Is the ground beneath your feet causing leakage?

Introduction

2022 leakage

Overall, 2022 was a challenging year for water companies across the UK. There was very little rainfall from January to October, which lead to 11 of the 14 Environment Agency areas being put into drought measures. Coupled with this July and August saw spikes in temperature that broke records and sent us all searching for ways to keep ourselves and our families cool. As might be expected, with this hot weather came a sharp increase in demand for all water companies as customers filled paddling pools, watered parched gardens and took extra showers to try to stay comfortable. However, when the temperatures returned to normal the consumption values did not fall to previous levels; in fact, typically across UK water companies, an average of 10% (figure.i) increase in leakage levels was observed when comparing figures from late June to mid-August.

Fast forward to December 2022 and leakage figures were hit even harder by a prolonged freeze-thaw event that saw minimum temperatures drop well below freezing for almost two weeks. The aftermath of this freezing weather left water companies with over 50% (figure.i) higher reported leakage as temperatures jumped overnight and pipes burst nationwide.

But could there be a common factor between these two high leakage events? In this blog I will be delving deeper (literally) to see what part soils can play in accentuating bursts during more extreme climatic conditions.

Figure i: typical UK water company leakage figures in 2022

Soils Background

In the UK we have a very interesting ‘soilscape’ due to there being high diversity in the component factors that create soils within a relatively small space. There are 5 key factors that come together to create soils, these are:

Climate
Organisms
Parent material (base rock)
Topography
Time

As each factor changes then so too does the soil type where the particular combination occurs.

Climate is in essence a combination of temperature and rainfall, and across the UK we actually have quite a varied climate, especially when considering seasonal rainfall levels between the north-west and south-east. Due to its nature, climate has the greatest effect on soil development as it alters weathering, abundance of organisms and the leeching of nutrients. Across the UK we also have many underlying rock types that make up parent material providing different nutrients to the soils, as well as changeable topography as you move from former glaciated areas to lower fluvial environments.

All this variation means that there are multiple soil types that react to climatic conditions in different ways. Most soil classification differentiates between particle size and the percentage of organic material within the soils. The Unified Soil Classification (USC) separates soils into three main categories:

Coarse-grained soils (CGS) which contain 50% or less of fine particles (sands)
Fine-grained soils (FGS) which contain more than 50% fine particles (clays)
Highly organic soils which are peat, muck, humus, or swamp soil (peats)

These categories are then subdivided by each soil’s liquid limit and the plasticity of the soil. This can be quite a complex system to use if you are relatively new to soils, but when looking at the UK I find that using the soil groups created by the National Soil Resources Institute (NSRI) at Cranfield University to be more understandable (figure.ii & figure.iii). Using a soilscape such as this allows you to see where similar soils can also be found across the UK and can be added as a GIS base layer if required.

Figure ii: NSRI soilscape map of England and Wales

Figure iii: NSRI soilscape categories

Soil types and transformations

Roughly speaking soils can be described as being sandy, clayey or highly organic (peaty). A healthy mix of the three would create loamy soils that are well drained, hold nutrients and have plenty of organic material. At each end of the spectrum, soils can encounter issues that cause different behaviours during climatic events.

Sandy soils have a high proportion of larger particulates (figure iv.) which do not tesselate well and leave space for water and nutrients to drain away freely. This means that highly sandy soils often dry out quicker and do not have as well established vegetation in the surface layers which can cause increased erosion.

Clayey soils on the other hand are made up of mostly fine particles (figure iv.) which form sheets with very little space in between. Due to this clay-based soils hold on to moisture more readily which can result in waterlogging and heaving of the soil. Soil heave occurs when the soil is saturated and expands as water fills all available space between the clay particles. Interestingly, once clay soils do dry out, for example during the summer of 2022, they can subside which creates deep vertical cracks in the soil and deforms the underlying soil horizons (layers).

Figure iv: typical soil particles and soil types (Credit: agriculturistmusa.com)

How does this affect water networks?

Unfortunately for water companies, pipes are installed underground and so interaction with the soil is unavoidable. If a pipe has been installed into a particularly clayey soil, then it may be subject to higher levels of stress than if it were installed into loamy or sandy soils. Clayey soils are affected by both extreme hot and dry weather as well as extreme wet and cold; during freeze thaw events the water particles in a waterlogged clay soil will expand and put external pressure onto the pipe. The pipe could also be moved by heave which will create even more strain, especially at the fittings. Whilst any bursts caused during the freezing may be minimalised by the expanding soil, on thawing this pressure will be alleviated and allow the leak to grow.

During drought conditions, clayey soils will eventually become dry and subside as there is no moisture, leading to cracking which deforms the deeper soil structure. This can put stress onto the water network as heavy pipes may drop as the water table falls, or become strained when the surrounding soil is deformed. Again, there is a risk that bursts may occur during the initial deformation during a drought, along with a further risk once the soil becomes wet after the drought ends. It may be in soils such as these that the 10% increase in leakage during the summer was mainly found.

Sandy soils tend not to experience as must deformation caused by climate as they drain quickly and have a looser structure. However, there is a risk that during burst events, the leakage from the pipe can wash away the looser soils and create hollows that over time, the pipe could subside into.

How might water companies use soil data to reduce the impact of climate related bursts?

It seems inevitable that extreme climatic conditions will become more prevalent in the years to come. Having knowledge of local soil types could be useful to UK water companies, as if a portion of network is installed in clayey soils, then this will increase the likelihood of bursts occurring during droughts and freeze thaw conditions, especially in rural areas where the mains are less likely to be covered in concrete/tarmac. By understanding where pipes are more likely to be strained and burst, there could be an improvement made to the incident planning process, which may help reduce customer supply interruptions. By monitoring climate data and focussing on these areas before events occur water companies could reduce the amount of reactive leakage work as proactive measures could be put in place.

Water companies can also track the soil moisture deficit (SMD) values in their areas to understand which DMAs could be most at risk. SMD is calculated as:

SMD = Rainfall – (Run-off + Demand + Evapotranspiration)

The Environment Agency publishes monthly reports on the water situation across England with detailed rainfall, groundwater and projected climate data that could be used as an early warning system. They also produce useful SMD maps which show the level of soil moisture deficit across the UK.

Conclusion

As part of my ongoing personal training at SME Water, I have focussed my annual assignment on the possible relationship between underlying soil types and burst rates during hot weather events like the summer of 2022. Whilst there have been many scientific investigations into how soils behave in droughts and the impacts this can have on farming and engineering, I am hoping to be able to come up with a simplified pragmatic way of calculating the possible effect on DMAs depending on which soils are present and what the climatic conditions are.

If the results are promising I am hoping that this may be a future component or flag that could be used as part of our Paradigm model, helping to distinguish where a DMA may experience higher or lower leakage during certain climatic events. Whilst predicting burst leakage is and may always be a dark art, I think for UK water companies to continue to drive down leakage figures, they will need to have a good understanding of all aspects of their network including the environment in which it has been installed.

16th March 2023

Sam Leese

Having worked for a large UK water company before joining SME Water, Sam has experience in leakage targeting and in delivering business change. He has a positive outlook and is developing a strong understanding of network hydraulics and analysis approaches.