Sliding Flood Gate for the Stephenson Street Flood Scheme

MM Engineering designed and manufactured the sliding flood gate across Corporation Road on the Stephenson Street flood scheme in Newport, delivered by Natural Resources Wales (NRW). The gate is 1.7 m high with a 9 m span. The road carries a high volume of heavy goods vehicles and abnormal vehicles, and the gate was designed for deployment by a single operative in high winds. It is manually operated, and it was flood tested on site.

The scheme

Stephenson Street is a £25m scheme, funded by the Welsh Government and delivered by NRW, that reduces tidal flood risk in Lliswerry on the east bank of the River Usk. Alun Griffiths (Contractors) Ltd built it and Arup designed it. Arup describes it as NRW's largest and most complex flood scheme to date.

The scheme provides about 2 km of raised defences, made up of flood bund, sheet pile wall and reinforced concrete stem walls. Its 700 m of sheet pile wall was installed to a depth of 10 m by silent press-piling. That kept noise and vibration down beside the River Usk, a Special Area of Conservation whose features include shad, a fish sensitive to vibration.

The defences raise protection to a 1 in 200-year standard, meaning a flood with a 0.5% chance of happening in any year, with allowances for sea level rise and the predicted increase in rainfall over the next 50 years. NRW says the scheme reduces flood risk to more than 2,000 homes and businesses. That figure includes climate change. The scheme's case study puts the present-day risk at more than 190 homes and 620 businesses, rising to 1,120 homes and 1,020 businesses by 2070.

Why Corporation Road needed a gate

Corporation Road, the access to industrial premises, passes beneath a railway overbridge on the line of the new defences. Left open, it would give flood water a flow path under the bridge. A wall would close the road, so the scheme closes the gap with a flood gate that stays open for traffic. The gate and its wing walls tie the defence into existing high ground. The gate is closed for maintenance and ahead of anticipated high tides. While it is shut, 0.7 km of new flood relief road between East Bank Road and Corporation Road provides alternative access and emergency egress.

Designing a 9 m gate for a working road

The scheme's published case study records what the gate was designed for:

• a 9 m span at a height of 1.7 m
• a road with a high volume of heavy goods vehicles and abnormal vehicles
• deployment by a single operative, including in high winds

Sliding gates are used on wide openings where a swing or lift gate is not practical. A 9 m swing leaf would sweep a quarter circle 9 m in radius each time it opened or closed, while a sliding leaf travels along the line of the defence and is stored to one side. Size also sets the wind load. The leaf is 10.3 m long and 1.75 m high, so it overlaps the 8.72 m clear opening at both ends. That is about 18 m² of steel for the wind to act on while one operative moves it.

How the gate works

The Corporation Road gate is a single steel leaf that lifts, slides and seals. To move it, the operator first raises the leaf onto four castors with a hand pump. Two hand winches, one to open and one to close, then pull it along a stainless steel guide rail. Once it is in place, the leaf is lowered onto the stainless steel threshold, compressing the base seal.

The leaf weighs almost 2.5 tonnes, but its calculated rolling resistance is about 30 N, roughly the weight of a 3 kg bag. That, with the hand pump and the winches, is what makes a manually operated 9 m gate practical for one person. MM Engineering's standard sliding gate is manually operated up to 6 m wide and 1 m high; larger gates are usually automated.

In a flood, the water presses the leaf against sealing posts fixed to both wing walls, so the flood itself pushes the ends of the leaf onto their seals. The leaf is a matrix of 100 mm S355 steel hollow sections, fully welded between two steel skins. The threshold, guide rail and sealing blades are stainless steel, and the steelwork is blast cleaned and painted to a C5 system for the tidal setting. The gate is designed for a 60-year life.

What the gate puts on the walls

At the design flood, water stands about 1.7 m deep against the closed gate. The calculations take it to 1.725 m, 25 mm above the gate's 1.7 m flood protection height. That is about 15 kN on every metre of the gate's width (unfactored), or roughly 130 kN across the opening, acting about 0.6 m above the threshold.

The wing walls either side were designed to take an even share of that load, and MM Engineering detailed the gate to suit. The leaf seats against sealing posts fixed to both walls, and packing pieces on its dry side limit its movement under load.

The leaf's weight takes a different route. A hinged leaf hangs its weight off its hinge post whenever it is open or moving, and an open leaf catches the wind like a sail. A sliding leaf has no hinges. Its weight goes straight down through its castors and base into the threshold, so the wing walls take the water load but not the leaf's weight. When it is open, the leaf parks alongside the defence between guide posts, which take the wind.

The trade-off is the threshold. It carries the leaf in both positions, so it runs 20 m for an 8.72 m opening, and the part across the carriageway carries the road's heavy goods traffic. The concrete sill beam beneath it was designed for a five-axle articulated lorry with a maximum axle load of 100 kN.

Result

Construction of the 2 km of defences began in February 2023 and finished in spring 2025. The scheme was officially opened in October 2025. It received the Bill Ward Sustainability Award at the 2025 ICE Wales Cymru Awards.

The scheme's case study, written by the project team from NRW, Arcadis and Arup and published by Water Projects Online in July 2025, states: "The gate was manufactured by local Port Talbot-based firm MM Engineering." Its supply chain list credits the design and manufacture of the 9 m floodgate to MM Engineering, and floodgate design assistance to Pebble Engineering Ltd.

MM Engineering sizes each gate from the clear opening, the flood level and the loadings it must carry, traffic included. It also supplied flood gates, flood glazing and flood barriers for the Belfast Tidal Flood Alleviation Scheme. See our sliding flood gates, or talk to our engineering team about a wide opening on your site.