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