Stormwater control on building sites

By - , Build 109

Putting stormwater control systems in place early in a project can save significant time and costs, and prevent rework. How much you have to do will depend on your site.

What can happen in building sites – here, the neighbour’s retaining wall foundations have been exposed, and sediment-laden water has flowed from the site.
Stormwater has been redirected from the neighbour’s subsoil field drain clear of the building site.

Uncontrolled stormwater from or across a building site can be a nuisance, or even dangerous, to those on site and on adjoining property. It can make surfaces slippery, block council drains, or cause earth and cut faces to slip, damaging completed work or nearby property. Contaminated or muddy, silt-laden stormwater should not be allowed to leave a site without having some form of treatment to remove solids and get the water as clean as possible.

Builders and designers take responsibility

Responsibility for controlling site stormwater and for any damage it causes ultimately lies with the property owner. But most owners rely on their building and design professionals to adequately design and manage a site to ensure that stormwater does not cause any problems.

New Zealand Building Code Clause E1 Surface water provides for the control of stormwater during siteworks, in particular E1.2 and E1.3.1. In addition, Clause F5 Construction and demolition hazards applies. There appear to be no Acceptable Solutions that relate specifically to the short-term needs of site works.

In addition to Building Code requirements, stormwater control is often a condition of resource consents or local council bylaw consents and/or permits. There are also common law principles regarding the retention and protection of adjoining properties.

Stormwater control starts with design

The effective control of stormwater starts at the design stage. The proposed stormwater control system should be part of the building consent documentation. With clever planning and design, any work undertaken and materials used to catch site water during construction can easily be used for the completed stormwater drainage system.

The work needed to control stormwater will depend on the scope of the work being undertaken, the amount of time the site will be disturbed, the season, the land contours, across-land flow paths and the site’s location relative to the road. If there is a natural water flow on or adjacent to the site, specific care needs to be taken to avoid damage to the waterway.

During the planning stage of a project, you should contact your local and regional councils. They may have specific requirements that need to be considered when establishing budgets and work programmes. It may also help to talk to the neighbours.

Stormwater has been redirected from the neighbour’s subsoil field drain clear of the building site.

Minimising stormwater effects on site

There are some basic steps that can be taken to minimise the effects of stormwater on a site. These include:

  • checking council records or the project information memorandum to see if there are any existing stormwater systems on site
  • limiting the amount of natural ground that is disturbed during site works
  • controlling vehicles on site
  • creating a working platform with site concrete or compacted basecourse
  • placing building materials (framing, bricks, fill, top soil) close to where they will be used
  • covering building materials in plastic or polythene
  • covering cut faces and banks with polythene/tarpaulins (in particular, until retaining walls are constructed and ready to be backfilled) before rain starts
  • checking adjoining properties and roads to ensure that there is no stormwater being directed onto the site
  • listening to weather forecasts.

Stopping water from leaving the site

Depending on the site and the scope of work, greater steps may need to be taken to stop contaminated or silt-laden water leaving the site. This could include:

  • laying perforated drainage pipe in a filtering sock in trenches and directing to a suitable sump
  • early installation of the stormwater systems
  • properly designed silt ponds/traps
  • installing appropriately placed sumps
  • backfilling retaining walls with free draining material, perforated pipe and filter cloth as soon as possible.

In the absence of any specific council requirements, typical sump details can be found in Figures 8 and 9 of Building Code E1/AS1. The type of sump used will depend on the size of the site (the Acceptable Solution has some limitations) and the amount of expected rainfall.

What can happen in building sites – here, the neighbour’s retaining wall foundations have been exposed, and sediment-laden water has flowed from the site.

On larger sites, it may be necessary to create storage ponds that act as sumps. Such requirements are usually controlled through the District Plan or resource consents rather than the Building Code. In many cases, the design of this type of control system will require the input of a suitably qualified engineer.

Keeping the water clean

There are several ways to keep site stormwater relatively clean, and it is preferable to have the plan in place before rain starts:

  • Lay site concrete with a fall to one point where a sump can be installed so water can be collected and disposed of easily.
  • Turn up the bottom of tarpaulins/polythene when laying them over slopes. This will form a channel to stop water from spilling onto the ground. Water can then be directed to an onsite sump and discharged to an appropriate outfall.
  • Ensure perforated pipes in trenches are protected with an appropriate filter cloth to stop large particles entering the pipe. This pipe can be laid to fall to the sump.

Where site stormwater creates a nuisance, councils have several enforcement options available through their bylaws, the Building Act and the Resource Management Act. A little careful planning and common sense can avoid receiving fines or having to repair council services or reinstate adjoining property.

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Articles are correct at the time of publication but may have since become outdated.

What can happen in building sites – here, the neighbour’s retaining wall foundations have been exposed, and sediment-laden water has flowed from the site.
Stormwater has been redirected from the neighbour’s subsoil field drain clear of the building site.

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