The Colchester SWMP is within the administrative area of Colchester Borough Council and is ranked fifth within the county in terms of properties at risk of surface water flooding. Colchester is also recognised nationally as a Flood Risk Area (FRA) by the Environment Agency. The study area is approximately 88km2 and comprises of Colchester town, along with the surrounding settlements of Wivenhoe, Stanway, Beacon End and Lexden.

The topography of the catchment varies between areas of high ground located in the South-western and North -western portions of Colchester town through to areas which are at a lower elevation, which are typically confined to the river corridors. The bedrock geology is dominated by London Clay, which is exposed along the sides of the river valley and at the ground surface in some areas. In some locations this is overlain by drift deposits that are composed of the Lowestoft Formation, head deposits, alluvium and river terrace deposits.

The River Colne dominates the catchment area, flowing in a South-eastern direction through the centre of the town. Of the four main tributaries that discharge into the River Colne, the Salary Brook and Roman River are located within the study area. In addition to these, there are a number of smaller tributaries, creeks and brooks, particularly within the tidal reaches of the watercourses. The River Colne converges with the Blackwater Estuary at Mersea Island and Brightlingsea, and is tidally influenced up to the East Mill Sluice, which is located to the East of the town.

11 CDA’s have been identified within the study area and around 940 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 1,630 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 Colchester 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%	179	30	0	209
5%	306	62	0	368
3.33%	406	80	0	486
1.33%	791	120	0	911
1%	940	152	0	1092
0.50%	1375	208	1	1584
1% CCL**	1307	200	0	1507
1% CCU***	1628	230	2	1860

* 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	NCOL_001	Old Hythe	16	38
2	NCOL_002	The Hythe	63	101
3	NCOL_003	Abbey Gate	79	164
4	NCOL_004	Lexden	1	1
5	NCOL_005	St Annes	48	68
6	NCOL_007	Mile End	35	76
7	NCOL_008	Parsons Heath	113	232
8	NCOL_009	Highwoods	3	4
9	NCOL_012	Balkerne Hill	22	35
10	NCOL_013	Hythe Quay	23	41
11	NCOL_015	Wivenhoe	33	51