The Clacton on Sea SWMP is within the administrative area of Tendring District Council and is ranked ninth within the county in terms of properties at risk of surface water flooding.

The catchment is characterised by a valley topography with areas of high elevation in the North-east and North-west, falling to the South onto the floodplain of the Holland Brook. The area is underlain by Clayey Loams, Sandy Loams and Clay to Silt, which vary from relatively freely draining (Sandy Loams) soils to those that impede flows (Clayey Loams). The urban extent comprises a mix of permeable and impermeable land cover. Apart from the coastline along the Eastern boundary, this is enclosed by a larger rural upper catchment, that is categorised mainly as grass and parkland.

The Holland Brook and Pickers Ditch run through to the Eastern extent of Clacton on sea. The Holland Brook is the largest watercourse within the study area, at approximately 10km in length and flows North to South, combining with Pickers Ditch, East of Clacton on Sea, before discharging to the sea at Holland on Sea. Picker’s Ditch is approximately 7km in length from its tributary with the Holland Brook to its upstream extent, North-west of Clacton on Sea.

3 CDA’s have been identified within the study area and around 250 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 580 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 Clacton on Sea 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%	30	0	0	30
5%	-	-	-	-
3.33%	80	5	0	85
1.33%	192	8	0	200
1%	246	14	0	260
0.50%	402	19	0	421
1% CCL**	429	19	0	448
1% CCU***	577	29	0	606

* 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	CLT_001	Valley Road	80	163
2	CLT_002	South West Clacton on Sea	62	176
3	CLT_003	North Clacton on Sea	72	211