Under Salt Marsh’s production designer explains how to flood a set

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The Sky Atlantic series, Under Salt Marsh, takes place in the made-up Welsh town of Morfa Halen. As a massive storm approaches, it disrupts the investigation of a disturbing crime.

Jackie Ellis, a former detective now teaching, is shocked to find the body of her eight-year-old student, Cefin, who seems to have drowned. The case is being investigated by Detective Eric Bull, Jackie’s ex-partner, but their relationship is strained. They haven’t spoken in three years, ever since Bull suspected Jackie’s sister in the disappearance of Jackie’s niece, Nessa.

Okay, so things are really tense – we need to figure out what happened to Cefin, and fast! There’s this massive storm that’s been building up throughout the whole show, and it’s about to hit, potentially washing away any clues at the crime scene and just causing total mayhem. The team could’ve used those quick pop-up water tanks you sometimes see on set, but production designer Stevie Herbert and her crew went all out and built their own huge tank – it had to be seriously strong to handle everything they were doing!

Building a tank was incredibly stressful, as she told TopMob. It’s not something people do every day, and it presented a lot of challenges. Obviously, you can’t actually flood real towns in Wales for filming and risk causing damage, so they constructed an outdoor set – a replica of the main shopping area in Morfa Halen – on the backlot of Dragon Studios in Bridgend.

The tank measured 50 by 50 meters and had steel walls standing 1.2 meters high. Workers sealed it using wood, glue, and expanding foam, and then covered the exterior with tarmac for an extra watertight barrier.

Filming the flood scenes took a full week. According to a source, Dragon Studios handled all the logistics of bringing in the water, which arrived in massive tanks and was pumped onto the set using large hoses. The water level was carefully controlled – rising from as little as 20 centimeters to almost a meter – and filming was scheduled around these changes, as well as complex stunt work involving vehicles and actors. Managing the sheer weight of the water required significant preparation, including reinforcing structures and preventing leaks.

The filmmakers built a rain system that covered the entire set. To simulate realistic flooding, they used pumps to create a current in the water tank, making debris and objects move around. They also constructed rooms that could slowly flood. These rooms were built on hydraulic lifts with metal mesh floors, allowing water to rise gradually and fill the space.

As a huge fan of the show, I always knew creating the underwater scenes would be a massive challenge! It’s surprisingly tricky working with water, especially when it’s deep – apparently, things either sink to the bottom or become these unexpected obstacles on the set floor. I heard they actually had the set decorators wearing full waders alongside the props team, and they even had to work with professional dive teams to get everything right!

We’re able to make things appear to float, even if they’d normally sink, using lightweight items like plastic chairs, bowls, and pots. We carefully removed anything that could cause issues or break, and took apart any parts with moving pieces. To create a realistic, weathered look, we’re using spray paint to simulate mud and dirt.

The materials were submerged in water for a week to ensure they didn’t release any harmful substances. According to Herbert, the water was then carefully released back into the regular water supply. They made sure everything was safe by using clean steel, natural building materials like wood and plywood for the walls, and only water-based paints.

The fifth episode of Under Salt Marsh sees the town of Morfa Halen cut off by rapidly rising floodwaters, and a new murder discovered. As people try to escape to safety, the killer remains unknown. The season culminates in a powerful storm, and it’s a race against time to catch the murderer before it’s too late.

You know, as a movie fan, I always appreciate when filmmakers really nail the natural world. Herbert – a visual effects guy, I gather – was saying it’s incredibly tough to make things like storms and floods look real. He explained the biggest hurdle wasn’t just showing the chaos, but making the audience feel it – the fear, but also the sense of what everyone involved could lose. It’s about emotional impact as much as visual spectacle, and that’s what makes it truly believable.

The final episode of Under Salt Marsh will premiere on February 27 on Sky Atlantic and NOW at 9pm.

Don’t forget to add ‘Add Under Salt Marsh’ to your watchlist on the TopMob app! Download TopMob now to get daily TV recommendations, exclusive features, and more.

Authors

Laura RutkowskiCommissioning Editor

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2026-02-27 21:35