The effects of flooding on the area of Lewes

The effects of flooding on the area of Lewes




Lewes is the county town of East Sussex, which is situated within the South Downs. The Lewes area is of far less elevation than the surrounding South Downs area, with a variety of chalk hills that canvas’ a total area of 620km2. The river Ouse runs through the town on the lowest land, with a confluence of the Winterbourne Stream on the east, which flows southward (Fig. 1)

Figure 1: Map of Lewes area with river cutting through the area


The Ouse valley is a wide, flat plain; heavy rainfall in the river’s catchment area can bring severe flooding to nearby settlements (Ekwell, 1988). For example, the town of Naburn Lock, south of York experienced unusually heavy rain, which lead to water-logged soils and flooding in October 2000 (Willers, 2000). This report aims to analyse the Lewes area and study the effects of flooding within the area using GIS (Geographic Information System) technology. This report will also look at the effect of flooding on urban areas, including roads and buildings.




The Digital terrain model of the area was obtained using Digimap, which is a web mapping and online data delivery service, allowing access to spatial data and maps. Data of the sea levels, and river flooding were also obtained from SeaZone, which gave important data on the effects of flooding.


ArcGIS was used to analyse the data and with the use of ArcMap, maps that displayed the raw data were produced. This was done by inputting the digitally downloaded data into ArcMap as layers, which created maps.


The main type of data was raster type, which was further classified into six classes to manage the data in an improved layout. Using the raster calculator, which is a spatial analyst tool, illustrative demonstrations of the flooding events at different flood levels (1,2 and 5 m) could be produced.






The main data was the Digital Terrain Model (DTM) named TQ40west, which is classified as metadata; a kind of data that gives information of other data having scale, source and relevance to be understood while used in GIS (Dempsey, 2011). The file is also an Ascii raster file (Table 1), as an Esri grid, with cell sizes of 10, 10. These cells obtain information of the Lewes area within them.


Table 1: Illustration of raster data (TQ40west) properties


Using the DMT the topography of the Lewes area could be mapped out. It had been classified into 6 classes, which clearly shows the floodplain characteristics of the area e.g. the river has the lowest elevation within the area (Fig. 2).


Figure 2: The topography and elevation of the Lewes area


Figure 2 demonstrated which areas are at higher risk to flooding, as the areas with low elevation are more likely to flood. Figure 3 also presents similar information showing how slope contributes to flooding, as the steepness of the land affects the speed of surface runoff, causing increased flooding.

Figure 3: The slope and steepness of the Lewes area



The DMT was classified into 6 classes. Each class took up a certain area of space, which could be calculated to understand how much of the Lewes area differed in its elevation (Table 2).


Table 2: Area of the 6 classes of the DMT


Table 2 shows the amount of area in each class e.g. most the Lewes area in in low lying land as 41.02% of the area is 2m or below, meaning much of Lewes is at risk of flooding.


There are various land uses surrounding the Ouse River, which may be affected by flooding, including roads, building and water networks (Fig. 4)


Figure 4: Base map of Lewes with building, roads and water networks



With GIS, it is possible to calculate the areas that can be affected by flood events at different levels of flooding (1m, 2m and 5m).  Table 3 shows that with increased flood elevation there is an increase in the land affected.


Flood Elevation Percentage of affected area (%) Affected Area (Hectares)
1 metres 2.6 130.45
2 metres 11.31 567.2
5 metres 28.05 1406.55
Total Area   501501

Table 3: Percentage of area affected by different flood elevation


Effects of 1m flooding:


Figure 5 shows the effects of 1m flooding, as seen the consequences are minimal, with little impact on urban areas occurring. When compared to GoogleEarth images (Fig. 6) the 1m flooding would mostly affect pasture zones.


Figure 5: Effects of 1m flooding on Lewes area


             Figure 6: Satellite images of Lewes



Effects of 2m flooding:


As seen in figure 6 there are significantly more risk with 2m flooding than there is with 1m flooding. 2m flooding would affect more land by 8.71%, with the western part of the map being affected, which have the urban areas. This would mean that buildings and roads would be affects which could cause significant damage.


Figure 7: Effects of 2m flooding on Lewes


The southern part of the map is also affected, however due to the southern areas being mostly pasture there will be little social and economic impact in those areas.


However, the environment and ecosystems within the area may be largely affected.

Water networks are largely flooded too, which would have an impact in the quality of water in the area, due to sediments being washed into them.


Effects of 5m flooding:


There is noticeably extensive flooding at 5m, with a larger area of buildings roads and water networks being flooded. This could have large social impacts as hospitals may be flooded, reducing the quality of life. Economic impacts may also occur as businesses may need to shut down, and road closures may reduce commerce transported from the town.


There are also environmental impacts that may occur due to such flooding, such as landslides and erosion of the surrounding cliff hills, which are a soft sediment so is more susceptible to erosion.


Figure 8: Effects of 5m flooding in Lewes




In general, the areas that will be most affected by the flooding is low-lying areas, with the first class in the elevation model having the most flooding occurring. It can also be seen that the low-lying areas are situated closer to the river, which would increase the impacts of a flood event in Lewes. As seen in figure 2 much of Lewes is on low-lying land, as it is situated in a low-slope gap in within chalk hills, which means it is more likely for the area to flood.


The main drive of flooding is the climate and heavy rainfall. In attempts to reduce flood risk preventative approaches can be put in place, such as using statistical data to establish the frequency and probability of heavy rain which contributes to flooding (Nelson, 2015). GIS can also be used to produce models of the area, which analyses the extent of potential flooding. Using this information, mitigation can be put into place e.g. producing safe routes.




In summary, the use of GIS gives a broader understanding of how flooding can affect the area of Lewes. It uses data to determine potential flooding which can further be used to prevent the impact of flooding by using mitigation.

Word count: 1062



Ekwall, E. English River Names (Oxford University Press: 1928). Waite, Alice. Exploring the Yorkshire Ouse (Countryside Productions: 1988)


Willers, D. Water levels of River Ouse rise in York – flood warning in place at Naburn Lock. York press 2000


Dempsey, C. (2011). Metadata ~ GIS Lounge. [online] GIS Lounge. Available at: [Accessed 26 May 2017].


Nelson, S., (2015). Flooding Hazards, Prediction & Human Intervention. Tulane University. New Orleans, Louisiana.


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