Problem characterisation – Isle of Wight

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 Isle of Wight

The graph below illustrates the combined results of the 2020 BRAVA assessment for all 20 wastewater systems on the Isle of Wight. It shows how many wastewater systems have a risk under each of the 14 planning objectives. For example, for the risk of internal flooding, 14 wastewater systems are in band 0 (not significant), there is 1 in band 1 (moderately significant), and none in band 2 (very significant). 5 were ‘not flagged’ for inclusion (i.e. screened out at the risk based catchment screening stage of the DWMP).

The graph illustrates that storm overflow performance, Dry Weather Flow Compliance and nutrients 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 5, 8 and 11.

The wastewater system with the highest number of planning objectives in band 2 (very significant) is Sandown with 7 objectives in band 2.   This is 1 of 3 wastewater systems in our operating area with this level of risks for 2020, so Sandown is a high priority for future investment to reduce these risks. Godshill has 3 band 2 risks, and Wroxall, Roud, St Helens and Brighstone each have 2 objectives in band 2. All other catchments have fewer risks.

We are focusing our attention on Sandown in the first cycle of the DWMP as it has such a large number of risks. The specific risks and the causes of risk for Sandown are explained in the summary of the problem characterisation and is available to download from the link in the table below.

Future risks in the Isle of Wight

The 2050 BRAVA results help us to identify the future challenges for drainage and wastewater management on the Isle of Wight.  These are:

(a) Growth

New homes, businesses, roads and other infrastructure is planned in many locations across the Isle of Wight, mainly in the coastal settlements and in Newport. The new development is anticipated to be more than 2000 homes by 2050.

This additional development may mean that our current permits for wastewater treatment quality might be exceeded by 2050 without further investment. However, many of these areas are serviced by the Sandown treatment works and the BRAVA results show that the risks at Sandown need investment in the shorter term to address the current risks.

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 the St Helens wastewater system. This means further investment will be needed in the future to increase the capacity of our treatment works to accommodate the additional flow from new homes and businesses.

New development on the Isle of Wight might put additional pressure on the internationally designated habitats sites in the Solent, so solutions will need to be found to ensure that development is nutrient neutral.

A map of the Isle of Wight 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. 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 the Isle of Wight 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 Sandown, Brighstone, St Helens and Chillerton where there is already a very significant risk from rainfall related flooding. All these wastewater systems are located around the coast of the island. The flooding risks will increase by 2050 unless measures are taken to manage and reduce these risks.  The BRAVA results indicate that the risk of storm overflow discharges for 6 of the wastewater systems is likely to increase to very significant, unless further action is taken.

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 Sandown wastewater system.  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 flood volumes generated by rainfall over the area covered by this wastewater system could increase from 58656m3 in 2020 to over 99435m3 by 2050.

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.

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.

We are progressing the Sandown wastewater system during this first round of the DWMP into the investment planning stage.  We have included a map of the Sandown system and an explanation of the causes of risks for Sandown below. Information on the other wastewater systems on the Isle of Wight will be added soon.

System Ref Wastewater system Wastewater system map Information on causes of risks
SAND Sandown Sandown Map Sandown Causes of Risks
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