Soils: Their Forgotten Role in Natural Flood Management
Yog Watkins is Senior Land Officer and Farming Advisor at the Westcountry Rivers Trust. He engages and delivers advice to farmers through one-to-one visits, development of whole farm plans and running workshops; and presents to wider stakeholders including private companies, community interest groups and students.
There is a growing interest in the use of natural flood management to slow the flow of water through river catchments. Yog Watkins calls for more attention to be paid to the importance of soils in holding back the water, and reports on a preliminary trial of a soil conditioner that may offer some help.
For several years now we have seen a gradual shift away from the unsustainable but long-established paradigm of eat more, buy more, produce more. Whilst this has been a very slow process, there have been several catalysts in the UK that have given sustainable land management a boost in momentum. It’s unfortunate that it takes an ‘act of God’ or other catastrophic event to generate this momentum, but so long as it’s sufficient to influence Government and those holding the purse strings, so be it.
The floods at Tewkesbury (2007), on the Somerset Levels and along the Thames (2013/14), and in Cumbria (2015) have brought catchment hydrology to the fore. Governments have been forced to confront the issues involved, gain an understanding of the causes, and work on future mitigation. Natural flood management isn’t a new concept but it has leapt up the priority list, and is now being used in many areas to ‘slow the flow’ and recreate a more naturally functioning catchment. Essentially, natural flood management works with the landscape to reduce the rate at which rainwater reaches the wider river system by holding water in the landscape for longer, using natural features and processes as well as mitigation techniques such as re-meandering and leaky dams.
Frequently a bottom-up movement for change is more effective and sustainable than a top-down approach, and within natural flood management an often-cited example is Pont Bren. Ten farmers from upland mid-Wales came together to look at alternatives to their existing farming model, which was both financially and environmentally inefficient. Their high input, intensive methods were not offering the returns they were looking for, and they needed to work more with their landscape and stop fighting against their geography. One of the more fundamental shifts was to introduce more trees back into the landscape via hedges and small copses. Whilst their initial intention was to offer shelter for livestock, studies showed that their actions not only served this purpose well but also offered additional benefits such as the interception of water run-off within the catchment, leading to reductions in the peak flows of the local watercourses. Whilst I have not been party to the specifics of the soil type, soil condition and underlying geology of Pont Bren, it does seem that compaction of the soil is likely to have played a part in the run-off that had been occurring, and the role of soils has been overlooked.
We very often overlook the most important part of a process as we attempt to tackle the easier options first. For example, much time and energy has been devoted to the ‘recycle’ element of ‘reduce-reuse-recycle’, while the more effective and useful ‘reduce’ and ‘re-use’ have been inadvertently bypassed. Similarly, soils play a pivotal role in catchment hydrology, yet the more obvious soft engineered solutions are often looked at first, therefore missing a valuable opportunity. Soil condition should always be the first option to be investigated. While there will be sections of landscapes that offer few opportunities for flood attenuation due to their soil type and geology, in most cases well-structured and functioning soils will deliver a greater effect upon flood attenuation than soft engineering solutions such as leaky dams. These more conventional natural flood management options may simply be applying a sticking plaster to a much larger problem.
Well-structured soils are a classic ‘win-win’ scenario for the environment and farmer. Landowners will see improved yields, greater windows of opportunity for cultivations, and increased grazing time at the beginning and end of the year. Local watercourses will see reduced peak flows in the hydrograph and more consistent flows through prolonged dry spells, and there will be greater aquifer replenishment, all of which would benefit river ecology. Whilst well-structured sandy soils have a much greater absorption capacity than heavy clays, whatever the soil’s texture and topsoil depth a good soil structure will allow swifter downward movement of water through its profile, as opposed to the increased surface water flow in poorly structured soils.
Soils are often described as saturated, when in fact they are not. If you are able to investigate the whole of the soil profile you will often find layers of compaction that inhibit the flow of water through the profile. If a layer of compaction sits at around 10 cm, but above a further 30cm of well-structured topsoil, the operational capacity will only be the top 10cm instead of the full 40cm. In periods of heavy rain this topmost layer can become saturated and cause overland surface flow, whereas in the same rain event, but with the compaction removed, the soil’s capacity would be significantly better and so it would take much longer for the full soil profile to become saturated and cause overland surface flow. Assuming the soil has a 25% air capacity (for the full 40cm profile) it would be capable of temporarily storing 10cm of rainfall. The same principle applies at whatever depth the compaction is found, even right to the surface. It is not unusual (especially on light sandy soils) for a bare soil surface to become ‘capped’ (or ‘slaked’), which effectively makes the soil behave like an impermeable surface, and overland surface flow can start very soon after the rainfall starts.
There shouldn’t be a one-size-fits-all approach to catchment management and flood attenuation. Natural flood management is a combination of measures that work together to mitigate against high flow-rates, but the functioning of the catchment’s soils should always be addressed first. In some cases, the natural water-holding capacity of the soil will be negligible, so other options should then be utilised. However, there will be many situations where good soil structure can make a significant difference, and measures should be taken to improve its function, thus avoiding the need for additional and unnecessary interventions.
Soil organic matter plays a major role in soil health and hydrology. A soil that has a high porosity can have inherently low organic matter levels, but reducing these levels further through land management (overworking, continuous cropping etc.) can increase the risk of erosion and reduce its porosity and nutrient cycling. Soil organic matter helps a soil to process nutrients, moving them from the unavailable state to a plant-available form, whilst also giving the soil greater structural stability. This stability allows water to flow through the profile and reduces the risk of slumping or capping- both of which can increase surface run-off and influence flooding - along with the additional benefits to the land owner.
Capping of soils effectively means that their capacity to hold water is completely bypassed. A 5ha field that is capped but has 30cm of porous topsoil, would behave significantly differently to an uncapped field during a two-hour, 50mm rain event. If the field were completely capped then this field would generate 900,000l of surface water. If the soil were healthy and functioning (without the cap) this field would be able to absorb the water and allow it to transition slowly to a watercourse through sub-surface flow, or recharge the local aquifer.
Even where this water travels through shallow sub-surface flows, the delay in reaching the watercourse can be crucial in attenuating the flood peak and flattening out the hydrograph. It is important to note though that not all delays can be helpful. Attenuating sub-catchment flows in isolation has the potential to cause problems at particular points within the wider catchment. For example, flood water attenuated for longer periods in a sub-catchment downstream of un-attenuated catchments can mean that the flood waters reach the pinch points of confluences etc. at times that exacerbate the flows at these confluences and downstream. This is always a risk and modelling of the whole catchment can be important to assess this risk.
Novel approaches to improved soil management need exploration to assess their efficacy in improving soil condition (see Soil Conditioning Experiment details, below). Also, the approach to soils needs to be the same as that used for dealing with diffuse water pollution from agriculture. Essentially, this is the gradual amalgamation of many small pollution incidents towards a level of pollution that has a detrimental effect upon water quality downstream. Addressing the individual small incidents contributes to the improvement of water quality in the catchment. Soils and their impact on hydrology are very similar: every field, or part field, that has an improved soil structure will have a positive influence on water flow. At an individual level this may be minimal, but collectively at a catchment scale this can lead to significant reductions in volumes of rainwater reaching the watercourses shortly after rainfall, lowering the risk of flooding and making the landscape more resilient.
In the spring of 2017, the government released £15 million to trial and demonstrate the impacts of natural flood management. This money is very welcome, and the projects being funded should be thoroughly investigated to ascertain best practice and the long-term sustainability of the options being used. Natural flood management is a multi-faceted tool capable of bringing significant benefits, but we need to make sure that we work through our options for delivery in a logical order, with the most appropriate tools used in the most suitable areas.
Farmers and landowners are under increasing pressure to make their businesses viable as we see fluctuations in the prices paid by the consumer. When you are under pressure you tend to deal with the immediate priorities in an attempt to stay afloat, whilst long term strategic and sustainable issues soon drop down the list. As soil plays such a vital role in our environment, we the consumers need to ask ourselves: should we be offering farmers financial incentives to carry out soil mitigation work? Or do we pay a fairer price for our food and give them the money to reinvest into their businesses and our landscape?
Iorwerth (Yog) Watkins, Feb 2019 -
LINKS AND FURTHER INFORMATION:
Pont Bren project - Woodland Trust
Soils and natural flood management manual
Catchment Based Approach (CaBA) partnerships have been set up all over the country to take a more holistic approach and deliver work to improve water quality, based upon their area’s specific pressures. East Devon CaBA (jointly coordinated between Westcountry Rivers Trust and Devon Wildlife Trust) has produced a manual that helps explain the importance of soils in the natural flood management toolbox. Richard Smith (author, Environment Agency) explains the dynamic relationship between soil and water, and how our diverse landscapes have varying abilities to regulate rainwaters’ transition from cloud to sea.
South West Water in collaboration with a group of regional conservation charities, including the Westcountry Rivers Trust, have initiated one of the largest and most innovative conservation projects in the UK, called the ‘Upstream Thinking Initiative’. The latest £11million programme focuses on 11 catchments across Devon and Cornwall between 2015-20; this represents an expansion of the £9 million 2010-2015 programme.
WRT’s work aims to improve raw water quality through a collaborative approach which sees landowners informed and assisted in the protection of river catchments. Tailored one-to-one advice and farm plans are supported by a capital grant scheme.
If we can determine which pressures are exerting negative impacts on our aquatic ecosystems and identify where they are coming from, then we can develop a programme of tailored and targeted catchment management interventions to remove these sources and disconnect their pollution pathways.
Soil conditioning experiment
As part of the Upstream Thinking project, a simple trial was carried out using ZEBATM, a soil conditioning product that has the potential to help minimise both leaching and soil erosion. ZEBATM is a biodegradable starch polymer added to soil in a prill form, with the ability to absorb and retain water over 400 times its original volume: it therefore has the potential to reduce leaching of soluble nutrients by keeping them close to a plant’s roots, where they can then be used when the plant requires them. In sandy soils these nutrients need to be added little and often, as over-applying can mean nutrients pass through the soil profile before they can be utilised by the growing plant. By swelling, ZEBATM can also have a direct impact on the soil’s structure; fracturing the soil surface and reducing soil slumping, which can lead to reduced drainage and further problems for the plants.
In spring 2016, after discussions with a local farmer, two soil profiles were obtained from an actively managed field, representative of the area’s sandy soils and land use. The profiles were obtained by pushing household ‘soil’ pipes through the soil profile until a 75cm core was contained within the pipe. These cores were then housed in a cold frame so that the volume of applied ‘rainfall’ could be accurately monitored. As each core was 75cm deep, any drainage from the base of these samples would be beyond the reach of the majority of plant roots and captured in vessel’s below.
The top 18cm of the soil cores were then individually extracted and inverted to replicate the action of ploughing. Prior to being returned to the pipes both samples had one teaspoon of Ammonium Nitrate fertiliser added, while sample A also received one teaspoon of ZEBATM. Once back in the pipe, ten spring barley seeds were planted into each pipe and the samples watered.
To mimic the rainfall and to avoid excessive capping of the soil surface, the ‘rainfall’ was applied using known volumes of water via a perforated tin. This allowed the slower application of the rainfall, though not so slow that the applications took as long as an average rain event of the same magnitude. For example, the addition of 20mm of rain water equivalent would take less than five minutes. Large volumes of rain water were added to fully test ZEBATM against very extreme conditions that might become more common with climate change.
One of the more interesting initial observations was the surface of the soil on both samples: that of sample A (ZEBATM) was much more broken up than sample B. Despite the use of the rain applicator, sample B’s soil surface had started to become capped, which is often the case with this soil type. Slumping of the soil was expected following the ‘ploughing’, but there was a considerable difference between the samples. Sample A had dropped on average 15mm below the edge of the pipe, whereas sample B had slumped to 25mm (a 14% reduction in the soil profile).
Once the application of rainfall had been completed the time taken for the water to be absorbed below the surface was then measured to determine the infiltration rates. Sample B took 5 minutes 3 seconds longer to absorb the 20mm than sample A. During the second test, sample A absorbed 40mm of rainfall before sample B had absorbed 20mm of rainfall. In fact, it took an additional 1min 30 seconds before the initial 20mm has been fully absorbed.
What became apparent is that there is a significantly greater volume of water draining through the soil profile, despite the presence of ZEBATM. This was probably due to the improved soil structure, which accelerates drainage, and because the ZEBATM would have become saturated and unable to retain any more rain water.
As the two samples were above ground and held within a cold frame the study was conducted in a very artificial environment. Whilst this allowed a greater control on inputs it did mean that evapotranspiration rates were likely to be higher, particularly with the improved summer weather in 2016. If the rainwater was held up in the top few inches of the sample by impeded drainage below then this water would become more vulnerable to evaporation and thus there would be reduced percolation through to the reservoirs below.
The apparent improved soil structure in the ZEBATM plot is a significant advantage on these vulnerable soils. If this can be replicated at the field scale, there could be meaningful impacts on the wider environment.