The Chelmsford SWMP is within the administrative area of Chelmsford City Council and is ranked sixth within the county in terms of properties at risk of surface water flooding. Chelmsford is also recognised nationally as a Flood Risk Area (FRA) by the Environment Agency. The study area includes the city of Chelmsford and the surrounding settlements of Boreham, Broomfield, Chignal, Galleywood, Great Baddow, Little Waltham and Springfield.
The topography varies as it is located at the junction of four significant river valleys, with the higher ground being located along ridge lines to the North and South. The bedrock geology is dominated by mudstone (the London Clay Formation), whilst the surface geology varies in line with the topography, with Lowestoft Formation on the ridgelines and a combination of river terrace deposits and glacial deposits adjacent to the rivers and on natural flood plains.
There are several watercourses within the study area, with the largest being the River Chelmer which flows entirely through Essex, originating in Uttlesford and flowing south through the study area. It continues flowing to the east after joining a major tributary, the River Can. It then flows east through the borough and into Maldon & Heybridge until it meets the River Blackwater near Maldon. It discharges into the North Sea via the Blackwater Estuary.
12 CDA’s have been identified within the study area and around 760 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 1,500 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 Chelmsford 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
|AEP*||Residential †||Non-Residential ‡||Critical Service §||Total|
* 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.
|No.||CDA Ref.||CDA Name.||1% AEP||1% AEP+CCU|
|1||NCLF_001||St Andrews South||21||75|
|2||NCLF_002||St Andrews North||8||22|
|6||NCLF_006||Lawns and Spring||12||21|
Page updated: 12/09/2022