Question: *Star question* What are some examples of more sustainable methods for reducing flood risk, and how do they work? - Arran C, live chat

Josh Wolstenholme answered on 16 Jun 2020:

Good question from Arran!

There are lots of different methods of reducing flood risk, but what do we mean by sustainable in this context? It could be ensuring that healthy river habitats are maintained or improved upon, built from ethically sourced materials and reducing the consumption of finite resources or emissions used to produce the management methods.

I think that the most sustainable is improving soil health and regenerating moorland/peat bogs. First, soil health. Healthy soil contains capillaries/pockets where water can be stored, when rain hits the ground, it is able to infiltrate through the soil and seep into the ground, being transported to a nearby stream or river via throughflow or groundwater. When you get heavy machinery on this ground, the air bubbles are compressed, and they can hold less water – the ground gets more compacted. This means that the soil can hold less water as there’s less space! At the surface, when the soil has reached its maximum capacity, water will runoff as overland flow, carrying soil and sediment with it into the river. This is bad for flood management as it increases the amount of water travelling to the output of the catchment in a given time.

Moorland and peat bogs can also store a huge amount of water, but again, there is poor soil health and the land is often used for grouse shooting and it would be a loss to the owner to regenerate it (in terms of income).

These are more indirect methods, where they try to reduce the risk before the flood. There are also more direct methods like working with natural processes or natural flood management that work holistically in small (<10km2) catchments for flashy small-medium storms. These can be found in the river as 'leaky dams' which are supposed to activate when the river gets to a certain height, storing the water in the upper catchment and slowing the flow downstream. Rivers can also be diverted to overflow sites which will store the water during times of higher flow when it has rained, working on the same principle as slowing the flow. We can also plant more trees to increase the roughness of the surface of the floodplain, reducing the overall speed that the water is travelling. By making the landscape more complex and varied, the easiest path for water to flow down becomes more difficult.

All of these methods would be classed as more sustainable as they use the landscape to their advantage to reduce cost, improve heterogeneity of the land and emulate natural processes rather than building huge concrete walls. Having said that, there is a time and a place for huge concrete walls, especially when you want to protect important land such as homes and power plants.