Identifying the national economically preferred option

As the scheme nears the end of its design life, it is crucial to update and seek to improve the scheme where possible while maintaining the same high standard of flood protection. Simultaneously, we must ensure that the scheme aligns with environmental considerations and benefits both people and wildlife.

The scheme comprises six primary water level control structures (sluice gates), and decisions about their future are now needed. A holistic approach is essential, as actions taken in one location may impact on a wider area.

Following our mitigation review, we created a revised shortlist of options for updating the current scheme. The full revised shortlist includes Option Yellow or ‘Do nothing' which is a requirement of the appraisal process to give us a baseline to compare other options against. Option Yellow is considered to be non-viable. This Option would lead to an unacceptable increase in flood risk, if the gates at the main structures; Viaduct Sluice, Island Barn Sluice, Molember Sluice, remained closed when river flows were high.

For each option, we have reassessed the changes in water levels, environmental and flood risk benefits, cost, carbon footprint and maintenance implications. We are asking your views on Options Red, Blue, Pink and Orange. Option Purple offers the same outcomes for the river in terms of water levels as Option Red, but at greater cost and in a less sustainable way with a higher carbon footprint. Although Option Purple is a viable option to maintain the current level of flood risk protection it not considered an option we would seek to progress as the preferred option for the Scheme. Therefore, Options Yellow and Purple are not a part of the final choice for the Scheme.

The new revised shortlisted options

Option Purple - Do minimum to the Lower Mole Scheme 

We would only carry out reactive maintenance and repairs as the structures gradually failed. There would be similar levels of plant and animal habitat along and in the river. The water levels would remain similar to the current situation. 

Option Red - Maintain the current water levels and habitat throughout the Lower Mole Scheme

We will keep the gates at Island Barn and Viaduct. There will be a combination of gates and fixed weirs at Molember. Plant and animal habitat along and in the river will not change.

Option Blue - Reduce changes in the water level and improve habitat by using fixed weirs and building a rock ramp

We will keep the gates at Island Barn, replace the gates with fixed weirs at Molember. We will also remove the gates at Viaduct and build a new rock ramp upstream. The fixed weirs and rock ramp would minimise the change in water levels compared to Options Pink and Orange. There could be some increases in plant and animal habitat along and in the river.

Option Pink - Reduce changes in the water level and improve habitat by building two rock ramps

We will keep the gates at Island Barn. We will also remove the gates at Molember and Viaduct and build new rock ramps upstream of both structures. The rock ramps would minimise the change in water levels compared to Option Orange. There could be some increases to plant and animal habitat along and in the river.

Option Orange (national economically preferred option) - Reduce the water levels and potential habitat improvements beyond what is currently along the Scheme

We will keep the gates at Island Barn. We will also remove the gates at Molember and Viaduct. The river would start to become a more natural environment. There would be a greater increase to plant and animal habitat along and in the river for at least 2km upstream of the Scheme.


Identifying the national economically preferred option

The national economically preferred option is identified using the decision rules set out in the appraisal guidance which consider the cost and benefits of each option. For this Scheme Option Orange is the national economically preferred option as it has the highest benefits and the lowest costs.

Find out more information about the Governments appraisal and the option selection process guidance at

https://www.gov.uk/guidance/fcerm-appraisal-guidance(External link)

https://www.gov.uk/guidance/flood-and-coastal-erosion-risk-management-appraisal-guidance/8-compare-and-select-the-preferred-option(External link)

Costs and benefits

When evaluating the various options, several costs need to be considered. The Government appraisal process allows us to understand the costs and benefits over the entire lifetime of each option, including long-term maintenance expenses.

The economic aspect of our appraisal produces a benefit cost ratio (BCR). The BCR is the ratio of the benefits of a scheme option, expressed in monetary terms, relative to its costs, also expressed in monetary terms. To facilitate direct comparison, we use the estimated whole life present value costs and benefits for each of the options. This includes the costs for the design and construction of any changes to the scheme which are made now, as well as future operation and maintenance costs over a 100-year period. Within our cost estimates we include potential future costs, such as ongoing maintenance and the replacement of gates and electrical equipment as they reach the end of their working life. We also include a risk allowance to allow for increases in costs that cannot be foreseen at this time. 

The benefits of the Scheme that we quantify include prevention of flooding to the area surrounding the Lower Mole Flood Alleviation Scheme and improvements to the natural environment, including opening up the river to the passage of fish and carbon savings. 

The national economically preferred option is identified using the decision rules set out in the appraisal guidance which consider the cost and benefits of each option. The benefits, costs and benefit cost ratio of the options are shown in the table below. For the Scheme, Option Orange is the national economically preferred option as it has the highest benefits and the lowest costs which means the highest benefit cost ratio. 

Option 

Benefits (£M) 

Costs (£M)  

Benefit Cost Ratio 

Purple 

559.8 

52.3 

10.3 

Red 

560.2 

43.1 

13.0 

Blue 

568.7 

40.2 

14.2 

Pink 

571.5 

42.1 

13.6 

Orange (national economically preferred option)

571.7 

38.8 

14.7 


Flood risk

One of our priorities is to maintain the standard of protection the scheme currently offers against flooding into the future (1:100 or 1% annual probability), taking into account the impacts of climate change. All the options that propose changes to the scheme maintain the current standard of protection.

For all of the options where structures are removed and greater capacity created in the river channel, for the larger magnitude or rarer flood events (1:1000 or 0.1% annual probability) there would be a reduction in the risk to flooding compared to the present-day situation. You can find out more about day to day water levels by visiting our combined 'water quality, levels and understanding impoundments page'.

Updating the flood protection figures

In our correspondence about the Lower Mole Flood Alleviation Scheme, we have been referencing a figure of approximately 3,000 homes and businesses protected from flooding. This estimate was established a decade ago, based on the number of properties that would be at risk in a 'do nothing' scenario over the following 100 years. However, the area at risk of flooding in the 1980s—before the Scheme was constructed—differs from the area that would be affected today if the gates were not operational and remained closed.

Our current flood risk modelling indicates that if the gates at Viaduct Sluice were not to open today, floodwaters would primarily exit the river on its eastern side, near the railway line, and flow northwards, flooding properties between Esher and Thames Ditton. This would impact approximately 1,300 properties. We have calculated that Options Red, Blue, Pink, and Orange would protect these 1,300 properties for the next 100 years.

Water level

The options to update the scheme may impact the present day water level in the main channel. Some of the options lead to changes in the water level and depth within the river channel while other options will not result in any change. For further information on water levels please visit our combined 'Water quality, levels and understanding impoundments page'.

Whole life carbon

When appraising potential scheme options, carbon is a key consideration within the government guidance. We must carefully balance the needs of communities alongside tackling the global climate emergency.

We have used our carbon modelling tool to carry out some initial calculations on the carbon footprint of our options. There are two components of our carbon calculations. The first is called Capital Carbon, this is the carbon associated with construction activities to update the scheme in the near future.

The second is Future Carbon, which considers carbon potentially produced during the future operation of the scheme. It includes aspects like the ongoing maintenance of structures. The two numbers combined gives us the total Whole Life Carbon. The Whole Life Carbon figures will contribute to the appraisal of each option. The table below shows the whole life carbon estimates for each option.

Option

Purple

Red

Blue

Pink

Orange (national economically preferred option)

Whole Life Carbon (tCO2)

8,243

7,995

7,581

7,702

7,321

For more information, please see our 'Understanding carbon’ page.

Water environment

This is subdivided into three keys areas:

Fish passage describes how fish move up and downstream, as well and in and out of the river system. This is important to allow different species and life stages to utilize specific habitats, move away from disturbance events and be able to subsequently recolonize affected areas. Some species (such as eels) need to migrate upstream and downstream to complete their life cycle. For more information, please see our 'Understanding fish passes’ page.

Biodiversity describes the variability or richness in flora and fauna, also called biological diversity. For further detail visit our 'Improving the environment on the Lower Mole’ page.

Water quality describes the condition of the water in terms of its favourability for species to survive. This includes temperature, dissolved oxygen, fine sediment load as well as contaminants such as phosphates, nitrates and heavy metals. For further detail visit our combined 'Water quality, levels and understanding impoundments’ page.

For more information about the appraisal process and how funding works please follow the links on the right hand side of this page.


For more information about the appraisal process and how funding works, please follow the links on the right hand side of this page.

As the scheme nears the end of its design life, it is crucial to update and seek to improve the scheme where possible while maintaining the same high standard of flood protection. Simultaneously, we must ensure that the scheme aligns with environmental considerations and benefits both people and wildlife.

The scheme comprises six primary water level control structures (sluice gates), and decisions about their future are now needed. A holistic approach is essential, as actions taken in one location may impact on a wider area.

Following our mitigation review, we created a revised shortlist of options for updating the current scheme. The full revised shortlist includes Option Yellow or ‘Do nothing' which is a requirement of the appraisal process to give us a baseline to compare other options against. Option Yellow is considered to be non-viable. This Option would lead to an unacceptable increase in flood risk, if the gates at the main structures; Viaduct Sluice, Island Barn Sluice, Molember Sluice, remained closed when river flows were high.

For each option, we have reassessed the changes in water levels, environmental and flood risk benefits, cost, carbon footprint and maintenance implications. We are asking your views on Options Red, Blue, Pink and Orange. Option Purple offers the same outcomes for the river in terms of water levels as Option Red, but at greater cost and in a less sustainable way with a higher carbon footprint. Although Option Purple is a viable option to maintain the current level of flood risk protection it not considered an option we would seek to progress as the preferred option for the Scheme. Therefore, Options Yellow and Purple are not a part of the final choice for the Scheme.

The new revised shortlisted options

Option Purple - Do minimum to the Lower Mole Scheme 

We would only carry out reactive maintenance and repairs as the structures gradually failed. There would be similar levels of plant and animal habitat along and in the river. The water levels would remain similar to the current situation. 

Option Red - Maintain the current water levels and habitat throughout the Lower Mole Scheme

We will keep the gates at Island Barn and Viaduct. There will be a combination of gates and fixed weirs at Molember. Plant and animal habitat along and in the river will not change.

Option Blue - Reduce changes in the water level and improve habitat by using fixed weirs and building a rock ramp

We will keep the gates at Island Barn, replace the gates with fixed weirs at Molember. We will also remove the gates at Viaduct and build a new rock ramp upstream. The fixed weirs and rock ramp would minimise the change in water levels compared to Options Pink and Orange. There could be some increases in plant and animal habitat along and in the river.

Option Pink - Reduce changes in the water level and improve habitat by building two rock ramps

We will keep the gates at Island Barn. We will also remove the gates at Molember and Viaduct and build new rock ramps upstream of both structures. The rock ramps would minimise the change in water levels compared to Option Orange. There could be some increases to plant and animal habitat along and in the river.

Option Orange (national economically preferred option) - Reduce the water levels and potential habitat improvements beyond what is currently along the Scheme

We will keep the gates at Island Barn. We will also remove the gates at Molember and Viaduct. The river would start to become a more natural environment. There would be a greater increase to plant and animal habitat along and in the river for at least 2km upstream of the Scheme.


Identifying the national economically preferred option

The national economically preferred option is identified using the decision rules set out in the appraisal guidance which consider the cost and benefits of each option. For this Scheme Option Orange is the national economically preferred option as it has the highest benefits and the lowest costs.

Find out more information about the Governments appraisal and the option selection process guidance at

https://www.gov.uk/guidance/fcerm-appraisal-guidance(External link)

https://www.gov.uk/guidance/flood-and-coastal-erosion-risk-management-appraisal-guidance/8-compare-and-select-the-preferred-option(External link)

Costs and benefits

When evaluating the various options, several costs need to be considered. The Government appraisal process allows us to understand the costs and benefits over the entire lifetime of each option, including long-term maintenance expenses.

The economic aspect of our appraisal produces a benefit cost ratio (BCR). The BCR is the ratio of the benefits of a scheme option, expressed in monetary terms, relative to its costs, also expressed in monetary terms. To facilitate direct comparison, we use the estimated whole life present value costs and benefits for each of the options. This includes the costs for the design and construction of any changes to the scheme which are made now, as well as future operation and maintenance costs over a 100-year period. Within our cost estimates we include potential future costs, such as ongoing maintenance and the replacement of gates and electrical equipment as they reach the end of their working life. We also include a risk allowance to allow for increases in costs that cannot be foreseen at this time. 

The benefits of the Scheme that we quantify include prevention of flooding to the area surrounding the Lower Mole Flood Alleviation Scheme and improvements to the natural environment, including opening up the river to the passage of fish and carbon savings. 

The national economically preferred option is identified using the decision rules set out in the appraisal guidance which consider the cost and benefits of each option. The benefits, costs and benefit cost ratio of the options are shown in the table below. For the Scheme, Option Orange is the national economically preferred option as it has the highest benefits and the lowest costs which means the highest benefit cost ratio. 

Option 

Benefits (£M) 

Costs (£M)  

Benefit Cost Ratio 

Purple 

559.8 

52.3 

10.3 

Red 

560.2 

43.1 

13.0 

Blue 

568.7 

40.2 

14.2 

Pink 

571.5 

42.1 

13.6 

Orange (national economically preferred option)

571.7 

38.8 

14.7 


Flood risk

One of our priorities is to maintain the standard of protection the scheme currently offers against flooding into the future (1:100 or 1% annual probability), taking into account the impacts of climate change. All the options that propose changes to the scheme maintain the current standard of protection.

For all of the options where structures are removed and greater capacity created in the river channel, for the larger magnitude or rarer flood events (1:1000 or 0.1% annual probability) there would be a reduction in the risk to flooding compared to the present-day situation. You can find out more about day to day water levels by visiting our combined 'water quality, levels and understanding impoundments page'.

Updating the flood protection figures

In our correspondence about the Lower Mole Flood Alleviation Scheme, we have been referencing a figure of approximately 3,000 homes and businesses protected from flooding. This estimate was established a decade ago, based on the number of properties that would be at risk in a 'do nothing' scenario over the following 100 years. However, the area at risk of flooding in the 1980s—before the Scheme was constructed—differs from the area that would be affected today if the gates were not operational and remained closed.

Our current flood risk modelling indicates that if the gates at Viaduct Sluice were not to open today, floodwaters would primarily exit the river on its eastern side, near the railway line, and flow northwards, flooding properties between Esher and Thames Ditton. This would impact approximately 1,300 properties. We have calculated that Options Red, Blue, Pink, and Orange would protect these 1,300 properties for the next 100 years.

Water level

The options to update the scheme may impact the present day water level in the main channel. Some of the options lead to changes in the water level and depth within the river channel while other options will not result in any change. For further information on water levels please visit our combined 'Water quality, levels and understanding impoundments page'.

Whole life carbon

When appraising potential scheme options, carbon is a key consideration within the government guidance. We must carefully balance the needs of communities alongside tackling the global climate emergency.

We have used our carbon modelling tool to carry out some initial calculations on the carbon footprint of our options. There are two components of our carbon calculations. The first is called Capital Carbon, this is the carbon associated with construction activities to update the scheme in the near future.

The second is Future Carbon, which considers carbon potentially produced during the future operation of the scheme. It includes aspects like the ongoing maintenance of structures. The two numbers combined gives us the total Whole Life Carbon. The Whole Life Carbon figures will contribute to the appraisal of each option. The table below shows the whole life carbon estimates for each option.

Option

Purple

Red

Blue

Pink

Orange (national economically preferred option)

Whole Life Carbon (tCO2)

8,243

7,995

7,581

7,702

7,321

For more information, please see our 'Understanding carbon’ page.

Water environment

This is subdivided into three keys areas:

Fish passage describes how fish move up and downstream, as well and in and out of the river system. This is important to allow different species and life stages to utilize specific habitats, move away from disturbance events and be able to subsequently recolonize affected areas. Some species (such as eels) need to migrate upstream and downstream to complete their life cycle. For more information, please see our 'Understanding fish passes’ page.

Biodiversity describes the variability or richness in flora and fauna, also called biological diversity. For further detail visit our 'Improving the environment on the Lower Mole’ page.

Water quality describes the condition of the water in terms of its favourability for species to survive. This includes temperature, dissolved oxygen, fine sediment load as well as contaminants such as phosphates, nitrates and heavy metals. For further detail visit our combined 'Water quality, levels and understanding impoundments’ page.

For more information about the appraisal process and how funding works please follow the links on the right hand side of this page.


For more information about the appraisal process and how funding works, please follow the links on the right hand side of this page.

Page last updated: 15 Aug 2024, 10:53 AM