The cost of soil degradation in England and Wales
Appendix L: Soil in an urban environment
Soil in the urban environment
The ecosystem value of soils in the urban environment is easily over looked yet as important as elsewhere. Table 1 outlines the locations that soil may be found in many towns and cities, its value and the pressures it is exposed to. To better illustrate the soil function and the cost of soil degradation in the urban environment a case study example using Milton Keynes as its focus is described below.
Table 1. Soil within the urban environment
Location / Value / PressuresParks and sports ground / Recreational value
Biodiversity
Health and wellbeing feeling
Education
Potential soak away areas / Compaction
Erosion of pathways
Pollution from exhaust emissions, deposits in rainfall
Woodlands / Recreational value
Biodiversity
Health and wellbeing feeling
Education
Potential soak away areas / Compaction
Erosion of pathways
Roadside verges and roundabouts / Corridors for wildlife
Potential soak away areas / Diffuse pollution from traffic exhaust emissions and herbicides
Pavement edges / Potential soak away areas / Compaction
Gardens / Food production
Recreational value
Biodiversity
Health and wellbeing feeling
Potential soak away areas / Competition from other uses e.g. parking, patios and extension of properties
Herbicides and pesticides
Allotments / Food production
Recreational value
Biodiversity
Health and wellbeing feeling
Potential soak away areas / Competition from other uses e.g. housing and infrastructure
Contamination from road traffic and the use of herbicides and pesticides / Mean annual willingness to pay to keep allotments £78.93, £35.64 and £79.43, and for local residents were £41.66, £46.14 and £283.57, in Wye, Ashford and London, respectively (Perez-Vazquez et al., 2006)
Part of urban landscaping / Health and wellbeing feeling
Potential soak away areas / Compaction
Diffuse contamination from traffic fumes and herbicides/pesticides
City farms / Food production
Educational
Potential soak away areas
Cemeteries / Social and cultural value
Habitat and biodiversity
Case Study – Milton Keynes
Since its conception in 1967 Milton Keynes has grown from a population of 60,000 to 222,000 in 2006 and is set to reach 350,000 by 2031[1]. The development of a new town afforded a unique opportunity to evaluate aspects of the development[2]. Between 2001 and 2021, the plan is that 45,000 new houses will be built in Milton Keynes.
In 1992, the Parks Trust was created as an independent charity to own and manage the strategic open spaces in Milton Keynes. The primary objective of the Parks Trust is to “provide, maintain and equip parks, gardens, landscape areas, woodlands, open spacing, playing fields and recreational amenity spaces within Milton Keynes for the benefit of the inhabitants and visitors to the area”[3]. Much of the land owned by the Parks Trust is within the floodplains of the river catchments; the Ouse, the Ouzel and the Loughton Brook.
Milton Keynes has been transformed from a predominantly rural to an urbanised area. Increased development is putting continued pressure on agricultural land. Before 1967 81% of the present Milton Keynes was under agricultural use (7200 ha out of 8900 ha). In 2003 0.5% of total employment in the area was in agriculture or related sectors. By 2005 this had reduced to 0.4%[4].
Within this urban environment two ecosystem services provided by soil are particularly threatened: 1) soil water storage and associated flood prevention, and; 2) carbon sequestration.
Sustainable Urban Drainage Systems (SUDS)
Soil sealing and the consequential lose of ecosystems goods and services, is the predominant soil degradation process caused by urbanisation. Urban development alters the hydrological behaviour of a catchment. Increased impermeable area produces larger quantities of runoff in shorter time periods leading to higher peak flow rates[5].The Milton Keynes master plan envisaged a 1 – 1.5% increase in runoff due to proposed urban development. In order to reinstate equivalent lost goods and services they must be engineer back into the urban landscape.
Sustainable urban drainage systems (SUDS) represent an engineered landscape developed to simulate healthy natural hydrological processes and to act as checks/balances to the disruption of existing on site drainage systems caused by housing developments. SUDs include the use of water butts, permeable paving, green roof, vegetated sidewalks, swales, detention and wetland basins, and retention ponds. The concept is that water passes through swales, detention basins and wetlands and into final retention pond. It is stored there before being released to a local drainage ditch outside the development site.
When the water exceeds capacity of ‘source control’ it is safely stored and treated in larger SUDS features integrated within public open space until the flood threat has passed. Such measures also contribute to the provision of green space, visual amenity and promoting wildlife. Combining SUDS with public open spaces optimises use of space within high density housing developments.
In Milton Keynes large balancing ponds and canal systems have acted as SUDS at a city-wide scale and managed surface waters. Most new developments in Milton Keynes now focus on SUDS to mimic preceding natural drainage. Oxley Park is a case in point.
Oxley Park SUDS: The Oxley Park development is located in an area which used to be predominantly natural vegetation and agricultural land. Land use change has resulted in the loss of ecosystem functions including changes in surface water regulation and ground water recharge via natural processes and pressure from human occupation. The natural soil of the Oxley Park area is Hanslope, a slowly permeable calcareous clayey soil prone to slight seasonal water logging and low storage capacity. The Future Forecast of flood damage[6] places the Milton Keynes area as ‘medium’ increased risk (Table 2). The SUDs should minimise the risk of flooding to residents. Insufficient water in the future is also of concern with serious levels of water stress predicted for the Milton Keynes area[7]. Carefully designed and managed SUDs will improve both retention and detention of water in the local area.
The estimated capital cost of different SUDS options proposed for the Oxley Park scheme are presented in Table 3.
Table 2. Summary of scoring of flood risk indicators for areas of high growth and high flood risk (from Halcrow, 2008[8])
Flood risk indicators / Milton KeynesPresent day inherent flood risk / Moderate
Climate change inherent flood risk / High
Fluvial flood risk / Moderate
Tidal/Coastal flood risk / N/A
Flood risk from groundwater / Low
Surface water flooding / Low
Residual risk of flooding / High
Level of structural mitigation / Moderate
Table 3. Capital and maintenance cost of SUDS options.
Component / Unit capital cost / Unit annual maintenance costInfiltration trench / £55-£65/m3stored volume (Wallingford, 2004) / 5 to 10% of capital cost (US Environmental Protection Agency, 1999). Rehabilitation may be required every 5 to 15 years. Rehabilitation cost run from 15-20% of original capital cost.
Filter drains / £100-£140/m3 stored volume / 2-12% of capital cost per annum
Soakaway / >£100/m3 stored volume / Shouldn’t require any maintenance.
Permeable pavements / £35-£40/m2 permeable surface
Infiltration basin / £10-£15/m3 detention volume
Detention basin / £15-£20/m3 detention volume
Wetland / £25-£30/m3 detention volume
Retention pond / £15-£20/m3 detention volume
Swales / £10-£15/m2 swale area
Filter strip / £2-£4/m2 filter strip area
Green roofs / -
Carbon sequestration
As Milton Keynes expands, the services provided by nature must be replaced with expensive technologies to purify water, purify air, treat sewage and sequester carbon in new ways[9]. Prior to development Milton Keynes was primarily an agricultural area. Under agricultural land provides considerable potential for carbon dioxide mitigation through carbon stored in the soil. Urbanisation reduces the soils ability to sequester carbon in this way and leads to losses of stored carbon during the development process. A suggested option for urban areas to readdress this is the use of trees.
Trees in the urban environment, including individual trees, spinneys and woodland play many important ecosystem roles and cover all servicing groups i.e. provisioning, regulating, cultural and support. The presence of trees in the urban environment also preserves soil function in the urban environment. Trees protect the soil from erosion (willow is a particularly useful species for this), help combat compaction through root development, maintain flood regulation services of the soil and helps reduce diffuse contamination of the soil from airborne pollutants. Soil biological functions are also maintained by the presence of trees.
Changes in soil organic carbon in land pre- and post- urban development have been estimated for the Fairfield development in Milton Keynes. The Fairfield area is located on the western edge of Milton Keynes is earmarked for development. It is a 345 ha plot. Land use change would see permanent pasture, with some hedgerows, converted into an urban area with 60% impervious surfaces (see Table 4). Calculations of carbon storage pre- and 30 year post- development estimate 36% to 48% carbon stored in the urban area compared to pre-development levels (Table 5[10]).
Based on the provision of this information, a typology of the typical causes of soil degradation in urban areas (Table 6) and the impacts of that degradation on ecosystem service provision has been provided (Table 7). A series of mitigating measured to help improve levels of carbon in organic soils has been provided in Table 9 for increasing carbon sequestration for the Fairfield area, Milton Keynes and the impacts of a variety of landuses and landuse options on ecosystem functions has also been qualitatively described by Defra (Table 10). Based on these sources, a set of proposals for the loss of soil functionality experienced as a result of soil degradation due to urban development has been summarised in Table 8.
Table 4. Distribution of impermeable areas by land use in Fairfield (Working Group Report, 2008[11])
Table 5. Comparison of carbon storage pre- and post- development of Fairfield area
Table 6. Summary of causes of soil degradation in urban areas
Soil degradation process / Degradation processErosion
Compaction / · By treading and vehicles
· Use of soil scrapers during construction phase
Loss of organic matter / · Lost during conversation from agricultural land to urban
· Reduction in amount of carbon sequestered due to sealing under roads, driveways, buildings and other impermeable surface or structures.
· Oxidation through exposure releases greenhouse gasses, CO2 and NO2
· Under anoxic condition e.g. in storage heaps, anaerobic decomposition of organic matter releases methane
· Urban development releases large amounts of CO2
· Removal, storage and reinstatement of the soil affect carbon storage potential in many different ways.
· Working Groups Reports: estimated changes in carbon sequestration for Fairfield development in Milton Keynes, pre urbanization 21,715 total tonnes C, post urbanization 10,450 total tonnes C
Loss of biodiversity
Diffuse contamination / · From traffic, industry, burning of fossil fuels
Surface sealing / · Pressures from urban development and expansion, pressure for new housing, infrastructure and businesses.
Table 7. Potential benefits of Allotments and SUDS in Milton Keynes
Services and goods / Impact/EffectProvisioning
Allotments / · There are around 2500 allotment plots in Milton Keynes. Expansion plans include provision for 20 plots per 1000 population. People pay between £8 and £80 a year to rent a standard size allotment plot of 253 square metres (Harrison, 2008).
Regulating
SUDS / · Regulation of water, encourages natural groundwater recharge. The estimated cost savings due to SUDS is approximately £11k. Financial benefits of approximately £30/year/household to the residence through avoidance of annual payment of storm water disposal changes to the sewerage undertaker (Lamb Drove, Cambridgeshire example).
· Estimated 1 – 1.5% increase in runoff due to proposed urban development in Milton Keynes
· Pollution attenuation at source reducing end of pipe measures at catchment scale. SUDs must be maintained according to best practices in order to successfully achieve this.
· Reduction of the urban heat island effect.
Cultural
SUDS / · Maintaining urban ecology
· Provide a habitat for wildlife in urban watercourses
· Access to nature on site in urban development
· Feeling of safety and protection against natural hazards
· Amenity and recreation value
· Aesthetic and landscape value
· The quality of the local environment can affect property values. Land values and house prices located adjacent to SUDs may demand a 10% differential (HR Wallingford 2003a)
· Educational value
Supporting
Water cycling
SUDS / · Soil formation
· Nutrient cycling
· Water cycling
· Habitat
· Sympathetic to the environmental setting and the needs of the local community
Table 8. Summary of potential remedial measures for soil degradation in Milton Keynes
Soil degradation process / Remedial measuresErosion / · Subsoiling
Compaction / · Woodland planting (Nisbet & Broadmeadow, 2003)
Loss of organic matter / · Encourage green spaces in urban development; trees
· Wetland management: Stony Stratford Nature Reserve on northern side of Milton Keynes
Loss of biodiversity / · Encourage green spaces in urban development
Diffuse contamination / ·
Surface sealing / · SUDs
Table 9. Recommendations for increasing carbon sequestration (R = Relevance for the Parks Trust, B = Benefit level for C stocks increase).
Table 10. The ecosystem services provided by a variety of land uses and development options (from: Defra, 2007[12])
Draft reportPage 1Cranfield University
[1] Milton Keynes Council (2007)
[2] Milton Keynes Partnership (2005)
[3] EA (2005)
[4] Milton Keynes Population Bulletin 2005/2006
[5] Cambridge (2004)
[6] Defra (2008)
[7] Environment Agency (2007)
[8] Halcrow (2008) Regional flood risk appraisal for south east England: summary. Report for south east England Regional Assembly
[9] Schaefer et al. (2004)
[10] Working Group Report (2008)
[11] Working Group Report (2008)
[12] Defra (2007)