Types of flood risk
There are multiple types of flooding and all of these must be considered as part of a flood risk assessments, including national and local flood risk designations.
Fluvial flooding can come from:
- Main rivers
- Ordinary watercourses
From a planning perspective main rivers are under the authority of the Environment Agency and ordinary watercourses are under the authority of the Lead Local Flood Authority (LLFA).
For main rivers all land will fall into one of the following designations:
- Flood Zone 1. An estimated less than 1 in 1000 year return period (development is normally acceptable) and an annual probability of <0.1%. Note that if no flood zone is labelled on the mapping system, then it is flood zone 1.
- Flood Zone 2. An estimated between 1 in 100 and 1 in 1000 year return period and an annual probability of 1% - 0.1%.
- Flood Zone 3a. An estimated between 1 in 30 and 1 in 100 year return period and an annual probability of 3.3 (recurring)% - 1%.
- Flood Zone 3b (functional flood plain). An estimated greater than 1 in 30 year return period (most development is unacceptable) and an annual probability of greater than 3.3 (recurring)%.
Further information on flood zones is available at Flood risk and coastal change - GOV.UK.
For ordinary watercourses all land within Leicester will fall into one of the following designations:
- Ordinary watercourse flood extent 1. An estimated less than 1 in 1000 year return period (development is normally acceptable) and an annual probability of <0.1%. Note that if no OW flood zone is labelled on the mapping system, then it is OW flood zone 1.
- Ordinary watercourse flood extent 2. An estimated between 1 in 100 and 1 in 1000 year return period and an annual probability of 1% - 0.1%.
- Ordinary watercourse flood extent 3a. An estimated between 1 in 20 and 1 in 100 year return period return period and an annual probability of 5% - 1%.
- Ordinary watercourse flood extent 3b. An estimated greater than 1 in 20 year return period and an annual probability of greater than 5%.
Pluvial, also known as surface water flooding is significantly more complicated to estimate and model than fluvial flooding.
For surface water all land will fall into one of the following Environment Agency designations:
- Surface water very low risk. An annual probability of <0.1%.
- Surface water low risk. An annual probability of 1% - 0.1%.
- Surface water medium risk. An annual probability of between 3.3 (recurring)% - 1%.
- Surface water high risk. An annual probability of greater than 3.3 (recurring)%.
In addition to this we have defined additional local pluvial flooding constraints:
Critical drainage areas (CDA’s)
These are the catchment areas for the corresponding surface water flooding hotspot. Surface water runoff within a CDA, is likely to contribute to surface water flooding in the associated surface water hotspot. The CDA designation itself, does not indicate whether an area is at significant risk of flooding. Developers need to refer to this information in flood risk assessments and when designing SuDS.
Surface water flooding hotspots
These are areas where properties have been identified as most at risk to surface water flooding within the associated CDA. This is based on cluster analysis modelling, which considers:
- Surface water flood risk under a variety of rainfall return periods (ranging from 1:5 to 1:1000, with climate change allowances applied to some return periods)
- Surface water velocity
- Surface water depths
- UK flood hazard rating as outlined in Flood risk assessment guidance for new development - GOV.UK.
Where a surface water flooding hotspot does not reside within a CDA, it is considered a risk due to the size of the property, its associated impermeable surfaces and its classification as critical infrastructure. For example, the surface water flooding hotspots associated with the city’s hospitals.
Both the CDA’s and surface water flooding hotspot originate from surface water flood risk modelling undertaken as part of our Surface Water Management Plan (2012). For further information visit Flood risk studies.
Flooding from groundwater
Groundwater flooding occurs when water comes out of the ground due to the underlying geology, which can cause flooding. Groundwater flooding often occurs after extensive periods of heavy rainfall but can arise from upwelling springs of water. Low-lying areas, where the water table is closer to the surface, or where underlying soil and bedrock are vulnerable to becoming saturated, pose greater risk of groundwater flooding.
You can view the map of ground water susceptibility in Leicester in Appendix G of the Strategic Flood Risk Assessment (2022). Instructions on how to obtain a copy of this are available on the Flood risk studies page.
Flooding from sewers
Sewer flooding can occur due to blockage, collapse or surcharging (where the volume of water entering the sewer exceeds its capacity) causing water to back up in the sewer system. This can lead to water overflowing from gullies, drains and manholes causing flooding.
Severn Trent Water STW are responsible for the provision of sewerage services, which includes the management of public sewers, (both foul water and surface water) and for the provision of water supply in Leicester. As required by the Water Industry Act 1991.
They also offer services to support planning applications such as developer enquiries. Get more details on the Severn Trent website.
Flooding from artificial sources
This is flooding that occurs because of infrastructure failure or human intervention. Sources of artificial flooding include reservoirs, canals, ponds and other artificial structures. The probability of a structural failures are low but the impacts can be significant.
Flooding from reservoirs is managed by the Environment Agency. You can check the long term flood risk for at GOV.UK.
Flooding from canals is managed by the Canal and Rivers Trust in Leicester and further information is available at: Canal and River Trust.