Test and Itchen River Basin Catchment

Our DWMP sets out our priorities for the Test and Itchen River Basin Catchment.

• Reducing the number of spills from the 62 storm overflows which together currently spill around 1,000 times per annum.
• Separating or attenuating excess rainwater in the sewer networks to reduce the risks of flooding and frequency of storm overflow discharges, especially in Portswood, Harestock and Stockbridge.
• Investigating the potential opportunity to designate the River Itchen at Riverside Park as an inland bathing water .
• Reducing nutrient discharges from the WTWs across the river basin to prevent deterioration of the Solent and River Itchen internationally designated habitat sites.
• Planning for potential significant development at Winchester, Romsey and the Mayflower Quarter in Southampton.
• Improving the resilience of our networks and treatment works to prevent pollution incidents, particularly in Harestock Amport and Barton Stacey.
• Reducing the risk of sewer blockages by increasing sewer jetting and targeting customer campaigns to reduce the amount of fat, oil and grease (FOG) and non-flushables in the sewer network around Kings Worthy, Basset, Harefield and Townhill Park.
• Reducing the risk to groundwater by reducing leakage from our sewers in the Barton Stacey, Andover, South Wonston, Kings Worthy and Easton areas.

We've worked with a wide range of organisations with responsibilities for drainage, flooding and protection of the environment while developing our DWMP. The organisations we worked with in the Test and Itchen River Basin Catchment include:

• Hampshire County Council
• Basingstoke and Deane Borough Council
• East Hampshire District Council
• Eastleigh Borough Council
• Fareham Borough Council
• Southampton City Council
• Test Valley Borough Council
• Winchester City Council
• The Environment Agency
• Natural England
• The Test and Itchen Catchment Partnership and member organisations including:
• Hampshire and Isle of Wight Rivers Trust
• Wessex Rivers Trust

Working together to co-create the DWMP is important. Our drainage and wastewater systems are often interconnected with the systems managed and operated by others and affect the natural environments within the catchment.

A wide range of issues and concerns have been raised and discussed throughout the development of the DWMP for the Test and Itchen. We're progressing these issues through the development of the DWMP as set out in our investment needs for the Test and Itchen. Furthermore, we commit to working with others to co-develop and co-deliver schemes that meet multi-organisational objectives and which benefit the environment, our customers and communities.

We developed and ran a series of activities between 2020 and 2022 as we prepared our DWMP for the Test and Itchen. You can find the dates and purpose of the various webinars, workshops, and meetings on individual wastewater systems and our interim consultation below.

The regionally-based webinars presented and discussed issues and information relevant across the whole of our operating region. You can view the presentations used in the webinars on our Who we’re working with page.

The presentations we discussed at the Test and Itchen workshops are below.

Workshop 1
Held in September 2020, participants discussed the findings of the risk based catchment screening (RBCS) and proposed additional planning objectives for the DWMP. Workshop slides.

Workshop 2
Held in May 2021, participants:

1. discussed the results from the BRAVA risk assessments and the proposed investment strategy for the wastewater catchments within the River Basin
2. identified the generic options that should be explored to address the identified risks, and
3. discussed which wastewater catchments to progress through the Options Development and Appraisal stage of the DWMP. Workshop slides.

Workshop 3
Held in March 2022, we reviewed and discussed the draft investment programme for the Test and Itchen River Basin Catchment. This included the types of investment, priorities and timing for investment needs and the wider opportunities arising from the proposed investment in terms of partnership projects and catchment-wide solutions providing multiple benefits. Workshop slides.

You can also view the findings from our interim consultation.

Risk based catchment screening for the Test and Itchen Catchment

Risk based catchment screening (RBCS) is a process that uses existing, readily available data. This helps us to identify where there is a current and/or potential risk or vulnerability in the sewer catchment to future changes, such as new residential development or changes in climate. This enables effort to be focused on these catchments during the development of the DWMP in order to understand these risks in more detail and why they're likely to occur.

The RBCS involves the assessment of each sewer catchment against 17 indicators set out in guidance published by Water UK. Water companies can add additional indicators to ensure that other important issues are highlighted at this early stage in the development of the DWMPs. We've included an additional metric on customer complaints as this provides a flag for catchments with ongoing or outstanding concerns.

Find out more about the risk based catchment screening process.

You can download the RBCS results for the Test and Itchen River Basin Catchment below.

Download the RBCS for the Test and Itchen Catchment

The Baseline Risk and Vulnerability Assessment (BRAVA) is an important step in the development of our DWMP. It helps us to understand current system performance and future vulnerabilities. This means that we can identify the investment needed to manage and reduce the risks to band 0 (no significant level of risk).

A BRAVA assessment has been completed for each of the wastewater systems in the Test and Itchen Catchment that were flagged during the RBCS. A risk assessment is completed for all 14 planning objectives.

The output of the BRAVA shows:

• the current risks and issues in each wastewater system within the Test and Itchen catchment – providing a baseline from which we can assess future risks
• the future risks in 2030, 2035, 2045 and 2050 (where the methodology currently allows us to assess the future risks) so we can understand how the current risks may change without additional investment
• the key issues behind the future changes in risk, including:
• a deterioration in the condition of our wastewater systems
• climate change – including the increasing frequency and severity of droughts and storms
• growth and urban creep
• a combination of any or all of these that are relevant in the catchment being assessed.

You can download the BRAVA results for the Test and Itchen River Basin Catchment below.

Download the BRAVA Summary for the Test and Itchen Catchment

Notes

1. In the BRAVA results table, “not flagged” means no risks were identified in the initial RBCS using the nationally set criteria. Wastewater systems not flagged were screened out and did not progress to the BRAVA stage. “Not applicable” means the planning objective wasn't relevant within the wastewater system. For example, where a system has no storm overflows, this will be marked as “Not applicable”.
2. Please check the DWMP glossary for any unexplained acronyms.

The Problem Characterisation stage of the DWMP uses the results from the Baseline Risk and Vulnerability Assessment (BRAVA) to explore the causes of risks and the primary drivers. A technical summary provides information on our approach to the Problem Characterisation stage.

Current risks in the Test and Itchen Catchment

The graph below illustrates the combined results of the 2020 BRAVA assessment for all 31 wastewater systems in the Test and Itchen River Basin Catchment. It shows how many wastewater systems have a risk under each of the 14 planning objectives. For example, for the risk of internal flooding, 22 wastewater systems are in band 0 (not significant), six are in band 1 (moderately significant), one is in band 2 (very significant) and two were ‘not flagged’ for inclusion (i.e. screened out at the RBCS stage of the DWMP).

[insert BRAVA results graph]

This graph illustrates that nutrients are the main concern in this river basin based on the BRAVA results for 2020, followed by flooding in a 1-in-50-year storm event, storm overflow and annualised flood risk. This is illustrated by the brown bars being the highest for planning objectives 11, 4, 5 and 7.

The wastewater systems with the highest number of planning objectives in band 2 (very significant) are Barton Stacey and Woolston, each with five objectives in band 2. Millbrook, Portswood and West Wellow each have three band 2 risks. All other catchments have fewer risks.

The specific risks and the causes of risk for each of the wastewater systems are explained in the summary of the problem characterisation for each system. These are available to download from the link next to the name of each system in the table below.

Future risks in the Test and Itchen Catchment

The 2050 BRAVA results help us to identify the future challenges for drainage and wastewater management in the Test and Itchen River Basin Catchment.

(a) Growth

There are several wastewater catchments where new homes, businesses, roads and other infrastructure are planned. The main areas identified for potential new development in the Test and Itchen Catchment are:

• Millbrook
• Chickenhall Eastleigh
• Fullerton
• Romsey
• Harestock

The BRAVA results show that the additional homes and businesses will increase the risks of non-compliance with our Dry Weather Flow (DWF) permits from the Environment Agency in nine of the 31 wastewater systems. This includes Millbrook, Chickenhall Eastleigh, Fullerton, Morestead Road Winchester, Harestock, Ivy Down Lane, Oakley, Overton and Ludgershall. This means further investment will be needed in the future to increase the capacity of our treatment works to accommodate the new homes and businesses.

The additional development may mean that our current permits for wastewater treatment quality might be exceeded by 2050 without further investment in six wastewater systems, including Romsey, Whitchurch, Whiteparish and Evans Close Over Wallop.

New development in the Test and Itchen Catchment might put additional pressure on internationally designated habitat sites such as the Solent and Dorset Coast. So, solutions will need to be found to ensure that any development is nutrient neutral.

A map of the Test and Itchen Catchment showing the estimated future growth in each wastewater system is shown below. The technical summary explains how we've considered population growth and urban creep in our DWMP.

(b) Climate change

Climate change will bring greater variability of our weather with warmer wetter winters and hotter drier summers. The impacts we will see will be more intense summer storms that exceed the capacity of the drainage and wastewater networks and cause localised flooding. Hence, the risk of flooding from sewer systems is increasing due to climate change. The technical summary explains how we've considered climate change in our DWMP.

We'll work with partner organisations, such as Hampshire County Council and the Environment Agency, who have responsibility for flooding and drainage, to consider options and develop opportunities to find solutions that reduce the risks from flooding.

We'll need to adapt our wastewater systems to operate in future climates. There will be an increasing need to slow the flow entering our sewer networks so the systems can carry the water without flooding homes and businesses and/or without causing discharges from storm overflows. Preventing additional rainfall from entering foul sewer networks, including combined sewer networks where possible, could delay the need to upgrade and enlarge the vast underground network of sewers.

Climate change is expected to have an impact on the risk of flooding in several wastewater systems, especially Woolston, New Alresford, Ivy Down Lane, Oakley, Whitchurch, West Wellow, Ludgershall and Barton Stacey. The flooding risk will increase by 2050 unless measures are taken to manage and reduce these risks.

The BRAVA results indicate that there's a very significant risk of storm overflow discharges for five systems in 2020 (Millbrook, Portswood, Redynch, West Wellow and Woolston), although no other systems will move into a higher risk band for storm overflows by 2050.

The map below shows the potential impact of climate change, urban creep and growth on the risk of flooding in a 1-in-20-year storm event for the wastewater systems. We followed Water UK’s capacity assessment framework to apply a 20% uplift to rainfall forecasts to assess the potential increases in flood volumes shown on the map. Several systems are shaded red (Portswood, Millbrook, Woolston, Chickenhall and Morested Road, Winchester) to indicate that there could be more than 1,000 properties at risk from sewer flooding by 2050. For example, the rainfall flood volumes in Portswood could increase from 6,317 m3 in 2020 to 12,525m3 by 2050. In Woolston, they could increase from 43,687m3 in 2020 to 63,073m3 by 2050.

Urban creep was estimated using the approach developed by the UK Water Industry Research (UKWIR) report on Urban Creep in sewerage systems (2010).

The map highlights that we'll need to adapt to climate change. Adaptation will mean considering long-term sustainable options, such as reducing the volume of rainwater entering the sewer network. This approach may provide the capacity within the wastewater system to allow for future growth, therefore reducing both the need for significant increases in the capacity of the existing wastewater systems and reducing discharges from storm overflows.

[insert flooding volume map]

Investment planning for each wastewater system

We used the BRAVA results and our understanding of the causes of risks and drivers to propose an investment strategy for each of the wastewater systems. Find out more information on how we determined the investment strategies. These strategies help us to target the wastewater systems that need further investment to reduce the potential risks to customers and the environment. We've produced a table that lists the proposed investment strategy for each catchment.

We used a risk-based approach to identify the wastewater systems that we need to progress in this first round of DWMPs. For these systems, we'll develop an investment plan. Our technical summary sets out how we selected the systems to take forward.

The table below lists the wastewater systems in the Test and Itchen River Basin Catchment that we’re progressing further during this first round of the DWMPs into the investment planning stage. We've included a catchment map and an explanation of the causes of risks for these systems (see links in the table below).

We'll publish the maps and causes of risks for the remaining wastewater systems in the Test and Itchen when available.

The Programme Appraisal stage of the DWMP follows the Options Development and Appraisal (ODA) process. The ODA process identified the preferred options for investment in our wastewater systems to reduce the current risks as well as the risks up to 2050.

The Programme Appraisal brings the investment needs for each wastewater system together into an investment needs programme for the Test and Itchen Catchment. We look across the river basin catchment to review the investments needed, the timing of these needs and how they combine to reduce the risks to our customers and the environment.

We also look at the wider risk reduction that each option provides across all the planning objectives. Some actions, like separating rainwater from wastewater sewers, could reduce risks under several planning objectives such as storm overflows, external flooding, bathing water quality, shellfish water quality and good ecological status. You can find the details of the method for prioritisation in our Technical Summary on Programme Appraisal.

The investment needs for the Test and Itchen River Basin Catchment include:

• reducing the number of spills from the 62 storm overflows which together currently spill around 1,000 times per annum
• separating or attenuating excess rainwater in the sewer networks to reduce the risks of flooding and frequency of storm overflow discharges, especially in Portswood, Harestock and Stockbridge
• investigating the potential opportunity to designate the River Itchen at Riverside Park as an inland bathing water
• reducing nutrient discharges from the WTWs across the river basin to prevent deterioration of the Solent and River Itchen internationally designated habitat sites
• planning for potential significant development at Winchester, Romsey and the Mayflower Quarter in Southampton
• improving the resilience of our networks and treatment works to prevent pollution incidents, particularly in Harestock Amport and Barton Stacey
• reducing the risk of sewer blockages by increasing sewer jetting and targeting customer campaigns to reduce the amount of fat, oil and grease (FOG) and non-flushables in the sewer network around Kings Worthy, Basset, Harefield and Townhill Park
• reducing the risk to groundwater by reducing leakage from our sewers in the Barton Stacey, Andover, South Wonston, Kings Worthy and Easton areas.

Investment Needs for the Test and Itchen Catchment

We progressed 12 wastewater systems through the ODA stage in the first cycle of the DWMP. These 12 wastewater systems serve a population of around 529,000 which is approximately 94% of customers in this catchment.

We extrapolated the investment needs for these 12 systems to the other 18 systems in the Test and Itchen River Basin Catchment. This provides an estimate of the total investment needs required to reduce the risks in all our wastewater systems in the catchment to band 0 (not significant). This concept of “Band Reduction” and full details of the process for extrapolation are explained in the Technical Summary on Programme Appraisal.

A graph to illustrate the extrapolation of investment needs across the whole river basin catchment is shown in figure 1.

Figure 1: Test and Itchen: Extrapolated Investment Needs and Risk Band Reduction

The Programme Appraisal for the Test and Itchen identified the following needs.

• The 12 wastewater systems serve a population of around 529,000 which is approximately 94% of customers in this river basin catchment.
• A total of 87 band reductions are required in 2020 across the 14 planning objectives for the 12 wastewater systems.
• By 2050, this will increase to 105 band reductions due to the impact of climate change, growth and creep.
• The options identified to date would cost around £437 million for the 12 systems and are expected to provide 21 band reductions by 2050 (the options do not result in a band 0 for all risks).
• The Test and Itchen has 30 systems which require 129 band reductions in 2020 and 158 band reductions by 2050 in order to achieve band 0 across 14 planning objectives.
• Extrapolating the investment needs for all the systems in the Test and Itchen will cost around £1.27 billion for a population of 563,000. This illustrates the scale of investment needed to get to band 0 by 2050.

These investment needs provide indicative costs that allow us to understand the level of funding required to reduce the risks. The funding hasn't been secured at this stage. The DWMP informs the development of our 5 yearly Business Plan which is submitted to our economic regulator, Ofwat, to agree on how we should invest the money received from our customer bills.

Consultation on the Test and Itchen River Basin Catchment

We held a preliminary consultation on our draft DWMP in September and October 2021. The purpose of the consultation was to gain feedback and advice from our customers and organisations on our developing Plan.

We specifically asked about:

• our Strategic Environmental Assessment Scoping Report
• our selection of wastewater systems to take forward into the Options Development and Appraisal stage of the DWMP in the first cycle, and
• our developing plans for each of the 11 river basin catchments.

Our report on the initial DWMP consultation is now available.

When we consulted on the developing plans for the Test and Itchen River Basin Catchment, you highlighted key issues that we needed to take into consideration.

• Engagement in the DWMP has helped engender a better understanding of the risks and issues.
• Tidal ingress into surface water systems has not been looked at or addressed. With climate change, more and more sewer outfalls will be under sea level and won't be able to discharge effectively. This issue is likely to increase inland surface water flooding and there are already significant issues in the Millbrook system.
• Millbrook has been identified as a regeneration area.
• Lenham, and all systems that discharge into the Itchen Estuary, should be progressed.
• Storm overflow releases must be reduced and the quality of effluent improved to help improve water quality.
• Surface water and foul systems should be separated. Partnership funding is needed to implement sustainable drainage systems.
• Barriers between the engineering aspects and environmental concerns need to be broken down to develop ‘green’ plans. A strategic level, holistic view beyond Southern Water infrastructure should also be developed.

How we responded to the issues raised during the workshops and the preliminary public consultation is set out in our Register of Stakeholder Comments.

We held a full 12-week public consultation on the draft Regional (Level 1) DWMP between Monday 13 June and Monday 05 September 2022. Our Statement of Response and a report and analysis on the consultation is published on the Have your say page.