Risk/opportunity:(from the Climate Change Risk Assessment for Scotland 2012):
FL7/24/27 Flooding of non-residential property

Narratives: Flooding and infrastructure, Climate change risks to society and our capacity to adapt

SCCAP theme: Buildings and infrastructureSociety

SCCAP objectives:
B2: Provide the knowledge, skills and tools to manage climate change impacts on buildings and infrastructure

Latest figures

2010-2012: 39 (of around 1800) wastewater assets have experienced historical flooding issues (2.17%).

An additional 390 wastewater assets lay within SEPA’s 0.5% (1in 200 year) Annual Probability indicative coastal and fluvial flood boundaries (21.7%).

At a glance
  • Wastewater treatment works (WWTW) tend to be located at the bottom of catchments near watercourses, and thus may be at flood risk
  • Some Scottish wastewater assets have already experienced flooding, and others may be at risk now and in the future due to climate change.
  • Until 2002, water assets were managed at regional and local levels, with case-by-case management.  From 2006 Scottish Water has undertaken flood risk assessments at a national level, followed by interventions where required 
  • Flooding of WWTWs is unlikely to impact directly on service (as receiving watercourses are likely to be in spate during floods so the relative environmental impact from flooded WWTWs is reduced). Therefore this indicator does not measure impact on customers
  • The Scottish Water Asset Register includes wastewater treatment works, water treatment works and pumping stations. The approaches to, and methodology for, any assessment of asset flood risk undertaken, or commissioned by, Scottish Water are broadly similar

As the sole provider of wastewater treatment to around five million customers dispersed over a wide geographic area, Scottish Water has responsibility for more than 1800 wastewater treatment works, as well as for standalone wastewater pumping stations (approximately 600) and sludge treatment centres (27).   The treatment of wastewater may be vulnerable to climate change because of the potential for flooding of localised assets (Scottish Government, 2011).

Flooding of wastewater assets may arise as a function of inundation via fluvial, coastal or pluvial sources. Typically, short term loss of treatment due to flooding would occur during high environmental flows in watercourses. This is recognised in the licences by which Scottish Water operate. Failure of provision of the treatment function arises principally due to the impact upon critical equipment (particularly electrical) rather than due to impact on the process. The core function of wastewater treatment works can however also be affected. For example, when filter beds become submerged and subsequently, when the water depth recedes, biomass can be washed out. This takes time to re-establish, which leaves the wastewater treatment works at risk of reduced performance and can also cause a reduction in final effluent quality. Enabling quicker recovery through appropriate siting of control panels and so on would be the first measure, and only if a site is repeatedly experiencing such difficulties may it be appropriate to ‘defend’ the site.

Understanding this risk is important for Scottish Water, in order to inform both central strategic objectives and local asset management plans.  However, it should be noted that the impact of loss of treatment on the community or the environment is likely to be limited, as wastewater treatment works are designed to operate on a 90-95% percentile within their licence agreement. Transient failures do occur without flood risk, and would tend to have a higher impact during low flow conditions in the watercourse – during a flood, the relative impact on the environment of temporary loss of treatment would be much less.  

It is therefore worth noting that asset resilience and service resilience are two different concepts. Temporary loss of treatment at a wastewater treatment works will not usually impact customer service levels. By their nature, wastewater treatment works are generally located at the bottom end of catchments adjacent to watercourses, and upstream impacts on customer service following a flood event would be unusual. Additionally, the receiving watercourse would be in spate, impacted by land runoff and so on, such that the relative impact from the wastewater treatment asset would be low.  Service resilience through the management and reduction of customer flood risk is a key factor in Scottish Water’s 2015-21 regulatory business plan (Scottish Water, 2014).

Related indicators

BW5 Water treatment works in areas at flood risk

Wastewater assets may be vulnerable to fluvial, pluvial and coastal flooding. At present, 429 wastewater assets are noted either as having experienced historical flooding issues or as falling within SEPA’s 0.5% annual probability indicative coastal and fluvial flood boundaries.  This represents around 24% of wastewater assets.

To help understand the risk, Scottish Water maintains a register of critical assets vulnerable to flood risk. This database identifies the asset by name, as well as presenting data on fluvial, pluvial and coastal flood hazard probabilities and peak water levels determined through the flood risk assessment process. Information on both proactive options to mitigate flood risk and reactive options for repair or replacement of specific critical/vulnerable equipment is also included.

Scottish Water identifies wastewater treatment works lying within SEPA’s stated 0.5% Annual Probability indicative coastal and fluvial flood boundaries, but not previously affected by flooding as ‘Amber Risk’. Those assets previously affected by flooding are categorised as ‘Red Risk’.

A Flood Risk Assessment was undertaken in 2012 for 42 wastewater assets, identified from a total of more than 1800 as either being critical assets (in terms of operational provision) or PFI assets. Of these 42, 1 had previously experienced flooding and was included in the 39 noted above. The remaining 38 assets were also assessed. Local asset planners and managers were informed of the outcomes of this flood risk assessment, and have responsibility for risk mitigation through a coordinated programme of capital investment and maintenance. Scottish Water is now developing a more risk-based approach that focuses on critical assets in order to prioritise the management of flood risk. This determines related activities set out in their business plan with the aim of ensuring customer service and infrastructure resilience.

Between 2006 and 2010, Scottish Water undertook assessments of flood risk. Raw water storage, raw water pumping stations, water treatment works, treated water pumping stations, wastewater pumping stations and wastewater treatment works were examined in a desk based study using SEPA’s 1:200 flood risk maps (coastal & fluvial), and Scottish Water’s anecdotal records of surface water flooding of assets. This information was consolidated into an asset flood risk register.

Following the introduction of the Flood Risk Management (Scotland) Act 2009 and the subsequent release of SEPA’s National Food Risk Assessment 2011, Scottish Water was able to update the way in which it assessed the flood resilience of its asset base. In 2012, and with the authorisation of the Water Industry Commission, Scottish Water commissioned the development of an asset selection methodology(Scottish Water, 2012a), with the aim of better understanding the risk exposure and intervention options for a number of wastewater (and other) assets. This methodology considered wastewater treatment works that had either been flooded, or were located within SEPA’s 0.5% annual probability indicative coastal and fluvial flood boundaries (but had not already been flooded).

Additionally, and at a service resilience level, 42 wastewater treatment works were identified as ‘critical’ from which it was possible to determine that only one of these also fell into the category of having previously been flooded.  At the time of this study, 39 wastewater assets were known to have experienced flooding issues. A flood risk assessment (Scottish Water, 2012b) was therefore carried out for the 42 ‘critical and PFI’ assets and for an additional 38 assets known to have experienced flooding.  A further 205 ‘amber risk’ wastewater treatment assets were also identified via the screening process, and are kept under review.  

t is widely accepted that climate change will bring an increase in sudden and intense rainfall, that coastal flooding will become more frequent and more severe, and that we will experience sea-level rise (Scottish Government, 2009). More specifically, climate change will influence catchment response through an increase in peak rainfall intensity, thereby also increasing peak fluvial flows. With respect to coastal flooding, climate change will bring an increase in both sea levels and in storminess, making those assets located nearer Scottish shores more susceptible to waves and breaches of defence crest levels. This overtopping of coastal defences may result in increased flood risk due to ponding (as influenced by topography) or may result in structural problems due to the ingress of seawater from coastal spray.

As noted above, Scottish Water is now developing a more risk-based approach that focuses on critical assets in order to prioritise the management of flood risk.


The data for 2010-2012 offer a baseline for assessing the number of wastewater treatment works located in areas at risk of flooding.  However, they do not indicate the direct impact to communities.


There are no additional data to allow for analysis of trends.

For the 39 wastewater treatment works known to have previously experienced flooding and the additional 41 wastewater assets deemed ‘critical and PFI assets’, the outcome of the flood risk assessments is not known to the project team. However, it should be noted that the 41 additional ‘critical and PFI assets’ may not necessarily have been (or be) at risk of flooding.


Dixon and Tawn (1997). Estimates of extreme sea conditions: spatial analyses for the UK coast. POL report 112.

Environment Agency (2011). Coastal flood boundary conditions for UK mainland and islands.

SEPA (2010) Technical Flood Risk Guidance for Stakeholders, Version 6.0. Ref SS-NFR-P-022.

SEPA (2011) National Flood Risk Assessment. Available at: http://www.sepa.org.uk/flooding/flood_risk_management/national_flood_risk_assessment.aspx

Scottish Government (2009). Scotland’s Climate Change Adaptation Framework (SCCAF). Water Resource Management Action Plan. Available at http://www.gov.scot/Resource/Doc/295166/0091322.pdf

Scottish Government (2011). Scotland’s Climate Change Adaptation Framework (SCCAF). Energy Sector Action Plan. Available at: http://www.gov.scot/Resource/Doc/175776/0114907.pdf

Scottish Water (2012a). Asset Flood Resilience, Asset Selection Methodology, SW Ref: 401191-0000-20-GEN-0001

Scottish Water (2012b). Asset Flood resilience, Flood risk assessments. Hydrological and hydraulic modelling strategy to assess flood hazard at asset sites. SW ref: 401191-0000-20-GEN-0003

Scottish Water (2014). Strategic Projections and Business Plan 2015-2021 http://www.scottishwater.co.uk/about-us/publications/strategic-projections

This indicator was compiled by Professor Lynne Jack and colleagues at Heriot Watt University with input from Scottish Water.  The data on assets at flood risk was provided by Scottish Water.

Katherine Beckmann, Heriot-Watt University / CXC contributed to this indicator.