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, in partnership with the Environment Agency, we are in the initial stages of developing a flood infrastructure scheme known as the River Itchen Flood Alleviation Scheme. This 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 the 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, see what to do before, during, and after a flood.

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, the drainage network can be overwhelmed, leading to localised surface water flooding.

Who is responsible for managing the risk?

As a lead local flood authority, we have 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 so water that falls on the highway flows into the drainage network. Gullies are inspected on a maintenance schedule however, if you believe a gully needs clearing, please report it to us at blocked drains.

We 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, we comment on and request 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 you can use at 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.

See what to do before, during, and after a flood.

Fluvial flooding usually occurs when significant rainfall falls for an extended period of time. This results in the river's 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. This is due to the high coverage of the city in impermeable surfaces.

Other factors that can influence the risk of flooding include:

• High groundwater levels
• Tidal ingress
• 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
• Blighmont Crescent Stream

The Environment Agency:

• Manage main rivers by carrying out maintenance, improvement, or construction work, where needed, to manage flood risk
• Has the responsibility to issue flood alerts for any main river in Southampton

The risk of flooding from ordinary watercourses is our responsibility. 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. See what to do before, during and after a flood.

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?

As the lead local flood authority (LLFA), we 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 challenges of groundwater flooding is that traditional methods of managing flood risk may not be effective against it.

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 it. Use the Environment Agency’s Groundwater Vulnerability Map by finding your location on the map, 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. See what to do before, during, and after a flood.