Preliminary flood risk assessment

5.1 Summary

The EA have produced and released a number of datasets to assist LLFAs to prepare their PFRA report. The following datasets have been released to help LLFAs to identify potential future flood risk areas;

Flood Map (River and the Sea)

Flood Zone 3 identifies the natural flood plain as having a 1% or greater annual probability of fluvial flooding or 0.5% or greater annual probability of tidal flooding.

Flood Zone 2 identifies the natural flood plain ignoring the presence of flood defences, as having between a 1% and 0.1% annual probability of fluvial flooding or between a 0.5% and 0.1% annual probability of tidal flooding in any year.

Flood Map for Surface Water

Flood Map for Surface Water (1 in 30)

The map shows the 1 in 30 year probability of a rainfall event that produces flooding greater than 0.1m depth.

Flood Map for Surface Water (1 in 30 deep)

The map shows the 1 in 30 deep year probability of a rainfall event that produces flooding greater than 0.3m depth.

Flood Map for Surface Water (1 in 200)

The map shows the 1 in 200 year probability of a rainfall event that produces flooding greater than 0.1m depth. Stockton Borough Council has 9,200 residential properties that could be affected along with 2,000 non residential properties.

Flood Map for Surface Water (1 in 200 deep)

The map shows the 1 in 200 deep year probability of a rainfall event that produces flooding greater than 0.1m depth. Stockton Borough Council has 1,500 residential properties that could be affected along with 300 non-residential properties.

Areas Susceptible to Surface Water Flooding (AStSWF)

• AStSWF (Less)
  • The map shows areas which are 'less susceptible' to surface water flooding. Stockton Borough Council has a potential 14,500 properties that could be affected along with 3,500 non residential properties.
•  AStSWF (Intermediate)
  • The map shows areas which have a 'intermediate susceptibility' to surface water flooding. Stockton Borough Council has a potential 3,300 residential properties that could be affected along with 900 non residential properties.
• AStSWF (More)
  • The map shows areas which are more susceptible to surface water flooding.

Area Susceptible to Groundwater Flooding (AStGWF)

AStGWF - the map shows places where emergence is more likely to occur.

Stockton on Tees Borough Council considered the above scenarios produced by the EA to determine potential future flood risk. Following a discussion with our partners, details provided in section 5.2, it was agreed that Areas Susceptible to Surface Water Map best represents the Borough's local conditions and will be considered for future flood risk management. Stockton on Tees Borough Council's surface water management plan is currently being prepared and therefore no specific detailed locations for future flood risk areas have been identified.

Stockton Borough Councils indications of areas that may be susceptible for future flood risk are taken from the Areas Susceptible to Surface Water Map produced by the EA identifying the main source of flooding as surface water run off caused by natural exceedance. The map only provides a general indication of areas which may be more likely to suffer from surface water flooding.

The areas below have been identified in the risk assessment as having a higher risk of possibly being affected by Surface Water Flooding:

• Yarm-on-Tees 
  • Several residential properties and non residential properties could be affected by surface water flooding. Annex 7 includes a map showing the areas with the three bandings from 'less' to 'more' susceptible to surface water flooding.
• Fairfield
  • Several residential properties and non residential properties could be affected by surface water flooding. Annex 7 includes a map showing the areas with the three banding from 'less' to 'more' susceptible to surface water flooding.

5.2 Locally Agreed Surface Water Information

The Environment Agency made available two national datasets showing surface water flooding to LLFA's:

• Areas Susceptible to Surface Water Flooding (AStSWF)
• Flood Map for Surface Water (FMfSW)

Areas Susceptible to Surface Water Flooding

The AStSWF dataset contains one rainfall event and the map shows areas susceptible to surface water flooding with three susceptibility bandings; less, intermediate and more. The map has been produced using a simplified method that excludes, underground sewerage, drainage systems, smaller over ground drainage systems and buildings. The map only provides a general indication of areas which may be more likely to suffer from surface water flooding.

Flood Map for Surface Water

The FMfSW contains two rainfall events, divided into two depth bandings; 1 in 200 rainfall and 1 in 200 rainfall deep, as well as 1 in 30 rainfall and 1 in 30 rainfall deep. The FMfSW was developed using 5m by 5m grids. The elevation data was processed at a 2m resolution to produce a bare earth digital terrain model (DTM) before buildings were re-added with an arbitrary height of 5m. It does not show flooding that occurs from overflowing watercourses, drainage systems or public sewers caused by catchment wide rainfall events or river flow.

Both maps were reviewed with an EA representative and it was agreed that Areas Susceptible to Surface Water flooding best represents Stockton Borough Council (figure 3 (PDF, 423 KB)). This will be considered for future flood risk management. Annex 2 (Future Floods) reports details of potential future floods and consequences.

5.3 Impacts of Climate Change and Long-term Developments

The Evidence

There is clear scientific evidence that global climate change is happening now. It cannot be ignored.

Over the past century around the UK we have seen sea level rise and more of our winter rain falling in intense wet spells. Seasonal rainfall is highly variable. It seems to have decreased in summer and increased in winter, although winter amounts changed little in the last 50 years. Some of the changes might reflect natural variation, however the broad trends are in line with projections from climate models.

Greenhouse gas (GHG) levels in the atmosphere are likely to cause higher winter rainfall in future. Past GHG emissions mean some climate change is inevitable in the next 20-30 years. Lower emissions could reduce the amount of climate change further into the future, but changes are still projected at least as far ahead as the 2080s.

We have enough confidence in large scale climate models to say that we must plan for change. There is more uncertainty at a local scale but model results can still help us plan to adapt. For example we understand rain storms may become more intense, even if we can't be sure about exactly where or when. By the 2080s, the latest UK climate projections (UKCP09) are that there could be around three times as many days in winter with heavy rainfall (defined as more than 25mm in a day). It is plausible that the amount of rain in extreme storms (with a 1 in 5 annual chance, or rarer) could increase locally by 40%.

Key Projections for Northumbria River Basin District

If emissions follow a medium future scenario, UKCP09 projected changes by the 2050s relative to the recent past are:

• winter precipitation increases of around 10% (very likely to be between 0 and 23%)
• precipitation on the wettest day in winter up by around 11% (very unlikely to be more than 24%)
• relative sea level at Tynemouth very likely to be up between 7 and 38cm from 1990 levels (not including extra potential rises from polar ice sheet loss)
• peak River flows in a typical catchment likely to increase between 8 and 13%

Increases in rain are projected to be greater near the coast than inland.

Implications for Flood Risk

Climate changes can affect local flood risk in several ways. Impacts will depend on local conditions and vulnerability.

Wetter winters and more of this rain falling in wet spells may increase river flooding in both rural and heavily urbanised catchments. More intense rainfall causes more surface runoff, increasing localised flooding and erosion. In turn, this may increase pressure on drains, sewers and water quality. Storm intensity in summer could increase even in drier summers, so we need to be prepared for the unexpected.

Rising sea or river levels may increase local flood risk inland or away from major rivers because of interactions with drains, sewers and smaller watercourses.

Where appropriate, we need local studies to understand climate impacts in detail, including effects from other factors like land use. Sustainable development and drainage will help us adapt to climate change and manage the risk of damaging floods in future.

Adapting to Change

Past emission means some climate change is inevitable. It is essential we respond by planning ahead. We can prepare by understanding our current and future vulnerability to flooding, developing plans for increased resilience and building the capacity to adapt. Regular review and adherence to these plans is key to achieving long-term, sustainable benefits.

Although the broad climate change picture is clear, we have to make local decisions against deeper uncertainty. We will therefore consider a range of measures and retain flexibility to adapt. This approach, embodied within flood risk appraisal guidance, will help to ensure that we do not increase our vulnerability to flooding.

Long Term Developments

It is possible that long term developments might affect the occurrence and significance of flooding. However current planning policy aims to prevent new development from increasing flood risk.

In England, Planning Policy Statement 25 (PPS25) on development and flood risk aims to "ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. Where new development is, exceptionally, necessary in such areas, policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall."

In Wales, Technical Advice Note 15 (TAN15) on development and flood risk sets out a precautionary framework to guide planning decisions. The overarching aim of the precautionary framework is "to direct new development away from those areas which are at high risk of flooding."

Adherence to Government policy ensures that new development does not increase local flood risk. However, in exceptional circumstances the Local Planning Authority may accept that flood risk can be increased contrary to Government policy, usually because of the wider benefits of a new or proposed major development. Any exceptions would not be expected to increase risk to levels which are "significant" (in terms of the Government's criteria).

5.4 Relevant Local Information

Stockton Council developed a Local Climate Impacts Profile of Stockton on Tees for inclusion in its Climate Change Action Plan 2009-2020. Predictions within the Profile are drawn from the North East Climate Change Adaptation Study, which uses EARWIG (the Environment Agency Rainfall and Weather Impacts Generator) developed for the Environment Agency by Newcastle University and University of East Anglia, and uses a 5km resolution which provides a much more detailed projection for local regional areas.

Predictions for the Stockton area suggest that by the 2050s, we could experience 1.5o C to 2.5o C increase in temperature and up to 20% more winter rainfall and 30% less summer rainfall.

Although projected average annual rainfall shows a decrease of around 8.6%, it is expected that rainfall will become increasingly seasonal, with greater winter rainfall and a reduction in summer rainfall. Winter rainfall could increase by 12.7% Average summer rainfall could decrease by 33.2%.

Much of winter precipitation could be delivered by intense events. Flash flooding might become more frequent and flooding of the River Tees flood plain might worsen. The continuous rise in sea level will lead to higher risk of coastal erosion and flooding.

5.5 Future Major Developments

There are a number of new and proposed major developments within the Borough of Stockton on Tees listed below.

• Northshore Development: (E) 445757 (N) 519138
• Queens Park North Development: (E) 444581 (N) 520249
• Bowesfield North Phase 1 & Phase 2: (E) 444440 (N) 516910
• Boathouse Lane Development: (E) 444630 (N) 518150
• Tall Trees Hotel Development: (E) 441224 (N) 510541
• The Rings, Ingleby Barwick Development: (E) 443659 (N) 514499
• Wynyard Park Development: (E) 444045 (N) 526549
• Finchale Avenue Development: (E) 445687 (N) 524083