Problem characterisation – New Forest

Current risks in the New Forest Catchment

The graph below illustrates the combined results of the 2020 BRAVA assessment for all 16 wastewater systems in the New Forest 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, 13 wastewater systems are in band 0 (not significant), 1 is in band 1 (moderately significant), 0 in band 2 (very significant) and 2 were ‘not flagged’ for inclusion (i.e. screened out at the risk based catchment screening stage of the DWMP).

The graph illustrates that storm overflows 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 and 11.

The wastewater system with the highest number of planning objectives in band 2 (very significant) is Slowhill Copse Marchwood with 5 objectives in band 2. All other systems have 2 or fewer band 2 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 New Forest Catchment

The 2050 BRAVA results help us to identify the future challenges for drainage and wastewater management in the New Forest 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 New Forest catchment are:

• Slowhill Copse Marchwood (2000+ Homes)
• Pennington (1000+ Homes)
• Ashlett Creek Fawley (1000+ Homes)

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 a number of the wastewater systems, including Slowhill Copse Marchwood, Brockenhurst and Flexford Lane Sway. 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.

New development in the New Forest 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 development is nutrient neutral.

A map of the New Forest catchment showing the estimated future growth in each wastewater system is shown below.

 

(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 incorporated climate change into our DWMP.

We will 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 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 increase the risk of flooding in the Pennington wastewater system to significant by 2050.   The risk of storm overflow discharges is forecast to increase to significant for Ashlett Creek Fawley wastewater system although 8 other systems already have a very significant risk in this river basin as illustrated by the BRAVA results.

The map below highlights the potential impact of climate change on each of our wastewater systems. It shows the impact of climate change, urban creep and growth on the risk of flooding in a 1 in 20 year storm.  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 the flood volumes are relatively small in the wastewater systems, but the 20% increase in rainfall actually almost doubles the flood volumes. It demonstrates 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.

The table below lists the wastewater systems in the New Forest 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 New Forest will be published when available.