Programme appraisal

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

The Programme Appraisal stage brings all the investment needs identified during the ODA stage together into a programme for each river basin catchment. This means we have been able to see the range of investments needed to meet our planning objectives and address the risks identified in the DWMP for the short, medium and long-term. We have also been able to consider the priorities and timing for investment to achieve the outcomes we want for our customers and the environment. By considering the wider benefits, we can identify the best value and the most cost effective options. In some cases, a single solution, such as surface water separation, can address a range of planning objectives (POs).

We explored the option for investment for the Sandown system in the Isle of Wight during the first cycle of the DWMP. A full list of the Investment needs for Sandown is below.

The list has been prioritised using the risk reduction score. The details of the method for prioritisation can be found in our Technical Summary on Programme Appraisal.

A summary of the investment needs for these systems is: 

  • Surface water separation 
  • Increase capacity of the Wastewater Treatment Works (WTW).  
  • Optimisation or extension of site to allow for a DWF up to 47371m3 required due to growth in catchment (Permit Review Required) 
  • Construction of new gravity sewer and pumping station 
  • Upsizing of existing gravity sewers.  
  • Surface water separation 
  • Increase in online storage. 

We extrapolated the costs for Sandown to the other 19 systems in the Isle of Wight to understand the total investment needs. The process for extrapolation is set out 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:  Isle of Wight: Extrapolated Investment Needs and Risk Band Reduction


The programme appraisal has identified the following for the Isle of Wight:

  • The two wastewater systems serve a population of around 131,000 which is approximately 90% of customers in this river basin catchment.
  • A total of 17 band reductions are required in 2020 to reduce the risks across the 14 planning objectives for Sandown – the one wastewater system on the island that was considered in the first cycle of DWMP.
  • By 2050, this requirement will stay the same at 17 band reductions even taking climate change, growth and creep into account. Although the risks will increase, this is not visible where the risk is already in the highest risk band 2
  • The options identified to date would cost around £61 million for Sandown and is expected to provide 8 Band reductions in 2050 (the options do not result in a band zero for all risks)
  • On the Island, there are 20 wastewater systems which need 72 band reductions in 2020 and 78 by 2050 in order to achieve band 0 across all 14 planning objectives.
  • Extrapolating the investment needs for all the systems in the Island will cost around £724 million for a population of 144,000. This illustrates the scale of investment needed to get to band 0 by 2050 for all 14 planning objectives.


You can view the list of the Investment Needs for the Isle of Wight river basin catchment.

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