Floodplains and river wiggling
As summer drew to a close, the Tame Valley Wetlands Team (TVW) climbed out of the river Blythe and hung up their waders for another year. The summer found the team restoring habitats and natural river processes from Hampton in Arden to where the Blythe joins the river Tame, northeast of Coleshill.
This year was all systems go to complete most of the river-based works for the Blythe Alive Again project. In the background permits, designs, maps, machinery and materials have all been organised. Landowners, farmers, regulators and cows have all been consulted, culminating in a busy summer across a 4km2 mosaic of grasslands, woodlands and wetlands.
The river Blythe flows through a wide and low-lying floodplain and historically would have spilled out during rainfall events. Past techniques of straightening, dredging and removing woody debris resulted in the river becoming entrenched in its channel, no longer able to spill out. At the time, this was believed to be the best approach, to get the water away as fast as possible. While these old techniques might have solved a problem of flooding in the immediate locality, it then increased the burden of water downstream. More recent understanding of river dynamics has developed methods to help to slow the water down, reducing extreme flooding overall. A river that is wiggly, therefore longer, and connected to the floodplain has a larger storage capacity.
Picture a straight river channel with other straight rivers joining to it. When there is high rainfall, all these channels will flow fast, and the water is carried downstream all at once. This is called a peak flow and often results in flooding. In a slower flowing system, with bends, floodplain connectivity, and habitats designed to help store the water, the peak is much lower, and the water takes more time to flow through.
The knock-on effects of the old techniques created a dramatic change in the landscape. Wet woodland and grassland became scarce with fewer areas of open water. A reduction in habitat resulted in huge biodiversity losses, contributing to the UK being one of the most nature-depleted countries in Europe.
River restoration work aims to repair this historical damage, meaning otters, brown trout, willow tit and numerous other species benefit from a more natural environment.
Across the Blythe Alive Again project area, over 20 new scrapes (shallow pools) have been created. Differing from a pond, a scrape might not contain water all year round. They often have muddy edges and, while they may not be obviously wet in summer, they are usually still a little damp. Left to vegetate naturally they are an important habitat often supporting plant species lost from the wider landscape. All of this means there is a great opportunity for invertebrates to breed through the spring, a superb food source for young birds such as lapwing and curlew, which are often associated with our farmlands but are becoming more and more scarce.
As we’ve learnt, the river needs some help to be able to spill out of its channel. The simple answer is lowering the banks. Naturally, a river would create this kind of feature by eroding the banks leaving an undercut. When this earth falls in, the result is a lowered riverbank and a narrowing of the river channel. The TVW team have copied this process to help speed up the recovery of the river.
'In-channel' work such as creating berms, woody debris and flow deflectors all help to mimic natural river processes, where trees and branches naturally wedge in the riverbed. These in-channel restrictions create areas of fast and slow water. Fast water helps to clean the riverbed, vital to help species breed and to improve oxygen content. Slower areas are where the silt drops, creating sediment for plants. A healthy river system has a mixture of flow types, and this is what we try to replicate through our river restoration work.
It is important to understand these features have been designed to help the river connect to the floodplain. This means there is localised flooding of surrounding fields. Working in partnership with landowners and farmers is a vital part of this work to ensure flooding is in appropriate locations and not negatively impacting on the farmed landscape. Designs are computer-modelled to ensure that there is no impact on any nearby properties.
Collaborative working to carry out our river restoration work ensures the river and its habitats for wildlife are repaired and protected in a way that is sustainable for the future.
Simon Watts
River restoration by Jacob Loughran