The Maldon SWMP is within the administrative area of Maldon District Council and is ranked eighth within the county in terms of properties at risk of surface water flooding.

The study area is largely low lying, with elevations below 10m Above Ordnance Datum (AOD) in the majority of the South and East of the catchment, including the town of Heybridge and along the River Blackwater. The elevations increase to the North and West of the catchment where elevations exceed 70m AOD. The bedrock geology of the region is primarily clay with sand and gravel superficial deposits.

The River Blackwater dominates the catchment, flowing in a South-easterly direction and separating the towns of Maldon to the South and Heybridge, to the North’ Tributaries of the River Blackwater include the rivers Chelmer and Ter to the North-west, Catchpole and Spicketts Brooks to the East of Heybridge and Limbourne Creek and Mundon Wash to the South-east of Maldon.

5 CDA’s have been identified within the study area and around 420 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 920 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 Maldon 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%	18	6	1	25
5%	66	7	1	74
3.33%	117	9	2	128
1.33%	311	23	3	337
1%	417	37	3	457
0.50%	678	67	3	748
1% CCL**	640	64	3	707
1% CCU***	922	85	3	1010

* 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	NMLD_001	Maldon	8	13
2	NHEY_002	Heybridge Wood	36	121
3	NHEY_004	Langford	0	0
4	NHEY_006	Jacobs Farm	2	3
5	NHEY_007	Great Totham	67	113