In 1990, when the photographer Gideon Mendel left his native South Africa for London, he deposited a number of boxes in his friend’s garage in Johannesburg for safekeeping. These boxes contained, among other things, negatives and transparencies from Mendel’s harrowing first few years as a photojournalist during the struggle against apartheid in the mid-80s.

Over the next three decades, Mendel became renowned for his intimate, socially engaged photography, documenting the effects of the HIV/Aids crisis and climate breakdown. For one project, from his Drowning World series, he has gathered nearly 2,000 water-damaged family photographs picked up on his journeys through flooded communities in the US, India and elsewhere.

Three years ago, Mendel got a call about water damage to some of his own work. “My friend in Johannesburg got in touch and said: ‘We’ve still got your boxes but I’m a bit worried about the condition – one of the boxes has been rained on.’” When Mendel retrieved the box, his guilt at neglecting his archive turned to excitement. “I spent a lot of time carefully separating [the negatives and transparencies] and getting the images scanned,” he says, “and some of them turned out to be very interesting.”

In this image, taken at a mass funeral in Duduza township outside Johannesburg in 1985, the damage forms a frame within a frame, cropping the image to highlight the protester. The effect is “amazing”, says Mendel, even if the context was sobering: the young activists being buried were killed when an undercover agent gave eight people booby-trapped hand-grenades, which exploded when the pins were pulled. Funerals such as this often led to further outbreaks of violence, and more funerals.

“The original images have an almost radioactive energy from that period; they carry all the weight of history,” says Mendel. “And then you have this new layer, the effects of water and time and mould on the images” – they speak to Mendel of the deterioration of memory, and the fading of hope in post-apartheid South Africa – “which creates a much more complex and nuanced statement than the photograph can make on its own. It takes the image into a whole new dimension.”

Freedom Or Death by Gideon Mendel is published by GOST (£35)