Earthworks & stream diversion

Land recontouring and stream diversion require careful management to avoid or mitigate impacts on the environment.

Earthworks

Earthworks expose the soil to the forces of gravity and rain/water which accelerate natural erosion processes. Sediment and other contaminants may enter water and cause environmental damage, unless preventive measures are put in place:

Minimise disturbance: Fit land development to land sensitivity. Some parts of a site should never be worked and others need very careful working. Watch out for and avoid areas that are wet, have steep or fragile soils or are conservation sites or features. Adopt a minimum earthworks strategy - ideally only clear areas required for structures or access.

Stage construction: Carrying out bulk earthworks over the whole site maximises the time and area of soil that is exposed and prone to erosion. Construction staging, where the site earthworks are undertaken in small units over time with progressive revegetation, limits erosion.

Protect steep slopes: Existing steep slopes should be avoided. If clearing is absolutely necessary, runoff from above the site should be diverted away from the exposed slope to minimise erosion. If steep slopes are worked and need stabilisation, traditional vegetative covers like top soiling and seeding may not be enough.

Protect watercourses: Existing streams, watercourses, and proposed drainage patterns need to be mapped. Works that cross or disturb the watercourse may require resource consent.

Stabilise exposed areas rapidly: The ultimate objective is to fully stabilise disturbed soils with vegetation after each stage and at specific milestones within stages. Methods are site specific and can range from conventional sowing through to straw mulching. Mulching is the most effective instant protection.

Install perimeter controls: Perimeter controls above the site keep clean runoff out of the worked area – a critical factor for effective erosion control. Perimeter controls can also retain or direct sediment-laden runoff within the site. Common perimeter controls are diversion drains, silt fences and earth bunds.

Employ detention devices: Even with the best erosion and sediment practices, earthworks will discharge sediment-laden runoff during storms. Along with erosion control measures, sediment retention structures are needed to capture runoff to allow the sediment generated to settle out. Ensure that the control measures used are appropriate for the project and adequately protect the receiving environment.

Experience and training: A trained and experienced contractor is an important element to the success of any project. These people are responsible for installing and maintaining erosion and sediment control practices. Such staff can save project time and money by identifying threatened areas early on and putting into place correct practices. At each earthworks site, there should be a person whose responsibility it is to oversee the erosion and sediment control practices. It is often useful for Taranaki Regional Council monitoring staff to meet with this person for a pre-construction meeting, for regular inspection visits, and a final inspection.

Make sure the plan evolves: An effective site erosion and sediment control plan is modified as the project progresses from bulk earthworks to project completion. Factors such as weather, changes to grade and altered drainage can all mean changes to planned erosion and sediment control practices. Update the erosion and sediment control plan to suit site adjustments in time for the preconstruction meeting and regular monitoring inspections, and make sure it is regularly referred to and available on site.

Assess and adjust: Inspect, monitor and maintain control measures. Assessment of controls is especially important following a storm. A large or intense storm will leave erosion and sediment controls in need of repair, reinforcement or cleaning out. Repairing without delay reduces further soil loss and environmental damage. Assessment and adjustment is an important erosion and sediment control practice and must figure in the erosion and sediment control plan. Assign responsibility for implementing the erosion and sediment control plan.

Stormwater from earthworks

You don’t need a resource consent to discharge stormwater from earthworks to land or water if you can meet these conditions:

  • The discharge is not to a significant wetland.
  • The earthworks area is less than a hectare.
  • The volume of disturbed material is less than 3000 square metres.
  • The area is stabilised as soon as possible after the earthworks are finished.
  • The runoff does not contain a lot of silt.
  • There are unlikely to be significant effects on water quality.

If the earthworks extend over a larger area (1ha to 8ha) they are still permitted if the volume of disturbed material is less than 24,000 cubic metres and work does not place in winter (1 May to 31 October).

If you can’t meet these requirements, talk to a consents officer on 0800 736 222 or email consents@trc.govt.nz. You should also check whether your District Council has requirements relating to earthworks.

Diverting rivers & streams

You won’t need a resource consent to realign or modify a stream or river if these conditions are met:

  • The upstream catchment is no more than 25ha.
  • The modification or realignment does not restrict fish moment or have adverse effects on aquatic life or instream habitat.
  • The work doesn’t result in significant erosion, scouring or deposition.
  • The drainage channel cross section (depth times width) does not exceed four square metres.
  • The length of stream being modified does not exceed 200m (may include previous realignments since October 2001).

The Council must be informed at least two days before work begins.

If these conditions can’t be met, you’lll need to apply for a resource consent. The Council will take into account environmental, physical and cultural considerations.