Making Space for Water

Following the devastating flooding in the Derwent and Amber valleys in Derbyshire, there needs to be a focus on how we as a society can work with the natural world to identify solutions to the problems we face.

Following the devastating flooding in the Derwent and Amber valleys in Derbyshire, here are some thoughts on how we as a society can work with the natural world to identify solutions to the problems we face.  

People regularly talk about building on flood plains being a bad idea.  This is true for the people who buy the houses or businesses concerned.  However, it’s more complicated than that.  Rainfall is not distributed evenly across a catchment.  You get much more rainfall in the uplands than the lowlands, so for Derbyshire, that’s on the hills of the Peak District and Peak fringe.  But all of this water flows downhill. Once the rainfall has happened, it’s in a closed system, being removed by rivers and streams.  Realistically, once water is in a river, it’s very difficult to control.  The only control you have is to hold it back completely, using dams (which have no real relevance in managing flooding), or holding it into the river, using flood defences.  And thinking back to the closed system, this water will go somewhere, and that somewhere is downstream.  So flood defences, which may protect an “asset” (community, factory, road, power station, farmland) will stop water leaving a river, increasing the volume of water in the river and making flooding more likely downstream. 

The Environment Agency base their decision making on how high to build them by looking at the percentage likelihood of flooding, and describe it as the chances of it happening in any given time period.  So, a 1 in 100 year flood just means that there is a 1% chance of it happening.  Summarising hard flood defences, which do work in protecting our towns and cities, they only work to the point that they were designed for (a 1 in 50 year flood, for example) , at which point they stop working and you get “over-topping”. 

Climate Change predictions (which are being borne out in reality) are that we will get more intense rainfall, meaning that 1 in 100 year events will happen much more regularly.  So, in Derbyshire in 2019, we have had more rainfall than average, but on fewer days, meaning that rainfall events are more intense, making over-topping more likely.  Matlock is viewed by the EA as a 1 in 100 flood risk, but the town centre has flooded at least 5 times in the last 60 years.

 

Flood risk managers are looking at two key characteristics on a graph relating rainfall to the likelihood of flooding.  The first is the peak flow, which is the highest level that the water reaches and the second is the lag time, which is the gap between the rain falling and when it all reaches a certain point in the river network.  

 

And this is where managing water within the landscape comes in, and the best from a wildlife point of view is using Natural Flood Management. Every decision that people take within the landscape changes the chances of flooding downstream, particularly when multiplied by millions of people and thousands of land managers. Keeping water in the landscape, works generally in two ways:

  1. Making space for water (creating volume for water to sit in  and reducing peak flow);
  2. Slowing the flow (lengthening the lag time between rainfall and peak river flows).

 

The simplest one of these two is Making Space for Water.  This can be about creating ponds that hold water temporarily and the work being done in the uplands by the Moors for the Future Partnership and Yorkshire Peat Partnership, to block gullies, is about making space for water, physically stopping water from leaving the hill.  Tens of thousands of small dams have been built across the Pennines.  In the lowlands you can physically construct dams and allow floodplains to reconnect with rivers, flooding farmland and allowing water back into rivers when the danger of flooding has passed. 

 

But it’s not all about big places to hold water.  Possibly the most important space for water is virtually invisible, in soil. In the uplands, peat should be about 97% water.  The restoration work being done there by the Moors for the Future Partnership to rewet the peat does store water within the peat and is critically important.  But this is permanent holding of water and bogs in pristine condition do not have much capacity to store much water at periods of high flood risk, (in Derbyshire, blanket bogs are not in good condition yet and so there is still capacity for them to hold water).  Flood risk reduction is primarily about temporary storage of water and lowland soils do that very well.

Flooding

Apologies, but let’s talk numbers on it. 

Normal soils can hold about 25% of their volume as water. So, a cubic metre of soil can hold about 250 litres of water. 

A soil in good condition, with pores of different sizes and plenty of organic matter (lots of earthworms!) holds water in the top 30cm.  This means that to hold its 250 litres of water, you would need to have about 3.33 square metres of ground.

A soil in typical condition, with very little organic matter, compacted by heavy farm machinery and livestock, will hold water in the top 10cm.  So to hold your 250 litres of water, you need to have 10 square metres of ground. 

A hard surface, such as a building roof, road or car park holds no water within it at all.  It will hold water on it, until it either flows away (into drains or rivers) or evaporates. (Generally, this holding of water is called surface-water flooding).

Note: Soil infiltration only really works when the surface is covered in vegetation; bare soils wash off into rivers.  This reduces the amount of good quality soil to hold water and reduces the capacity of rivers to hold water.  Dredging, often spoken about as a cure for flooding, is another symptom of the problem of managing soil and land.

When you extrapolate this to field level, a 25 hectare field in good condition can hold about 18, 750,000 litres of water (or over 18,750 cubic metres).  A field in typical condition could hold about 6250 cubic metres.  And a 25 hectare field, converted into a housing estate of 400 houses, with roads, parking etc, would store very little rainfall, moving it into the water network very quickly.  This is an issue as the water will flow much more rapidly within the drainage system than it does as overland flow on fields, meaning that it enters the river much more quickly, significantly reducing the lag time.  And this also shows why “don’t build in floodplains” is too simplistic.  Because this water moves more quickly into the floodplain, and has restricted places to go, the water level rises, effectively making the floodplain larger, bringing houses into it that weren’t before.  

Flooding

(C) Scott Petrek

The speed that water enters rivers in the landscape is called slowing the flow.  Increasing the roughness of the surface slows the flow of water into rivers, however you do it.  Increasing surface roughness also improves infiltration rates, allowing more time for water to percolate into the soil.  

Trees provide hundreds of layers through their leaves, intercepting water and increasing the time it takes for it to reach the ground.  And if they are in a wood, their trunks divert water flowing down a hillside.  But it needn’t be trees.  Hedgerows and drystone walls both slow the flow.  Short grass is better than tarmac, long grass is better than short grass, scrub, bracken, brambles are all great at slowing the flow of water.  And the best plant of all is a tiny plant that lives at the top of the Derwent catchment, in blanket bogs, sphagnum moss.  This is over 97% water and has a huge amount of space between its stems, storing lots and lots of water.  You can tell where it has grown in the past, as it forms peat because of the water-logged conditions.  It is being reintroduced to many upland catchments by lots of organisations.  (There’s lots of information about sphagnum on the Moors for the Future website www.moorsforthefuture.org.uk/sphagnum

sphagnum moss

(C) Mark Hamblin/2020VISION

So why is this good for nature?  Most of the things that we can do to make space for water, also make space for nature. A big unmanaged hedgerow, great for thrushes and species like dormice, intercepts rainfall and improves infiltration.  Tidiness, devastating for wildlife, also reduces lag time and increases peak flow.      Rewilding has great potential to help with this.  Wetlands throughout the catchment store masses of water, and if they are managed by beavers...  Allowing nature to control regeneration improves soil health and increases surface roughness; grass is longer, trees and scrub plant themselves randomly rather than in rows and at a whole range of heights and ages.  Habitat complexity both slows the flow and provides more space for nature.  

We know that hard flood defences are critical, and they worked really well in Matlock itself, but reducing the height of floodwaters by a couple of centimetres will stop homes flooding and septic tanks overflowing and landscape flood management can do that. It’s time for policy makers to focus on the benefits that working with nature and the natural world can have.