Preliminary flood risk assessment

3.1 Introduction

The PFRA is prepared in accordance with the final guidance from the Environment Agency which was published in December 2010. The PFRA is a high level risk assessment that identifies areas where the risk of flooding is considered to be significant and warrants further investigation. The closer investigations will include the production of flood risk maps, flood hazard maps and flood risk management plans. The latter stage does not apply to Stockton, as there are no 'significant' flood risk areas.

3.2 Data Collection

In order to prepare the PFRA a large amount of information gathering was conducted from a variety of sources. Stockton Borough Council contacted a number of organisations to share data for the preparation of the PFRA, these included; NWL, Cleveland Fire and Rescue Services, Cleveland Police, Emergency Planning, Environment Agency, Planning department, highways department.

The available data sources and information collated by Stockton Borough Council to prepare this PFRA report are listed below.

Stockton Borough Council Data

Stockton Borough Council: Strategic Flood Risk Assessment, (SFRA)

JBA Consulting was commissioned to undertake a review of the existing Tees Valley Strategic Flood Risk Assessment (SFRA) published in 2007. The level 1 SFRA for Stockton-on-Tees Borough Council was prepared in accordance with current best practice, Planning Policy Statement 25 Development and Flood Risk (PPS25). A level 2 SFRA follows on from volumes I, II and III which formed the level 1 SFRA. The Level 2 SFRA provides an assessment of flood risk at key development and regeneration locations within the borough of Stockton-on-Tees. The level 1 & 2 SFRA were both published in June 2010.

Surface Water Management Plan, (SWMP)

The surface water management plan is currently being prepared by Stockton Council. A large data collection exercise was conducted in order to prepare the SWMP. The collated data from various sources helped to prepare this PFRA report.

Highways Flooding Reports

A number of highway officers held varied information on past flood events. The information was collected by officers during their general duties.

Direct Services Reports

Direct Services have kept records of all flooding incidents reported by members of the public on an internal system. The information recorded varies most have very little detail about the flooding incident while some records have a response provided from the technician who attended the incident giving more detail about the event.

Environment Agency

Indicative Flood Map

The indicative flood map identifies national flood risk areas, based on the definition of 'significant' flood risk described by Defra. Stockton Borough Council has no significant flood risk areas.

Flood Map (River and the Sea)

The flood map is a multi-layered map which provides information on areas that could be affected by flooding from river or the sea. It provides information on flood defences and for the major defences it shows the areas that will benefit from those flood defences. The multi-layered map provides the following data:

• Flood zone 3 - Areas of land at risk of flooding The natural flood plain ignoring the presence of flood defences, as having a 1% or greater annual probability of fluvial flooding or 0.5% or greater annual probability of tidal flooding.
• Flood zone 2 - Areas of land at risk of flooding 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 defences - the location of linear raised flood defences such as embankments and walls.
• Flood storage areas - land designated and operated to store flood water are shown in a separate polygon layer.
• Areas that benefit from flood defences - land that may benefit from the presence of major defences during a 1% fluvial or 0.5% tidal flood event. These are areas that would flood if the defence were not present, but may not flood because the defence is present. Areas benefiting from flood storage areas may be remote from the flood defence structure.

Flood Map for Surface Water

The Flood Map for Surface Water shows areas where surface water would be expected to flow or pond. The map includes two rainfall events, one with a 1 in 30 and the other with a 1 in 200 chance of occurring in any year.

Areas Susceptible to Surface Water Flooding

The map shows areas susceptible to surface water flooding. The map provides three bandings from 'less' to 'more' susceptible to surface water flooding. The 'more' band will be useful to help identify areas which have a natural vulnerability to:

• flood first
• flood deepest
• and/or flood for relatively frequent, less extreme events (when compared to the other bands)

Area Susceptible to Ground Water Flooding (AStGWF)

Areas Susceptible to Groundwater Flooding is a strategic scale map showing groundwater flood areas on a 1km square grid.

National Receptors Dataset

The national receptors dataset is a database of social, economic, environmental and cultural receptors including residential properties, schools, hospitals, transport infrastructure and electricity substations.

Historic Flood Map

The Historic Flood Map shows the combined extents of known flooding from rivers, the sea, and groundwater.

Tees Catchment Flood Management Plan

The Tees CFMP aims to reduce risk of flooding from rivers, groundwater, sewers and the sea to people, property, and the built and natural environment by supporting and implementing government policies.

Northumbrian Water

Northumbrian water have provided two datasets; however there is a data sharing protocol in place due to its sensitivity.

Sewer Flooding Location (Pipe Length)

The dataset provides sewer flooding location where properties are on the DG5 sewer flooding (1 in 10, 2 in 10 and 1 in 20 year) registers. The pipe length reference where hydraulic incapacity has been identified in storm conditions has also been provided.

Extreme Events

The dataset provides locations were past extreme flooding events have occurred

Cleveland Fire and Rescue Services

Historic Flooding Records

Cleveland Fire and Rescue Services have provided information on recorded past flooding incidents. The information varied however the majority of the records included precise geo-referenced locations and some records included actions carried out by the duty officer.

Cleveland Police

Historic flooding records

Cleveland Police have provided information on past flooding incidents that they have recorded, however the data provided varies and with very little detail of the incident recorded.

British Waterways

British Waterway's canal network

British Waterways provided detailed GIS information on their canal network, including the location of canal centrelines, sluices, locks, culverts, etc. British Waterways did not have any records of historic breaches or overtopping events that have occurred within the Borough's boundary.

3.3 Data Quality

The historic flood records and information collated from all sources was found that the quality of data varied. The majority of the flood records and information did not hold a detailed account of the flooding incident. However the majority of the records held by Cleveland Fire and Rescue Services included precise geo-referenced locations.

3.4 Data Storage and Restrictions

The data can be accessed and shared through the councils Geographical Information System (GIS). The data can be viewed by a series of maps through GIS.

All data is stored on a secure server which is backed up daily and can be easily restored if an emergency occurred. The server has restricted user permissions and can only be accessed by those with necessary permissions. Stockton Borough Council has entered an agreement with partner organisations and must adhere to these data security measures to ensure that sensitive data is held in a secure manner.

The Environment Agency licences some of its data and includes the following disclaimer:

We do not promise that the information supplied to you will always be accurate, complete or up to date or that the information will provide any particular facilities or functions or be suitable for any particular purpose".