Most developments, big or small, require some level of earthworks. That might be scraping the grass away and digging foundations, putting in a retaining wall or larger cut and fill operations. In all cases you need to think about where all the dirt (“silt and sediment”) will go, and how you’ll manage it.
Why should we manage sediment??
We value a number of things about our natural environment. New Zealand has a variety of freshwater fish species and some are found only in New Zealand. These species need to be able to breath effectively and be able to move freely within waterways in order to reproduce. New Zealanders and visitors to our country also value swimming and recreation in our rivers and beaches; and simply visiting and viewing natural environments. We also take water from rivers for water supply and irrigation. Its important that we minimise the amount of sediment that goes into waterways, to maintain the quality of the water.
What is erosion and sedimentation?
Erosion is a process where soil or sand is eroded by wind or water and transported to another location. Rainfall, soil type, slope of the land and how much land is disturbed all influence how much erosion occurs. Sedimentation is where soil or sand particles suspended in water drop out and settle on land. Erosion and sedimentation can change the shape of watercourses and wetlands and affect the health of streams and aquatic life. They can also affect the operation of infrastructure and block pipes, leading to flooding. It can even be a hazard to vehicles if dirt is left on the road.
What is erosion and sediment control?
Sediment and erosion control are processes or devices that reduce the impact of erosion and sedimentation. It’s important to minimise the amount of dirt that leaves a property. This can be achieved by working in stages and leaving the remainder of the property in grass or vegetation as long as possible; and quickly stabilising (such as regrassing) the area after the earthworks are complete. Diverting clean water around the work area and a stabilised vehicle entry (often metalled) are also important techniques.
However, some dirt will always leave the property and that is where sediment controls come into play. Typical sediment controls on residential developments include silt fences and filter socks. A silt fence is a temporary barrier of woven geotextile fabric and is used to intercept run off, reduce its speed and impound sediment laden run off. The purpose of a filter sock is to intercept, filter and contain 'dirty water' on earthwork sites.
It is important that silt fences are dug into the ground properly and it is a common sight to see these flapping in the wind and letting sediment through. Installed properly, they can be really effective at capturing sediment. For bigger developments, special ponds are used. These need to be designed by your Engineer. There are also a number of other techniques for larger sites, but these aren’t necessary for smaller developments.
Any controls need to be maintained often so that they continue to work properly. Bad weather will affect how well the controls work and they should be checked after heavy rain and additional controls installed prior to bad weather if necessary.
What do I need to do on my small building site?
If you do have a resource consent for earthworks, Council will require erosion and sediment controls to be put in place as a condition of resource consent and often consent conditions will also require your contractors to attend a meeting before works begin where Council staff will check the controls. Even where resource consent is not required for a development, District Plans often still requires silt and sediment control. In Auckland you’re required to comply with Guidance Document 05 (“GD05”) which outlines best practice for erosion and sediment control. You can access this document online.
Who is responsible for erosion and sediment control?
Ultimately, you as the property owner are responsible for any runoff from development on your property as well as any people doing work on your property. Council can also take enforcement action if suitable controls are not in place and clean up may even be more expensive than installing the correct controls in the first place.
Need some help?
Please contact us if you need resource consent for earthworks or need to prepare an Erosion and Sediment Control Plan as part of your resource consent conditions. We can prepare your resource consent application and we have a network of contacts to advise us on specific sediment and erosion control matters.
Disclaimer As with all our blogs, the information detailed here is general in nature and meant as a preliminary guide only. This should not be substituted for your own investigations or use of your own professional’s. Planning Plus is not liable for any errors or omissions.
Jo Michalakis is a Senior Planner at Planning Plus and has over 14 years of resource management experience. Jo has a wide range of experience, including public and private sectors and has worked at both Auckland Council and Gisborne District Council.
Her planning work experience includes residential, commercial, infrastructure, natural environment, heritage and community projects throughout the North Island of New Zealand. She has experience in policy development, preparation of assessments of environmental effects and processing resource consents.