Earthworks are undertaken to create areas of level land that can be used for living, business and recreation, and the required gradients for paths and roads that enable people to get from place to place. Earthworks are integral to the construction of foundations and buildings. Earthworks are part of most development projects undertaken at the small and large scale, including small scale projects by individuals.
Managing earthworks is a complex issue. This is due in part to the wide range of activities covered by the term 'earthworks', as well as the wide range of effects generated. It is also further complicated by:
- the overlap in jurisdictional responsibility between regional and territorial authorities under ss 30 and 31 of the RMA
- the role of the Building Act 2004 in controlling site works (including sedimentation) and stability of earthworks and structures through the issue of building consents
- the considerable technical/engineering component in determining the effects of proposed earthwork activities and appropriate management methods.
Earthworks: issues and effects
Earthworks involve revealing, disturbing, removing or depositing soil/earth which results in the creation of exposed surfaces. These surfaces are created by activities such as land contouring for urban development and roading, quarrying, filling operations (including cleanfills), clearing vegetation, forestry, agriculture and horticulture.
If not managed well, earthworks have the potential to cause significant adverse effects on the environment.
Effects on water quality and the ecosystems of water bodies
Erosion is a process by which the surface of land is gradually eroded by water or wind. The erosion process produces sediment runoff or air borne dust.
Sediment runoff is a natural occurrence. All land areas have a natural sediment runoff or discharge load that varies primarily with rainfall, geology and land use. Sediment runoff will also vary with factors such as storm intensity and duration, soil type, slope length and angle, and surface cover.
Exposing land surfaces through earthwork activities can increase sediment loads that are discharged to water bodies and the coastal marine area above normal levels. This can result in significant adverse effects on receiving environments and their habitats. In particular, an increased sediment load discharged to watercourses can affect water quality and the ability of aquatic organisms to survive and/or migrate.
The effect of increased loads of sediment discharged to waterbodies will vary as different waterbodies and habitat types have differing capacities to cope with elevated levels of sediment.
Aside from the immediate effects associated with single sites, one of the most significant impacts of accelerated sediment discharge is the cumulative effect of discharges from multiple sites over an extended time period. Without appropriate management, there is likely to be significant and long-term adverse effects on the streams, estuaries and harbours into which the catchment discharges. A broader catchment based analysis and management approach is needed in these situations.
Effects on local amenity
Earthworks can have adverse effects on amenity values, including -
- Visual Impacts - earthworks involving cut and fill have the potential to affect the visual qualities in the immediate area, including natural landscapes and views. Large areas of fill have the potential to block views, while large cuts can create a 'scar' or a visually dominant face.
- Dust from earthwork activities can have a potential effect on amenity values at a local scale. The level of dust generated by earthworks is dependent on a number of matters including soil characteristics, rainfall, wind and method of excavation.
- Noise is an indirect effect associated with earthworks (e.g. operation of heavy machinery) and the degree of management is dependent on local conditions. For example, machinery operating near a residential area may require more stringent controls to be applied than a similar operation in an isolated rural environment.
- Soil deposited as a nuisance - where earthworks result in the movement of vehicles to and from a site there is potential for soil to be deposited in an indiscriminate and uncontrolled manner in transit (e.g. soil falling off tyres, soil being blown off uncovered loads).
For the most part, adverse effects on amenity values will be temporary and generally restricted to the time required to complete the earthworks. The closer the proximity of earthworks to sensitive activities (i.e. residential), the greater the potential for adverse effects on local amenity values.
The effectiveness of on-site mitigation measures will be a key factor in determining the significance of effects on local amenity value and managing the adverse effects on these values.
Hazards - land instability and flooding
Earthworks can create or worsen hazard potential (i.e. flooding and land instability) and careful management is required to avoid this. For example, filling parts of a known overland flow path is likely to impede runoff and worsen flooding upstream, potentially enlarging the area affected by inundation. Instances of land instability may be created where excavations under cut a hillside, or where excavations result in un-retained or bare hillsides.
Earthworks are also often a part of physical works undertaken to manage the effects of hazards. The cumulative effects of earthworks on hazards may be significant.
Effects on significant natural land forms
A majority of earthworks involve permanent removal of soil from one area and relocation to another. Consequently, earthworks have the potential to change the underlying landform of an area, depending on their scale. Some landforms are valued for their natural character or landscape qualities, and earthworks can potentially compromise these values.
Earthworks that result in the modification of landform patterns can have an adverse impact on the visual coherence of an area through the degradation or, in some cases, removal of a natural landform. For example, levelling coastal dunes can give them an unnatural appearance (and interfere with their function), or cutting a track across a hillslope can create a highly visible 'scar' and can lead to other effects, such as erosion and those related to land instability.
Earthworks involving significant natural landforms can have an irreversible impact on their visual and cultural qualities. In some circumstances mitigation measures may not be sufficient to manage the adverse effects, with avoidance being a more appropriate option.
Effects on vegetation
The effects of earthwork activities on vegetation will depend on the type, extent and values of the vegetation cover being removed or modified. The removal of vegetation, particularly indigenous vegetation, can result in the loss of habitat and visual amenity. In addition, exposed soil can cause other effects such as erosion, increased surface water and sediment runoff and dust nuisances.
When vegetation cover is removed, the ongoing and long-term effects will depend on the replacement cover and how quickly the exposed ground is stabilised. While hard surfaces (e.g. roads or buildings) can help to avoid effects such as dust and stability, replanting areas subject to earthwork activities is a key mitigation measure to manage both the short and long term effects of earthworks.
Effects on archaeological and heritage sites
Earthwork activities have the potential to alter, disturb, modify or destroy heritage or archaeological sites, regardless of whether they are scheduled in regional or district plans or identified on the New Zeeland Archaeological Association database.
The Historic Places Act 1993 (HPA) is the primary statutory framework for protecting known and probable archaeological sites (refer s10 HPA). RMA implementation (i.e. plan provisions and resource consents) should complement HPA requirements (e.g. by reference to protocols, etc).
Earthworks on contaminated land
Earthwork definitions in regional and district plans usually refer to soil disturbance of clean soils. Undertaking earthworks on contaminated land has the potential to give rise to a number of adverse environmental effects, including contaminated sediment and air discharges and may require/lead to the need to dispose of any excavated contaminated material. Specialist expert advice should always be obtained in respect of contaminated sites prior to any works taking place.
The disturbance of contaminated land may require consent to discharge contaminants under s15 of the RMA. It is also covered by the National Environmental Standard on Assessing and Managing Contaminants in Soil that affect Human Health. A thorough and robust assessment must be carried out where earthworks are proposed on a contaminated site. In particular, care is required when exposing contaminated soil to different environmental conditions and when contaminated material is disposed. Chemical and biological solutions may be available to either neutralise or make contaminants inert.
Earthworks may be temporary activities but effects can be permanent
Earthworks are often temporary activities associated with the construction phase of the urban development process or another land development activity (i.e. primary production land use conversion). There are a range of practical techniques that can be employed to manage the effects of the activity. In certain circumstances (e.g. fill areas for the disposal of soil or other uncontaminated material, unstable ground) or cuts there may be permanent effects such as instability, visual and flooding. In these situations, careful consideration of mitigation and the appropriateness of the earthworks is required. In particular, consider whether the earthworks are needed, or whether the proposed development can be designed so as to minimise the earthworks proposed.
Effects of earthworks can be time and seasonally dependent
The time of year, seasonal weather conditions, and duration of earthworks influence the magnitude of the effects generated. These environmental characteristics vary greatly around the country and should be considered when assessing the effects of earthwork activities. In particular, plan provisions and resource consent conditions should recognise local seasonal conditions. For example, erosion rates and related potential sediment runoff loads will be greater during months of higher rainfall.