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 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 (Refer to Surface Water Quality Guidance Note).
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 significant and long-term adverse effects on the streams, estuaries and harbours into which the catchment discharges are likely. A broader analysis catchment based approach is needed in these situations.
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Effects on local amenity
Earthworks can have adverse effects on amenity values, including -
- 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 sensitive activities (i.e. residential) to earthwork activities, 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.
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Hazards - Land instability and flooding
Earthworks can create or worsen hazard potential (i.e.: flooding and land instability) and careful management is required. 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 often a part of physical works undertaken to manage the effects of hazards. The cumulative effects of earthworks on hazards may be significant.
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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, depending on their scale, to change the underlying landform of an area. 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.
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Effects on vegetation
The effects of earthworks on vegetation depends 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 worked over areas is a key mitigation measure to manage both the short and long term effects of earthworks.
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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 NZ Archaeological Association database.
The Historic Places Act is the primary statutory framework for protecting known and probable archaeological sites (refer s.10 HPA). RMA implementation (ie. plan provisions and resource consents) should complement HPA requirements (e.g. by reference to protocols, etc).
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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 s.15 of the RMA. 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.
Refer to MfE Guidance on Contaminated Land for more information.
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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 employed to manage the effects of the activity. In certain circumstances (eg. fill areas for the disposal of soil or other uncontaminated material, unstable ground) or cuts the effects such as instability, visual and flooding may be permanent and so careful consideration of mitigation and the appropriateness of the earthworks is required. In particular are the earthworks needed or can the proposed development be designed so as to minimise works proposed are matters that may need to be considered?
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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.
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