Preliminary flood risk assessment

4.1 Flooding Sources

Stockton Borough Council collated all the data and information collected from partner organisations and the council's internal services. The flooding events across the Borough of Stockton came from a variety of flood sources; however the source of flooding was unknown or not recorded in many cases. LLFAs must consider specific sources of flooding as part of the PFRA report, a summary of each source of flooding considered is included below.

Surface water flooding

Surface water runoff is rainwater (including snow and other precipitation) which is on the surface of the ground and has not entered a watercourse, drainage system or public sewer. Surface water flooding generally occurs when heavy rainfall exceeds the capacity of local drainage networks and water flows across the ground. In the preparation of the PFRA, Stockton Borough Council has obtained information on surface water flooding incidents from a number of sources.

Groundwater flooding

Groundwater is water which is below the surface of the ground and in direct contact with the ground or subsoil. Groundwater is most likely to occur in areas underlain by permeable rocks, called aquifers. These can be extensive, regional aquifers, such as chalk or sandstone, or may be more local sand or rivers gravels in valley bottoms underlain by less permeable rocks.

Flooding from Drainage Systems

Flooding from drainage systems occurs when the flow entering a system, such as an urban storm water drainage system, exceeds its discharge capacity and it becomes blocked or it cannot discharge due to a high water level in the receiving watercourse.

Canals and Ordinary Watercourses

Flooding from an ordinary watercourse occurs when it cannot accommodate the volume of water draining from surrounding land. British Waterways provided detailed information on their canal network. The natural tidal limit of the Tees previously extended to Low Worsall. The Tees Barrage now maintains upstream water levels at around mean high water throughout the tidal cycle.

4.2 Historic Flood Events

This section identifies areas that have suffered from past flooding events that have had significant harmful consequences for human health, economic activity, cultural heritage and the environment. The maps included in Annex 6 (Summary maps for past floods), identifies areas where past flooding incidents have occurred. The map has been produced using 1km by 1km grid squares therefore if one or many incidents have occurred within a grid square then this is highlighted to represent a past flood incident area.

Yarm-on-Tees

An intense storm hit Yarm on 31st January & 1st February 1995 and flooding was caused by surcharge from surface water drainage system serving the Town Centre. The flooding affected gardens, car parks, the Skinnery Housing Site and the High Street but did not affect any properties. Cleveland Fire and Rescue Service managed to contain the flood water at acceptable levels within Yarm High Street.

Lustrum Beck

Lustrum beck has a record of flooding dating back to 1771 with the worst flood occurring on the 29th of March 1979 which included substantial flooding of land, residential and commercial properties. Further floods occurred in April, June, October, and November 2000 all of which affected properties along Hartburn Avenue and the area around Browns Bridge. It was reported that properties along Darlington Road, Burnside Grove, Oxbridge Lane, Bishopton Road, Wrensfield Road, the Adult Training Centre, Newtown Methodist Church, Kingdom Hall and Wrensfield School were also affected by flooding during the severe event in November 2000.

Lustrum Beck is classed as a Main River and is the responsibility of the Environment Agency. However, given the number of housing developments creating additional surface water run off built over the years on floodplain land that would have once stored much of the floodwater it was agreed to be appropriate to include this area within the PFRA report.

Halidon Way, Billingham

Over recent years there have been several instances of flooding in and around Halidon Way in Low Grange, Billingham. The most notable of which was the event which occurred in March 1979, resulting in 68 dwellings being flooded, some to depths in excess of 1 metre.

Following the 1979 event, a number of works were implemented to reduce the impact and risk of repetition. The work comprised the construction of a new trash screen at the entrance to the culverted section of Cowbridge Beck, sealing the pedestrian subway on Neasham Avenue and constructing an earth bund around the perimeter of the Billingham Campus playing field.

During heavy rainfall in 2003, several properties of Halidon way, Billingham suffered from internal flooding. It was reported that the flooding seemed to originate from two areas surface water run-off from within the curtilage of Oakdene School, other water running off Neasham Avenue down into Halidon way.

Leven Bridge, Low Lane (A1044)

In November 2004, the River Leven burst its banks from continuous heavy rainfall and flooded carriageway and nearby houses including Cross Keys (PH). Leven Bridge was closed by police on the night of the 2/3 November due to water flowing across the road. This past extreme event is included in the PFRA report due to the event having a significant harmful consequence for human health. The River Leven is classed as Main River and is the responsibility of the Environment Agency.

Fairfield, Stockton-on-Tees

An intense storm hit the Borough of Stockton on Tees on the 21st July 2010; one area hit particularly badly was Fairfield, where several properties suffered from internal flooding. The area affected by flooding included Victoria Road, Blackburn Close, and Logan Drive. It was reported that the flooding occurred due to the drainage exceeding its capacity within the area.

In September 2009, surface water flooding across Stockton Borough occurred along with drainage exceeding its capacity in several places across the Borough. One area hit hard which included internal flooding was Fairfield. It was reported that properties on Victoria Road, Logan Drive, Blackburn Close suffered flooding.

Stockton Borough Council are aware that Northumbrian Water are currently carrying out works to alleviate the flooding within the area. However, at the time of writing this report the works were not complete.

Port Clarence

In March 1999 substantial flooding occurred due to heavy rain and peak flows unable to pass through Holme Fleet culvert, which is located to the north of Port Clarence. It was reported that the culvert was blocked at the time by material which had entered the access chambers.

On the 8th November 2000 between 2-4am an intense storm hit the area of Port Clarence were approximately 16 properties suffered from internal flooding with flood water reaching ground floor level. The properties affected included Holly Terrace, High Clarence, Palm Terrace and Laburnum Grove, Port Clarence. There were also a number of properties that suffered flooding to their gardens within the Port Clarence area. It was reported that the flooding occurred due to Holme Fleet Beck bursting its banks due to heavy rainfall.

Following these events Stockton Borough Council desilted the culvert and improvements were made to the inlet. In spring 2006 a pre-feasibility study was completed and concluded that the risk presented from tide locking of the outfall is insignificant. It was concluded that future operations and maintenance activities along Holme Fleet should be focussed on keeping the culvert free from blockage. In April 2005 Holme Fleet Beck was classified as Main River and became the responsibility of the Environment Agency.

4.3 Consequences

The consequences of flooding can have a positive and/or negative outcome. Wildlife and agricultural areas may have a positive benefit from flooding as it may result in more fertile land and habitats for wildlife. The severity of a flood will impact on the consequences for human health, economic activity, infrastructure, the community and environment. The Flood Risk Regulations require consideration of significant harmful consequences on:

• human health
• economic activity
• environment (including cultural heritage)

This definition of 'significant' has been defined by legislation and areas with more than 30,000 people at risk of surface water flooding have been designated as Indicative Flood Risk areas. Further information regarding the criteria for Indicative flood risk areas is provided in section 6.1.

The Tees Valley Authorities agreed that flooding is deemed locally significant if one of the following is affected by flooding:

• 5 or more residential properties
• 2 or more businesses
• 1 or more critical services
• 1 or more transport links (Impassable for 10 Hours or more)

The records listed in Annex 1 (Past Floods) are what Stockton Borough Council considered to be locally significant and not in line with the definition of significant provided by Defra.