Stormwater Drainage for Sloped Land

Kelso Grove

Stormwater Drainage for Sloped Land


New Zealand’s landscape is dominated by hills, gullies and mountain ranges, which is why developing stormwater drainage solutions for sloping land is a common challenge.

Land on an incline is easily eroded by heavy rainfall and fastflowing stormwater, but the right drainage solution can help manage flows and minimise damage.

Identifying the right drainage solution for sloped land and limiting erosion is important for health and safety, access and ecological reasons. Without the right drainage, erosion can lead to flooding, it can block roads and cause access issues, and it can wash away the nutrient-dense topsoil needed to support healthy plant growth.

Once soil erosion takes place in an area, it is more likely to reoccur – so getting your drainage right the first time is the best protection.



Typically, any slope of more than 5% can be considered a steep slope. Developing a drainage solution for sites which necessitate gradients above 5% requires proper assessment of the site.

Spending time on planning and assessment will also ensure the installation phase of your project runs smoothly – and can be delivered on time and on budget.

Depending on the scale of your project you may need to engage geotech and engineering consultants. As a first step, you can talk to your local drainage experts and get initial guidance around your options. Talk to one of the P&F Global team today.

Recommended steps for this first phase are:

  • Performing a geohazard assessment
  • Developing site-specific plans
  • Identifying any water features, such as streams, wetlands, lakes, or reservoirs, that are within your site boundaries or connected to it, and could contribute to water flows in heavy rain events
  • Identifying the requirement for any civil or geotechnical mitigation measures
  • Planning for site-specific reclamation and revegetation strategies

During this planning phase, another consideration to keep in mind is:

  • Evaluating the track record of any suppliers and contractors. If you’re unsure of your options, seek advice from your local drainage experts.



The two most important considerations in selecting culvert pipe for your project are:

  • meeting the necessary regulatory requirements
  • determining which pipe is best suited to your drainage needs: the right culvert should be fit for purpose now, and should have the capacity to meet your drainage needs for the medium to long-term.


Your local council and/or water authorities will have their engineering standard list and criteria that will need to be factored into any decisions about which culvert pipe to install. Some councils and water authorities are still defaulting to traditional pipe choices (such as concrete pipes) for civil projects, but will look favourably at other fit-for-purpose options if those options meet their technical and certification criteria.

An example of an alternative to concrete pipe that meets all the specifications and criteria is EUROFLO SN8 twin wall culvert pipe. It has already been used in major civil projects around New Zealand including by Waka Kotahi NZ Transport Agency (read the full SH6 case study below) and it meets NZ Standards:

  • EUROFLO pipe is designed in New Zealand, manufactured in Europe, and the civil grade SN8-rated pipe is AS/NZS5065 certified for 250mm OD to 1200mm OD. It is suitable for gravity drainage.
  • It also holds certification to meet the AS/NZS4404 Land Development and Subdivision Infrastructure Standard, the AS/ NZS2566 Buried Flexible Pipelines Standard, and performs satisfactorily according to the HO-HN-72 NZTA Bridge Design Load Capacity certification.


The reduced weight of HDPE pipe options like EUROFLO make it much easier and safer to manoeuvre on sloped sites and in challenging site conditions while not compromising on strength.

Pipeline comparison for 10 metres

  • Concrete Class 4 – 1150mm OD/1050mm ID
    Requires 4 pipes @ 2500kg ea, Total weight 10,000kg
  • Euroflo SN8 – 1200mm OD/1030mm ID
    Requires 2 pipes @305kg ea



Getting your drainage solution right for your current needs is important, but you also want a solution that meets your future needs and minimises any requirement for maintenance or repairs.

EUROFLO HDPE pipe has an expected design life of 100 years.

The strong, flexible qualities of HDPE pipe also deliver superior in-ground performance, connect easily to existing infrastructure and provide higher resilience to ground movement – a critical factor in a seismically active country where earthquakes have caused significant damage to concrete pipes in the past.


Cost is another important consideration. When comparing pipe pricing, remember to factor in the full product costs including installation and working life.

The ease and speed of installation of an HDPE pipe versus concrete can cutdown implementation costs by as much as 30-60% (see featured case study).

Faster installation results in reduced traffic management, reduced road closures, reduced freight costs, reduction or elimination of machinery hire, reduced labour – and lower overall costs.

Alongside the costs savings, faster installation results in reduced public disruption, and the ease of installation and reduced machinery requirements lead to reduced risks for contractors on site and increased safety across whole projects.

Choosing a durable, flexible pipe with an expected design life of 100 years will also create significant medium and longterm savings.


  • Length
  • Size range
  • Weight
  • Flow rates and velocities
  • Friction loss
  • Abrasive resistance
  • Durability
  • Ease of installation – this is even more critical with sloped sites: is it easy to manoeuvre/ lightweight enough to
    handle in challenging site conditions?
  • Ease of connection to manholes, head walls and existing infrastructure
  • UV stability (in above ground application)
  • Strength/durability
  • Life expectancy
  • Efficient jointing process
  • Ground stability
  • Is the pipe rigid or flexible?
  • Suitability for ground conditions e.g. soil acidity, ground movement, steep grade
  • Smooth exterior wall or ribbed exterior wall
  • Smooth interior wall lining
  • Pipe load rating
  • Availability of stock
  • Does the pipe meet NZ standards and approvals?



During the installation phase, considerations to keep in mind are:

  • Minimising disturbance to the land and damage to existing vegetation
  • Minimising construction traffic on site (this can be supported by lighter weight pipe choices such as (HDPE)
    pipe, where less heavy machinery is required for installation)
  • Minimising stockpiling and storage areas on site during the project (this can be supported by choosing rapid install pipe options like EUROFLO)
  • Minimising disruption to surrounding towns, roads, houses and residents (again, this can be supported by
    choosing rapid-install pipe options like EUROFLO)
  • At the conclusion of the installation, maximising soil restoration to restore permeability.

Taking the time to identify the right products and materials, ensure compliance with engineering standards, and to follow good installation practices, will guarantee that you develop a drainage solution that is fit for purpose now and in the future.

Download a copy of the white paper here