Flooding risk

Tidal flooding is the temporary inundation of coastal areas during exceptionally high tides or storm surges. This type of flooding usually occurs over a short period of time across high tide however, it can quickly cause significant impacts to low lying coastal areas.

Tidal flood risk is increased when a number of factors coincide, these can include:

• Spring (high) tides
• Strong coastal winds
• High river levels (tidal locking effect)
• Low atmospheric pressure
• Significantly high rainfall

What is the risk in Southampton?

Tidal flooding is the greatest flood risk facing the city with approximately 10% of the city being identified as at risk. As the effects of climate change are realised, the amount of properties and businesses at risk is likely to increase.

Currently, there are no formal raised flood defences in the city, and where defences do exist they are in private ownership and offer varying degrees of protection. However, Southampton City Council in partnership with the Environment Agency is in the initial stages of developing a flood infrastructure scheme known as the River Itchen Flood Alleviation Scheme which aims to reduce the risk of tidal flooding to large areas of Northam, St Marys and Chapel.

Who is responsible for managing the risk?

The Environment Agency has a strategic overview of all sources of flood risk which means they coordinate various high level flood risk management activities across the county. As part of this, the Environment Agency is responsible for the management of tidal flood risk including the implementation of flood risk management activities such as maintenance (excluding privately owned assets) and building of flood defence assets.

How can you prepare?

There are many steps you can take to both reduce your risk of flooding and to keep yourself safe in the event of a flood. 

• What to do before, during and after a flood
• Property Flood Resilience

Surface water flooding occurs when rainfall is unable to soak into the ground or enter a local drainage network resulting in water pooling on the ground.

What is the risk in Southampton?

Due to the urban nature of the city, Southampton is particularly vulnerable to surface water flooding. This type of flooding is often very difficult to manage as it can happen anywhere following heavy rainfall. The city has an extensive drainage network to manage surface water, however when rainfall is particularly intense or prolonged, it is possible for the drainage network to be overwhelmed, leading to localised surface water flooding.

Who is responsible for managing the risk?

Southampton City Council as a Lead Local Flood Authority has the responsibility to manage surface water flood risk and we do this in a number of ways:

• Our highways partners, Balfour Beatty Living Places clear and maintain highway gullies to enable water that falls within the confines of the highway to flow into the drainage network. Gullies are inspected on a maintenance schedule, however if you believe a gully needs clearing please report it to us on our blocked drains webpage
• The Flood Risk Management team hold a bi-monthly meeting with our highways partners, Balfour Beatty and Southern Water to identify and investigate areas which may benefit from further investigation, intervention and in some cases, surface water management schemes
• As a statutory consultee for planning applications, the Flood Risk Management team comment on and request for the implementation of Sustainable Drainage Systems (SuDS) in new major developments across the city to help manage surface water

How can you prepare?

There are many different types of resilience and resistance measures that can be implemented into your home or business to manage the flood risk to your property. There are also some steps you can take to both reduce your risk of flooding and to keep yourself safe in the event of a flood.

• What to do before, during and after a flood
• Property Flood Resilience

Fluvial flooding usually occurs when significant rainfall falls for an extended period of time. This results in the rivers water level rising to the point where it exceeds capacity and overflows onto surrounding land.

What is the risk in Southampton?

There are a number of classified Main Rivers and ordinary watercourses in the city that contribute to the risk of fluvial flooding.

There are many factors that influence the risk of fluvial flooding. As Southampton is an urban environment the amount of water that enters a river, and speed of which it enters, is much higher than a more natural environment due to a high coverage of impermeable surfaces. Other factors that can influence the risk of flooding include high groundwater levels, tidal ingress and blockages within a river channel which reduce a river's capacity.

Who is responsible for managing the risk?

The responsibility for managing fluvial flood risk in the city is shared between the Environment Agency, Southampton City Council and in some instances, riparian landowners. 
Southampton has a number of classified Main Rivers which are the responsibility of the Environment Agency, these include the River Itchen, River Test, Tanners Brook, Holly Brook, Rolles Brook, Monks Brook and Blighmont Crescent Stream.

• The Environment Agency manage Main Rivers by carrying out maintenance, improvement or construction work where needed to manage flood risk
• The Environment Agency has the responsibility to issue flood alerts for any Main River in Southampton

The risk of flooding from ordinary watercourses is the responsibility of Southampton City Council. Ordinary watercourses are classified as small streams, ditches and watercourses that are not Main Rivers.

In some instances, the responsibility for maintaining a watercourse lies with the riparian landowner. If you own a property or land which is adjoining, adjacent or with a watercourse you will be responsible for managing the risk of flooding. As a riparian landowner you have certain rights and responsibilities for a watercourse. 

How can you prepare?

There are many steps you can take to both reduce your risk of flooding and to keep yourself safe in the event of a flood, these are explained on the following webpages: 

• What to do before, during and after a flood
• Property Flood Resilience

Flooding from groundwater can happen when the level of water within underlying rock or soil (known as the water table) rises. When the water table rises and reaches the ground level, water can seep through the surface and flooding can occur.

Groundwater flooding usually occurs following heavy or prolonged rainfall and can last weeks or even months causing significant damage and disruption.

What is the risk in Southampton?

Our current understanding of groundwater flood risk is limited due to the complexities of mapping flows and understanding where groundwater is likely to emerge. There are believed to be a number of perched water tables within the city which increases the likelihood of groundwater flooding, with issues usually arising following periods of persistent rainfall.

Who is responsible for managing the risk?

Southampton City Council as the Lead Local Flood Authority are responsible for overseeing management of the risk of flooding from groundwater.

As the LLFA we continuously aim to improve our knowledge of groundwater flood risk in the city through data collection and analysis. As part of this we ask that if you believe you have experienced groundwater flooding please report it to us to improve our understanding of the issue.

As it is a complex matter there are many studies being conducted nationally with the aim to improve the understanding of groundwater, however one of the many challenges of groundwater flooding is that many traditional methods of managing flood risk may not be effective against groundwater flooding.

How can you prepare?

Knowing the risk of groundwater flooding in your area is a good first step to understanding your flood risk and determining any measures you could take to reduce you risk. Use the Environment Agency’s Groundwater Vulnerability Map by finding your location on the map and clicking the Landscape tab and searching the Geology and Soils layer.

Managing groundwater flooding is complex however there are many steps you can take to both reduce your risk of flooding and to keep yourself safe in the event of a flood, these are explained on the following webpages: 

• What to do before, during and after a flood
• Property Flood Resilience