The Newt Ditch Marlow Flood Alleviation Scheme

The Newt Ditch Marlow Flood Alleviation Scheme aims to reduce flood risk to 125 residential and non-residential properties in this flood prone area around the Newt Ditch in eastern Marlow.

We're committed to managing flood risk and this is one of the various projects aimed at managing flood risk across the county.

Marlow has a history of flooding and roads prone to flooding include:

• Dedmere Road
• The Croft
• Quarrydale Drive
• Fieldhouse Lane

A primary cause of flooding is the lack of capacity in the culverts that drain into the Newt Ditch. A culvert is a structure (a pipe or channel) which allows water to flow under a road, pathway or similar.

Our scheme is focused around the culvert under Dedmere Road from Fieldhouse Way through to the open channel section either side of the A404.

We've carried out video camera surveys of the current culverted drainage network which runs under Dedmere Road, Quarrydale Drive and the footpath leading to the A404.

Surveys show this existing culvert is blocked in several places. The cause of the blockages include silt and debris build up, in growing tree roots and intrusion by utility services pipes.

The most recent flooding occurred between 26 October and 2 November 2022 in and around Fieldhouse Lane.

Our operations team have been supporting the local community impacted by the flooding by:

• clearing out the soakaways and realigning the kerbs in the area around Fieldhouse Lane
• reviewing opportunities for additional improvements, including installing a new soakaway close to Fieldhouse Lane (this work should be completed shortly after Christmas)

If you've been flooded during this recent event, we would like to understand what happened and how you were affected. This will help us finalise our plans to reduce flood risk in the area.

Complete our flood questionnaire

The project team have continued to work hard in developing a viable flood alleviation scheme for East Marlow.

Over the last 6 months our work has focused on:

• the completion of the A404 culvert clearance works and survey work of the oil separator, pipes and manholes
• the completion of topographical surveys of extended project area east of the A404
• updates to the hydraulic modelling
• the production of an updated economic appraisal
• the development of outline design drawings
• carrying out of additional environment surveys, including ground-based bat roost assessment, bat activity surveys, badger walkover survey, and a preliminary ecological appraisal

Due to the current economic climate and the increasing costs of materials and labour, we carried out an updated economic appraisal of the scheme's scope.

The appraisal found that we would not be able to secure sufficient government funding to continue with the original scope.

We've now developed a revised scheme. The cost is significantly lower and gives the project the best opportunity to secure government funding.

The properties that would have benefited from the original scheme will continue to benefit from a reduction in flood risk under the revised scheme, as flood volume levels will be reduced across the area.

The key elements of the revised scheme are:

We will:

• divert utility services that cross through within the pipeline, including making localised repairs to the pipe where necessary
• construct a new manhole at the junction of Dedmere Road and Newtown Road
• carry out jetting work to clear the existing pipe

We will:

• carry out jetting work to clear existing pipe
• replace a short section of pipe to tie in with the new manhole at the Dedmere Road and Newtown Road junction

We will:

• replace the pipe running from the end of Quarrydale Drive to The Acre and through the alleyway behind - the new pipe will be laid under the public footpath
• provide new manholes to improve maintenance access to the pipe, including where the pipe crosses The Acre

We will remove the existing oil interceptor arrangement, which was discovered at the end of Savill Way/Gunthorpe Road.

An alternative treatment of pollutants will be provided by designing the open channel in this area to function as a swale.

We will:

• open out the existing pipeline east of the alleyway behind The Acre and replace this with an open channel
• modify the open channel to provide pollution control. This may involve particular planting or riverbed material
• construct retaining walls on the north bank, due to space constraints. We currently plan to use vegetated bagwork

The existing ditch will be cleared, with alterations to the bank profiles and landscaping works.

No modifications are proposed to the existing A404 culvert.

We will clear the Newt Ditch as far as the siphon structure, which is part of the drainage pond serving Globe Business Park. This will provide a discharge for water in the A404 culvert and upstream.

We will also clear debris the pipes of the siphon.

Elements that we've had to remove from the scope due to their impact on costs are:

• works to replace or pass flow beneath the siphon
• a new widened footpath (reinstatement with a gravel surface is now proposed)
• new lighting to footpaths

There may be opportunities to secure funding for these improvements through other routes outside of the flood alleviation project.

We've been working with the Environment Agency (EA) to assess the flood risk of flows from the Newt Ditch moving east beyond the A404, downstream of the immediate project works area.

We have agreed with the EA to carry out a detailed flood risk modelling assessment. This will improve our understanding of how the water moves from the east of the A404 to the River Thames.

We expect this assessment to take around 9 months due to:

• the necessity for topographical surveys over 3km, stretching to the River Thames at Bourne End
• the complexity of the modelling work being undertaken

From January 2023 we will engage with landowners east of the A404 on the surveys we need to undertake along the watercourse down to the River Thames in Bourne End.

To secure government funding we plan to submit an updated business case report in March 2023.

We're currently working on detailed design alongside a flood risk modelling assessment.

Once the flood risk modelling assessment is complete, we will engage with stakeholders ahead of planning submission. We expect this to take place in autumn 2023.

We will submit a planning application to Buckinghamshire Council at the end of 2023. This aims to ensure we have planning consent obtained by spring 2024.

We expect construction to start in early 2025 and to complete the work later that year.

If you have any questions or you need a printed copy of the flood questionnaire, email us at [email protected]