Problem characterisation – Medway

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.

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), 9 are in band 1 (moderately significant), 4 in band 2 (very significant) and 12 were ‘not flagged’ for inclusion (i.e. screened out at the risk based catchment screening stage of the DWMP).

Graph of BRAVA results for the Medway RBC

The graph illustrates 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 5 objectives in band 2.  Three wastewater systems have 4 planning objectives in band 2. These are Gravesend, Crowborough and Staplehurst.  Six wastewater systems have 3 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. These are:

(a) Growth

There are several wastewater catchments where new homes, businesses, roads and other infrastructure is 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, including 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.

Map of estimated growth in the Medway RBCMap of estimated growth in the Medway RBC

(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 have considered climate change in our DWMP.

We will 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 will 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 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 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 will need to adapt to climate change.  Adaptation will mean considering  long-term sustainable options, such as reducing the volume of rain water 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.

Map of climate change impacts in the Medway RBC

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.  The strategies help us to target the wastewater systems that need further investment to reduce the potential risks to customers and the environment.  We have 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 will develop an investment plan.  Our technical summary sets out how we have 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 have included a catchment map and an explanation of the causes of risks for these systems (see links in the table below).

The maps and causes of risks for the remaining wastewater systems in the Medway will be published when available.

System Ref Wastewater system Wastewater system map Information on causes of risks
TUWS Tunbridge Wells South Tunbridge Wells South Map Tunbridge Wells South Risks
GRAV Gravesend Gravesend Map Gravesend Risks
PAWD Paddock Wood Paddock Wood Map Paddock Wood Risks
CRRM Redgate Mill Crowborough Crowborough Map Crowborough Risks
TONB Tonbridge Tonbridge Map Tonbridge Risks
LIMP Oxted Oxted Map Oxted Risks
MOTN Motney Hill Motney Hill Map Motney Hill Risks
TUWN Tunbridge Wells North Tunbridge Wells North Map Tunbridge Wells North Risks
STAP Staplehurst Staplehurst Map Staplehurst Risks
HORS Horsmonden Horsmonden Map Horsmonden Risks
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