs for the upstream catchments (Stort, Beane and Rib) is very
different. After 30 hours, the flows on the downstream tributaries are returning to baseflow
levels whereas, the flows on the Stort, Beane and Rib are reaching their peak. The
hydrographs for the upstream catchments are much flatter and the high flows are sustained
for longer before they start to decrease (30 hours on the Stort).
Of all the tributaries, the Pymmes Brook has the highest peak in flows (at 74 m³/s), but it is
very short-lived. As a result, the average flow on the Pymmes Brook (11 m³/s) is less than half
that on the Stort (25 m³/s) as the flows remain high for much longer in this catchment. The
lowest peaks are on the Ching and Turkey Brook (18 and 20 m³/s respectively). The tributary
with the lowest peak in flows on the Thames is the Colne (66 m³/s). All of the Thames
tributaries are less responsive than even the upper catchments in the Lee basin, with the
peak flows not occurring until over 150 hours on the Thame, Kennet and Cherwell.
Interaction between the River Lee and its major tributaries The timing and volume of flow from the tributaries in the Lee basin coincides with the changes
in flows observed on the River Lee. The early peaks and high discharges from the Pymmes
and Salmons Brooks at 17 hours result in a rapid increase in flow downstream on the
navigation channel at Lea Bridge and a peak in flow at 20 hours. Flows on the RLFRC peak
77
first downstream at Walthamstow after 26 hours. This is much earlier than any other node
point on the RLFRC and is a result of the early peaks on the Turkey Brook and Ching
between 18 and 21 hours. The peaks on the Stort, Rib and Beane are followed by gradual
increases in flow at the upstream nodes on the RLFRC after 30 hours.
Figure 2.36 shows that the Stort has the greatest percentage contribution to the total volume
at the A406 (located on the LFRC just downstream of the confluence of the Pymmes Brook
and the River Lee - see figure 2.28). This is however a proportionate contribution in relation to
the area of the catchment. As the Pymmes Brook and Salmons Brook catchments are
downstream of the A406, they are not included in this section of the analysis. The A406 was
chosen because downstream of this point, the River Lee divides into the RLFRC and the Lee
navigation channel.
Figure 2.36 Catchment area (as a percentage of the total BSM area) plotted against contribution to volume at the A406 (as a percentage of the total volume) for the 1% AEP event
Figures 2.37 and 2.38 show how the volume contributions from the River Lee and its major
tributaries, to the total volume at the A406 (RLFRC), change over the course of a 1% AEP
flood event. In the early stages of the event (20 –30 hours), the contribution from the major
tributaries is small compared to that from the River Lee. Of the tributaries, the largest
percentage contribution is 12% from Cobbins Brook, whereas 50% of the volume at the A406
is from the River Lee. However, the volumes from the River Stort, Beane and Rib gradually
increase and their percentage contributions to the volume at the A406 between 40 – 50 hours
are 31%, 16% and 13% respectively. They increase further between 50 and 60 hours and
together, contribute to 94% of the total volume at the A406. As a result, the high flows on the
0%
5%
10%
15%
20%
25%
30%
Beane Rib Ash Stort Cobbins Turkey Small Lee
Modelled tributaries
Volu
me
cont
ribut
ion
(%)
Percentage of modelled area Percentage volume contribution
78
RLFRC continue up to 60 hours. After 40 hours the River Turkey, Small Lee and Cobbins
make a minimal contribution towards the total volume.
Figure 2.37 Breakdown by source of volume at the A406 (RLFRC) for a 1% AEP modelled event
Figure 2.38 Maps showing the contribution of the catchments to a 1% AEP modelled flood volumes at the A406 (RLFRC) for the four main time intervals
When we compare the modelled event with data from observed events, the timing of the
peaks on the tributaries in relation to each other and those on the River Lee are similar. For
example the Pymmes Brook and Salmons Brook are the first to peak in all cases. Also, the
50%
11%10% 12%
7%
3%
4%
3%
Time Interval20 - 30 hours
FRC at A406
41%
12%3% 7%
19%
6%
6%
6%
Time Interval30 - 40 hours
FRC at A406
24%
6%0% 1%
31%
9%
13%
16%
Time Interval40 - 50 hours
FRC at A406
1%
0%0% 0%
38%
6%
29%
26%
Time Interval50 - 60 hours
FRC at A406
Key to Catchments
Beane
Rib
Ash
Stort
Cobbins
Turkey
SmallR Lee
Other
79
peak downstream at Low Hall on the RLFRC occurs after the downstream tributaries but
before the upstream tributaries in the observed flood events. This same pattern occurs on the
RLFRC at Walthamstow in the BSM.
The upper catchments appear to be more responsive in the observed flood events than
predicted by the BSM. The hydrographs for the Beane and Rib especially are not as flat and
the flows fall fairly rapidly after the rainfall event. However on the Stort, in October 1993, flows
remained above 20m³/s for over 35 hours. Although it was the last river to peak in both the
1993 and 2000 flood events, the peak on the Stort precedes the peak on the Rib in the BSM.
However, although this may occur during a basin-wide rainfall event, it may not happen for all
events.
The longest time to peak in the BSM is on the RLFRC in the middle Lee, just upstream of its
confluence with Cobbins Brook. This is due to the late discharge of the peak flows from the
upstream catchments, namely the Beane, Rib and Stort.
The amount of flow on one tributary compared to another and the volume they each
contribute to the total at Low Hall, will differ depending on both the preceding catchment
conditions and the nature of the rainfall event. For example, in the Autumn 2000 flood, large
areas of upstream parkland upstream in a number of the lower lee tributaries, including
Salmons Brook, became saturated and behaved like impermeable surfaces. As a result,
these rivers reacted very quickly to further rainfall and recorded very high flows.
Also, the ratio of flows in the navigation channel and the RLFRC varies for different scales of
flood event, due to the interaction between them. For example, there are a number of sidespill
weirs on the east bank of the navigation channel that feed a small flood relief channel known
as the western flood relief or overflow channel. This drains directly into the RLFRC south of
the William Girling Reservoir (just upstream of report location 4 on figure 2.29).
The hydrological response of the Lee basin is very different to that of the Thames basin.
Firstly, all the rivers in the Lee basin respond much quicker to a rainfall event. This is a result
of a number of factors such as catchment size, topography, geology and land use (especially
the degree of urbanisation). The peak flows on the Thames are much higher than on the
RLFRC (730 m³/s and 140 m³/s respectively) and remain higher for longer after a rainfall
event.
In conclusion:
• The combination of manmade surfaces, steep catchments and clay soils means
watercourses in the Lower Lee respond rapidly to rainfall and are liable to sudden
80
flooding after storms. This has particularly adverse consequences for areas at the
confluence of the lower Lee tributaries with the River Lee.
• There is a slower response time at Feildes Weir compared to that of the lower Lee
tributaries discharging into the RLFRC. This is supported by the shape of the catchment,
with larger run-off areas within the middle and upper Lee.
• Catchment size becomes smaller and more urban in nature towards the downstream end
within the Lower Lee.
• Flows from the lower Lee tributaries and the upper Lee tributaries, based on the BSM and
the observed flood events, do not combine to cause flooding. Peaks from the Pymmes,
Salmons, Turkey and Cobbins Brooks have all passed through either Low Hall (RLFRC)
or Lea Bridge (Navigation channel), before the peak flows arrive from the Rib, Beane and
Stort.
• The highest peaks downstream on the RLFRC occur when there are exceptionally high
peaks on any of the lower Lee tributaries. This was seen in the flood event in October
2000 when peak flows on Cobbins Brook reached 76 m³/s compared to just over 8 m³/s.
Flows at Low Hall almost reached 146 m³/s in 2000 (almost bank-full capacity) compared
with 80 m³/s in 2001. Flooding occurred on Cobbins Brook but not on the Lee itself.
• The peaks in flow on the downstream tributaries all occur in quick succession, which can
result in large volumes of water arriving in a short space of time further downstream
where the Navigation channel and the RLFRC meet. This can lead to flooding problems.
• The Lee basin is a complex system with many controls on flow and a fair amount of
interaction between channels. Flow routes change depending on the scale of the flood
event and preceding catchment conditions can affect the response of the tributaries.
Therefore it is very difficult to predict the timing and volume of flows that will arrive
downstream.
81
The London rivers The basin covers an area of approximately 1,100 km2. The major rivers within it are the
Quaggy, Ravensbourne, Wandle, Hogsmill, Beverley Brook, Brent, Crane, Roding, Beam and
Ingrebourne (see figure 2.39). With the exception of the Hogsmill, all of these rivers
discharge into the Thames downstream of its tidal limit at Teddington weir. There is no
hydrological interaction between the individual rivers; the only interaction is between the
individual rivers and the River Thames. This means that each river experiences some degree
of tidal influence, although this varies depending on location, topography and defences.
Compared with the rivers in the Lee and Thames basins, the London rivers are relatively short
in length. The catchments are all situated in the London clay basin and drain heavily
urbanised areas within the M25. Because of this, the amount of rainfall run-off is relatively
high, with little water infiltrating into the soil. This makes the catchments very responsive.
Surface water and sewer flooding are also important contributors to flood risk in London.
Following urban development, the majority of the watercourses (particularly in South London)
have been heavily modified. Some rivers have been straightened or culverted in concrete
channels to increase their conveyance. This has further increased their rate of response to a
rainfall event, which can lead to flash flooding, especially during the summer. There are flood
alleviation schemes on some sections of the London rivers to increase the level of protection,
notably on the Ravensbourne, Brent and Quaggy.
82
Figure 2.39 The London rivers
Hydrological characteristics of the London rivers (a) Analysis of previous flood events Figures 2.41 and 2.42 illustrate how the magnitude, concentration and timing of a rainfall
event across the London rivers basin can affect the timing and volume of flow on each of the
major rivers, and the extent of flooding that this can lead to. The flood events of 1993 and
2000 are used as examples. It is important to understand the factors that caused particular
flood events in order to appraise potential flood risk management options.
Figure 2.40 shows the location of selected flow gauges and the corresponding rain gauges.
These were chosen to show how the different rivers across the basin responded to varying
levels of rainfall. (In some cases, the selection was restricted by the availability of good quality
data for a particular flood event. Not all gauges are used for all three events).
83
Figure 2.40 Location of selected flow gauging stations and corresponding rain gauges in theLondon rivers basin
84
The data recorded at each gauge during the flood event is shown in figures 2.40 and 2.41.
For each catchment, the rainfall plot is shown on the left, and the corresponding hydrograph
is on the right. All the graphs cover the same time period and are grouped depending on
whether they are in north, south or west London.
October 1993 The rainfall plots in figure 2.41 show two periods of heavy rain between the end of September
and the middle of October 1993. The first rainfall event was from the 30th September to the
2nd of October and the second was overnight on the 12th/13th of October. The earlier rainfall
event was more significant in the catchments to the south of the River Thames (Hogsmill,
Beverley Brook, Wandle, Ravensbourne and Quaggy) and resulted in higher flows. The
second event produced flows of a higher magnitude on the Brent and Crane to the west of
London and the Roding to the north (see figure 2.41).
The highest flows were on the Brent and Roding where flows increased to over 40 m³/s on the
13th October. Both of these rivers have two large peaks in flow whereas the other rivers
(particularly the Hogsmill, Wandle, Ravensbourne and Quaggy) are more flashy with
numerous smaller peaks. Both the peaks on the Roding are of a similar magnitude, whereas
the first peak on the Brent is about half the size of the second peak. This is because more
rainfall fell over a shorter space of time to the west of London during the second rainfall event.
Of all the London rivers, the Roding was the slowest to respond to both of the rainfall events.
The Ingrebourne and Beam were also less responsive than the south London rivers. Flows on
the Crane also stayed high for longer periods of time. During the second rainfall event, flows
at Marsh Farm on the Crane stayed above 10 m³/s for 28 hours and the peak was
approximately 15 hours after the peak on the Brent. Flows on the Brent at Costons Lane also
fell quicker and remained above 30 m³/s for 18 hours.
The response of the different groups of rivers to the two rainfall events was very different.
However, all the south London rivers behaved in a similar way and the timing of the peaks
and troughs of the Beverley Brook, Hogsmill and Wandle all followed the same pattern. The
Ingrebourne, Beam and Roding to the north of the Thames were less responsive and all
showed one major peak per rainfall event. To the west, the Brent and Crane showed slightly
different characteristics to each other. The Brent was more responsive with higher peaks in
flow.
October 2000 September and October 2000 were unusually wet. This prolonged period of rainfall followed
by heavy rain on the 28th/29th October resulted in large peaks in flow on all the London rivers
85
Figure 2.41 Rainfall data and hydrographs for selected locations in the London rivers basin,during the October 1993 flood event
86
87
Figure 2.42 Rainfall data and hydrographs for selected locations in the London rivers basin,during the Autumn 2000 flood event
88
89
(especially on the Brent and Roding) and extensive flooding on the River Roding (see figure
2.40).
On the Lower Roding, the flows were the highest since 1947. They reached over 70 m³/s at
Redbridge, compared to 35 m³/s in 1993. Defences on the river Roding at Wanstead in NE
London were overtopped by flows that exceeded the design standard. As a result, over 200
properties were flooded. Elsewhere, the Roding defences prevented flooding to several
hundred properties. Existing defences on the south London rivers also prevented flooding to
any properties.
There were two subsequent rainfall events on the 1st/2nd November and 5th/6th November,
however these did not cause any flooding.
The south London rivers all showed a similar response to the rainfall events, with a series of
rapid peaks and troughs in flow. The flows on the 28th/29th October were of a similar
magnitude to the peaks recorded on the 1st/2nd November and during earlier events in
October (see figure 2.42).
The Roding, Brent and Crane were less responsive to the rainfall and over the course of the
month (5th October to 14th November 2000) had a smaller number of peaks in flow than
witnessed on the south London rivers. The largest peak was on the 28th/29th October.
(b) Modelling results No BSM has been run for any of the London rivers, therefore we have used the 1% AEP flow
results from detailed hydraulic modelling (e.g. Section 105 mapping). However, no models are
available for the River Brent.
The hydrographs for the modelled event are much smoother than the observed events for all
the rivers. This may be a result of the way that the rainfall event was represented in the
model, with a static storm as opposed to rain tracking across the catchments. Catchment
conditions are also likely to be different for a 1% AEP event. The time to peak is still very
short (under 6 hours for the south London catchments) and flows also recede very quickly. As
a result, the duration of the flood event is much shorter in comparison to that for the rivers in
the Thames and Lee basins. Peak flows range from just over 160 m³/s on the Lower Roding
to 15 m³/s on the Quaggy. This is a similar range to the Lee tributaries, but much lower than
the Lee itself and rivers in the Thames basin.
In conclusion:
90
• The rivers in the London basin are a lot more responsive than in the Thames and Lee
basins. There are a number of reasons for this:
a) Compared with the rivers in the Lee and Thames basins, the London rivers are
relatively short in length and have a steeper topography in their upstream reaches
(particularly in south London).
b) The catchments are all situated on clay soils and drain urbanised areas with
expansive man-made impermeable surfaces. Because of this, the amount of rainfall
run-off is relatively high, with little water infiltrating into the soil.
c) The majority of the rivers have been straightened or culverted in concrete channels to
increase their conveyance.
• As a result, the London rivers are susceptible to flash flooding, especially in the summer.
• The London rivers all behave in a very similar way following a rainfall event. The peaks
and troughs in flow are very rapid.
• The highest flows recorded during the observed flood events were on the Roding (75m³/s
at Redbridge in 2000), closely followed by the Brent (52 m³/s at Costons Lane in 2000). In
the 2000 flood event, flows on the other London rivers did not rise above 30 m³/s.
• During both of the observed events, the south London rivers all respond in a very similar
way. The numerous peaks and troughs in flow all occur at approximately the same time.
• For the 1% AEP modelled event, peak flows reach 161m³/s on the lower Roding. The
second highest flows are on the Ravensbourne (68 m³/s).
Overall conclusions (Thames, Lee and London) This section has described how the magnitude, concentration and timing of a rainfall event
across the London rivers basin can affect the timing and volume of flow on each of the major
rivers, and the extent of flooding that this can lead to. Through the use of both observed and
modelled flood event data we have shown how differently the rivers in the Thames, Lee and
London basins react to rainfall in terms of scale and duration of response. This is highlighted
in figure 2.43 below. It allows a comparison of hydrographs for the modelled 1% AEP flood
event for the River Thames (at Kingston), River Lee Flood Relief Channel (at Walthamstow)
and the River Ravensbourne (south London catchment).
The graph shows the large difference in flows on the Thames compared to those on the Lee
and London rivers. Peak flows at Kingston are over 10 times higher than on the
Ravensbourne. The time to peak at Kingston is over 150 hours and the flows take almost 200
hours to recede. The London catchments are much more responsive. Flows on the Lee
increase fairly quickly but are sustained for a longer period of time than the London
catchments. However, after 100 hours, the flows have returned to baseflow levels.
91
Figure 2.43 A comparison of the 1% AEP modelled flows for rivers in the Thames, Lee and London basins
We have also shown the importance of understanding the interactions between the rivers and
their tributaries in both the Thames and Lee basins, during large-scale events. The nature of
these interactions can affect flows downstream and the degree of flooding that occurs. This is
particularly evident in the lower Thames, near the confluence of the Wey and Mole with the
River Thames, where widespread flooding occurred in January 2003.
Thames The main tributaries of the Thames include the Kennet, Colne, Wey and Mole. The extensive areas of chalk and limestone mean the Thames basin generally responds slowly to rainfall. Flows on the River Thames increase with distance downstream. For a 1% AEP modelled flood event, the peak flow at Oxford is just over 300m³/s, whereas at Kingston, flows increase to over 700m³/s. This is a result of the inputs from the tributaries. The timing of the peaks at Oxford, Reading at Windsor follow in succession, however they all occur later than the peak at Kingston. This is due to the early peaks in flows on the tributaries downstream of Windsor (Colne, Wey and Mole). The Wey has the
92
highest flows of all the Thames tributaries (approximately 140 m³/s) and is also one of the most responsive catchments. The amount of flow on one tributary compared to another and the volume they each contribute to the total at Kingston, will differ depending on both the preceding catchment conditions and the nature of the rainfall event. We have demonstrated that in order for a Thames basin-wide event to occur, widespread and prolonged rainfall (several waves usually) is required. Catchment-scale events can occur with less extreme conditions, but are less likely to result in flooding downstream on the Thames. Lee The upper Lee is mainly rural. The river character changes below Hertford, where it becomes navigable and significantly increases in size with the inflows from the main tributaries (Rivers Stort, Ash, Rib and Beane). The lower Lee (downstream of Feildes Weir) flows through highly urbanised areas and is a heavily modified system. The main stem consists of three principal channels - the Old River Lee, the Flood Relief Channel (FRC) and the Lee Navigation. The major tributaries are Salmons Brook, Pymmes Brook, Turkey Brook and the Ching. The interaction between the main channels and volume of flow in each varies between flood events and is difficult to predict. The hydrological response of the Lee basin is very different to that of the Thames basin. Firstly, all the rivers in the Lee basin respond much quicker to a rainfall event. This is a result of a number of factors such as catchment size, topography, geology and land use (especially the degree of urbanisation). Secondly, the flows are lower. The highest flows on the RLFRC for a 1% AEP modelled flood event are 140 m³/s. There is a clear distinction between the response of the catchments in the upper Lee (upstream of Feildes weir) and those in the lower Lee. The downstream tributaries (Pymmes Brook, Salmons Brook and Ching) drain smaller, steeper and more urbanised catchments and are therefore very responsive to rainfall events. The hydrographs for the upstream catchments are much flatter and the high flows are sustained for longer before they start to decrease. London The major rivers in London are the Quaggy, Ravensbourne, Wandle, Hogsmill, Beverley Brook, Brent, Crane, Roding, Beam and Ingrebourne. With the exception of
93
the Hogsmill, all of these rivers discharge into the Thames downstream of its tidal limit at Teddington weir. There is no hydrological interaction between the individual rivers. Compared with the rivers in the Lee and Thames basins, the London rivers are relatively short in length. In London, clay soils and expansive man-made impermeable surfaces mean the amount of rainfall run-off is relatively high and fast, with little water infiltrating into the soil. Both these factors result in the London rivers being far more responsive than those in the Thames and Lee basins. The London rivers all behave in a very similar way following a rainfall event. The peaks and troughs in flow are very rapid. For the 1% AEP modelled event, peak flows reach 161m³/s on the lower Roding. The second highest flows are on the Ravensbourne (68 m³/s). In general, the flows on the London rivers are lower than on other rivers in Thames region.
94
2.8 Natural and Historic Environment
Flood risk management activity has a direct impact on the natural environment. There is a great deal of
scope to benefit the environment, for example by creating of wetlands and restoring floodplains. But, the
environment also presents us with constraints. Thames region has many internationally important wildlife
sites that are protected by European law and may deteriorate if, for example, the hydrological regime was
disrupted by more frequent flooding.
Recent national and European guidance has both highlighted and strengthened the links between flood
risk management and the environment by promoting the need to work with natural processes in order to
achieve environmental benefits.
‘Making Space for Water’ (Defra, July 2004) promotes the greater use of rural land use solutions such as
creating more wetlands and encourages the use of ‘softer’ solutions including the realignment of defences.
In this way, flood risk management activities will contribute to:
• maintaining the overall integrity of Natura 2000 and Ramsar sites;
• the Public Service Agreement (PSA) target to have 95% of all SSSIs in ‘favourable condition’ by 2010;
• meeting Biodiversity Action Plan (BAP) targets;
• implementing the actions in the England Biodiversity Strategy.
We have also set up national minimum targets for the creation wetland habitat to make sure that flood risk
management solutions are consistent with biodiversity needs.
Realigning and restoring rivers will also help to meet the objectives of the Water Framework Directive
(WFD). The WFD provides a major opportunity to improve the water environment and promote the
sustainable use of water to benefit both people and wildlife. The WFD aims to protect and enhance
aquatic ecosystems and associated wetlands. So, there are strong links between the WFD and both the
Habitats and Birds Directives, which together protect over 25 sites within Thames region.
2.8.1 Designated sites There are a large number of designated sites within the region, with varying levels of protection and status
(see table 2.2 below). Some are internationally important, while others are significant at a national or local
level. They have all been selected for the quality of their habitat and the species they support. However,
only the internationally important sites will be considered in this CFMP, as these are the only sites large
enough to potentially present significant opportunities or constraints with regard to flood risk management.
95
Site Designation Description Status No. in Region
Ramsar Wetlands International 3
SPA Special Protection Areas International 5
SAC Special Areas of Conservation International 21
SSSI Sites of Special Scientific Interest National 451
NNR National Nature Reserves National 17
LNR Local Nature Reserves Local 75
County wildlife sites Local wildlife sites Local Over 5,000
Table 2.2 Hierarchy of designated sites
Sites of Special Scientific Interest (SSSI) are the best examples of our natural heritage of wildlife habitats,
geological features and landforms. An SSSI is an area that has been notified as being of special interest
under the Wildlife and Countryside Act 1981. Sites that are internationally important are also designated
as SACs and SPAs. The total number and area of SSSIs within each policy unit is shown in Table 2.3
below. If a policy unit does not contain any SSSIs it does not appear in the table. A map showing the
locations of all the SSSIs can be found in Appendix B (Figure B4).
Policy Unit
No. of SSSIs located within
or partially within the policy unit
Total area of SSSIs located
within the policy unit
(km2) Addlestone Bourne, Emm Brook, The Cut 11 24.70
Beam 2 0.73 Beverley Brook 3 7.44 Brent 4 1.28 Byfleet and Weybridge 2 0.50 Colne 15 5.75 Colne tribs & Wye 32 14.70 Crane 1 0.19 Guildford 1 0.04 Hoe Stream 5 15.71 Hogsmill 4 2.00 Ingrebourne 6 5.45 Kennet 63 31.29 Loddon 21 24.36 Lower Lee 6 3.60 Lower Lee tributaries 10 18.07 Lower Mole 1 0.42 Lower Thames 14 29.58 Luton 6 1.35 Middle Lee & Stort 16 7.83 Middle Mole 10 19.32 Middle Roding 2 0.74 Ock 14 3.72 Oxford 6 1.19
96
Pinn 1 2.43 Ravensbourne 5 1.44 Rural Wey 37 53.03 Sandford to Cookham 41 9.32 Swindon 8 1.36 Thame 37 15.53 Upper & Middle Blackwater 11 22.79 Upper Lee 11 1.84 Upper Mole 6 2.37 Upper Roding 7 9.98 Upper Thames 108 33.90 Wandle 9 5.48 Windsor & Maidenhead 8 6.16
Table 2.3 Number and area of SSSIs within each policy unit
The Convention on Wetlands of International Importance especially as Waterfowl Habitat (Ramsar
Convention) was adopted in 1971 and came into force in December 1975. In the UK, the first Ramsar
sites were designated in 1976 and consequently gained a status of wetlands of international importance.
The initial emphasis was on selecting sites of importance to waterbirds within the UK, and consequently
many Ramsar sites are also Special Protection Areas (SPAs). This is the case for all three Ramsar sites in
Thames region.
Special Protection Areas and Special Areas of Conservation are established under the EC Birds Directive1
and the Habitats Directive2 respectively, and together form the Natura 2000 network. The Birds Directive
applies to birds, their eggs, nests and habitats. The Directive says that Member states must take
measures to preserve a sufficient diversity of habitats for all species of wild birds to maintain populations
at ecologically and scientifically sound levels. SPAs are areas of the most important habitat for rare and
vulnerable birds, listed in Annex I to the Birds Directive, and for regularly occurring migratory species
within the European Union.
The main aim of the Habitats Directive is to promote the maintenance of biodiversity by requiring member
states to take measures to maintain or restore natural habitats and wild species at a favourable
conservation status, introducing robust protection for those habitats and species of European importance.
SACs are areas which best represent the range and variety of habitats and (non-bird) species within the
European Union listed on Annexes I and II to the Directive.
The number and area of SACs and SPAs within each policy unit is shown in Table 2.4.
1 Council Directive 79/409/EEC adopted in 1979. www.jncc.gov.uk
97
Policy Unit
No. of SACs within or partially
within the policy unit
Total area of SACs in the policy
unit
No. of SPAs within or partially
within the policy unit
Total area of SPAs in the policy unit
Addlestone Bourne, Emm Brook, The Cut 1 11.66 1 23.26
Beverley Brook 2 7.43 0 0.00 Byfleet and Weybridge 0 0.00 1 0.12 Colne 0 0.00 1 0.65 Colne tribs & Wye 2 9.15 0 0.00 Hoe Stream 1 12.99 1 15.21 Hogsmill 1 0.03 0 0.00 Kennet 4 2.06 0 0.00 Loddon 0 0.00 1 17.70 Lower Lee 0 0.00 1 2.21 Lower Lee tributaries 2 12.13 1 1.76 Lower Thames 2 18.10 2 10.36 Middle Lee & Stort 1 0.08 1 0.78 Middle Mole 1 8.76 1 0.04 Middle Roding 1 0.52 0 0.00 Ock 2 0.82 0 0.00 Oxford 1 0.69 0 0.00 Rural Wey 4 28.70 3 38.01 Sandford to Cookham 4 2.33 0 0.00 Thame 2 3.72 0 0.00 Upper & Middle Blackwater 1 5.40 1 21.90
Upper Lee 0 0.00 1 0.04 Upper Mole 1 0.07 0 0.00 Upper Roding 1 6.94 0 0.00 Upper Thames 2 3.04 0 0.00 Wandle 2 0.34 0 0.00 Windsor & Maidenhead 2 5.25 0 0.00
Table 2.4 Number and area of SACs and SPAs within each policy unit
Protected areas are given particular protection under the WFD. This applies to the Natura 2000 sites that
contain one or more habitats or species that directly depend on the status of water. All of the SPAs and
just over half of the SACs in Thames region meet this criteria and are classed as water dependent
conservation areas. The opportunities and constraints related to these sites are, therefore, considered
when forming flood risk management policy (UK Technical Advisory Group on the Water Framework
Directive: Guidance on the Identification of Natura Protected Areas, March 2003).
In this CFMP, we have assessed the protected sites that are within the floodplain and are also surface
water dependent (i.e. their hydrological regime is not linked to groundwater supplies) to understand the
potential implications for flood risk management policy. This was done using information presented in the
2 Council Directive 92/43/EEC adopted in 1992
98
corresponding SSSI Water Level Management Plans (WLMPs) and is discussed further in Section 3.3.
Figure 2.44 shows the location of these sites.
Thames region has a wealth of designated sites of varying types and importance. A proportion of these are water dependent and have the potential to influence, and be influenced by, flood risk management policies. Any policy or associated activity will need to support the site conditions or else mitigate any negative impacts.
99
Figure 2.44 Water dependent conservation areas (SACs and SPAs) considered in this CFMP and areas of strategic opportunity for wetland creation
100
Figure 2.45 Location of existing wetland BAP habitats in Thames region
101
Thames Lee London Total
BAP Habitat Total (km2)3
Inside floodplain
(km2)4
as a % of the floodplain
area5
Total (km2)
Inside floodplain
(km2)
as a % of total floodplain
area
Total (km2)
Inside floodplain
(km2)
as a % of total floodplain
area
Total (km2)
Inside floodplain
(km2)
as a % of total
floodplain area
Fen 85 7 1 3 2 2 2 1 1 90 10 1
Reedbed 18 15 2 1 1 1 8 6 8 27 22 2
Floodplain grazing marsh 108 89 10 7 7 7 8 7 10 123 103 10
Wet woodland 135 3 0 33 0 0 13 0 0 181 3 0
Mudflats 0 0 0 0 0 0 1 0 0 1 0 0
Total 346 114 13 44 10 10 32 14 19 422 138 13
Table 2.5 Area of BAP wetland habitat in the plan area and the 0.1% AEP fluvial floodplain
3 Total area of BAP habitat within the plan area 4 Floodplain refers to the 0.1% AEP fluvial floodplain within the plan area 5 Area of BAP habitat within the 0.1% AEP fluvial floodplain as a percentage of the total floodplain area
102
2.8.2 Landscape Areas of Outstanding Natural Beauty (AONB) are areas of attractive and unspoilt countryside
designated under the National Parks and Access to the Countryside Act 1949. Areas are
designated solely for their landscape qualities for the purpose of conserving and enhancing
their natural. There are 37 in all, covering about 15% of England. Thames Region contains
large parts of 4 AONB, which in total cover 27% of the Region (see Figure 2.44). The largest
is the Chilterns (95% of its total area is within Thames Region), followed by the Cotswolds
and North Wessex Downs.
The Countryside Character Programme is a joint initiative by the Countryside Agency and
English Nature. A series of descriptions that analyse the area’s character and identify the
main forces for change have been produced as part of the initiative. There are 20 Countryside
Character Areas (CCAs) in Thames region.
Character Areas are complemented by Natural Areas (identified by English Nature). Natural
Areas (NAs) are zones which reflect the geology, natural systems and processes and the
wildlife in different parts of England. Each Natural Area has a unique identity resulting from
the interaction of wildlife, landforms, geology, land use and human impact. They are an
important tool for dividing up national biodiversity targets and also provide the best means for
establishing habitats and species targets for local Biodiversity Action Plans (see section
2.8.3).
A list of the CCAs in the plan area and the corresponding NAs is provided in Appendix G,
along with a description of their characteristics.
Thames region is covered by Countryside Character Areas (CCAs) and also has some designated AONBs. It is unlikely that the impact of any flood risk management policy would have a major impact on the valued characteristics of these areas. However, as a minimum requirement any policy and associated activity will need to avoid detrimental impacts, and seek to enhance the landscape. 2.8.3 Biodiversity Action Plans (BAPs) and biodiversity Opportunities to increase the benefits for the environment are not just restricted to designated
sites. The South East England Biodiversity Forum (SEEBF) produced a map in 2004 showing
areas of strategic opportunity for biodiversity improvement. These are broad indicative areas
of greatest regional-scale potential for enhancement, restoration and re-creation of given
habitats. A number of key strategic habitats are included, however we are only interested in
the areas of potential wetland creation. These are shown on figure 2.44. We have built on the
SEEBF map using local knowledge, to include the whole of Thames region.
103
The UK Biodiversity Action Plan was launched in 1994 in response to the conference on
biological diversity in Rio de Janeiro (1992). It contains action plans for 391 species and 45
habitats of conservation concern. As well as having national priorities and targets, there are
also 162 Local Biodiversity Action Plans (LBAPs) in England, Scotland and Wales.
Habitats identified within BAPs that are directly linked to potential flood risk management
policies are wet woodland, reedbed, chalk rivers, fens and floodplain grazing marsh. The
location of the existing BAP wetland habitat in Thames region is shown on figure 2.45. The
total area of each habitat within the Thames CFMP area is listed in table 2.5.
Wet woodland is the most expansive in terms of total area however, only 2% is within the
0.1% AEP fluvial floodplain. In comparison, 84% of the existing floodplain grazing marsh is
found within the floodplain, primarily in the Upper Thames. Total length was calculated for the
BAP chalk rivers (as opposed to area) so these results have not been included in the table.
The total length of BAP chalk river within the plan area is 964km. The total for the Thames
basin is 695km, for the Lee basin it is 260km and for London, the total is 9km.
In line with the commitments within Making Space for Water to identify targets, Defra have
issued high level targets, with those relating to biodiversity being covered by high level target
4:
• 4A - ensure no net loss to habitats that are covered by the BAP and seek opportunities
for environmental enhancements
• 4F - create new areas of habitat especially where the impacts of climate change and sea
level rise are most immediate
The national target for habitat creation imposed by Defra is 200ha of BAP habitat per year of
which at least 100ha should be saltmarsh or mudflat. The regional contribution to this target
for 2006/7 is 3ha. The figure viewed nationally as being a reasonable contribution for Thames
region is around 20ha. Flood risk management activities have an important role to play in
helping to achieve these targets. This CFMP will look at opportunities where wetland creation
in terms of flood storage or river restoration could be sustainable options for the future.
The majority of internationally designated sites in Thames region are classed as water dependent conservation areas and are given particular protection under the WFD. Flood risk management activities may have either a positive or negative effect on the status of these sites and the implications of this need to be considered. Opportunities for environmental enhancement are not just restricted to designated sites. Future flood risk management activities offer great potential to create new BAP wetland habitats across the region. This may be achieved for example through more regular inundation
104
of undeveloped floodplain. Using the information in this section, our policy appraisal will identify areas where this may be possible.
2.8.4 Fisheries
Fish are widely recognised as a valuable indicator of the quality of our rivers. Fish diversity
and abundance is largely governed by good water quality, quantity and availability of suitable
habitat for them to successfully breed, shelter and feed.
We have a duty under the Salmon and Freshwater Fisheries Act 1975 to maintain, improve
and develop freshwater and migratory fish populations in rivers, lakes, transitional waters and
coastal areas out to 6 miles. New additional legislation is expected by 2010 to extend our
powers to protect and sustain fresh water and migratory fish populations, which will have
implications for how we and others manage rivers.
The Thames Regional Fisheries Strategy was launched in 2007. The aims of the strategy are
to:
• increase fish populations by improving the water environment for fish;
• increase angling participation;
• improve the economic and social value of Fisheries.
This flagship document identifies cross-cutting issues affecting fisheries and outlines some of
the key actions needed to help address them. The main issues include excessive habitat
modification due to wide-scale urban development, waterways and flood risk management,
variable water quality and vulnerable water resources. Other issues include non-native
invasive species such as signal crayfish, illegal fishing and poor fisheries management
practices.
The rivers in Thames region provide a wide variety of aquatic habitat which in general terms
support diverse and abundant fish populations, however many rivers still contain poor
numbers of fish because of barriers to migration, degraded or heavily modified habitat,
extremes of high and low flows, fluctuating water levels, and variable water quality. Some of
these rivers have great potential for improvement when planning any new flood risk
management scheme or maintenance work.
Many of the watercourses, in particularly within London, are heavily modified, with concrete
river walls and structures such as culverts, weirs and bridges, often with limited space for
vegetated buffer zones before development begins. The development of the Thames River
Basin Management Plan, as guided by the WFD, will help us identify priority areas within
105
Thames region to improve water quality and habitat for fish. When used in conjunction with
the CFMP we have stronger evidence to guide investment that will improve water quality,
protect and create habitat for fish, and incorporate measures to improve fish migration
through the installation of appropriate multi-species fish passes. For example, this information
can assist us identify priority areas where we should invest resources negotiating land
through the Planning and Compulsory Purchase Act 2004 to allow naturalising of river banks.
2.8.5 The Historic Environment English Heritage is a public body with responsibility for all aspects of protecting and promoting
the historic environment in England. This includes historic buildings, monuments and areas,
and archaeological remains. English Heritage recognises the need for today’s built
environment to be adapted to become more resilient to unavoidable climate change over the
next 20 to 40 years.
'Scheduling' is the process that provides legal protection for nationally important sites and
monuments. Only deliberately created structures, features and remains can be scheduled.
The Secretary of State must be informed about any work which might affect a monument
above or below ground.
The schedule has about 18,300 entries (about 31,400 sites). There are over 1,800 Scheduled
Ancient Monuments (SAMs) located throughout Thames region, with concentrations to the
West, particularly in the Marlborough Downs (almost 300). 126 SAMs (7%) are within Flood
Zone 3 (1% AEP) and 164 (8%) are within Flood Zone 2 (0.1% AEP). 50 of these are found in
the upper Thames floodplain. There are other, smaller concentrations in the lower Thames
and Wey.
In London, the Greater London Archaeological Advisory Service (part of English Heritage) has
defined a number of Archaeological Priority Zones (APZs) which have particular
archaeological interest. Councils have defined APZ’s within their development plans where
planning policies relating to archaeology will be applied more rigorously than elsewhere.
Some of the APZs are extensive for example the whole of the Lee Valley downstream of the
M25 and the London Borough of the City of London.
World Heritage Sites are defined in the World Heritage Convention as ' places of 'outstanding
universal value from the point of view of art, history, science or natural beauty'. A Heritage
World Convention was drawn up and adopted by UNESCO in 1972, to identify cultural and
natural properties throughout the world whose protection would be of concern to the
international community.
There are six cultural World Heritage Sites in Thames Region:
106
• Blenheim Palace (near Oxford)
• Avebury (near Marlborough)
• Maritime Greenwich (London)
• Royal Botanic Gardens, Kew (London)
• Tower of London (London)
• Westminster Palace, Westminster Abbey and St. Margaret’s Church (London)
The four sites in London are located within the tidal floodplain, and are currently protected to
a 0.1% AEP standard of protection due to the presence of the Thames Barrier, tidal walls and
embankments. The other two sites are not considered to be at risk from fluvial flooding.
although both contain some areas of the 1% AEP floodplain. At Avebury, the steep local
topography means the stones are on land outside of the floodplain. At Blenheim Palace, the
floodplain (both 1% and 0.1% AEP) is constrained within the lake. The palace itself is not in
the floodplain.
In summary Thames region contains international, national and local scale historic assets.
The assessment identifies that a proportion of these are vulnerable to flooding. At the CFMP
scale it is the internationally recognised sites that have been screened and scoped. All other
SAMs are protected through the land use planning system and would therefore be fully
assessed at a local scale if any interventions arising from the CFMP could impact on a SAM.
Policies and responses for adaptation and mitigation to climate change (including flood risk)
may have an impact on the historic environment. The non-renewable character of historic
features and the potential for their damage and loss should always be taken into account
when adaptation and mitigation responses are being planned and executed. English Heritage
is committed to working with others to avoid or minimise any adverse impacts, while
delivering the necessary changes.
Thames Region contains a large number of nationally (SAMs) and also some internationally important (World Heritage Sites) archaeological sites. A small percentage of these are within the 0.1% AEP and may be vulnerable to both increases and decreases in water level. 2.8.6 Surface Water and Groundwater Quality Groundwater is very important within Thames Region, providing around 40 percent of public
water supplies with chalk forming the predominant aquifer. Groundwater is also an important
source for private water supplies, domestic use, industry and farming. In addition, river and
stream flows and wetland habitats are often heavily dependent on groundwater seepage and
springs, especially during the drier months and in the upper reaches of the catchment. As a
result, groundwater is fully utilised over much of Thames Region and therefore both the
107
quantity and quality of groundwater is extremely important in maintaining these resources.
Groundwater is vulnerable to pollution from surface activities since aquifers make up two-
thirds of the land surface in this densely populated region.
Runoff from agricultural land is often a source of nutrients (such as nitrate and phosphate)
which can cause eutrophication, leading to excess plant/algal growth and undesirable effects
on the ecology, quality and uses of the water. Nitrates and pesticides can also contaminate
groundwater, making it unsuitable for public supply.
Organic pollution reduces the amount of oxygen dissolved in waterbodies, which can have
major impacts on aquatic life. It can run off yard areas or fields when slurry, manure, sewage
sludge or manufactured fertilisers are applied. Farm gateways and tracks can assist nutrients
and sediment to drain straight out of fields and into watercourses. Currently, 22% of rivers
and 22% of groundwaters are impacted by nutrients (phosphate) from land. With increasing
intensity of storms predicted with climate change, this could become more of a problem.
Approximately a quarter of rivers in Thames region are at risk from failing Water Framework
Directive objectives as a result of the effects of urban run-off. Most rivers in the region are
affected, with the worst impacts in London and the major cities. Typical pollutants in urban
runoff include organic waste, pesticides, fertilisers, hydrocarbons, nutrients and sediment. In
Thames Region the issue is acutely experienced in the London area given the large extent of
urban land use. It is also an issue to varying degrees in most other towns and cities across
the region and is closely linked to the transport infrastructure, particularly roads.
29% of rivers in Thames region are impacted by sewage related point discharges. Increases
in the quantities of rainwater and/or wastewater due to population growth will put further
pressure on sewage treatment works and could result in more frequent failures of discharge
consent standards.
Diffuse pollution is strongly linked to land use activity. In Thames region, diffuse pollution pressures on river water bodies account for around 90% of them being at risk of not achieving good status under the Water Framework Directive. Diffuse source pollution pressures are also significant for groundwater bodies, also accounting for around 90% of them being at risk of not achieving good status. Diffuse pollution (rural), diffuse pollution (urban and transport) and point source pollution have all been identified as Significant Water Management Issues (SWMIs) in the Thames River Basin District6.
6 ‘Water for Life and Livelihoods. River basin planning: summary of significant water management issues, Thames River Basin District’ (Environment Agency, July 2007).
108
2.9 Communities and the local economy The Thames CFMP area includes all or parts of 29 London boroughs, eight unitary authorities
and 40 district councils. In terms of regional government, the plan area is covered mainly by
the South East England Regional Assembly (SEERA) the East of England Regional Assembly
(EERA) and the Greater London Authority (GLA). All these organisations have a role in
planning the levels and location of future development. The Thames CFMP has an important
part to play in informing both local and regional government of the need to take both current
and future flood risk into account.
2.9.1 Population The Thames region covers less than 10% of the land area of England and Wales, but
contains nearly a quarter of the population (approximately 12 million people). Even without
London, the South East has approximately twice as many people per hectare than the UK
average. Besides London, major urban areas like Reading and Oxford have some of the
highest population densities in Thames region. There is a continuing pressure for new
development, in some places, on a very large scale.
Since 1945 the distribution of the population has changed significantly, with rapid expansion
of urban areas such as Swindon, Oxford, Reading, Basingstoke and Bracknell. In 2000
London’s population was estimated to be more than 7.4 million people and will rise to 8.1
million by 2016. To meet this planned population growth, (circa 700,000 people over 15
years) a baseline target of providing new houses has been set, amounting to a minimum of
457,950 new properties in London (1997-2016). A large percentage of the new development
is planned in the lower Lee valley (the 2012 Olympics site) and in the tidal floodplain of the
River Thames (Thames Gateway). The South East England Regional Assembly (SEERA)
propose to develop between 25,500 and 32,000 new houses each year from 2006 until 2026.
The exact locations for this new development have yet to be confirmed. If the properties are
built within the floodplain, there will be a large increase in the number of people at risk from
flooding. As well as the localised flood risk associated with the new development, more
urbanisation could also increase the risk of flooding in other parts of the catchment.
Amongst the population at risk to flooding, the most vulnerable social groups are likely to
include the long-term sick, elderly, single parents and those suffering financial deprivation.
These factors are summarised per enumeration district using the SFVI (Social Flood
Vulnerability Index)7. The data show that areas of highest social vulnerability are
concentrated in and around London, the Lower Lee and major urban areas further west of the
region, for example Oxford and Reading. This is discussed further in section 3.3.2.
7 Compiled by the Flood Hazard Research Centre (FHRC) at Middlesex University
109
2.9.2 Communications and industry Thames region generates more than a quarter of the England and Wales’ Gross National
Product (GNP) and has a similar percentage of all construction work. Table 2.6 shows the
regional share of UK imports and exports for London and the South East. The economy of the
South East is an important part of the UK economy as a whole. New industries, such as
information technology, biotechnology and advanced engineering, are at the forefront of the
economy, with 88% of the working population in the region being employed in the service
sector. South East
(exc. London) London Combined Total
(SE + London) UK
Exports as a percentage of UK exports
All exports 13.8 13.9
27.7 100
To the EU 13.9 13.6 27.5 100
To non-EU countries
13.6 14.3 27.9 100
Imports as a percentage of UK imports All imports
22.5 17.1 39.6 100
From the EU 26.1 12.9 39.0 100
From non-EU countries 18.4 21.7 40.1 100
Table 2.6 Regional share of UK export and import trade, 2002. Source: HM Customs and Excise
Trends suggest that the economy in Thames region will continue to increase in importance
both within and outside its boundaries. The number of people living here will increase, putting
even more pressure on the population density. These changes could potentially make both
society and the economy more vulnerable to flooding.
The Thames region covers less than 10% of the land area of England and Wales, but contains nearly a quarter of the population. The region has some areas that are particularly heavily urbanised and densely populated. Even without London, the South East has approximately twice as many people per hectare than the UK average. The economy of the South East is an important part of the UK economy as a whole with Thames region generating more than a quarter of the Gross National Product. New industries, such as information technology, biotechnology and advanced engineering, are at the forefront of the economy, with 88% of the working population in the region being employed in the service sector. As part of the South East of the UK, the population is expected and encouraged to further increase in Thames region. The number of people and the infrastructure required to support this growth will increase the pressure on resources, particularly land.
110
