WORKSHOP REPORT 4

Soil Erosion – Causes, Consequences and Prevention

Report of a Workshop held at the United Utilities office, Thirlmere, the Lake District

29 July 2003

Contents

Background

Soil erosion – causes, processes and implications – Marianne McHugh (NSRI, Cranfield University)

Process of soil erosion

Causes of soil erosion

Implication of soil erosion

Is there a soil erosion problem in the UK?

Soil erosion - management to prevent and remediate - Philip Bull (ADAS)

Causes

Prevention and remediation

Conclusion

Diffuse erosion in upland catchments in Northern England - Richard Johnson (University of Central Lancashire) and Jeff Warburton (University of Durham)

Introduction

Catchment scale studies

Sediment budget

Summary –

Field trip

Footpath erosion

Tree planting

Erosion caused by agricultural activity

Discussion group feedback

Is erosion always a problem? If not, what are the indicators of a problem that needs to be tackled

To what extent do leisure pursuits and agriculture contribute to erosion damage? Where do we need to focus erosion control?

What are the most challenging problems from a water company’s point of view?

Which remediation methods are seen as most cost-effective?

Where are the gaps in our knowledge? What research needs do we have?

Summary

Acknowledgements

Author contacts

References

Background

This workshop was held at United Utilities office at Thirlmere, Keswick, Cumbria and focused on the causes, consequences and prevention of soil erosion. The workshop was prepared for local land managers and project officers and included formal talks, informal discussions and a field visit to see some examples of erosion problems and restoration work.

Soil erosion – causes, processes and implications – Marianne McHugh (NSRI, Cranfield University)

Process of soil erosion

When vegetation is removed or damaged, soil can be depleted over time as forces, including the weather and traffic, act on the bare soil surface. Although soil erosion is a natural process, that has been quantified, albeit with some difficulty, it is the accelerated (unnatural) losses of soil that are of concern.

Erosive forces can be classified in two ways. Describing them as either an initiator or maintainer of erosion, is a useful classification when targeting remediation of eroded areas, and minimising the creation of further erosion. Alternatively they can be classed as natural (can’t be controlled) or accelerated (can be controlled).

Causes of soil erosion

Natural causes

These include erosion due to water (rainfall and groundwater), wind, frost heave, and combinations of these elements. Some examples are given below:

  • Waterlogging of peat may cause vegetation to be lost. The peat is then prone to crusting over or being washed or blown away.
  • The natural breakdown of unstable soil or headward retreat of upland streams can cause erosion. Streams creeping uphill may breach intact bogs, causing loss of water and soil.
  • Gully erosion can also be caused by natural processes and once started can be difficult to control. Soil at the gully edges is subject to desiccation and therefore vulnerable to removal by wind, frost and water.
  • Once exposed, frost can break up peat or mineral soil, destroying existing soil bonding; the thawed soil can be highly unstable and prone to removal by water or wind.

Accelerated causes

Human activities such as walking, shooting, draining, driving, military exercises, burning and parking can disrupt vegetation and accelerate erosion, as can grazing by animals such as sheep, cattle and rabbits, and industrial activities such as water abstraction, forestry, mining or pipeline installation.

  • Grazing animals, particularly sheep, may carve out hillside niches for shelter (sheep scars). These expose the soil, making it susceptible to removal by water and wind. Soil loss may be exacerbated if grazing has kept the vegetation short. As sheep scars get larger, localised slope failure (slippage) may occur.
  • The soils on frequently used footpaths will inevitably become exposed and subject to erosive forces. The path is likely to become muddy and as people tend to avoid the mud, the path continually widens.
  • Preparing and draining soil for tree planting and, later, tree felling itself can all result in exposed, degraded soil that is then subject to erosive forces.
  • Fire caused by out-of-control moor burns, or by carelessness, can destroy vegetation, peat and the seed bank. Any soils that are left are subject to erosive forces.

Erosion is often caused by natural processes that have been accelerated by non-natural activities. Damage caused by ATV is currently a major problem about which operators are invariably unaware. This needs to be addressed, for example, through the establishment and circulation of guidelines for ATV users.

Implication of soil erosion

The implications of soil erosion can be divided into onsite and offsite factors.

Problems onsite include:

  • Erosion scars which have limited aesthetic appeal
  • Reduced vehicular or pedestrian access as unvegetated surfaces become muddy. This affects people working or recreating in the area.
  • Poaching and erosion which reduces the amount of vegetation available for grazing.
  • Reduction of soil reserves. Soil should be treated as a resource of which there is a limited supply, not least because upland soils are an important source of plant nutrients, and also act as a carbon reservoir and can sequest atmospheric pollutants.

Offsite impacts of upland soil erosion include:

  • Loss of soil nutrients into watercourses causing eutrophication.
  • Increased risk and propensity for flooding: a surface devoid of vegetation does not absorb rainfall as well as vegetated ground and the excess rainfall run-off can contribute to the risk of flooding.
  • Poor aquifer recharge as, again, a lack of vegetation reduces rainfall infiltration.
  • Sedimentation in reservoirs. This effectively reduces the size of the reservoir and is very costly to remove.
  • Increased likelihood of pollutants entering the water including pathogens, chemicals, heavy metals and fluvic and humic acids. The latter cause water discoloration.
  • A decline in fish stocks due to increased siltation and pollutants
  • Sediment deposition on roads causing a safety hazard, blocking drains, or even burying archaeological features or affecting homes.

Is there a soil erosion problem in the UK?

A comprehensive survey of 400 sites across the upland areas of England and Wales revealed that erosion is widely distributed. The worst affected areas are the Pennines and mid and South Wales, whilst the Lake District, North York Moors, Cheviots and Bodmin are the least affected.

As a rule, a site from which there is a risk of erosion reaching a watercourse, should become high priority for remediation. In the uplands, there is always a high risk of the eroding soil reaching a watercourse therefore upland sites are high priority.

Soil erosion - management to prevent and remediate - Philip Bull (ADAS)

Causes

The causes of soil erosion were dealt with in the previous talk, but some of the major causes in the uplands are reiterated below:

  • Livestock – high stocking rates result in short and bare vegetation and poaching around gateways or feeders
  • Vegetation management such as burning or bracken spraying,
  • Draining of moorland
  • Traffic e.g. ATV
  • Recreational trampling

Prevention and remediation

Livestock

Appropriate stocking rates for the species and type of land should be adopted. This is not an exact science but Countryside Stewardship Scheme (CSS) and Environmentally Sensitive Areas (ESA) recommendations are useful guides. Off-wintering of livestock in sheds or onto drier, lowland sites is desirable but not always possible. Grant aid has been available for building sheep sheds, especially under European Union Objective 5b schemes, but this is no longer so.

If possible, feeders should be placed in areas not prone to poaching and away from watercourses. Hard standing can be provided around feeders and gateways; the cost of this may be offset as less land is poached. Feeding areas can be rotated; the new location should be at least 250m away. Feed blocks cause less damage than fodder but are not always the appropriate feed for the stock. Hay scattered around the feeder or gateway may prevent poaching, but can be blown away.

Vegetation management

When burning heather, the heather burning code should be followed (MAFF 1992) or the Muirburn Code (Phillips et al) Burns should be kept small (<1ha) and only 20-30meters wide. Length will depend on the terrain but not usually more than 150 metres and often less. Steep slopes and blanket bog should be avoided. A rotation should be established, firebreaks should be considered and experienced people should be present. Fires should not be too hot.

If bracken spraying is to be carried out, then it must be on a site on which vegetation will naturally recolonise. Steep slopes should be avoided and the size should be restricted so as not to cause mass destruction over a large area. Grazing should be restricted as new vegetation develops.

Drains (Moorland grips)

These were grant aided until the mid 1980s with the intention of draining the adjacent land and so improve productivity. Not only were they ineffective, but sheep and grouse tended to get stuck in them. Some drains naturally block up but others are prone to erosion, in these cases blocking is an option.

Where drain erosion is severe it may not be possible to repair the whole drain. Targeting the highest part of the drain or where it is flat is recommended. Blocking materials include heather bales, straw, plastic piling or timber posts. Heather bales tend to break down and stop working after a relatively short period. Timber piling is expensive and heavy and labour intensive. Plastic piling is lighter and can be transported by hand or by tractor. Another option is to dig a turf adjacent to the grip, 2-3 times the width of the drain, and dump it in the grip. A tracked excavator on wide tracks is required. The method is effective and the resulting hole becomes a pool and so adds to the biodiversity of the area.

Vehicles

ATV routes should be varied so that vegetation is not broken and ruts are not created.

Footpath erosion

Stone pitching can be carried out in rocky upland areas, or gravel can be used to repair footpaths. The stones should be pitched at an angle that is comfortable for walkers, otherwise they might be inclined to walk off the path. Locally produced material is preferable. Large flagstones from old industrial buildings have been used to good effect across blanket bog in the Peak District. Further information on footpath restoration is given in Davies et al. (1996)

Re-vegetating eroded areas

Natural re-vegetation is sometimes possible. Options for Calluna re-establishment include seed, brash, litter, plugs or pots. Litter can be gathered from below Calluna plants, brash can be salvaged from areas that have been cut for firebreaks or as an alternative to burning. Seed of local provenance is recommended wherever possible. It can be harvested or bought. Hessian or coir impregnated with seed is an option. The fibre stabilises the soil until the Calluna is established. Pots are between 60-70cm, need to be hand planted and at a cost of approximately 90p are relatively expensive. Plugs are cheaper especially since they can be mechanically planted. Plugs or pots might be an option where the site is open to public scrutiny. Other species such as dwarf shrubs or native grasses and sedges can also be established. Seed can be collected locally using a harvester machine or bought. Common cotton grass is useful as a peat stabiliser because of its deep rooting system. Some non-native species such as perennial rye grass seed may be used as a nurse crop whilst native species establish. Soil nutrients and pH should be measured prior to re-vegetation. Fertiliser use might be appropriate but should be used with care; it is not generally recommended.

Further details of these restoration approaches for blanket bog are given in Adamson and Gardner (2002) and (2003).

Conclusion

Simple control measures can be taken to protect land and benefit wildlife. Remedial measures are expensive and not necessarily successful.

Diffuse erosion in upland catchments in Northern England - Richard Johnson (University of Central Lancashire) and Jeff Warburton (University of Durham)

Introduction

Recent governmental policy positions have drawn attention to the problems of soil erosion in the uplands. These have highlighted that there is much that is still unknown about upland systems, and have consequently stimulated new work. Relevant documents include:

  • Water framework directive (2000) - forces people to consider diffuse pollution from small (<10ha) upland catchments. This will be enforced by 2015.
  • Defra (2001): Draft soil strategy for England - highlights the fact that soil erosion is a natural process that includes the removal and transportation of particles. It can have on and off-site impacts. In the uplands, the different causes of erosion need to be objectively quantified.
  • Defra/ADAS (2002): Diffuse pollution from agriculture. Sediment is considered to be a pollutant and problems can be created downstream from the source of the erosion. Environmental monitoring will be needed to assess the implementation of the policy.

A range of techniques can help us understand upland erosion and sediment dynamics e.g. sediment budgets and event reconstruction. Some of the catchment scale studies in which these have been used, are described below.

Catchment scale studies

Before carrying out a catchment scale study, there are some general points to consider:

  • What are the characteristics of the catchment e.g. is the substrate mineral or organic?
  • What are the type of processes involved e.g. are they geomorphological, hydrological or meteorological, are they influenced by humans or animals, and how do they interact?
  • What are the diffuse and multi-point catchment sediment sources, e.g. slopewash or small channel bank failures?
  • How do the process dynamics and system linkages interrelate?
  • What is the characteristic scale, in space and time, of upland erosion?

Catchment investigation techniques are used to determine the causes and impacts of erosion and to enable appropriate management solutions to be developed. Reconstruction of hydro-geomorphic events is one method. For example, Johnson and Warburton (2002a) investigated the impact of a large flood event in a Lake District stream. This established the rainfall-runoff relationship, the source and transfer of eroded sediment, and the long-term significance of high magnitude- low frequency geomorphic events as agents of erosion in upland catchments. Another method is the sediment budget in which individual processes, system components, and their interaction are monitored in some detail. Examples of how sediment budgets can be used are given below.

Sediment budget

A sediment budget is a quantitative statement of the rates of sediment production, transport and discharge of detritus. The three elements needed to construct a sediment budget are:

  • Recognition and quantification of transport processes
  • Recognition and quantification of storage elements
  • Identification of linkages amongst transport processes and storages

Sediment balance can be expressed as an equation:

I + ΔS = O
inputs + change in storage = outputs

Slope failure may occur on a hillside. The substrate may be washed into a hill stream; this is the input. It may be deposited (stored) on the stream bed and possibly upon adjoining floodplain segments. All sediment exiting the stream system is an output. This process of recognising system elements and their interactions can be applied to individual streams or to whole catchments.

The typical upland catchment sediment system can be split between the hillslope system and the channel system as shown in the flow diagram (Figure 1). Pink boxes are sediment production and transfer processes primarily operating in the hillslope sub-system. Green boxes are channel sediment transport processes, whilst blue boxes are stores at different points of the sediment cascade. Inputs are from the hillslope, channel margin and channel bed locations. This sediment is either transferred through or stored in the system (e.g. upland channels, lakes and reservoirs).

Figure 1: Typical upland catchment sediment system

Some examples of sediment budgets are given below:

Iron Crag – Northern Lake District

Research at Iron Crag (e.g. Johnson and Warburton, 2002b) provides an example of an actively eroding gully system for which a year long sediment budget can be constructed. It comprises gully and hillslope mineral sediment sources (inputs to the channel) from bare areas at the top of the hill. A main water channel which transports supplied and stored sediment, and an active fan area at the base of the crag into which sediments are deposited (storage) or are carried on into the stream system (outputs).

The techniques to measure sediment in order to construct a sediment budget are intense but simple. They include Gerlach troughs that trap sediment as it moves by surface wash and gravitational processes down the hillslope. Nets are placed under the crags to collect rockfall. In the channel there is a detailed series of cross-sections to measure deposition and erosion from the channel, whilst down on the fan system, there is an array of pegs to measure deposition and erosion.

The sediment budget can be constructed to investigate the impact of specific meteorological events e.g. freeze-thaw, summer dryness or rainstorms, and for a whole year to give an annual sediment budget in terms of tonnes per year. What became clear at Iron Crag is that most of the sediment was sourced from the channel bed and banks. Also, although the level of activity was high, much of the material was not being transported very far; it was being deposited in the fan area (194t a-1) and relatively little was leaving the catchment (46t a-1). This is typical of small catchments.

Wet Swine Gill - Northern Lake District

Wet Swine Gill is a tributary of the River Caldew. Hillslope failure occurred in the catchment of the gill between 4th January 2002 and 28th March 2002, the exact date is not known but it was probably coincident with the flooding in the River Caldew catchment in the first week of March. The failure occurred in an area of burnt heather (on 4th January 2002) on a fairly steep slope immediately below a drainage grip. The effects of the hillslope failure were that surface organic material and sub-surface mineral sediments were released into the channel system of Wet Swine Gill. Water and fine sediment was transferred into the River Caldew, whilst along the channel erosion and sediment storage occurred. An unstable sediment source has therefore been produced; this creates a greater, more widespread (diffuse) future sediment transfer potential in the catchment. Since the event, sediment has continued to erode off the hillslope into the channel and in some places deep incisions have developed up to 2m deep. These have become drainage channels in their own right. Thus, there is a major instability in the landscape caused by the one-off event.