The Brentwood SWMP is within the administrative area of Brentwood Borough Council and is ranked third within the county in terms of properties at risk of surface water flooding.

The topography of the area ranges from approximately 100m Above Ordnance Datum (AOD) in the North and central regions to approximately 10m AOD in the South. The land use is predominantly greenfield and farmland, with the main urban expanses of Brentwood and Hutton being located in the centre of the study area. Other notable towns include Ingatestone, Doddinghurst and Blackmore. These urban areas are made up of both commercial and residential development.

There are four key watercourses which drain the catchment area: the River Wid, the River Ingrebounre, the River Roding and the River Mardyke. The River Wid is the main conveyance through the study area and is located on the Eastern boundary. The River Ingrebourne drains the South-western portion of the area and is located west of Brentwood. The River Roding is located in the North-west and flows in a South-westerly direction, joining the River Thames via Barking Creek. Finally, the River Mardyke drains the south of the area via numerous smaller tributaries. Other watercourses include, the Ingatestone Hall Brook, the Stondon Hall Brook and the Ingrebourne Brook in the west.

10 CDA’s have been identified within the study area and around 1,460 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 2,160 residential properties should the upper limit of 40% be considered to account be considered to account for future climate change. There is a recorded history of flooding incidents within the area that corroborates the LLFA’s decision to conduct a more detailed study.

Map of Brentwood 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%	416	27	0	443
5%	689	49	0	738
3.33%	825	63	0	888
1.33%	1279	101	0	1380
1%	1465	117	0	1582
0.50%	1916	152	1	2069
1% CCL**	1831	143	0	1974
1% CCU***	2158	175	2	2335

* 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	NBTW_001	Hutton	52	67
2	NBTW_002	Shenfield	124	180
3	NBTW_003	Thrift Green	63	72
4	NBTW_004	Hutton Mount	8	16
5	NBTW_005	Brentwood	67	95
6	NBTW_006	Pilgrims Hatch	16	25
7	NBTW_HOR01	Horndon Industrial Park	76	137
8	NBTW_HOR02	West Horndon	9	11
9	NBTW_ING01	Ingatestone	16	30
10	NBTW_ING02	Mountnessing	98	125