The Harlow SWMP is within the administrative area of Harlow District Council and is ranked second within the county in terms of properties at risk of surface water flooding. Harlow is also recognised nationally as a Flood Risk Area (FRA) by the Environment Agency.

The Harlow district covers an area of over 30km2 within the Stort Valley and borders Epping Forest to the South and Hertfordshire to the North, with the River Stort forming a natural boundary between the two. Harlow is a former ‘New Town’, a concept conceived in the 1940’s in response to the need for additional housing following post-wartime destruction in London and the South-east to reduce overcrowding in London. 

There are several watercourses within the study area, with the largest being the River Stort which flows along the Northern boundary. The River Stort is a tributary of the River Lea and rises in Langley Hills near Clavering in Essex. The higher ground in Harlow is located to the South-east and falls North-west towards the River Stort. The bedrock geology in the area is primarily clay with sand and gravel superficial deposits.

9 CDA’s have been identified within the study area and around 1,680 residential properties identified as being at risk of surface water flooding during a 1 in 100 year storm, or 1% Annual Exceedance Probability (AEP). This has potential to increase to around 2,390 residential properties should the upper limit of 40% be considered to account be considered to account for future climate change. There is a history of surface water flooding within the area that reinforces the LLFA’s decision to conduct a more detailed study here.

Map of Harlow Study Area

1) The SWMP Study Area represents the extent of the LLFA’s hydraulic modelling and detailed modelling data can be made available within this area. The Risk of Flooding from Surface Water (RoFfSW) mapping should continue to be referenced for an indication of surface water flood risk outside of the SWMP Study Area boundary.

2) A Critical Drainage Area (CDA) is a discrete geographic area (usually a hydrological catchment), within the SWMP Study Area where multiple or interlinked sources of flood risk cause flooding during a severe rainfall event thereby affecting people, property, or local infrastructure.

Summary of Flood Risk across the Study Area

Table 1

AEP* Residential † Non-Residential ‡ Critical Service § Total
10% 374 17 0 391
5% 727 32 0 759
3.33% 922 36 1 959
1.33% 1485 60 5 1550
1% 1683 69 6 1758
0.50% 2173 91 6 2270
1% CCL** 2066 86 6 2158
1% CCU*** 2395 104 7 2506

* Annual Exceedance Probability (AEP) = Chance of a rainfall event occuring, or being exceeded in any one year.
** Climate Change Lower (CCL) = Lower Climate Change increase, taken as 20% additional rainfall.
*** Climate Change Upper (CCU) = Upper Climate Change increase, taken as 40% additional rainfall.

 Residential Property = a building used, or is suitable for use as, a dwelling.
 Non-Residential Property = property other than residential, such as office buildings, shopping centres, businesses, churches, hotels, hospitals, schools or government buildings.
§ Critical Service = includes services such as power, water, sewer, wastewater treatment, communications, education and emergency medical care.

Table 2

      Residential Properties
No. CDA Ref. CDA Name. 1% AEP  1% AEP+CCU
1 NHLW_01 Sumners 44 55
2 NHLW_02 Kingsmoor 125 156
3 NHLW_03 Stewards 19 55
4 NHLW_04 Latton Bush 67 91
5 NHLW_05 Brays Grove 238 362
6 NHLW_06 Nettleswell 32 52
7 NHLW_07 Victoria Gate 7 14
8 NHLW_08 Rivermill 35 49
9 NHLW_09 Old Harlow  182 225

Page updated: 12/09/2022

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