Hi everyone, I am Kieran and I have just become an Ambassador for the Rivers Trust. Influenced by some of my day-to-day work as a graduate scientist at BMT, I have decided to write my first blog about flood management, in particular natural flood management (NFM). I hope you enjoy the read!
Natural Flood management
NFM attempts to restore or emulate the natural functions of catchments. Such natural processes have been shown to compliment traditional engineering measures. Modelling results commissioned by the Rivers Trust in 2017 demonstrated that adding a range of NFM measures showed a significant reduction in flood peaks on large rivers may be reduced.
Natural flood management is generally focused on slowing down the flow of water through a catchment, through four underlying mechanisms:
- Increasing storage: creating temporary storage areas which will fill up with water during a flood event and empty slowly.
- Increasing catchment and channel roughness: this ‘slows the flow’ by increasing the resistance to surface and in-channel water flow.
- Increasing losses: increasing the amount of water that drains (infiltrates) into the ground or is lost back into the atmosphere via evapo-transpiration.
- De- synchronising peak flows from tributaries: Slowing down one tributary compared to another, so they do not peak at the same time, can significantly reduce flood peaks downstream.
Whole catchment flood management
As with all flood management it is important to understand how the changes you make will affect the surrounding areas. The last thing you want to do when you protect one area is to increase damage in somewhere else. Therefore, it is important to think about the whole system.
The diagram that I painted below can hopefully help visualise some of these flood management processes. I originally drew this figure was to conceptualise how the benefit of NFM techniques can be assessed whole catchment models, relating to some of the work I do at BMT. I have added numbers to the diagram which relate to examples of natural flood management methods described below. I find it interesting to think about how each technique is trying to apply some of the four mechanisms for slowing down flow I listed in the previous section.
- Log (leaky) dams, with woody debris in channels helps create attenuation ponds to help reduce flood peaks downstream.
- Remeandering of the stream increases the distance the water has to travel, which helps to slow the flow of water.
- Reforestation, means there are more trees that intercept rainfall, to increase evapotranspiration.
- Floodplain reconnection to increase storage of floodwaters in natural areas where flooding will not cause damage and could benefit local wildlife by enhancing habitats.
Although this list ends here, there are plenty of other examples of NFM such as wetlands, ponds or basins to hold excess water. As well as this you can adopt NFM methods in coastal environments including salt marshes, sand dune management and beach nourishment.
Benefits of NFM
A great example of the effectiveness of NFM comes from my colleague Duncan Kitt’s PhD research into a river restoration scheme in the New Forest. This scheme applied floodplain reconnection, re-meandering and log dams which resulted in a 21% reduction of flood peak magnitude. In addition to the main goal of reducing flooding, NFM often also offers a wide range of benefits. This can include improved water quality, habitat creation which can help boost biodiversity as well as trees capturing carbon - making our local areas nicer places for everyone to enjoy!
Another benefit of some NFM methods, as long as there has been careful planning to fully understand the impacts, is that local communities can get involved. The BBC Panorama documentary "Britain’s Wild Weather" showed an inspiring community that built ‘leaky dams’ to slow down flow entering the Calder Valley. It would be great if we saw more of this in the future.
Overall, flood management needs to be innovative and appropriate for each catchment and area. This means a combination of different types of flood defences are likely to have the greatest benefit and avoid increasing damage elsewhere. I believe the wider range of possible benefits from nature-based flood protection means we should be looking to apply it as much as possible.