Project overview

What area does this project cover?

The Lower Mole Flood Alleviation Scheme is approximately 7.5km in total length, with the upstream extent in the Hersham/West End area, to where the River Mole joins the River Thames opposite Hampton Court.

How will you decide what works to carry out to the scheme, is there an assessment being carried out and what information does this contain?

We are carrying out an appraisal to determine our preferred option for the proposed works. This appraisal establishes a range of options for updating the scheme, and compares their economic viability (costs and benefits), technical feasibility (engineering difficulty) and environmental impacts to determine the best overall option. It also incorporates feedback we received from the public and other stakeholders.

Government money will partly fund the updating of the Lower Mole flood alleviation scheme; therefore the development and assessment of the options is in line with the Department for Environment Food & Rural Affairs (Defra) policy statement on Appraisal of Flood and Coastal Erosion Risk Management, which outlines the Flood and Coastal Erosion Risk Management Appraisal Guidance (FCERM-AG). This guidance sets out the steps that we need to follow throughout the project appraisal, which we must comply with in order to receive funding.

We start with a long list of options for the scheme and try to narrow them down to a short list that are considered viable options.

To evaluate options, the appraisal process looks at our project objectives (for example to continue to maintain the scheme, sustaining the standard of flood protection), what constraints there might be to developing options (for example, environmental constraints) and the 5 critical success factors set out in the government guidance: strategic fit and business needs; potential value for money; supplier capacity and capability; potential affordability; potential achievability.

This then provides us with a framework to work within when developing the short-list of options. In order to understand both the opportunities and constraints present, the appraisal process uses a range of information, for example, river modelling, cost estimates, economic benefits quantification, condition surveys of the assets along the Lower Mole, historical information, habitat surveys and feedback from stakeholders.

As the project moves from the long list to the short list, the level of detail we have increases, allowing us to understand the pros and cons of each option more, and to rule out those which will not be viable for economic, technical or environmental reasons.

All of the information gained during the appraisal process is taken into account when determining the preferred option for updating the scheme.

Will the scheme still offer the same standard of protection against flooding as it does now?

One of our priorities is to maintain the standard of protection against flooding the scheme currently offers to householders and businesses.

All the options considered will ensure a standard of protection of 1:100 ( the annual probability of flooding that has a likelihood of occurring once in a hundred years over the 100-year appraisal period) allowing for the expected impacts of climate change increasing river flows.

Does the possibility of decommissioning some of the assets mean that the. risk to flooding will increase?

No. Maintaining the current flood protection standard is a key objective of this project, and any option proposing asset removal will undergo a thorough assessment to ensure that it does not increase flood risk. In fact, the project aims to decrease the likelihood of flooding.

If installing sluice gates were to alleviate the risk to flooding, is it counter intuitive to remove them?

The sluice gates were installed during the construction of the channel for amenity and recreation purposes. The gates remain closed on a day-to-day basis to retain a fixed water level, but in times of high flow, they are opened to allow the water to pass. The condition of the sluice gates is starting to deteriorate due to their age, and we need to act to sustain the standard of flood protection for years to come. Should the sluice gates be removed, the flood relief channel would continue to convey flood flows as it was designed to do.

How long will the works take to complete and when are they scheduled to start?

We are not able to confirm what the timescales for the project currently are. It is important to note that this project is in its early stages as we are yet to select a preferred option.

An asset management plan for the Flood Alleviation Scheme was developed in 2017 showing that there are varying residual lives for the asset elements of the sluices and flood relief channel. The residual life of an asset is the amount of life remaining after economic life or technological life has been reached and before functional failure of that asset. The electrical elements and controls of sluices have the shortest expected residual life and are now due for replacement, with the sluice gates themselves nearing the end of their residual life, and this needs to be considered in the future planning.

Will all of the planned works take place at the same time?

Due to the area the scheme covers, it is likely that the works will be phased over a period of a few years. Although we are currently unable to offer a specific timeline, when we have more detail on the nature and scale of the works we will be able to share this information.

Some of the structures and areas of land along the scheme are owned by others. We have started discussions with those owners to understand if they have any concerns, and to work with them to move the project forward.

How will the water levels and flows within the River Ember and the River Mole be managed whilst the works are carried out?

The lowering of the water levels and the management of flows within the river whilst the works are ongoing will be carried out with both flood risk and environmental needs fully considered. Management of the levels and flows will be confirmed once the proposal is fully developed and the details of the works are finalised.

In the past, we have successfully managed water levels to carry out maintenance works to our sluice structures and the river channel. We can use the knowledge from previous works to help us manage water levels and flows during this project.

Will the Lower Mole be dredged?

Currently, dredging is not planned to be carried out as part of this project.

We consider each location carefully and only carry out dredging where we know it will make a difference to the management of flood risk. Understanding where dredging will, and won’t, reduce flooding is the key.

Dredging has many short and long term environmental impacts such as the escape of silt plumes into the water, reducing water quality, or the removal of gravels from the river which is an important substrate on which fish spawn. In addition, this substrate helps to reduce channel erosion as it absorbs the energy of the river.

In the past, dredging was carried out periodically along the Lower Mole. However, regular surveys of the depth of silt in the river, highlighted that the accumulation of silt within the engineered channel was not significant, and was not reducing the ability of this channel to convey flood flows. Therefore dredging is not required and not the best use of our resources.

A bathymetric survey was carried out in late 2019 to assess the profile of the riverbed to offer up to date information on the depth of any silt within the river.

Are you going to be working with others on this project?

Yes. We have already made contact with the owners of those structures (Wilderness sluice and Royal Mills sluice) which are not under our ownership, to understand if there is the opportunity to work together as part of this project.

We are also speaking with Elmbridge Borough Council, Surrey County Council, utility companies, businesses and developers along the length of the scheme. Furthermore, we are collaborating with leading environmental organisations such as Surrey Wildlife Trust, South East Rivers Trust, Natural England, and the Lower Mole Catchment Partnership.

By working with others, we hope to realise more benefits and achieve a better overall outcome.

Who will own the new riverbank?

A riparian owner is someone who has any watercourse within or adjacent to any boundary of their property. Where a watercourse is located between two or more property boundaries, each owner may be equally responsible.

The options under consideration will not change land ownership or boundaries to Environment Agency land, although under some of the options more land may be exposed.

Riparian owners are responsible for maintaining the river bed and banks within their section of the watercourse. It is their duty to minimise pollution and prevent obstruction to the water flow, including maintenance of the banks and fallen trees etc.

Further information on riparian ownership can be found on the gov.uk website by following this link;

https://www.gov.uk/guidance/owning-a-watercourse(External link)

What are the plans for the area around Wilderness?

Wilderness sluice is not owned by the Environment Agency. While we can still carry out work here to construct a fish pass, it does require the agreement of the structure owners. As part of the scheme, we have a legal requirement to provide fish passage so fish can migrate from the River Thames into the River Mole catchment.

The construction of a rock ramp fish pass at Wilderness sluice is included in all available options, subject to obtaining the necessary agreement from landowners. A rock ramp pass is a gently sloping river ramp up to the weir that fish can swim up and over.

Will any new sluice gates be as quiet as the existing?

Should there be a need to replace the existing sluice gates with new ones as part of this project, we will seek to reduce the noise levels or at the minimum, maintain the current level of noise that the water makes as it passes through the sluice structures.

How will for the scheme be funded?

The scheme will be partly funded by central Government through the use of Flood and Coastal Erosion Risk Management (FCERM) Grant-in-Aid (GiA).

As Government funding is limited and is spent for the benefit of the Nation, we are also exploring options for funding from other sources, to ensure the works can progress.

As options to update the scheme are still being developed, costs are not currently available for the options. These will be available as the appraisal progresses and the options are agreed on.

Following the appraisal process a business case is put forward to obtain government funding. The amount of funding will be determined by a funding calculator, which prioritises funding for projects which have the greatest benefit to people and the environment. Environment Agency projects must go through an appraisal process to fully understand the costs and benefits over the full lifetime of the schemes design, and long-term maintenance. Our economic appraisals produce both a cost-benefit ratio and a Partnership Funding (PF) score. The PF score determines the proportion of a projects costs which can be paid for using (FCERM GiA), with any shortfall in funding needing to be found from other sources. These other sources can include local authorities, water companies, private businesses and individual land or property owners.

Will the options appraisal include a financial projection of maintenance levels and costs?

It is standard practice as part of the development of a flood risk management scheme to assess the anticipated capital, operational and maintenance costs of all options and include these in the total expected cost when comparing options.

This includes maintenance of all parts of the flood alleviation scheme, including all walls, embankments, surface water outfalls, sluice structures and the flood relief channel and is in line with the FCERM (Flood and Coastal Erosion Risk Management) Appraisal Guidance, which is available online (https://www.gov.uk/government/publications/flood-and-coastal-erosion-risk-management-appraisal-guidance(External link)).

Has the Environment Agency already chosen the cheapest option without fully considering all factors as well as the potential costs in the event of a flood?

No option has been selected at this time.

Cost is only one of a number of factors we need to consider. In June 2019, we received feedback from the community requesting more information on our proposals. In response, we undertook additional work over the winter of 2019, including environmental and river depth surveys, to better understand the Lower Mole and explore potential mitigation measures. Although these surveys are typically carried out at a later stage in the project, we wanted to address public concerns regarding water levels and wildlife by collecting and sharing this information early. With the insights gained from these surveys, we are now able to move forward with our updated short-list and engage in detailed discussions with the community this summer.

How we spend public money is an important factor, however, we also review the amount of benefits an option will deliver and over what timescales those benefits occur. We call this our 'economic appraisal' and this assessment is key to selecting a preferred option in line with Government guidance.

Following an economic appraisal, we consider a range of factors, including environmental and amenity impacts, legal obligations, health and safety, and carbon implications, to select the option that provides the best overall value for money. While cost is not the only consideration, we strive to ensure that we are making the most cost-effective decisions for the project. Lower value for money options may require additional contributions to the project to achieve the desired outcomes.

Ultimately, our goal is to select the option that provides the best possible outcome for the community and the environment, while ensuring that we make cost-effective decisions for the project.

How is the preferred option selected?

We have carefully reviewed all feedback and suggestions received during our conversations with the community including those during out online engagement in early 2021, and our aim to incorporate feedback received into our options wherever possible.

Feedback from the community will be presented to the Project Board along with the costs and benefits drawn out through the appraisal process.

The Project Board is made up of senior decision makers at the Environment Agency, and they will make a decision on how to proceed with updating the scheme.

The Project Board will then review all this information to make a decision on the future of the scheme. This means the Project Board is not bound to just select the option which provides the best economic value for money. Once the Project Board has made this decision, we will come back to the community with more information on our next steps.

Will planning permission be required to update the scheme?

While the project is still in the appraisal stage, we do not anticipate that planning permission will be required. We will, however, be speaking to the Local Planning Authority to confirm whether any permissions or permits will be necessary.

Under Planning Legislation, certain types of building work may be considered Permitted Development and may not require planning permission. We will have a better understanding of the planning requirements once we have further information available. Additionally, we do not anticipate the need for any additional Compulsory Purchase of land, as the proposed updates are primarily focused on the existing scheme.

What measures will be put in place to reduce the impact to local residents whilst the works are taking place?

As the options are still being developed, we do not yet know for certain what the works will look like, or the type of machinery required. Once we have further information, we will consult with householders and businesses who may be affected to understand concerns and share what plans will be put in place to minimise disruption.

In the past, we have carried out maintenance works to the scheme which have required the use of large machinery in areas close to houses. We consulted and notified the households that were affected, and successfully used techniques to reduce noise, vibration and dust and prevented materials or debris entering the watercourse.

Are there plans to reinforce the river banks? If so, how would updating the scheme fit with any other flood alleviation plans in the area?

Bank repairs are part of all the options being considered, and may involve the reinforcement of certain sections along the river bank. These improvements aim to enhance the bank's structural integrity and reduce the risk of damage during high flow flood events.

Do the ground anchors and ties associated with the defences form part of the project? If so, how will these be approached, will they need to be replaced?

The ground anchors and ties associated with the channel structure around Molember Sluice are integral parts of the flood risk management structure in this location and have been considered as part of this project. The Lower Mole FAS Asset Management Plan (2017) assessed the condition of the flood relief channel that includes these structures.

This inspection showed no defects associated with the ground anchors and ties therefore there is no intention to replace these in the short term. However, it is important that we are able to gain access to the area of land where the anchors and ties are located to allow for any future maintenance and inspection, and we will seek to keep this area of land behind the defences clear from any development.


What area does this project cover?

The Lower Mole Flood Alleviation Scheme is approximately 7.5km in total length, with the upstream extent in the Hersham/West End area, to where the River Mole joins the River Thames opposite Hampton Court.

How will you decide what works to carry out to the scheme, is there an assessment being carried out and what information does this contain?

We are carrying out an appraisal to determine our preferred option for the proposed works. This appraisal establishes a range of options for updating the scheme, and compares their economic viability (costs and benefits), technical feasibility (engineering difficulty) and environmental impacts to determine the best overall option. It also incorporates feedback we received from the public and other stakeholders.

Government money will partly fund the updating of the Lower Mole flood alleviation scheme; therefore the development and assessment of the options is in line with the Department for Environment Food & Rural Affairs (Defra) policy statement on Appraisal of Flood and Coastal Erosion Risk Management, which outlines the Flood and Coastal Erosion Risk Management Appraisal Guidance (FCERM-AG). This guidance sets out the steps that we need to follow throughout the project appraisal, which we must comply with in order to receive funding.

We start with a long list of options for the scheme and try to narrow them down to a short list that are considered viable options.

To evaluate options, the appraisal process looks at our project objectives (for example to continue to maintain the scheme, sustaining the standard of flood protection), what constraints there might be to developing options (for example, environmental constraints) and the 5 critical success factors set out in the government guidance: strategic fit and business needs; potential value for money; supplier capacity and capability; potential affordability; potential achievability.

This then provides us with a framework to work within when developing the short-list of options. In order to understand both the opportunities and constraints present, the appraisal process uses a range of information, for example, river modelling, cost estimates, economic benefits quantification, condition surveys of the assets along the Lower Mole, historical information, habitat surveys and feedback from stakeholders.

As the project moves from the long list to the short list, the level of detail we have increases, allowing us to understand the pros and cons of each option more, and to rule out those which will not be viable for economic, technical or environmental reasons.

All of the information gained during the appraisal process is taken into account when determining the preferred option for updating the scheme.

Will the scheme still offer the same standard of protection against flooding as it does now?

One of our priorities is to maintain the standard of protection against flooding the scheme currently offers to householders and businesses.

All the options considered will ensure a standard of protection of 1:100 ( the annual probability of flooding that has a likelihood of occurring once in a hundred years over the 100-year appraisal period) allowing for the expected impacts of climate change increasing river flows.

Does the possibility of decommissioning some of the assets mean that the. risk to flooding will increase?

No. Maintaining the current flood protection standard is a key objective of this project, and any option proposing asset removal will undergo a thorough assessment to ensure that it does not increase flood risk. In fact, the project aims to decrease the likelihood of flooding.

If installing sluice gates were to alleviate the risk to flooding, is it counter intuitive to remove them?

The sluice gates were installed during the construction of the channel for amenity and recreation purposes. The gates remain closed on a day-to-day basis to retain a fixed water level, but in times of high flow, they are opened to allow the water to pass. The condition of the sluice gates is starting to deteriorate due to their age, and we need to act to sustain the standard of flood protection for years to come. Should the sluice gates be removed, the flood relief channel would continue to convey flood flows as it was designed to do.

How long will the works take to complete and when are they scheduled to start?

We are not able to confirm what the timescales for the project currently are. It is important to note that this project is in its early stages as we are yet to select a preferred option.

An asset management plan for the Flood Alleviation Scheme was developed in 2017 showing that there are varying residual lives for the asset elements of the sluices and flood relief channel. The residual life of an asset is the amount of life remaining after economic life or technological life has been reached and before functional failure of that asset. The electrical elements and controls of sluices have the shortest expected residual life and are now due for replacement, with the sluice gates themselves nearing the end of their residual life, and this needs to be considered in the future planning.

Will all of the planned works take place at the same time?

Due to the area the scheme covers, it is likely that the works will be phased over a period of a few years. Although we are currently unable to offer a specific timeline, when we have more detail on the nature and scale of the works we will be able to share this information.

Some of the structures and areas of land along the scheme are owned by others. We have started discussions with those owners to understand if they have any concerns, and to work with them to move the project forward.

How will the water levels and flows within the River Ember and the River Mole be managed whilst the works are carried out?

The lowering of the water levels and the management of flows within the river whilst the works are ongoing will be carried out with both flood risk and environmental needs fully considered. Management of the levels and flows will be confirmed once the proposal is fully developed and the details of the works are finalised.

In the past, we have successfully managed water levels to carry out maintenance works to our sluice structures and the river channel. We can use the knowledge from previous works to help us manage water levels and flows during this project.

Will the Lower Mole be dredged?

Currently, dredging is not planned to be carried out as part of this project.

We consider each location carefully and only carry out dredging where we know it will make a difference to the management of flood risk. Understanding where dredging will, and won’t, reduce flooding is the key.

Dredging has many short and long term environmental impacts such as the escape of silt plumes into the water, reducing water quality, or the removal of gravels from the river which is an important substrate on which fish spawn. In addition, this substrate helps to reduce channel erosion as it absorbs the energy of the river.

In the past, dredging was carried out periodically along the Lower Mole. However, regular surveys of the depth of silt in the river, highlighted that the accumulation of silt within the engineered channel was not significant, and was not reducing the ability of this channel to convey flood flows. Therefore dredging is not required and not the best use of our resources.

A bathymetric survey was carried out in late 2019 to assess the profile of the riverbed to offer up to date information on the depth of any silt within the river.

Are you going to be working with others on this project?

Yes. We have already made contact with the owners of those structures (Wilderness sluice and Royal Mills sluice) which are not under our ownership, to understand if there is the opportunity to work together as part of this project.

We are also speaking with Elmbridge Borough Council, Surrey County Council, utility companies, businesses and developers along the length of the scheme. Furthermore, we are collaborating with leading environmental organisations such as Surrey Wildlife Trust, South East Rivers Trust, Natural England, and the Lower Mole Catchment Partnership.

By working with others, we hope to realise more benefits and achieve a better overall outcome.

Who will own the new riverbank?

A riparian owner is someone who has any watercourse within or adjacent to any boundary of their property. Where a watercourse is located between two or more property boundaries, each owner may be equally responsible.

The options under consideration will not change land ownership or boundaries to Environment Agency land, although under some of the options more land may be exposed.

Riparian owners are responsible for maintaining the river bed and banks within their section of the watercourse. It is their duty to minimise pollution and prevent obstruction to the water flow, including maintenance of the banks and fallen trees etc.

Further information on riparian ownership can be found on the gov.uk website by following this link;

https://www.gov.uk/guidance/owning-a-watercourse(External link)

What are the plans for the area around Wilderness?

Wilderness sluice is not owned by the Environment Agency. While we can still carry out work here to construct a fish pass, it does require the agreement of the structure owners. As part of the scheme, we have a legal requirement to provide fish passage so fish can migrate from the River Thames into the River Mole catchment.

The construction of a rock ramp fish pass at Wilderness sluice is included in all available options, subject to obtaining the necessary agreement from landowners. A rock ramp pass is a gently sloping river ramp up to the weir that fish can swim up and over.

Will any new sluice gates be as quiet as the existing?

Should there be a need to replace the existing sluice gates with new ones as part of this project, we will seek to reduce the noise levels or at the minimum, maintain the current level of noise that the water makes as it passes through the sluice structures.

How will for the scheme be funded?

The scheme will be partly funded by central Government through the use of Flood and Coastal Erosion Risk Management (FCERM) Grant-in-Aid (GiA).

As Government funding is limited and is spent for the benefit of the Nation, we are also exploring options for funding from other sources, to ensure the works can progress.

As options to update the scheme are still being developed, costs are not currently available for the options. These will be available as the appraisal progresses and the options are agreed on.

Following the appraisal process a business case is put forward to obtain government funding. The amount of funding will be determined by a funding calculator, which prioritises funding for projects which have the greatest benefit to people and the environment. Environment Agency projects must go through an appraisal process to fully understand the costs and benefits over the full lifetime of the schemes design, and long-term maintenance. Our economic appraisals produce both a cost-benefit ratio and a Partnership Funding (PF) score. The PF score determines the proportion of a projects costs which can be paid for using (FCERM GiA), with any shortfall in funding needing to be found from other sources. These other sources can include local authorities, water companies, private businesses and individual land or property owners.

Will the options appraisal include a financial projection of maintenance levels and costs?

It is standard practice as part of the development of a flood risk management scheme to assess the anticipated capital, operational and maintenance costs of all options and include these in the total expected cost when comparing options.

This includes maintenance of all parts of the flood alleviation scheme, including all walls, embankments, surface water outfalls, sluice structures and the flood relief channel and is in line with the FCERM (Flood and Coastal Erosion Risk Management) Appraisal Guidance, which is available online (https://www.gov.uk/government/publications/flood-and-coastal-erosion-risk-management-appraisal-guidance(External link)).

Has the Environment Agency already chosen the cheapest option without fully considering all factors as well as the potential costs in the event of a flood?

No option has been selected at this time.

Cost is only one of a number of factors we need to consider. In June 2019, we received feedback from the community requesting more information on our proposals. In response, we undertook additional work over the winter of 2019, including environmental and river depth surveys, to better understand the Lower Mole and explore potential mitigation measures. Although these surveys are typically carried out at a later stage in the project, we wanted to address public concerns regarding water levels and wildlife by collecting and sharing this information early. With the insights gained from these surveys, we are now able to move forward with our updated short-list and engage in detailed discussions with the community this summer.

How we spend public money is an important factor, however, we also review the amount of benefits an option will deliver and over what timescales those benefits occur. We call this our 'economic appraisal' and this assessment is key to selecting a preferred option in line with Government guidance.

Following an economic appraisal, we consider a range of factors, including environmental and amenity impacts, legal obligations, health and safety, and carbon implications, to select the option that provides the best overall value for money. While cost is not the only consideration, we strive to ensure that we are making the most cost-effective decisions for the project. Lower value for money options may require additional contributions to the project to achieve the desired outcomes.

Ultimately, our goal is to select the option that provides the best possible outcome for the community and the environment, while ensuring that we make cost-effective decisions for the project.

How is the preferred option selected?

We have carefully reviewed all feedback and suggestions received during our conversations with the community including those during out online engagement in early 2021, and our aim to incorporate feedback received into our options wherever possible.

Feedback from the community will be presented to the Project Board along with the costs and benefits drawn out through the appraisal process.

The Project Board is made up of senior decision makers at the Environment Agency, and they will make a decision on how to proceed with updating the scheme.

The Project Board will then review all this information to make a decision on the future of the scheme. This means the Project Board is not bound to just select the option which provides the best economic value for money. Once the Project Board has made this decision, we will come back to the community with more information on our next steps.

Will planning permission be required to update the scheme?

While the project is still in the appraisal stage, we do not anticipate that planning permission will be required. We will, however, be speaking to the Local Planning Authority to confirm whether any permissions or permits will be necessary.

Under Planning Legislation, certain types of building work may be considered Permitted Development and may not require planning permission. We will have a better understanding of the planning requirements once we have further information available. Additionally, we do not anticipate the need for any additional Compulsory Purchase of land, as the proposed updates are primarily focused on the existing scheme.

What measures will be put in place to reduce the impact to local residents whilst the works are taking place?

As the options are still being developed, we do not yet know for certain what the works will look like, or the type of machinery required. Once we have further information, we will consult with householders and businesses who may be affected to understand concerns and share what plans will be put in place to minimise disruption.

In the past, we have carried out maintenance works to the scheme which have required the use of large machinery in areas close to houses. We consulted and notified the households that were affected, and successfully used techniques to reduce noise, vibration and dust and prevented materials or debris entering the watercourse.

Are there plans to reinforce the river banks? If so, how would updating the scheme fit with any other flood alleviation plans in the area?

Bank repairs are part of all the options being considered, and may involve the reinforcement of certain sections along the river bank. These improvements aim to enhance the bank's structural integrity and reduce the risk of damage during high flow flood events.

Do the ground anchors and ties associated with the defences form part of the project? If so, how will these be approached, will they need to be replaced?

The ground anchors and ties associated with the channel structure around Molember Sluice are integral parts of the flood risk management structure in this location and have been considered as part of this project. The Lower Mole FAS Asset Management Plan (2017) assessed the condition of the flood relief channel that includes these structures.

This inspection showed no defects associated with the ground anchors and ties therefore there is no intention to replace these in the short term. However, it is important that we are able to gain access to the area of land where the anchors and ties are located to allow for any future maintenance and inspection, and we will seek to keep this area of land behind the defences clear from any development.


Page published: 31 May 2023, 12:12 PM