1 Catchment Overview

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 Thames Region Thames as a % Wales 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 2,228,400 705,700 31.7 (tidal and fluvial) Population at risk from a 0.1% AEP flood 5,014,000 1,588,000 31.7 (tidal and fluvial) Area of internationally designated sites 14 within the 0.1% AEP floodplain (km²) 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 425,646,000 85,230,000 20 management (£) (England only) (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 Figure 2.2 Location of major urban areas in Thames region Thames region covers approximately 10% of the total area of England and Wales. Compared to other Environment Agency regions, Thames region: • has the smallest area of 1% AEP floodplain • contains the largest number of properties at risk of fluvial flooding 2.1 Definition and extent of the Thames CFMP 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. 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. 200 180 160 140 120 100 Height (m) Height 80 60 40 20 0 0 25 50 75 100 125 150 175 200 225 250 275 Distance (km) 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.

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