Dec 09, 2019 Last Updated 10:52 AM, Aug 14, 2019

Leeds flood alleviation scheme

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The threat of flooding is increasingly linked to climate change and expanding urbanisation. Leeds narrowly escaped a major disaster during the 2007 floods and since then there have been several localised flood events closing the train station, blocking roads and resulting in city centre residents being evacuated from waterfront properties.

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In order to protect businesses, homes, the transportation network and the city economy in the future, Leeds City Council needs to provide flood defences. A flood alleviation scheme (FAS) has been designed to protect the city centre from a 1:75-year flood and the planned project will involve the construction of flood defence interventions between Leeds train station and Knostrop Lock downstream of the city centre.

In 2014, the BAM Nuttall and Mott MacDonald joint venture (BMMJV) won the competitively tendered contract for the detailed design and construction of the defences.

BMMJV’s work includes:

• Constructing landscape defences such as low embankments, terracing and riverside walls at low points along the riverbank to maintain a consistent 1:75 standard of flood protection
• Removing Knostrop Cut Island to merge the canal and River Aire, which will create additional flood water storage
• Providing flood defences along a length of Holbeck which flows into the River Aire.

The benefits

The FAS will provide environmental improvements to the waterfront wherever possible, including landscaped waterfront seating areas.

The overarching aim will be to enhance the River Aire green corridor and to provide opportunities for biodiversity and leisure including cycling and walking, improving links for people and wildlife between the city centre and the wider Lower Aire Valley. A key aim of the project has been to identify and build in sustainable construction techniques.

Early in the construction phase, BMMJV identified a cost-, programme- and environmentally-beneficial alternative to the proposed handrailing solution. The contract required BMMJV to remove several types of existing cast iron and timber handrails and to replace on a ‘like-for-like’ basis. However, it was pointed out by the site team that once the existing railings had been removed, there would be no railings left to match and it would not enhance the built environment if identical railings were adopted along the defences. The site management team convinced planners to adopt this principle and to use the same type of railings throughout.

Having agreed this, BMMJV and Marshalls, supported by the Leeds City Council design team and Arup, investigated the substitution of the cast steel railings with Ferrocast railing posts which was put to, and accepted, by the council’s planning department. Ferrocast is a polyurethane mix of an engineering grade specification which was developed from its use in the mining, quarrying and North Sea oil industries. These harsh environments were perfect testing conditions to prove that it would be an ideal material for manufacturing street furniture that would withstand the rigours of the urban environment.

The durability of Ferrocast makes it the preferred material when manufacturing posts and rails for developments such as the Leeds flood alleviation scheme. It has a long, maintenance-free lifespan and helps to reduce ongoing maintenance costs.

Environmental benefits

• Ferrocast is produced in North Shields on Tyneside, a two-hour drive away as opposed to importing cast-iron fence posts from countries such as China, reducing the transportation CO2e from 12t to 0.5t – a reduction of 95%
• BMMJV, Arup and Leeds City Council were co-located in Arup’s Leeds office next to the site for four months to finalise the design and identify sustainability offerings
• Marshalls was brought into the procurement process early enough to offer its expertise which helped to achieve several environmental benefits, including utilising Ferrocast
• As part of BAM Nuttall’s ‘Beyond Zero’ ethos, the health and safety benefits of placing lightweight Ferrocast posts are obvious – no machinery is required to lift the posts into place, they can be easily carried by hand and minimise hand and back injuries
• Ferrocast is impenetrable by water meaning it won’t rust
• A ‘warm-to-the-touch’ material providing a vital aid for the elderly and those with frailties.

Economic benefits

• The cost of Ferrocast is approximately 10% cheaper than cast iron
• Because the Ferrocast is lighter, it can be installed by hand, saving cost on machinery and equipment hire
• The polyurethane mix is solid throughout meaning the posts are not affected by chips and scrapes, further reducing maintenance costs and adding to the whole-life cost reduction
• The posts have no residual value and, therefore, are not subjected to potential theft
• Ferrocast resin is bonded to a 50mm mild steel core which only requires a 75mm core cut into a concrete wall, whereas a cast-iron post needs a 150mm core, saving on coring and grouting costs.

Quality benefits

• The posts have movable connections and can, therefore, take out slight imperfections in alignment on site to tie into the flood walls
• Marshalls is based in Halifax, West Yorkshire, and manufactures on Tyneside, so it was easier to accommodate changes or order additional posts – the production and delivery cycle is only six weeks in the UK compared to 20 weeks from the Far East
• Replacement posts can be sourced quickly
• Emblems or logos can be cast into the Ferrocast mix to enable personalisation for Leeds City Council.

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