Bath River Avon Options Study: June 2015

WELCOME

Last October the Environment Agency and Bath & North East Somerset Council conducted a workshop to present options for the long term future of the Twerton and Pulteney sluice gates. At this workshop valuable discussion focused not only around the merits of each option, and which should be taken forward for a more detailed appraisal, but also flood defence across the city centre.

Since that time, further research work suggests that a separate approach to the subsequent phases may not be the most appropriate. Originally, the team had separated the issues in Bath into two specific questions: the first being the maintenance of the gates and the second the consideration of flood defences through the City. The study shows that modifications to Twerton gates in particular have the potential to positively reduce flood risk in the City. The results of a recent silt survey upstream of Pulteney Weir are also now available.

It is possible now to look at a more holistic scheme to reduce flood risk in Bath that includes work at both gates and also various sites in between the gates and upstream of Pulteney Weir. By considering a more comprehensive approach it is possible to achieve the potential wider benefits of the whole scheme allowing stakeholders and funders to influence and invest in the scheme as a whole.

The purpose of this drop-in session is to share the team’s findings and comprehensive scheme study, including future possibilities for hydropower, giving you an update on progress, seeking further views from you and giving you the opportunity to ask questions.

FLOOD RISK IN BATH SEPTEMBER 2014

1960’s flooding in Bath

Background Bath is at risk from river and surface water flooding. Following significant flooding in the 1960’s the Bath Flood Alleviation Scheme (FAS) was completed in 1974 to reduce the risk of flooding to the city. The Environment Agency (EA) is responsible for the management of flood risk of the scheme, as it is on a ‘main’ river. This includes inspection of the flood defences, requesting/enforcing maintenance by river owners and carrying out works as appropriate. Bath and North East Somerset Council (B&NES) is the Lead Local Flood Authority (LLFA) and is responsible for managing the risk of flooding from surface water, groundwater and ordinary watercourses.

The Flood Alleviation Scheme through Bath

Bath FAS significantly reduces flood risk but there are estimated to be 930 properties in Bath at flood risk from flooding in a 1in 100 annual probability event (1% chance of flooding occurring in any one year). This is predicted to increase to 1500-2000 properties with the impact of future climate change.

In 2004/05 we re-appraised the Bath FAS and the study identified a number of improvement options. However, none provided a sufficiently high benefit cost ratio to justify being wholly publically funded at the time. Solutions to reduce flood risk to the whole of Bath, such as upstream storage, have been investigated but have not been found to be viable. We have identified that improvements in certain parts of Bath might be possible if linked with regeneration, as this can assist with funding. Planning legislation states that development, including flood alleviation schemes, will not increase flood risk elsewhere. The Council, with advice from the EA, enforces this legislation.

Bath Quays Waterside project The Bath Quays Waterside project located downstream of Churchill Bridge, reduces flood risk to 103 existing properties and the Lower Bristol Road. This project would not have been financially viable without a link to regeneration work. The project does not increase flood risk up or downstream and would not stop other flood alleviation works going ahead elsewhere in the city.

Future work Improvement works to reduce flood risk in parts of the city where major regeneration is not planned is unlikely to be possible from public funding alone. This is because the costs, compared to the reduction in flood damages do not provide a

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sufficiently strong business case to attract public funding. The EA and B&NES are working together to re-evaluate options inside and outside of the new regeneration areas to ensure flood risk in Bath is managed as effectively as possible. This includes the consideration of the future roles and operation of Pulteney and Twerton gates.

Pulteney weir

Operation of Pulteney & Twerton Gates

Pulteney and Twerton gates form part of the Bath FAS. During low flows, these sluices maintain a water level upstream. Pulteney has a single radial gate. Twerton has a vertical and a radial gate spanning the river. A minimum water level is required for navigation purposes, as well as protecting the ancient foundations of buildings along the river corridor in the World Heritage area of Bath. However, during high flows, the sluices gates are opened. The operation is automatic and reacts to changes in river levels. This means that as river levels rise, the gates gradually open and allow flood flows to pass through Bath. The gates do not hold back flood water and then suddenly release it. Flood water arrives at the gate and continues downstream as it would do naturally.

Twerton Gates

Preparation for Flooding. Flooding is a natural phenomenon that can still occur where flood alleviation is in place. The EA and B&NES work in partnership with emergency services and communities to prepare for flooding. To find out if you are at risk of flooding please visit our website link below. http://apps.environment-agency.gov.uk/wiyby Communities, residents and businesses are encouraged to register for the EA’s Flood Warning service and to consider the community and personal flood plan guidance that is available on the EA’s website. Flood Warning registration link: https://fwd.environment-agency.gov.uk Community Plan guidance link: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/292939/LIT_5286_b9ff43.pdf Personal Flood Plan guidance link: http://apps.environment-agency.gov.uk/flood/151256.aspx

On the 23rd April 2015 the Environment Agency removed 350 trolleys, 40 bicycles, 2 motorbikes and 9 cars from the River Avon in Bath at Victoria Bridge. The debris was removed to improve navigation through the city.

Bath River Avon Options Study: June 2015 Silt surveys

A river channel survey was undertaken in 2015 between Bathampton Weir and Pulteney Weir to determine the current quantity of silt within the channel and how the channel sections have changed over time. Comparison was made to survey data from various dates from the 1930s.

This shows that the River Avon between Bathampton Weir and Pulteney Weir is ‘in regime’. This means that the channel cross section is relatively stable with minimal change over time. Any sediment in the river passes through the city with minimal deposition on the river bed. This is to be expected and occurs because coarse sediment is deposited at upstream structures, such as weirs, before reaching Bath. The velocities in the river are then high enough, especially during higher flows, to flush through any fine sediment that remains.

The graphs to the right show selected channel cross sections along with the profile at Grosvenor Bridge with the 1954 (black) survey compared against one from 2001 (red).

Between Pulteney Weir and Twerton Sluice Gates the Environment Agency undertake regular surveys of the river bed. These show minimal change in channel section over the years, with the flow area remaining similar to that provided during the Bath Flood Protection Scheme. In a detailed study in 2012 a recent survey was compared against previous data from 1985. This showed that of the 40 channel cross sections surveyed, 33 showed a larger channel section in 2012 compared with 1985, with an average increase in flow area of 14m2.

In some locations, such as Victoria Bridge, debris has entered the river changing the bed profile locally, but this on its own has little impact overall on the conveyance of the river.

The one area where there is softer bed material is just upstream of Pulteney Bridge on the east bank. However the bed profile does not appear to have changed since the Bath Flood Protection Scheme was completed in the 1970s.

If Pulteney Sluice Gate was having a major benefit in preventing upstream siltation then you would expect to see the largest flow depths on the east bank (the same side as the sluice gate) and no build up of silt on this bank upstream. As the opposite is apparent it is inferred that the sluice gate is doing little in preventing upstream siltation.

Flood risk in Bath has not changed over time due to siltation within the River Avon.

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Infomap 2015 Survey 1989 Survey (33m Upstream) Merrett 1998 Survey Cartographic 2001 Survey

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Infomap 2015 Survey 1989 Survey Merrett 1998 Survey Cartographic 2001 Survey

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Infomap 2015 Survey 1989 Survey (18m Downstream) Merrett 1998 Survey Cartographic 2001 Survey

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Cross-Section 22

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Typical Channel Cross Sections between Bathampton Weir and Pulteney Weir

GROSVENOR BRIDGE

PULTENEY BRIDGE LOOKING DOWNSTREAM

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TYPICAL CROSS SECTION BETWEEN PULTENEY WEIR AND TWERTON SLUICE GATES (CROSS SECTION 22)

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Bath River Avon Options Study: June 2015 Questions and answers from last workshop

Option Comments from previous stakeholder workshop Update from work undertaken since workshop

General Do Minimum • Would allow time to holistically review management of river and catchment.

• Must consider impact of climate change and full impact on residents.

• Study is now looking at wider river flooding issues. Catchment issues and benefits considered under other studies.

• Potential climate change impacts have been fully considered in appraisal of flood risk.

Other • Flood risk is most important issue

• Costs and maintenance also important

• Public realm at Pulteney more important than Twerton

• Consider the impact the lack of maintenance, dredging and bank clearance has had on flow

• Opportunity at both sites to improve biodiversity

• Opportunity for increased future river use

• Potential to meet amenity use in combined projects

• Need to consider potential solution’s impact on water supply/sewerage network – building resilience for the future.

• Need to consider structures at Pulteney and Twerton together

• Agreed

• Agreed

• Agreed and scheme at Pulteney will be led by landscape designers

• Has been assessed as part of assessment of benefits of work. Silt survey undertaken to assess benefits of dredging.

• Agreed

• Agreed

• All Pulteney options are including considerations of bringing more people to the river

• Options will only impact water levels during flood conditions, however any options to reduce ingress into the sewer system from the river will be investigated

• Structures being considered as part of combined study

Pulteney Refurbishment • Consider future maintenance and access • Has been assessed in the options Replace with sluice gate • Have respect for the setting

• Consider future maintenance and access • Option rejected at end of first phase of study

Replace with fixed crest weir

• Step back in time

• Potential for increased sediment risk.

• New weir can have various forms. Potential to extend existing weir

• Silt survey shows limited if any silt. Replace with lock gates • Concerns on technical failure

• Examples of suitable lock gates used elsewhere (e.g. Cardiff Bay) • Concern over safety of navigation above the weir • Public transport opportunities

• Option rejected at end of first phase of study

Infill the land • Seen as a risk to hydro opportunities

• No fish/biodiversity improvements

• Safety impacts on people

• Hydropower unlikely to be economically viable at Pulteney

• Option will include new fish and eel passes

• Safety concerns are very valid and being discussed. Option will also make it easier for people to get out of the water.

Others • Just remove platform over sluice and celebrate the engineering

• Heritage and aesthetic equally important

• Educational value of hydropower

• Solution should be innovative

• Must look to reduce flood risk

• Retain the canoe/boat rollers.

• Would be a missed opportunity

• Agreed

• This would be the main reason for a hydropower scheme at Pulteney

• Agreed

• Agreed. Analysis is showing this is possible.

• They are retained in all options. Twerton

Lowered retained water level

• Negative impact on hydro • Impact on navigation • Visual impact

• Option rejected at end of first phase of study

Others • Need to address safety

• Hydro links to heritage of the area (make the structure work for its living)

• Link with future regeneration

• Connection of Twerton to the river

• Potential for biodiversity improvements

• Not so visible

• Potential for improved public realm access

• Safety concerns considered lower at Twerton, however new scheme will include provision for preventing boats reaching the gates

• Hydropower being considered by BWCE, although the projects are sharing information

• Linked fully with the proposed regeneration sites

• Considered to be limited opportunities

• New fish and eel passes included in the design

• Agreed

• Considered to be limited opportunities.