Medway River Basin Catchment

Our DWMP sets out our priorities for the Medway River Basin Catchment.

• Reducing the number of spills from the 238 storm overflows which together currently spill around 4,300 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 Motney Hill, Ticehurst, Hildenborough, High Halden, Penshurst and Gravesend.
• Investigating the potential impact of wastewater discharges on the Swale and Medway Estuary and Marshes and identifying the requirements to achieve nutrient neutrality and Good Ecological Status/Potential.
• Planning for potential significant developments at Northfleet, Rochester Riverside, Hoo, Chatham Docks, Paddock Wood, Tudley, South Godstone, Ebbsfleet and the Invicta Park Barracks at Maidstone.
• Improving the resilience of our networks and treatment works to prevent pollution incidents, particularly in Tunbridge Wells, Crowborough, Hildenborough and Paddock Wood.
• Reducing the risk of sewer blockages by increasing sewer jetting and targeting customer campaigns to reduce the amount of FOG (fats, oils and grease) and non-flushables in the sewer network around Rochester, Gillingham and Chatham.
• Reducing the risk to groundwater by reducing leakage from our sewers in the Sittingbourne, Strood, Rochestester, Chatham, Gillingham and Rainham areas.

Overview of the Medway River Basin Catchment

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 Medway River Basin Catchment include:

• Kent County Council
• Surrey County Council
• Medway Internal Drainage Board
• Dartford Borough Council
• Gravesham Borough Council
• Maidstone Borough Council
• Medway Council
• Mid Sussex District Council
• Sevenoaks District Council
• Tandridge District Council
• Tonbridge and Malling Borough Council
• The Environment Agency
• Natural England
• The South Downs National Park Authority
• Tunbridge Wells Borough Council
• The Medway Catchment Partnership and member organisations including:
• The South East Rivers Trust
• The Kent Wildlife Trust
• Thames Water

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 Medway. We're progressing these issues through the development of the DWMP as set out in our investment needs for the Medway. 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 Medway. 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. The presentations used in the webinars are available on our ‘Who we’re working with’ page.

The presentations we discussed at the Medway 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 2
Held in April 2021, participants:

(a) discussed the results from the BRAVA risk assessments and the proposed investment strategy for the wastewater catchments within the River Basin

(b) identified the generic options that should be explored to address the identified risks, and

(c) discussed which wastewater catchments to progress through the Options Development and Appraisal stage of the DWMP.

Workshop 3
Held in March 2022, we reviewed and discussed the draft investment programme for the Medway 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.

The findings from our public and interim consultations are available on the ‘Have your say’ page of our website.

Risk based catchment screening (RBCS) is a process that uses existing, readily available data. From this, we can identify where there's 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 Medway River Basin Catchment below.

Download the RBCS for the Medway Catchment

The Baseline Risk and Vulnerability Assessment (BRAVA) is an important step in the development of our DWMP. This assessment helps us 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 Medway River Basin 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 Medway 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 performance of our sewerage 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 results of the BRAVA for the Medway River Basin Catchment below.

Download the BRAVA Summary for the Medway

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 was not 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 Medway Catchment

The graph below illustrates the combined results of the 2020 BRAVA assessment for all 77 wastewater systems in the Medway 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, 54 wastewater systems are in band 0 (not significant), nine are in band 1 (moderately significant), four in band 2 (very significant) and 12 were ‘not flagged’ for inclusion (i.e. screened out at the RBCS stage of the DWMP).

This graph shows that pollution, storm overflows and flooding due to hydraulic overload are the main concerns in this river basin, based on the BRAVA results for 2020. This is illustrated by the brown bars being the highest for planning objectives 2, 5 and 7.

The wastewater system with the highest number of planning objectives in band 2 (very significant) is Tunbridge Wells South. This system has five objectives in band 2. Three wastewater systems have four planning objectives in band 2. These are Gravesend, Crowborough and Staplehurst. Six wastewater systems have three of the planning objectives in band 2. 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 Medway Catchment

The 2050 BRAVA results help us to identify the future challenges for drainage and wastewater management in the Medway 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 Medway Catchment are:

• Aylesford
• Motney Hill
• Tonbridge
• Ham Hill

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 26 of the 79 wastewater systems. This includes Motney Hill, Aylesford, Gravesend, Ham Hill, Tonbridge, Tunbridge Wells North, Whitewall Creek and Tunbridge Wells South. 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 10 wastewater systems, including Tunbridge Wells North, Ditton, Wateringbury and Cranbrook.

New development in the Medway might put additional pressure on internationally designated habitat sites such as the Medway and Swale Estuaries. So, solutions will need to be found to ensure that development is nutrient neutral.

A map of the Medway catchment showing the estimated future growth in each wastewater system is shown below. The technical summary explains how we have 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. As a result of this, we could be seeing 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 have considered climate change in our DWMP.

We'll work with partner organisations such as Kent 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 Gravesend, Northfleet, Tonbridge, Tunbridge Wells South, Paddock Wood, Pembury and Staplehurst where there is already a very significant risk from rainfall-related flooding. The risk of flooding will increase in all wastewater systems by 2050 unless measures are taken to manage and reduce these risks. The risk of storm overflow discharges is forecast to increase for the East Grinstead and Grain wastewater systems by 2050, although 16 systems already have a very significant risk of discharges from storm overflows.

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 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. 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 and therefore reduce both the need for significant increases in the capacity of the existing wastewater systems and reduce discharges from storm overflows.

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 Medway 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 Medway when they're available.

Our approach to Options Development and Appraisal (ODA) is explained in a technical summary.

We commenced the ODA process at the river basin catchment (RBC) scale (level 2 planning). This enabled us to look across all the wastewater systems in the river basin and consider generic options that could work at the catchment scale, as well as those specific to a wastewater system.

These generic options are grouped into those that help tackle the risks 'at source’, those that help to improve the wastewater system ‘the pathway’, and those that protect or mitigate the impacts on the receiving waterbodies, ‘the receptors’.

This process helped to identify the types of options that could be used individually or in combination with other options to address the risks.

We held meetings with partner organisations to build upon the list of generic options relevant to each wastewater system. As a group, we identified and proposed unconstrained options to tackle the drivers and causes of risks identified during the Problem Characterisation stage of the DWMP. We then progressed these unconstrained options through the Options Development and Appraisal stage.

The options development involved evaluating each of the options in two stages. Firstly, to screen out unviable options to leave a set of ‘constrained’ options. Then, a second stage to reduce the list further and leave only potentially ‘feasible’ options (see the Options Development and Appraisal technical summary for full details of the process). The process for evaluating the benefits and how we've taken the environment into account is set out in our Strategic Environmental Assessment (SEA) Scoping Report and the SEA Progress Report.

Only feasible options with positive benefits proceeded to the costing stage. This resulted in the selection of the preferred options and confirmed whether each was ‘least cost’ or provided ‘best value’.

The feasible options column in the table below shows how we applied the process within each wastewater system. Beginning with the generic options through the appraisal stages, the table shows the point at which some options were rejected and why. If an option wasn't rejected, it was costed and became either the final best value or the least cost preferred option (see ODA technical summary for details of this process).

Our final preferred options are set out in the Investment Needs tables for each wastewater system. The accompanying map shows the location of the proposed options within the wastewater system.

Please check the DWMP glossary for any unexplained acronyms.

The options and investment needs are not committed funding but an identification of the needs for funding. We'll include these options in our future business plans as part of the Ofwat periodic review of water company funding to secure the investment needed to implement these options.

Please note: The areas highlighted for customer education in the investment needs maps are indicative areas of likely focus, as they were derived from historical incident data.

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 Medway Catchment. We look across the 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. The details of the method for prioritisation can be found in our Technical Summary on Programme Appraisal.

The investment needs in the Medway River Basin Catchment include:

• reducing the number of spills from the 238 storm overflows which together currently spill around 4,300 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 Motney Hill, Ticehurst, Hildenborough, High Halden, Penshurst and Gravesend
• investigating the potential impact of wastewater discharges on the Swale and Medway Estuary and Marshes and identifying the requirements to achieve nutrient neutrality and Good Ecological Status/Potential
• planning for potential significant developments at Northfleet, Rochester Riverside, Hoo, Chatham Docks, Paddock Wood, Tudley, South Godstone, Ebbsfleet and the Invicta Park Barracks at Maidstone
• improving the resilience of our networks and treatment works to prevent pollution incidents, particularly in Tunbridge Wells, Crowborough, Hildenborough and Paddock Wood
• reducing the risk of sewer blockages by increasing sewer jetting and targeting customer campaigns to reduce the amount of FOG (fats, oils and grease) and non-flushables in the sewer network around Rochester, Gillingham and Chatham
• reducing the risk to groundwater by reducing leakage from our sewers in the Sittingbourne, Strood, Rochestester, Chatham, Gillingham and Rainham areas.

Investment Needs for the Medway River Basin Catchment

We progressed nine wastewater systems through the ODA stage in the first cycle of the DWMP. These nine wastewater systems serve a population of around 481,000 which is approximately 51% of customers in this catchment.

We extrapolated the investment needs for these nine systems to the other 68 systems in the Medway 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 catchment is shown in figure 1.

[insert figure 1]

Figure 1: Medway: Extrapolated Investment Needs and Risk Band Reduction

The Programme Appraisal for the Medway identified the following needs.

• A total of 90 band reductions are required in 2020 to reduce the risks across the 14 planning objectives for the nine wastewater systems.
• By 2050, this requirement will increase to 102 band reductions due to the impact of climate change, growth and creep.
• The options identified to date would cost around £852 million for the nine systems and are expected to provide 59 band reductions in 2050 (the options do not result in a band 0 for all risks).
• The Medway consists of 77 wastewater systems which require 265 band reductions in 2020 and 326 band reductions by 2050 in order to achieve band 0 across the 14 planning objectives.
• Extrapolating the investment needs for all the systems in the Medway will cost around £2.61 billion for a population of 940,000. This illustrates the scale of investment needed to get to band 0 by 2050 for all 14 planning objectives.

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 Medway 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 Medway River Basin Catchment, you highlighted key issues for us to take into consideration.

• The impacts of climate change and the pressures of growth, and requirements of water and nutrient neutrality must be cross referenced within the DWMP.
• Significant growth is planned and there are concerns over groundwater pollution, nutrients and the condition of ageing assets with little detail provided on whether faults are mechanical, electrical or operational.
• Providing the infrastructure requirements for growth is a requisite as this is a risk to the delivery of housing targets.
• Groundwater ingress to the sewer systems increases hydraulic overload and egress from the sewers impacts the quality of ground and surface waters. These need to be addressed.
• The level of surface water entering the networks must be reduced. Surface and foul drains should be separated to reduce capacity and nutrient issues.
• More transparency is needed on:
• the timescales between identifying deliverables and actual delivery
• the locations of wastewater treatment sites/storm tanks/discharge points
• pollution events
• Sewer flooding and storm overflow discharges impact nutrient neutrality. We need to map where surface water enters sewers (as per Thames21) to identify opportunities to intercept flows and road runoff and treat through SuDS (Sustainable Drainage Systems). Funding should be available for SuDS implementation by third parties.
• Public education on fats, oil and grease and non-flushable items.

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.