'Car Bank' Removal Delivers Positive Change

February 25, 2021

During the Pearls in Peril project, one of the most eye-catching actions was the removal of 20 cars, plus rubble and timber, being used as a flood wall on the River Dee upstream of Braemar. New research shows that removing this barrier and reconnecting the river with the floodplain has caused many positive changes.

In 1984 the cars had been buried, then protected with corrugated iron sheeting, as a form of flood defence. Aside from the environmental impact of having these cars disintegrate into the landscape, the flood wall stopped the river from meandering which meant the in-river habitat was also poor quality. A straight section of river with no overhanging bankside vegetation caused swift water flows that reduced the complexity of the substrate leaving little area for fish or invertebrates to hunt or shelter. The floodwall also blocked the free connection between the river, a side-channel and the floodplain possibly reducing the floodplain’s ability to reduce peak flow events downstream.

In 2015 the cars were removed, and the bank lowered and reprofiled, with the aim of allowing the river to overspill into the floodplain and backwater side channel. Scientists from the James Hutton Institute measured habitat and floodplain water levels before, and three years after, the restoration to investigate the changes that took place – which were considerable! Not least because, within months of the work finishing, Storm Frank came along and caused large changes to the river shape.

Firstly, the river was able to overspill at a lower water height than before restoration and saturate the floodplain and side channel which may help lessen high flow events downstream (though it will take multiple sites to mitigate the effects of big flood events).

Next, a functioning floodplain stores water which helps in periods of high flow but can also reduce the impact of low flows in dry periods. In the years following the restoration work, this area was affected both by Storm Frank as well as low flows in the hot weather of 2018. Encouragingly, the water table was higher than before the restoration and indicates that by reconnecting the floodplain to the river, it retained more water in times of low flow.

Finally, by allowing the river to reshape itself, gravel deposition caused the river bed to rise by 1m in some places! By increasing the variability of substrate, more habitats should become available to fish and river life and we will be looking forward to seeing whether more fish are found in this area over the next few years.

An exciting aspect of this work is that its success comes from a combination of manually removing barriers and naturally occurring peak flow events. By enabling the river to overspill more easily, the flood events performed the heavy lifting of rebuilding habitat and reconnecting floodplains.

Re-connecting the river to historically blocked channels is one of the approaches we are taking to improve the habitat quality and function of the river. Unconstrained rivers that are allowed to overspill – in the right places – slow the river flow down and this helps to maintain a greater diversity of river bed substrates. This results in a range of habitats that can be used by river life all the way from algae up to fish.

Simply put, a greater variety of habitats should lead to more food that can feed more fish which ultimately gives our smolts the greatest chance of returning as adult fish.

It is very exciting to have our actions supported by peer-reviewed scientific research and we hope that this will help us to complete more work like this along the river.

Read all about the impacts in a paper published in the journal, Science of the Total Environment here: https://authors.elsevier.com/a/1cUTMB8ccoS24 (free access until 19/03/21)