GM Strategic Flood Risk Assessment

PPS25 (Development and Flood Risk), requires flood risk to be taken into account in all stages of the planning process.  Strategic Flood Risk Assessments are one of the main documents to be used as the evidence base for Local Development Frameworks (LDFs) and Sustainability Appraisals.

As Greater Manchester continues its renaissance, there is a significant challenge to delivering growth in urban areas, building sustainable communities, restoring degraded environments and providing high quality development that is economically and environmentally sustainable in the face of climate change and future flood risk. These challenges provide the basis for the Sub Regional SFRA.

Greater Manchester lies within a unique context, lying within major river catchments draining from the Pennines into the regional centre and then into the Manchester Ship Canal, flood risk from these rivers is a cross boundary issue which needs to be addressed at the sub regional catchment and cross boundary level.

Manchester like many major cities in the world including large parts of the South East and London is vulnerable to flood risk and climate change. However, directing new development away from existing communities is not a viable option. Nevertheless, flood risk is recognised as an important issue which provides both opportunities (e.g. with strong links to green infrastructure, for which AGMA is currently preparing a strategic Framework) as well as a challenge to building sustainable communities.

The SFRA, as a living process provides the vehicle for AGMA, Greater Manchester districts, the Environment Agency and the relevant utility company (United Utilities) to not only comprehensively assess flood risk in a consistent and cross boundary manner but having done this, to work together in a collaborative manner to manage risk and deliver water management solutions.  

Confidence Mapping

The Environment Agency (EA) holds national broad scale models of most watercourses and much of the coastline that are intended to broadly define the areas at risk of fluvial and coastal flooding in the UK. As part of more detailed river modelling studies undertaken by the EA (such as Strategic Flood Risk Mapping (SFRM) studies) Flood Zones have been refined along many watercourse reaches within the Greater Manchester area.

As part of the sub-regional SFRA, hybrid SFRA Flood Zones were created covering the entire sub region. The SFRA Flood Zones were created using flood outlines from detailed river models where available and in all other locations, broad scale model outlines were used. The SFRA Flood Zones presented the best available information at that time.

The Flood Zone Confidence Maps are intended to be used by planners as a tool for identifying areas of high, medium or low confidence in the data that has been used to derive fluvial Flood Zones across the sub-region. The confidence assigned to a Flood Zone demonstrates the level of detail and the number of assumptions made when deriving the Flood Zones. The maps can be
used to determine reaches of river where further, more detailed work is required to refine Flood Zones and therefore where resources should be directed.

The maps should be used when allocating development sites and for Development Control activities. For example, if a site is being considered for allocation and is shown to be in an area of Flood Zone that has low confidence, work should be instigated to quantify and define the risk at that location in more detail (see the Greater Manchester SFRA Level 2 Framework Document).

SuDs Map

As part of the sub-regional SFRA, a series of maps delineating the solid geology, superficial geology and groundwater vulnerability of the Greater Manchester area were presented. The purpose of the maps was threefold:

• Firstly, to demonstrate where geological conditions may be conducive to groundwater flooding;
• Secondly, to provide information regarding the general suitability of Sustainable Drainage Systems (SuDS) across the sub-region and
• To help planners in each of the ten districts in Greater Manchester to make decisions regarding the suitability of SuDS techniques into developments at an early stage.

To increase the usability of the mapping and the SFRA, a SuDS guidance map was created. SuDS are a sequence of management practices and control structures designed to drain surface water in a more sustainable fashion than some conventional techniques. Sustainable drainage is a concept that makes environmental quality and people a priority in drainage design, construction
and maintenance.

The SuDS approach includes measures to reduce surface water runoff at source, prevent pollution and provide a range of physical structures designed to receive the runoff. There are many SuDS design options that can be tailored to fit all types of development. They can also be designed to improve amenity and biodiversity in developed areas.